School Science Lessons
Physics Experiments - Electricity in motor vehicles
Updated: 2008-07-16
Please send comments to: J.Elfick@uq.edu.au
See also: Appendix Physics 40.9 Tools and equipment for electrical experiments
See also: 32.2.00 Circuit diagrams, electrical symbols
See also: Electrical hazards
See also: Electronics topics
See also: Interesting websites
Table of contents
32.5.0.0 Electrical equipment of motor vehicles
32.5.1.0 Motor vehicle circuits, series and parallel circuits, electric power
32.5.2.0 Motor vehicle wiring system, circuit diagrams
32.5.3.0 Motor vehicle battery
32.5.4.0 Motor vehicle electromagnets
32.5.5.0 Motor vehicle ignition system, electromagnetic induction
32.5.6.0 Motor vehicle generator (dynamo), charging system
32.5.7.0 Motor vehicle starter motor
32.5.8.0 Motor vehicle lighting systems and equipment
32.5.9.0 Motor vehicle auxiliary units, accessories, wiper, horn, fuel pump, gauges
32.5.1.0 Motor vehicle circuits, series and parallel circuits, electric power
32.5.1.1 Series circuits
32.5.1.2 Parallel circuits
32.5.1.3 Current through a bulb
32.5.1.4 Electric power
32.5.2.0 Motor vehicle wiring system, circuit diagrams
32.5.2.1 Wiring system diagrams
32.5.2.2 Technical and physical diagrams, ignition system
32.5.2.3 Technical and physical diagrams (a) ignition coil (b) fuel gauge
32.5.2.4 Wires, Lucas cable colour system
32.5.2.5 Fuses
32.5.2.6 Thermal circuit breaker
32.5.2.7 Electromagnetic circuit breaker
32.5.3.0 Motor vehicle battery
33.4.0 Dry cell
32.5.3.2 Lead acid battery secondary cell
3.87 Lead accumulator cell
32.5.3.3 Discharging and charging
32.5.3.4 Cell connections
32.5.3.5 Measure the state of charge of a battery
32.5.3.6 Mixing battery electrolyte
32.5.3.7 Capacity of a battery
32.5.4.0 Motor vehicle electromagnets
32.5.4.1 Magnetic fields and electromagnetism
32.5.4.2 Simple relay, horn relay
32.5.4.3 Cut-out
32.5.4.4 Electric Bell
32.5.4.5 Voltage Regulator
4.78 Cylindrical electromagnet
4.79 Horseshoe electromagnet
4.80 Test the strength of electromagnets
4.81 Magnetic field from electric current in a wire
4.82 Magnetic field inside an open coil, open solenoid
4.83 Electricity from a magnet and a coil
4.84 Make a simple electric motor
32.5.5.0 Motor vehicle ignition system, electromagnet induction,
32.5.5.1 Ignition system, primary circuit
32.5.5.2 Primary and secondary circuits
32.5.5.3 Self-induction, condenser (capacitor), principle of coil operation, ignition coil
32.5.5.4 Four cylinder ignition circuit
32.5.5.5 Spark plug, spark plug structure
32.5.5.5.1 Spark plug operating temperature, pre-ignition
32.5.5.5.2 Spark plug gap
32.5.5.6 Magneto, permanent magnet magneto, rotating magnet magneto, polar inductor magneto
32.5.5.7 Transistorized ignition system
32.5.6.0 Motor vehicle generator (dynamo), charging system
32.5.6.1 Generators
32.5.6.2 Alternating current, Graph of alternating current generated by an alternator
32.5.6.3 Conversion of alternating current to direct current
32.5.6.4 Voltage of a generator, field magnets, self-excitation
32.5.6.5 Voltage regulator
32.5.6.6 Current regulator, separate unit current regulator, compensated voltage control current regulator, testing current regulators
32.5.6.7 Motor vehicle alternator
32.5.6.8 Silicon diode rectifier, half-wave rectifier, full-wave rectifier, single-phase rectifier, three-phase rectifier
32.5.7.0 Motor vehicle starter motor, starters
32.5.7.1 Principle of the electric motor, shunt-wound motor, series-wound motor
32.5.7.2 Starter motor, starter circuit, compound-wound starter
32.5.7.3 Starter drives
32.5.8.0 Motor vehicle lighting systems and equipment
32.5.8.1 Sources of light
32.5.8.2 Headlamps, side lamps and tail lamps
32.5.9.0 Motor vehicle auxiliary units, accessories, windscreen wiper, horn, fuel pump, gauges
32.5.9.1 Motor vehicle horn, high frequency horn, wind tone horn
32.5.9.2 Flashing lamp direction indicators
32.5.9.3 Electric fuel pump
32.5.9.4 Balancing coil type fuel gauge
32.5.9.5 Water temperature gauge
32.5.0 Electrical equipment of motor vehicles
Components: 1. Generator (d.c.) or alternator as a source of electrical energy, with voltage regulator and current regulator, 2. Ignition and lighting switches, 3. Ignition coil, 4. Battery, 5. Current consuming appliances, e.g. light bulbs, horn, starter, 6. Cables and wire connectors.
32.5.1.0 Motor vehicle circuits, series and parallel circuits, electric power
In a motor vehicle the metal framework is used as a conductor for the earth return system where one terminal of the battery is earthed to the body of the vehicle. To form a circuit, the other terminal of the battery is connected to a component then to some metal part of the vehicle as earth.
32.5.1.1 Series circuits
See diagram 32.5.1.1
Series circuits are used for the battery, generator, ammeter, and circuits with a fuse. Series circuits are never used in lighting circuits because one burnt out lamp filament, or a break in a wire, would open the circuit and no lights in that circuit would function. Also, if two or more components of a circuit are connected in series, only the last component in the series can be earthed, so all other components must be insulated from the metal of the vehicle. If three resistances, 3 ohms, 6 ohms and 9 ohms are connected in series to a 12 volt battery, total resistance = 3 + 6 + 9 = 18 ohms, V = IR, I = V / R, so total current drawn from battery = V / R = 12 / 18 = 0.67 amps.
32.5.1.2 Parallel circuits
See diagram 32.5.1.2
The parallel circuits are used mostly because and the operation of one circuit does not affect the operation of any other parallel circuit. Also any circuit in parallel may be switched on or off without affecting the operation of any other circuit and a fault in any circuit need not affect another circuit. If three resistances, 3 ohms, 6 ohms and 9 ohms are connected in parallel to a 12 volt battery, V = IR, I = V / R, so current through 3 ohms resistance = 12 / 3 = 4 amps, current through 6 ohms resistance = 12 / 6 = 2 amps, current through 9 ohms resistance = 12 / 9 = 1.33 amps. Total current drawn from battery = 4 + 2 + 1.33 = 7.33 amps.
32.5.1.3 Current through a bulb
See also 7.10: Current, ampere
See diagram 32.5.1.3
All electrical devices consume a certain amount of energy per second and are rated in units of power, the watt, e.g. a lamp needs 42 watts, a radio needs 12 watts. Power = amperes x volts, I x V. The bulbs used in the lighting system of the motor vehicle are stamped with both the voltage and the power. The current flowing through a 12 volt 42 watt headlamp bulb = 42 / 12 = 3.5 amps.
32.5.1.4 Electric power
See diagram 32.5.1.4
The total power consumed = the sum of the powers consumed by each appliance. If two 42 watt headlamp bulbs, two 6 watt parking lamp bulbs, and three 6 watt tail lamp bulbs are connected in parallel to a 12 volt battery, the total power consumed = 42 + 42 + 6 + 6 + 6 + 6 + 6 = 114 watts and the total current drawn from the battery = 114 / 12 (I = W / V) = 9.5 amperes. The resistance of all the electrical connections in a motor vehicle should be as low as possible because the initial voltage from the battery is very low. The British unit 1 horsepower, 1 hp = 746 watts.
32.5.2.0 Motor vehicle wiring system, circuit diagrams
Lucas cable colour system, automotive cables, circuit protection fuses and circuit breakers
32.5.2.1 Wiring system diagrams
See diagram 32.5.2.1
As each unit in the electrical system must have its own circuit and means of control, all circuits are connected in parallel with the battery and have a separate switch. However most circuits share a common insulated conductor and common earth return circuit, e.g. a single large diameter cable connects the "insulated" terminal of the battery to the starter switch, and forms a part of every other circuit. The "live" terminal of the starter switch forms the first junction of parallel circuits, e.g. horn circuit, headlamp relay, car radio.
Wiring system circuit list S = switch (A) = optional ammeter. Horn circuit and lighting system do not require ignition system to be switched on.

Starter system, circuit 1 earth - battery starter S1 starter earth - - -
Horn, circuit 2 - starter S2 horn button earth - -
Charging system, circuit 3 - starter S(A)3 cut-out and
regulator 1		 earth - - -
- - - cut-out and
regulator 2		 generator earth - -
Lighting system, circuit 4 - starter S(A)4 light S1 tail light earth - -
- - - light S2 park light earth - -
- - - light S3 dip S1 dip full earth -
- - - - dip S2 dip full earth -
Ignition system, circuit 5 - starter S(A)5 ignition S1 fuel gauge earth - -
- - - ignition S2 fuel tank earth - -
- - - ignition S3 ignition coil distributor 1 earth -
- - - - - distributor 2 4 or 6 spark plugs 4 or 6 earth
- - - ignition S4 heater, demister - - -
Additional or optional circuits - - - - - - -
32.5.2.2 Technical and physical diagrams, ignition system See diagram 32.5.2.2 (a) (b)
32.5.2.3 Technical and physical diagrams
See diagram 32.5.2.3: (a) Ignition coil (b) Fuel gauge
Units such as the fuel gauge, horn button, and all the lamps have an earth wire connection. Units such as spark plugs, starter, generator, and fuel gauge tank unit do not have an earth wire because they are attached to metal parts of the vehicle. Current passes through all units to return to the battery through the metal parts of the vehicle. The engine and metal body are joined by a strap that conducts electricity.
32.5.2.4 Wires
See also: Lucas cable colour system in English cars | See diagram 32.5.2.4
Wires have distinguishing colours or number tags. Wires that run in the same direction are grouped together in a sheath of cotton or plastic. The groups of wires are linked by plug-in connectors and terminal blocks to keep the wiring compact. Colour systems are used to identify individual wires. Copper is used for coil windings and connecting cables because of its low electrical resistance, ease of working, and durability. Single, round copper wire is used in the windings. For connecting wires, stranded cables are used that are made of tinned copper and insulated with polyvinyl plastics braided with cotton. The outer covering of stranded cables may have different colours. The type of cable depends on the voltage drop along the length used. The current carrying capacity depends on the wire gauge. In Great Britain the standard wire gauge (S.W.G.) and the Birmingham wire gauge (B.W.G.) is used. In USA the Brown and Sharpe gauge (B. and S.) (American wire gauge) is used. The S.W.G. size refers to a single strand in a cable. Tables of cable specifications give information of number of strands, S.W.G. size of a strand, current capacity in amps, and suitability, e.g. general circuits, heavy duty starters. Cables must carry the maximum current likely to flow in that circuit without overheating. If a defect occurs in the insulation of a cable, current from the battery might bypass the normal circuit path to take a path of lower resistance for its return to the earthed terminal of the battery, i.e. a short circuit may occur. The current in this lower resistance short circuit is greater than in the normal circuit and may cause damage by overheating.
32.5.2.5 Fuses
See diagram 32.5.2.5
A fuse is a wire or strip of metal mounted in a fitting which melts, i.e. blows, when a certain maximum current passes through it, thus opening the circuit. Fuses in the automotive electrical systems are enclosed in glass tubes to reduce the risk of fire if fuse burns out. The fuse rating specified for a circuit is about three times the normal maximum current. However, headlamps and ignition are not protected by fuses because the sudden darkness would be a greater danger than overheated cables.
32.5.2.6 Thermal circuit breaker
See diagram 32.5.2.6
The thermal circuit breaker uses the heating effect of an electric current to cause a bimetallic arm to bend and open a pair of contacts. When the arm is heated by excess flow of a current it bends, opens the contacts, and breaks the circuit.
32.5.2.7 Electromagnetic circuit breaker
See diagram 32.5.2.7
The electromagnetic circuit breaker uses the magnetic effect of an electric current to open a pair of contacts. Current flowing through the series winding sets up a magnetic effect in the iron core. The armature is attracted towards the core, causing the push rods to push against the spring and open the contacts and break the circuit.
32.5.3.0 Motor vehicle battery
32.5.3.1 Primary cell | Dry cell
32.5.3.2 Lead acid battery secondary cell
See diagram 32.5.3.1 | See also: 33.4.1
The lead acid battery is a group of two or more electric cells connected in series. A 12 volt battery has six 2 volts cells. A 6 volt battery has three 2 volt cells. Secondary cells, which can be recharged many times are also called storage cells or accumulators. The chemical action can be reversed by passing a current through the cell in the direction opposite to the discharge current until the chemicals have been changed back to their original form, the cell is charged again. The 6 volt battery has three identical lead acid cells connected in series, so their voltage is added. The current is the same in all cells. Charging or discharging current must pass through all the cells in the battery. The positive and negative plates correspond to the positive and negative electrodes of a primary cell. These thin plates expose a large surface to the action of the electrolyte. The plates are constructed from a lead antimony grid filled with lead (IV) oxide, PbO, in the positive plate and a spongy form of pure grey lead (Pb) in the negative plate. Separators are insulators to prevent contact between the plates and allow circulation of the electrolyte, a dilute solution of sulfuric acid in water. The order of plates within the cell is negative plate, separator, positive plate, separator with the final plate in the series a negative plate. The voltage (EMF) of a lead acid cell varies from 2.3 volts when the cell is fully charged to 1.9 volts, when the cell is fully discharged. A voltmeter connected to the battery terminals of a discarded battery containing some liquid will still show some reading. When a lead cell accumulator is fully charged, the concentration of sulfuric acid is at maximum. When the accumulator is fully discharged, "flat battery", the concentration of sulfuric acid is at a minimum. A battery hydrometer is used to read the relative density (specific gravity) of the sulfuric acid in the electrolyte and check the charge of the battery. The density varies from about 1.28 in a fully charged battery to 1.15 in a discharged battery. The density of sulfuric acid purchased for use in accumulators is about 1.25 at 20oC.
32.5.3.3 Discharging and charging
Discharging --->
PbO, + 2H2SO4 + Pb ---> 2PbSO4 + 2H2O
<--- Charging
Lead (IV) oxide + sulfuric acid + lead ---> lead sulfate + water
When the battery is discharging, Lead (IV) oxide in the positive plate combines with sulfuric acid (sulfuric acid) in the electrolyte to form lead sulfate (lead sulfate) and water. Lead in the negative plate combines with sulfuric acid to form lead sulfate and water. How much lead sulfate is produced is directly proportional to how much current flows. The electrolyte not only takes part in the conversion of the stored chemical energy to an electric current, but also provides a low resistance path for the current through the cell. The battery is discharged when not enough sulfuric acid is left in the electrolyte for effective chemical action and most of the active materials, lead (IV) oxide and lead, in both sets of plates have been converted into lead sulfate. During discharge the electrolyte becomes weaker as the acid combines with the plates. The lead sulfate that has formed fills the pores of the plates so circulation of the electrolyte decreases and the voltage of the battery drops. The battery is fully discharged when the electrolyte cannot reach the remaining active material of the plates and react with it fast enough to maintain the working voltage and produce an effective current. When the battery is charging, current passes through the battery in the reverse direction to the flow on discharge to reverse the chemical reactions and reform the dark brown lead (IV) oxide on the positive plates and spongy lead on the negative plates. When the battery is fully charged, the specific gravity of the electrolyte is restored to its original value. If electricity is passed through the battery after it is fully charged then more electrical energy is being supplied to the battery than can be converted to chemical energy. The excess electricity decomposes the water in the electrolyte to form hydrogen and oxygen. This is called gassing.
32.5.3.4 Cell connections
See diagram 32.5.3.4
The lead acid cell has a voltage of two volts when in average working condition. Each cell has two terminals, the positive terminal and the negative terminal. Inside the cell, the positive terminal is connected to the positive plates and the negative terminal to the negative plates. The direction of conventional current flow is from the positive terminal to the negative terminal in the circuit outside the cell. The movement of current carrying particles, i.e. electrons, is in the opposite direction. In most of the electrical devices on motor vehicles it does not matter which way the current flows. In most motor vehicles the negative terminal of the battery is connected to earth, so the current flows from the insulated live side of the circuit down through the appliance to earth. However in motor vehicles using alternators and rectifiers a wrong connection can cause damage. A 6 volt battery has three 2 volt cells connected in series. To simplify connection, the centre cell is placed in the container the opposite way round to the end cells. Two complete 6 volt batteries form a 12 volt battery. Two batteries may be connected in parallel for heavy duty operation. Two identical batteries connected in parallel give the voltage of only one battery, but their capacity is double the capacity of each. An extra battery may be connected in parallel with the battery in a vehicle to start the engine. If a battery is put on charge with its negative terminal connected to the positive terminal of the charger, the battery may be badly damaged by reversal of the polarity of the plates. If the name plate is facing the observer, the positive terminal will be at the front right hand corner. Reverse assembly - the positive terminal will be at the front left hand corner. The polarity of each individual cell can be found with a moving coil permanent magnet voltmeter with its terminals marked. The positive post may be painted red, or becomes a dark chocolate colour after a very short period of use. The negative terminal always looks cleaner and remains light grey in colour. Each inter-cell connector becomes discoloured in the same way, one end dark brown and the other end light grey. The dark brown end is connected to the positive terminal of a cell, and the light grey end is connected to the negative terminal of the adjacent cell. A lead acid battery may self-discharge at the rate of 1% of its capacity per day.
32.5.3.5 Measure the state of charge of a battery
See diagram 32.5.3.1 (d)
Maintenance free batteries supplied to some modern motor vehicles under normal operation conditions will not need water addition within its average battery life. The vent caps remain sealed and the electrolyte cannot be tested as explained below. However the vent caps are accessible for inspection of fluid level in the battery if required when using the battery under abnormal operating conditions. The specific gravity, SG, of the electrolyte in each cell of a battery shows the battery's state of charge: Fully charged, SG = 1.280, Three quarter charged, SG = 1.240, Half charged, SG = 1.200, Quarter charged, SG = 1.160, Discharged, SG = 1.120. For battery testing, the hydrometer floats in a syringe. The electrolyte is drawn into a glass tube so that level can be observed in relation to the scale marked on the narrow stem of the hydrometer. Always take readings at eye level, i.e. your line of sight must be horizontal. The battery hydrometer indicates the state of charge of each cell. The electrolyte expands when heated and contracts when cooled so a reference temperature standard is needed. The state of charge of a battery is not a true indication of its internal condition if the battery has any bulging, cracking, leakage of electrolyte, or lifting of the cell covers at the positive post end. Check that you have enough electrolyte above the plates to make a hydrometer test then read the specific gravity of the electrolyte in each cell. If any reading is below 1.225, the battery has been recharged following the manufacturer's instructions. A low reading in one or more cells after the battery has been recharged indicates internal trouble. Test the ability of the battery to do the work required of it by drawing the normal starter motor current and measuring the voltage of the individual cells while they are discharging at a high rate with an accurate voltmeter. No cell should read less than 1.5 volts. As the cells of a battery are connected in series, each receives the same charging current and the same amount of current also flows through each on discharge. If the cell voltages do not fall below 1.5 volts while the starter is being operated and the specific gravity readings are at least 1.250, the battery is in good condition.
32.5.3.6 Mixing battery electrolyte
The electrolyte used in lead acid batteries is a solution of chemically pure sulfuric acid in deionized water. The efficiency and ultimate life of a battery depends on the purity of the electrolyte. Always use deionized water or, demineralized water or rainwater to maintain the level of electrolyte in the cells, topping up. Do not add tap water or water that has been in contact with metals, especially iron, because impurities cause secondary chemical reactions and the battery can self-discharge. Do not fill above an electrolyte marker or the top of the splash guards, just fill to cover the separators that extend upwards above the top edges of the plates. The electrolyte should be able to expand and rise when warmed without spilling out through the vent holes in the filler plugs. Sulfuric acid is highly corrosive and if spilt on the skin must be washed off immediately with plenty of water from a running tap. Apply baking soda solution to the affected area then wash off with water. Charge the battery at the normal rate until the specific gravity stops rising and the cells are gassing. If the SG is too high, draw off some electrolyte with the battery hydrometer syringe and replace it with deionized water or demineralized water or rain water. If the SG is too low, draw off some electrolyte and replace it with 1.300 specific gravity acid. Charge the battery for a further two hours after any replacement to mix the electrolyte thoroughly before taking another reading. Use a clean glass container for mixing the electrolyte. Keep a labelled jar of baking soda solution near by in case of spills on the skin. Put the water required into the container first then pour the acid slowly into the water while stirring.
BE CAREFUL! REMEMBER - ACID INTO WATER, NEVER WATER INTO ACID!
Let the electrolyte cool to room temperature before taking the final specific gravity reading. Keep the battery and its surrounding parts clean and dry to avoid corrosion of metal parts. Overcharging may cause loosening and shedding of the active material from the plate grids. The hot electrolyte may attack the separators and the life of the battery will be very short. A battery is left for a long time in a partially discharged state is harder to recharge to its original capacity because the fine crystalline lead sulfate may harden and become more dense, the battery becomes "sulfated". To avoid sulfation the generator must run long enough to restore the chemical energy used to start the engine and operate the lights and accessories while the engine is idle. The internal resistance of the battery is higher when cold, and a short run may not raise the electrolyte temperature sufficiently to lower its resistance, so the voltage regulator operates earlier in the charging cycle, and at a lower specific gravity than it would do normally. Do not allow the electrolyte level to become too low to expose the plates to the air because oxygen in the air will combine with the spongy lead of the negative plate to form a layer of lead (IV) oxide.
32.5.3.7 Capacity of a battery
Capacity of a battery refers to the quantity of electricity it can deliver. The ampere-hour is the quantity of electricity that flows in one hour at the rate of one ampere. Capacity in ampere-hours = current in amperes x time in hours. A battery with a capacity of 50 ampere-hours can deliver a current of 5 amperes for 10 hours, or 10 amperes for five hours, after being fully charged at the beginning and completely discharging at the end. However, the capacity of a battery is not the same for different rates of discharge if the battery has already discharged within the rated time. Batteries can be rated by measuring the ability to supply sufficient current to operate the starter, or operate the lighting system. A common standard rating is the 10 hour rating. It is the steady discharge current, in amperes, to reduce the cell voltage to 1.8 in 10 hours at 15oC, with specific gravity of the electrolyte 1.120 when fully discharged. The full theoretical capacity of a battery cannot be used because the discharge current that must diffuse into the plates fast enough to replace the electrolyte used in the production of an electric current. Lead sulfate clogs the pores of the plates and restricts the movement of the electrolyte so that chemical action is reduced, the voltage generated by the cell is lowered, and the useful current is reduced. Also, the resistance of the materials on the plates increases as the cells are discharged and the resistance of the electrolyte increases as the battery discharges and the acid content of the electrolyte falls. Both a 6 volt 50 ampere-hour battery and a 12 volt 50 ampere-hour battery will deliver a current of 5 amperes for 10 hours, so the stated capacity of 5 X 10 = 50 ampere-hours. The advantage the 12 volt battery is that it has twice the watt-hour capacity of the 6 volt, i.e. it stores twice the amount of electrical energy. The 6 volt battery delivers its 5 amperes at 6 volts but the 12 volt battery delivers the same current for the same length of time at 12 volts. Power from 6 volt battery, W = V x I = 6 x 5 = 30 watts. Energy capacity of 6 volt battery = Power x time = Watts x hours = 30 x 10 = 300 watt-hours. Power from 12 volt battery, W = V x I = 12 x 5 = 60 watts. Energy capacity of 12 volt battery = power x time = Watts x hours = 60 x 10 = 600 watt-hours. Thus, watt-hours = ampere-hours x voltage. These calculations show that the watt-hour capacity is equal to the ampere-hour capacity multiplied by the battery voltage. Watt-hour capacity = ampere-hour capacity v battery voltage.
32.5.4.0 Electromagnets in motor vehicles
See: 29.2.0 Magnetic fields | See: 30.3.0 Electromagnets
32.5.4.1 Magnetic fields and electromagnetism
Electromagnets are used to close or open a set of contacts (a switch) in an electric circuit and to provide a magnetic field that can be controlled automatically or manually.
32.5.4.2 Simple relay, horn relay
See diagram 32.5.4.2
A relay uses an electromagnet and an armature in a circuit to open or close another circuit. The horn relay has a single soft iron core wound with many turns of thin insulated wire. One end of the coil is connected to the battery and the other end is connected to earth through the horn button. The core has a spring loaded armature above it. One contact is on the armature connected to the frame then down to the battery terminal. The other contact is on a fixed support connected to the horn terminal. When the horn button is pressed, current flows through the winding around the soft iron core, then to the earthed base of the relay, then to the earthed terminal of the battery. The armature is attracted towards the core and closes the contacts. So when the contacts close, the horn is connected directly to the battery. By using the relay system, the horn button wire has to carry only sufficient current to energize the relay winding so the horn button contacts remain in good condition. Also, voltage drop in the main horn circuit is reduced because of the shorter length of cable required.
32.5.4.3 Cut-out relay
See diagram 32.5.4.3
A cut-out is a relay used on all types of d.c. generators. It connects the generator to the battery when the engine is driving the generator fast enough to charge the battery, and to disconnect or "cut-out" the battery from the generator when the engine is stopped or run at a slow idle. If the engine is running at any speed above a fast idle, the generator voltage will be higher than that of the battery. The generator and the battery oppose each other and the generator. Above the core and separated from it by a specified air gap is a flat, soft iron armature that carries a contact point. When the generator is not operating or is running at a low speed, the armature is held away from the winding core by a spring. When the engine is started, current from the generator flows through the voltage winding of the cut-out, creating a weak magnetic field. When the generator speed is increased, the voltage across its terminals also increases, and more current flows through the voltage winding. This creates a stronger magnetic field in the core that attracts the movable armature above it. When the voltage reaches the value for which the relay is set, the magnetism is strong enough to overcome the armature-spring tension and the armature is pulled towards the soft iron core. The contact point on the moving armature contacts a stationary contact point, thus closing the circuit between the generator and battery. Current flows to the battery, passing through the current winding of the relay in the direction that adds to the strength of the magnetic field established by the voltage winding. When the generator slows sufficiently or stops, current begins to flow back from the battery to the generator. In doing so it must pass through the current winding but in the reverse direction. Thus the magnetic field produced by the current winding opposes that of the voltage winding, which always remains the same, and the resultant magnetic field is no longer strong enough to hold the points closed. The armature-spring tension pulls the armature away from the core, and the points separate, breaking the circuit between the battery and the generator.
32.5.4.4 Electric Bell
See diagram 32.5.4.4
A piece of soft iron A which projects above the wooden base of the bell. Rods of soft iron B and C are screwed into A. Coils of insulated wire wound on wooden bobbins are placed over these rods and connected so that the polarity of one rod is opposite to that of the other. E is another projection raised above the base. H is a flat steel spring. D is a flat bar of soft iron fixed to H and carrying the hammer that strikes the bell. F is a brass post insulated from the base and carrying a contact screw with a tungsten point on the end of it at K. A corresponding tungsten contact point is carried on an extension of the spring H. J is a brass bar insulated from the base. Connections to the battery are made at J and E through a bell push. When the bell push is pressed, the circuit is completed and current
flows through the two coils to F, through the contact points, and back to the battery through H and E. The current magnetizes A, B, C, and D so that B and C attract D, which moves towards B and C causing the hammer to strike the gong. Just when the contact carried by D is pulled away from the fixed contact carried by F, and the circuit is broken. The current stops, so that A, B, C, and D are demagnetized and the spring pulls D back. The contacts touch again and the whole process repeats itself rapidly, causing repeated ringing of the bell and sparking between the contacts. If the adjusting screw at F is too far in, the bell will not ring because the contacts will be unable to part and current will flow without a break. If the adjusting screw at F is too far out, the contacts will not touch in the rest position, so that when the bell push is pressed no current will flow and as there is no attraction of D. The bell will not ring.
32.5.4.5 Voltage Regulator
See diagram 32.5.6.5 (a)
The voltage regulator is a cut-out relay with the contacts are held closed by the spring and no current winding. It has the typical voltage winding in the electromagnet, which pulls the contacts open to control the generator voltage. The term voltage winding always refers to a winding of many turns of fine insulated wire. Such windings, usually carrying a current of about 1 / 8 ampere, are sensitive to changes in the applied voltage. A voltage winding is always connected in parallel with (across) the generator or battery. The voltage winding is connected across the generator terminals. The contacts are in series with the field circuit, not the main charging circuit. The spring holds the contacts closed. The contacts are bridged by a resistance.
32.5.5.0 Electromagnetic induction, motor vehicle circuits
Electromagnet induction, primary and secondary windings, self-induction, condenser (capacitor), construction of ignition coils, ignition for multicylinder engines, creation and timing of the spark, tracking, gas tightness, electrode projection and reach, hot and cold plugs, spark plug gap, plug fouling, spark plugs and magneto ignition, operating temperatures and engine performance, magnetos, transistorized ignition system (high tension leads, horn press, turn signal switch, headlamp switch, ignition switch, hand brake warning lamp switch, windscreen wiper switch, headlamps, front side marker lamp, turn signal lamp, front park lamp (distributor, cap, rotor arm, contact breaker points (breaker contacts) (adjustable contact))
32.5.5.1 Ignition system, primary circuit
See diagram 32.5.5.1
The function of the ignition system is to provide an electric spark to ignite the compressed mixture of fuel vapour and air in the cylinder combustion chamber. It must do this at the correct instant in the combustion cycle in each cylinder of the engine. Also, the ignition system must provide a means of varying the instant of firing to suit the different operating conditions of the engine.
The components include the following:
1. A source of voltage to supply the electrical energy necessary to produce an electric spark, such as a battery, generator, or alternator
2. A coil to convert the low voltage of the battery to the very high voltage necessary to produce a spark at the spark plug gap.
3. A make-and-break mechanism, commonly called a contact breaker, to interrupt the flow of current through the primary winding of the coil. A make-and-break is a device that alternatively closes (makes) and opens (breaks) an electric circuit.
4. A device, commonly called an automatic advance unit, for varying the ignition timing to suit the operating conditions of the engine.
5. A distributor, in engines having more than one cylinder, to direct the spark to the right spark plug at the correct instant.
6. A spark plug for each cylinder.
7. A switch to operate the ignition system for starting and stopping the engine.
The coil ignition system depends on electromagnetic induction from a voltage created by the relative movement between a conductor and a magnetic field. Whether the conductor is moved in a stationary field, or the field moved past a stationary conductor, the result is an induced voltage in the conductor. In the ignition coil, the conductor is stationary and the field moves. The first need in the ignition coil is, therefore, a magnetic field, and an electromagnet is used to obtain this. In chapter 4 it was explained that an electromagnet could be produced by passing a current through a coil of insulated wire. When the coil is wound on a soft iron core, the electromagnet becomes much stronger, without any alteration to the number of turns of wire or how much current flows. The next need is that the field must move, and this is accomplished by switching the current on and off by means of the contact breaker. The ignition switch is closed and a magnetic field surrounds the iron core of the electromagnet. The winding of the electromagnet is called the primary winding. When the contact breaker points close, current flows through the primary winding, the magnetic field grows outwards from the core until it is fully built up, and the core becomes fully magnetized. When the contact breaker points open, current stops flowing and the magnetic field collapses, moving inwards to the point of origin as it does so.
32.5.5.2 Primary and secondary circuits
See diagram 32.5.5.2
To provide a voltage of the very high value necessary for ignition systems, a special kind of step-up transformer, called an ignition coil, is used. A second or secondary winding is wound on the same core as the primary winding. The primary winding consists of a few hundred turns of thick insulated wire, whereas the secondary winding is made up of many thousands of turns of fine insulated wire. The high ratio of the number of secondary turns to the number of primary turns produces a high voltage in the secondary winding. A magnetic field forms around a wire or a coil of wire when current flows through them. The direction or polarity of the magnetic field depends on the direction of the current in the wire. The magnetic field about a single wire appears instantly but the field about a coil of wire takes time to reach its final value.
32.5.5.3 Self-induction
See diagram 32.5.5.3
Each turn of wire in a coil influences the adjacent turns. When the current begins to flow, the encircling magnetic field grows outwards from the wires to through each coil turn. The growth of the magnetic field generates a voltage that opposes the voltage of the battery. This counter voltage is overcome by the battery after the short time needed to establish full current flow in the primary winding. All coil windings have the property of self-induction that slows the establishment of a full current in a coil and a full magnetic field. When the contact breaker opens to stop the flow of current, the magnetic field of the primary winding collapses and generates a voltage that tends to keep the primary current flowing across the contact breaker gap as a destructive arc that could erode the contacts. However, arcing at the contacts can be prevented by connecting a condenser across, in parallel with, the contacts.
32.5.5.3.1 Condenser (capacitor)
The condenser is an electrical capacitor that can store electric charge. It is made of strips of metallized paper impregnated with an insulating substance, rolled up tightly, and fitted inside a metal cylinder. A terminal at one end of the condenser is connected to the moving contact. The metal cylinder, acting as the other terminal is connected to earth. The condenser can release its stored energy to a connected circuit. The condenser that is connected across the contacts of the contact breaker provides an alternative path for the current. It accepts and stores an electrical charge from the primary coil and later releases it back into the circuit. This stored energy would otherwise cause destructive arcing across the contacts. So the current stops flowing, and the condenser immediately discharges itself through the primary coil in the opposite direction to the flow of induced current, reversing the polarity of the coil and increasing the rate at which the field collapses. This increases the voltage induced in the secondary winding to produce a voltage to fire the spark plugs. The growth of the primary current is slow but its decay is fast. The rate of movement of the collapsing field is greater than that of the growing field, so the voltage induced in the secondary winding at the break of the primary current is much higher than the voltage induced when the current is made. So the secondary voltage induced at break of current is used to cause a spark to jump the gap in the spark plug.
32.5.5.3.2 Principle of coil operation
1. When a current is passed through a coil of insulated wire wound round a soft iron core, the soft iron core becomes a magnet and its associated magnetic field grows outwards from the coil until it is fully established. 2. The growth of the magnetic field causes an opposing self-induced voltage in the coil that delays the full establishment of the current for a short time. (3) When the flow of current in the coil is interrupted, the magnetic field disappears by collapsing on the coil in the opposite direction to the direction in which it was established. A self-induced voltage arises again but now tending to maintain the current flow in the same direction and at the same strength as before. The self-induced voltage in the primary is now in the same direction as the battery voltage. The effect of this induced voltage is to create a small arc across the contact breaker points to keep the current flowing. (4) However, arcing is prevented by the condenser that allows the current to flow into it and charged it to stop the current quickly. (5) A secondary winding placed on the same core is subjected to the same movements of the magnetic field, and voltages are induced in it that depend on the strength of the field and on the rate of growth or decay of the primary current. The high voltage induced in the secondary winding at break of primary current is used to "fire" the spark plug. When the contacts close, the currents in both primary and secondary windings are influenced by the slow growth of the magnetic field generating a low voltage. When the contacts open, the condenser absorbs the voltages of the self-induction and so the magnetic field collapses instantaneously. As the speed of movement is accelerated, a high voltage is quickly generated by the secondary winding. The ignition coil has only three connections because one end of the secondary winding is connected inside the coil to one end of the primary winding. This common connection is made to the primary terminal, which is connected to the contact breaker terminal.
32.5.5.3.3 Ignition coil design
See diagram 32.5.5.3.3
At the centre of the coil is an iron core made of soft iron wires or flat iron strips called laminations. They are separately insulated to prevent any induced currents, eddy currents, from circulating within the core. Magnetic flux will cross from the ends of the iron core, across the air gap, to the iron outer casing. Layers of insulation are wound around the iron core then the secondary winding is wound around this insulation, e.g. 15,000 turns of fine enamelled 42 S.W.G. wire. After more layers of insulating material are wrapped over the secondary winding 200 - 400 turns of the primary winding are wound outside the secondary winding. The primary winding is outside the secondary winding to allow heat to be conducted away to the iron outer casing. The current flowing through the primary winding when the contacts are closed and the engine is at rest, or stalled, is called the stall current. It is the highest value of the current after the battery voltage has overcome the self-induced voltage in the primary winding. If the resistance of a 6 volt coil is 1.5 ohms, the stall current is 4 amperes. (I = V / R = 6 / 1.5 = 4). However, when the engine is running and the contacts are opening and closing rapidly, the average current is much less than the stall current. The primary or low tension wiring is similar to headlamp wiring, e.g. 14 S.W.G. copper wire. This wire is connected to a terminal on the side of the distributor assembly that leads to the contact breaker of the primary circuit. The secondary winding is made of fine wire, e.g. 42 S.W.G. The spark plugs leads are made mechanically strong and usually consist of several strands of wire for flexibility and strength. A very short spark occurs between the brass end of the rotor arm and the segments of the distributor cap so these surfaces act as electrodes and become eroded by constant sparking.
32.5.5.4 Four cylinder ignition circuit
See diagram 32.5.5.4
The cylinders fire in a sequence called the firing order as determined by a high tension distributor. The secondary terminal of the coil, or coil tower leads to the centre of the distributor cap containing brass conductors. The centre terminal contacts with the brass rotor arm rotating at the same speed as a cam that operates the contact breaker. The brass conductor on the rotor is in line with an outer electrode of the distributor cap at the instant the contacts break. The outer terminals of the distributor cap are connected to the spark plugs. Rotation of the rotor arm distributes the secondary output to the spark plugs in the correct firing order as the rotor arm meets each outer distributor cap terminal in turn as the contacts separate. Each cylinder of a four stroke engine fires once every two revolutions of the crankshaft. There is an interval of time from the instant the spark occurs until the ignited mixture is sufficiently burned to exert pressure on the piston head. So when the engine is running, the spark must occur before the piston reaches the top of the compression stroke. At idling speed, the spark occurs just before the piston reaches top dead centre. At higher engine speeds the spark occurs before top dead centre. The spark is "advanced" when it occurs early in the engine stroke and "retarded" when it occurs late in the stroke near top centre. The amount of mixture in the cylinder at the beginning of the compression stroke varies with throttle opening. At low engine speed, i.e. idling speed, if the throttle is opened fully a large quantity of fuel and air mixture enters the cylinder during the intake stroke which is compressed to a high pressure before firing. As combustion then occurs very rapidly, there is a short time interval between spark and maximum pressure, so the spark must be retarded so that maximum pressure will not occur before the piston reaches top dead centre. Above idling speed, when the throttle is nearly closed, it permits only a small amount of mixture to enter the cylinder, resulting in a lower pressure and so a much slower combustion. Therefore the spark must be advanced when the throttle is nearly closed so that maximum pressure will occur soon enough to deliver a useful thrust on the piston. If dust, oil or moisture accumulates on the surface of a high tension insulator, such as the distributor cap or the coil tower, the high tension current will begin to leak across the dirty surface to the nearest earthed metal. A fine track of carbon may become etched in the smooth surface of the insulator and the engine will not run well because the high tension current from the secondary winding will take the easier path to earth through the carbon track rather than through the spark plug gap. An insulator with this fault is said to be tracking and must be discarded, and a new coil, distribution cap, or rotor fitted. High tension insulators must keep have a smooth, glossy surface and be kept clean and dry. Tracking may take place along a crack in an insulator because it provides an easy path for the high tension current to follow.
32.5.5.5 Spark plugs, operating temperature, pre-ignition, spark plug gap
See diagram 32.5.5.5
Spark plug structure:
The spark plug ignites the air and fuel mixture in the combustion chamber by using a spark plug gap across which a current of electricity passes as a spark to ignite the mixture. Spark plugs have a corrosion resisting steel alloy body around an insulated central electrode. The electrode has a terminal cap at the top to connect to a high tension lead from the distributor. A second electrode that forms part of the earthed steel body of the spark plug has a small gap between it and the lower end of the central electrode. The body of the spark plug is screwed into the cylinder head. The spark jumps across the gap between the two electrodes.
32.5.5.5.1 Spark plug operating temperature, pre-ignition
Octane (C8H18) Octane number 16.1.1h  | 7.2.2.2 Tetraethyl lead
Pre-ignition can occur if the combustion chamber reaches a temperature of 800oC near the end of the compression stroke and before the spark occurs. Impure oil and carbon are good conductors of electricity under the high voltage generated in the ignition system so short circuiting of the insulator by oil, carbon or products of combustion can occur if the temperature of the insulator falls below 300oC. The optimum temperature range is 500oC to 600oC. The temperatures are high enough for any deposits on the spark plug to be burnt off but not so high as to cause pre-ignition.
32.5.5.5.2 Spark plug gap
If a spark plug is not gas tight, leakage of gas past a spark plug causes lowered gas pressure in the cylinder or damage to insulation from rising hot gases. The end of the spark gap is flush with the wall of the combustion chamber, depending on the reach of the plug, i.e. the length of the threaded part measured to the end of the body. If the reach is too short, the spark will be pocketed to cause difficult starting and misfiring at low speeds. If the reach is too long, the plug tip and the electrodes will project into the combustion chamber and become overheated to cause pinking, knocking, and, pre-ignition, resulting in loss of power. Protruding tip spark plugs run cooler at high engine speeds and run hotter at low engine speeds because the insulator receives more heat but is cooled more by the incoming fuel. Too large a spark plug gap can cause the sparking voltage of the plug to be too high overloading the secondary winding of the coil or preventing a spark from occurring. Too small a spark plug gap causes incomplete burning of the mixture and the gap may become fouled. The wear, corrosion, and erosion of the plug electrodes cause gap growth. Rapid gap growth is caused by operation at too high a temperature that accelerates the burning of electrode material. Also, fuel containing tetra ethyl lead deposit lead oxides and sulfates to cause pitting of the central electrode. Corrosion deposits on the insulator may short circuit the spark plug so that no spark occurs at the spark gap. Deposits of oil on the insulator and electrodes are caused by oil when the cylinder bores and piston rings are worn. Spark plugs should be cleaned to produce a smooth, polished insulator surface each side of the gap. After cleaning the spark plug, the spark gap must be checked with a feeler gauge.
32.5.5.6 Magneto
See diagram 32.5.5.6
32.5.5.6.1 Permanent magnet magnetos
Motors which do not use batteries in their ignition system, e.g. outboard motor, use a hand operated magneto to produce a high voltage to explode the petrol and air mixture. Some magnetos are used in high performance cars, vintage cars, farm vehicles, and aircraft. A magneto is an alternating current generator which uses a magnetic field from a permanent magnet. The armature carries two windings, primary and secondary, and voltage is induced in both as the armature is driven round by the engine. The voltage created in the secondary winding is not sufficient to cause a spark, and the necessary high tension output is obtained by opening the primary winding by means of a contact breaker at a predetermined instant, whereupon the primary field collapses and forces the main field to reverse direction very rapidly. By this action, practically the whole magnetic field of the magneto is made to cut the many turns of wire which make up the secondary winding, so creating a very high voltage. Two sparks per engine revolution are obtained from the wound armature type of magneto, and for a four cylinder four-stroke engine the armature would be driven at crankshaft speed. However, the distributor, which is built into the assembly of the magneto, must turn at half engine speed, and in this type of magneto suitable gearing is provided. The contact breaker with condenser is similar to that fitted to coil ignition systems.
32.5.5.6.2 Rotating magnet magneto
A voltage is induced in windings rotated in the magnetic field between the poles of a fixed magnet. The situation can, however, be reversed so that a rotating magnet produces the necessary flux changes in a fixed winding. The magnet, which is fitted with laminated shoes, revolves and, as the rotating armature does, induces voltage in the windings. The primary current acts as a restraining influence on the main field until the contact breaker interrupts the primary circuit, whereupon the rapid field reversal induces a high tension secondary current. The rotating cam and fixed contact assembly is similar to that of battery and coil ignition systems. A condenser, too, is fitted and can easily be removed for testing.
32.5.5.6.3 Polar inductor magneto
Neither the windings nor the magnet move. Two magnets are fixed in a closed magnetic circuit, with their n and s poles opposite each other. The shape of the laminated iron rotor is such that the portions of the magnetic circuit which carry the windings are alternately connected to the n and s poles as the rotor is turned. The manner in which the rotor, driven by the engine, produces an alternating flux through two independent windings. Here an alternator and a magneto are virtually combined in one unit, but in other respects the polar inductor magneto is similar to the rotating magnet type. In the magneto illustrated there would be six flux reversals per revolution of the rotor, any one of which could be used to produce a spark for ignition purposes by a suitably timed contact breaker assembly attached to the rotor shaft. A distributor would be necessary for multicylinder engines, but such magnetos have a very limited use in four-stroke automotive engines. They are more suited to the operation of two-stroke engines, where one spark is required for each revolution of the crankshaft. The flywheel magneto is found in motor-cycle and small two-stroke engines. When starting an engine with a battery system, the ignition is turned "on" by closing the ignition switch contacts, and "off" by opening the ignition switch contacts. However, this position is reversed in stopping an engine equipped with a magneto. Magnetos generate their own primary current, and the ignition is turned "off" by closing the ignition switch contacts so as to bridge the contact breaker in the magneto and thus prevent a sudden interruption of the current flow. The ignition is turned "on" by opening the ignition switch contacts to allow the magneto to function in a normal manner.
32.5.5.7 Transistorized ignition system
See diagram 32.5.5.7
A transistor may be compared with an electrical relay in that it is an electrical device which uses a small current to stop or start the flow of a larger current. Like the relay, the transistor has three terminals, two of which carry the main current with the third providing the "triggering" action. The emitter and collector of the transistor are in series with the primary winding of the coil, and the contacts feed the base through a resistor. The base carries only a very small current, about 1 ampere, and as there is no coil the circuit is not inductive and therefore no condenser is required at the contact breaker. By using transistors as a "relay" contact breakers can be reduced in size and weight because of the smaller currents they carry, arcing is no longer a problem because the inductance of the primary winding of the coil is isolated by the transistor, less expensive and more suitable metals can be used for the actual contacting surfaces of the contact breaker, larger primary currents can be used without adverse effects on the contact breaker and the efficiency of the coil at high speeds is improved.
32.5.6.0 Motor vehicle generator (dynamo), charging system
Generators (dynamo), inspection and maintenance, s (rotating field and pole shoe assembly, slip rings, brushes, fixed stator windings, three-phase alternating current, control unit, diodes so no cut-out unit, test: ignition lamp stays on after engine started and operating at 1000 rpm, alternator (a.c. generator) alternator regulator) silicon rectifier, selenium rectifier
The function of the generator is to supply current for the operation of the ignition system, the lighting system, and all other electrical parts when the engine is running and to replenish the battery after it has supplied current for starting and other purposes when the engine is at rest. An ideal charging system is one in which the battery "floats" across the output terminals of the generator, taking little or no charging current from the generator and supplying little or no current to external circuits. This is the goal of modem systems in which a generator or alternator of high output is virtually stabilized by a battery of quite small capacity. In older systems the battery must have a large capacity to provide the reserve energy necessary to carry the electrical loads in excess of the generator capacity. Nevertheless if the charging system is maintained in a serviceable condition there is no reason why the ideal state should not be approached as nearly as practicable in both old and new systems.
32.5.6.1 Generators
See diagram 32.5.6.1
A simple generator consists of a single loop of wire that can be rotated in a magnetic field between two unlike magnetic poles. A voltage is created in the loop and a current flows around the loop and through an external circuit. The loop is connected to the external circuit by circular rings of metal, called slip rings. A piece of conducting material. e.g. graphite, called a brush, presses against each slip ring and connects it to a meter. When the loop is rotates, the slip rings turn but keep in contact with the brushes in fixed position.
(a) The loop is moving parallel to the magnetic lines of force so it is not cutting them and so no voltage is generated and no current. The meter indicates zero current flow. (b) The loop rotates a quarter of a revolution to be moving at right angles to the magnetic lines of force so the voltage generated is at maximum because the rate of cutting the magnetic field is at a maximum. The meter indicates maximum current flow. (c) The loop rotates a quarter of a revolution to the vertical position to be moving parallel to the magnetic lines of force so it is not cutting them and so no voltage is generated and no current. The meter indicates zero current flow. (4) The loop rotates a quarter of a revolution to the horizontal position. Voltage and current are at a maximum but the current now flows in the opposite direction. During a full revolution of the loop the current rises to a maximum, falls to zero, rises again to a maximum in the opposite direction, and falls to zero again
32.5.6.2 Alternating current, graph of alternating current generated by an alternator
See diagram 32.5.6.2
Note the side of the loop marked "xy". The height of the graph line above the zero line of the graph shows how much current is flowing. The loop is not cutting the lines of magnetic force, no voltage is generated and the graph line is on the zero line. (b) The loop is moving at right angles to the lines of magnetic force, maximum current flows, the meter reads maximum to the right, and the graph line is at maximum position. (c) The loop has turned through a quarter of a revolution and is again moving parallel to the magnetic field. It is not cutting the lines of magnetic force so no current is induced in it. The meter reads zero again and the graph line is on the zero line. (d). The loop has turned through half a revolution. As the loop rotates still further the wire xy is cutting the lines of force from the opposite direction so the meter deflects to the left. However the loop has not yet rotated to a position where it is cutting the lines of force at right angles so the meter reading is between zero and maximum left. (e) The loop has continued to rotate so that the maximum number of lines of force are being cut. The meter indicates maximum current left. (f) The loop has not yet rotated to a position where it is cutting the lines of force at right angles, so the meter reading is between zero and maximum left. (g) The loop has completed one revolution and is back to the starting position shown in (a). The loop is not cutting the lines of magnetic force, no voltage is generated and the graph line is on the zero line. (h). A further revolution of the loop produces a similar rise, fall, and reversal of current direction for three revolutions. The graph of this current regularly reverses directions so it is called an alternating current, and the simple generator is acting as alternator. One direction in the graph is "positive" and the opposite direction as "negative" so "current to the right" is denoted positive (+) and "current to the left" is denoted negative (-), as shown above and below the zero line. However, the terms positive and negative are used just to distinguish between identical currents which flow in opposite directions.
32.5.6.3 Conversion of alternating current to direct current
See diagram 32.5.6.3
Commutator method. A simple form of commutator is made when the loop shown in the diagram is cut and the ends are attached to semicircular metal strips called segments. Each segment is supported by insulating material, and against each segment a piece of graphite or similar conducting material is placed as shown in fig. 91. These conducting pieces, called brushes, form the stationary contacts of what is, in effect, a rotary switching device. (a) The loop is moving at right angles to the magnetic field. The meter indicates maximum current flow through it and the loop in the direction VUXY. (b) The loop is not cutting a magnetic field and so no current flows in the circuit. (c) The loop is moving at right angles to the magnetic field. The meter indicates maximum current flow through it and the loop in the direction VUXY. It is now cutting the magnetic field in a direction opposite to that shown (a) but the electrical connections between the segments and the brushes are reversed at the time the loop passes through the position (b). (d) Further rotation of the loop is the same as (b) The reversal of connections between the brushes and the segments results in the second half of the current curve being above the zero line as in (c) instead of beneath it as in 32.5.6.2 (g).
32.5.6.4 Voltage of a generator
See diagram 32.5.6.4
When many loops are arranged at regular intervals so that as one loop moves away from the influence of the magnetic field another takes its place, the periods of current flow will be very close together and a more uniform current will flow in the meter. Each loop is connected to a pair of segments, so that the brushes provide a complete circuit for the coil when it is cutting across the magnetic field. This arrangement of segments is called a commutator. The winding is placed on a soft-iron core to increase the magnetic flux. The coils of wire are called armature coils. The magnetic field is produced by electromagnets and the coils rotating at high speed. The voltage of a generator depends on the number of turns of wire, the strength of the magnetic field, and the speed of rotation. The alternator used in many modern vehicles makes use of a diode rectifier (electronic switching device). In a two brush, two pole shunt-wound d.c. generator the armature shaft is supported by a ball race at the drive end bracket, and a composition bushing supports the commutator end of the armature. The two end brackets are spigoted into the yoke and clamped to it by two "through bolts". This type of construction makes the generator easy to dismantle for servicing or overhaul. The armature assembly The moving part of the generator, which carries the armature coils and the commutator, is called the armature assembly or armature. The part of the armature on which the coils are wound is called the armature core and is made of soft iron to provide an easy path for the passage of the magnetic flux from one magnetic pole to another. Since iron is a good conductor of electricity, local currents will flow within a solid iron core rotated within the influence of a magnetic field. Local currents of this type are called eddy currents. They can cause a serious loss of energy which appears as heat in the armature core. Eddy currents are prevented by building up the core from a large number of thin sheets of soft iron called laminations. The laminations are insulated on both sides and assembled so that slots appear when they are aligned by a key way on the armature shaft. The diagram shows at A, a single lamination and at B, a section through a wound armature core. The diagram shows an armature before it has been wound and the diagram the completed armature.
32.5.6.4.1 Field magnets
The magnetic field in which the armature rotates is usually produced by electromagnets as they provide the most convenient means of controlling the output of the generator. Each field magnet consists of numerous turns of small diameter copper wire, usually insulated with enamel or cotton, the whole being encased in a double layer of insulating tape for additional protection. Two or four such field magnets, each with an individual soft-iron core, are commonly used. Special care must be taken to replace each of these cores in its original position when reassembling a dismantled unit. The diagram illustrates a typical pair of field magnets opened out flat to show the relation between the direction of current and magnetic polarity. In a generator the n and s poles are diametrically opposite and face each other, with the armature between them. The windings of individual magnets are connected in series, and the whole group, of two, four, or six windings, is connected in shunt (parallel) with the armature brushes. Hence the name shunt wound generator, which is the only type of direct current generator
32.5.6.4.2 Self-excitation
All automotive generators have self-excitation, i.e. the magnetic field is energized without any form of external aid. This is explained as follows: 1. once energized by a current in the winding, the soft-iron cores of the field magnet system retain a small amount of magnetism after the current has stopped flowing. This weak magnetism is called residual magnetism and has the same polarity as was produced by the original current. A consequence of the fact that the residual magnetism is weak is that its polarity can readily be reversed. This can seriously affect the operation of the generator. 2. when the generator is not in operation the battery is automatically disconnected, and the field winding remains connected to the generator brushes and through them to the armature windings. (3) immediately the armature is rotated, its windings cut the weak residual magnetic field, which generates a very low voltage in them. Since there is only one path, a small current flows through both armature and field windings, and increases the strength of the field magnets. The now stronger magnetic field increases the voltage across the ends of the armature winding and so the current in the circuit increases. In this manner the generator excites its own magnetic field from the weak residual magnetism and builds up the armature voltage to a value in excess of that of the battery in a matter of seconds. (4) on completion of excitation, the cut-out, page 88, closes the charging circuit, thus permitting the generator to supply current to the external circuit and, under certain conditions which are explained on page 151, to charge the battery.
32.5.6.5 (d) Voltage regulator
See diagram 32.5.6.5 (d)
Control of generator: The automotive generator is subjected to wide variations in speed, and its output rises and falls accordingly. A voltage regulator and cut-out connects the generator and the battery when the generator voltage is the greater than the battery and disconnects them when the generator voltage is lower than that of the battery. Both units are combined in a control box or regulator with an additional unit called a current regulator. The control comes from the difference in intensity of the magnetic field in which the armature rotates. A weak magnetic field produces a lower voltage than a strong one. The voltage regulator consists of an electromagnet which is arranged to open a set of contacts inserted in the field circuit. The contacts are held closed by a spring which resists the pull of the electromagnet, and a resistance is placed across them to provide a path for a small field current when they are open. (a) illustrates the electrical circuit of the voltage regulator. The electromagnet winding is called the regulator voltage winding and consists of many turns of very fine insulated wire which carries a small current of about * ampere. It is connected like the field windings of the generator, in shunt with the armature and so is sensitive to changes in generator voltage. When the regulator contacts are held closed by the spring, there is an uninterrupted path for the field current. As the generator voltage rises with an increase in speed or with battery voltage, the current in the regulator voltage winding increases, with a consequent increase in the strength pull of the electromagnet. This continues until a point is reached at a predetermined voltage where the pull of the magnet exceeds the tension of the spring and the regulator contacts de opened. Separation of the contacts breaks the direct path for the field current, but an alternative path remains through the resistance. Hence the resistance is connected in series with the field windings of the generator whenever the regulator contacts are separated. This extra resistance in the field circuit reduces the field current to a very low value, and so the generator voltage is greatly reduced. A fall in generator voltage causes a fall in. the current in the regulator voltage winding (since it is connected across the generator), thus allowing the spring to overcome the pull of the weakened electromagnet and close the contacts. A direct path is again open to the field current which recovers strength, and the generator voltage rises. This cycle takes place at a rapid rate and the generator voltage is held at a constant value which is determined by the tension of the regulator spring and can be adjusted as required. Figure 99 illustrates a basic charging circuit with voltage regulator control. Current control The flow of charging current through the battery is the normal result of a generator voltage that is higher than the battery voltage. Under ideal conditions a correctly set voltage regulator could maintain the generator voltage at a level which would keep the battery adequately charged. However, operating conditions on a motor vehicle are far from ideal. For example, in a motor vehicle driven at night with all the lights on and the battery in a fully discharged state, the voltage of the battery would remain low and the generator voltage could not rise to the point where the voltage regulator would operate. So the current output of the generator could rise far in excess of its rated capacity and the armature windings would overheat and possibly burn out. To prevent such an occurrence and limit the maximum current output of the generator, an additional control, called a current regulator, is necessary.
32.5.6.6 (c) Current regulator
See diagram 32.5.6.5 (c)
32.5.6.6.1 Separate-unit current regulator
The separate-unit current regulator, three unit system, is similar to the voltage regulator except that its electromagnet is energized by the charging current instead of the charging voltage. The magnet winding consists of a few turns of heavy wire of the same gauge as used for the current winding of the cut-out. The unit is inserted in the charging circuit on the generator side of the cut-out.
32.5.6.6.2 (d) Compensated- voltage control current regulator
The compensated- voltage control current regulator, two unit system, system combines the separate-unit current regulator with the voltage regulator. The additional current winding compensates for abnormal conditions by causing the voltage regulator contacts to open at a lower voltage as the current output of the generator rises. Thus the charging current is prevented from exceeding the rated capacity of the generator. The charging current energizes the upper portion, while the lower portion is energized by any discharge current drawn through the feed to the ignition and lighting switch. Since the polarity of both portions of the compensating winding is the same as that of the voltage winding, the magnetic attraction on the regulator armature will be proportional to the sum of the ampere turns of each winding. The net effect is a reduction in regulator operating voltage to a value low enough to prevent overloading of the generator.
32.5.6.6.3 Testing current regulators
Testing includes starting the engine and checking the ammeter for normal rate of charge and checking any excessive sparking at the brushes when the engine is run at high speed. When an automotive generator is connected directly to a battery, its armature revolves as if it were in a shunt wound electric motor. This is one procedure to polarize the generator correctly. The polarity of the generator must be such that its voltage is applied in a direction that opposes the battery voltage. The generator is polarized by momentarily connecting it to the battery in the vehicle and bridging the cut-out with a jumper lead after the generator is installed on the vehicle and just before the fan belt is fitted. The generator will then have a polarity that is correct in relation to the battery in the vehicle at that time. The v-belt drive should be checked frequently for correct fan belt tension.
32.5.6.7 Motor vehicle alternator
See diagram 32.5.6.7: Two views of an alternator
In an alternator rectification is performed by silicon diodes in the alternator, instead of by a mechanical commutator. The main charging current is produced in the stationary windings (stator) of the alternator, so no sliding connections are needed to pass this heavy current. The use of the stator and diode rectifiers eliminate the need for a current regulator and cut-out in the alternator charging system. An alternator is made up of a 1. stationary winding assembly, the stator. 2. a rotating electromagnet, the rotor. (3) a slip ring and brush assembly. (4) a rectifier assembly. (5) two end frames. (6) a cooling fan. The stator has a cylindrical, laminated iron core with three sets of windings arranged so that a separate alternating waveform is induced in each winding as the rotating magnetic field cuts it. The rotor is an electromagnet consisting of a coil wound on an iron core on a shaft. When current is passes through the winding, magnetic poles are established at the ends of the iron core and shaft. An iron end piece is fastened on each side of the coil assembly so that projections on the end pieces interlace. These projections thus take on the same polarity as the ends of the shaft on which they are mounted, forming pairs of north and south poles around the periphery of the rotor. The rotor rotates within the stator with a very small air gap between the two parts so that a strong electric field cuts the stator windings and the maximum current is induced in the windings. The ends of the rotor coil connect to insulated slip rings mounted on the shaft. A current supplied from the battery passes through the brushes and slip rings to energize the rotor winding and produce the magnetic field. Direct current is needed to charge the battery so a three-phase, full wave rectifier is used. To prevent overheating, the rectifier is mounted in heat conducting metal called the "heat sink".
32.5.6.8 Silicon diode rectifier
See diagram 32.5.6.7
32.5.6.8.1 Half- wave rectifier
The silicon diode used in the rectifier is of the alloy junction type and consists of a thin wafer of silicon with one face alloyed with an impurity such as phosphorus and the other face alloyed with an impurity such as aluminium. The alloying process converts the faces of the wafer into semiconductors, separated by a "barrier" layer. Such an arrangement has a low resistance to the flow of current in one direction, but a very high resistance to a current flow in the opposite direction. The alternating current flowing in a positive direction then in a negative direction becomes pulsating direct current which always flows through the battery in the same direction, "half-wave" rectification. One half cycle of the alternator output is lost because the positive halves pass through the rectifier and around the circuit to charge the battery but the negative halves are suppressed. The rectifier prevents the battery from discharging through the alternator when the engine is stopped so a reverse current cut-out is not needed. If the battery connections are wrongly reversed the rectifier would be destroyed by the reverse current.
32.5.6.8.2 Full-wave rectifier, single-phase rectifier
The full-wave rectifier is connected in series with an alternator and a battery to allow full output to pass through to the battery. The current passes from the alternator in a clockwise direction and when the polarity of the alternator is reversed. Both the positive and the negative half cycles of current from the alternator flow through the battery in the same direction. Single-phase rectifiers may be used in motor bikes and battery charging units in garages.
32.5.6.8.3 Full wave rectifier, three-phase rectifier
Automotive alternators are constructed with a three-phase stator winding and six diodes are required to provide full-wave rectification of the current produced. To avoid damage to the alternator parts the charging current will not rise above the maximum safe output of the alternator irrespective of speed or electrical load. A voltage regulator adjusts the current flowing through the rotor windings so that the terminal voltage of the alternator remains the same even though the engine speed or the electrical load may be constantly changing.
32.5.7 Motor vehicle starter motor, starters
The starter motor is used to crank the engine. Other electric motors operate windscreen wipers, windscreen washers, and electric fans etc.
32.5.7.1 Principle of the electric motor
See diagram 30.07
The electric motor uses the magnetic effect of a current to produce movement. (a) A magnetic field exists around a conductor carrying an electric current. Reversing the direction of the current also reverses the direction of the magnetic field.
(b) If electric current passes through a conductor in a magnetic field, a force acts on the conductor, which can cause it to move. Lines of magnetic force from the two sources of magnetism interact so that a mechanical force arises which tends to move the conductor in the direction shown in the diagram.
(c) If a conductor is arranged as a loop so that current flows in opposite directions on each side of the loop, the loop moves with rotary motion through 90o then stops because the interaction of the two magnetic fields no longer exerts a force thrust on the conductors.
(d) If the loop is connected to the current source through a commutator and brushes, and additional loops are included as in a simple generator, the assembly turns with continuos motion.
(e) The assembly behaves like an electric motor.
32.5.7.1.1 Shunt-wound motor
The shunt-wound motor has its field windings connected in parallel with the armature windings, i.e. shunted across the armature windings. As each circuit is independent, current from the battery flows through each winding according to its resistance. A shunt motor has constant speed, whether or not it is running under a full load because it behaves like a generator when its armature is rotating at speed. The armature generates a voltage that opposes the voltage of the battery, and so limits the armature speed of the shunt motor. Shunt motors are used where a constant speed is required, e.g. driving a windscreen wiper.
32.5.7.1.2 Series-wound motor
The series-wound motor has its field windings connected in series with the armature windings. It is used to start an engine because of the wide range of speeds at which the armature can turn. The series-wound motor produces maximum turning effort, torque, immediately the motor is switched on. The armature speed of the series-wound motor varies with the mechanical load, low speed under heavy loads and high speed under light loads. The series-wound motor is not usually damaged by excessive loads of short duration.
32.5.7.2 Starter motor
See also: 32.5.6.4
A starter motor is a series-wound motor designed to produce a high torque and horsepower for its size. The current needed to produce a high power output is large and much heat is produced when the large current flows through the conductors. So the starting motor should never be used for more than 30 seconds, then given a minute of rest before further use to allow heat to be conducted away to the engine block through the casing of the motor. The windings are made from thick copper strips to allow to large currents to pass in the starter armature without excessive voltage drop. Since the current in a series circuit is the same in all parts of the circuit, the field windings carry the same current as the armature windings. Field windings are made of insulated copper strip similar to that used in the armature windings. The brushes and connecting leads must also be able to carry the same current so are made from copper carbon or copper graphite.
32.5.7.2.1 Starter circuit
See diagram 32.5.7.2
1. Main insulated cable from the battery to the starter switch, 2. Switch terminals and contacts. Most motor vehicles have an electromagnetic switch, 3. Cable from the switch to the starter, 4. Starter terminal, 5. Field winding, 6. Insulated brushes in contact with the commutator
See diagram 32.5.7.1 (g)
7. Armature windings with soldered connections to the commutator, 8. 9. Earthed brushes and earthed connection of the brush leads, see diagram 32.5.7.1 (f) 10. Metal casing of the starter mounted on the engine, see diagram 32.5.7.1 (g) 11., 12., 13. Engine, heavy connecting cable or strap from the engine to the frame, frame or chassis. (Not shown in diagram), 14. Battery earth strap connected to the frame and the earthed battery terminal, 15. The battery itself. The large starting current must pass through the cells and electrolyte of the battery.
32.5.7.2.2 Starter motor windings
See diagram 32.5.7.2
32.5.7.2 (b) Series winding has the current is the same in every part of the circuit, both internal and external.
32.5.7.2 (c) Series parallel winding has the current travelling through the field windings in two parallel paths, the field windings are in series with the armature windings and so that the starter is series wound and behaves like a series motor. Most starter motors have four brushes: two insulated and two earthed to reduce the current in each brush to half of the total starting current.
32.5.7.2 (d) The compound-wound winding has three of the four field poles series wound and the fourth pole with shunt winding consisting of many turns of fine wire. The shunt winding maintains the magnetic field at a more constant strength than in a series-wound motor. When the armature of a motor rotates at speed, a generator effect occurs and the armature windings generate a voltage which opposes the voltage of the battery which rises as the armature speed increases but never equals the voltage of the battery. In a shunt motor, the field windings are connected in parallel (shunt) with the armature windings direct to the battery. Regardless of the load on the armature, or its speed of rotation, the field current remains the same because battery voltage is applied to the constant resistance of the field coils so the strength of the magnetic field remains constant. In a series motor the field windings are in series with the armature windings so that when the armature current decreases at high armature speeds the field current, and therefore the strength of the magnetic field, also decreases. Thus the torque (turning effort) decreases at high armature speeds in the series motor but not in the shunt motor. In the compound-wound motor one of the field poles is energized by a shunt winding which gives the characteristics of a shunt motor to the basically series motor. This arrangement allows the torque of the starter motor to maintained at higher speeds so that the engine cranking speed is increased and also the starter armature cannot "race" when the mechanical load is removed.
32.5.7.2 Starter drives
To allow a small electric motor to start a large engine, the armature speed must be high, 1,000 to 1,500 rev / min), to develop enough power. So reduction gearing is needed between the starting motor and flywheel. Also the starter must engage a stationary engine while its armature is turning, and disengage when the engine is started. The Bendix type engagement mechanism depends on the principle of inertia to mesh the pinion with the ring gear. The positive mesh type engagement mechanism depends on an independent mechanical device to mesh the pinion with the ring gear. Both have a compression spring to absorb the shock of engagement.
Lucas cable colour scheme

Colour Circuit Letter No.
Blue Controlled by headlamp switch U 1
White Controlled by ignition switch (unfused) W 9
Green Controlled by ignition switch (fused) G 17
Yellow Charging system Y 25
Brown Unfused battery-fed circuits N 33
Red Circuits fed from side- and tail-lamp R 41
Purple Fused battery circuits fed from ammeter P 49
Black Earth circuits B 57
The battery voltage be can stepped up to operate a car radio by using a vibrator unit to produce a changing magnetic field from a pulsating direct current.