School Science Lessons
(UNPh32.6)
2024-07-26
Electricity in motor vehicles 2.
Contents
32.3.02 Electrical resistance
32.6.0.0 Electricity in motor vehicles
32.5.0.0 Motor vehicles
32.6.0 Electricity in motor vehicles
32.5.9.0 Motor vehicle auxiliary units, accessories, windscreen wiper, horn, fuel pump, gauges
32.5.5.0 Motor vehicle generator (dynamo), charging system
32.5.8.0 Motor vehicle lighting systems
32.5.7.1 Motor vehicle starter motor
32.5.6.7 Motor vehicle alternator
32.5.6.4 Motor vehicle generator, Voltage of a generator
38.5.01 Motor vehicle, Relays, magnetically operated switches, "make-and-break"
38.5.03 Motor vehicle, Reed switch, reed relay, "make-and-break"
32.5.0.0 Motor vehicles, automobiles, cars
11.0, Activated carbon (exhausts of motor cars)
13.2.8 Aerofoils, parts of an aircraft, (racing cars)
7.2.2.1 Aluminium, properties
5.5.0 Alloys (racing cars)
16.2.6 Casein and caseinates (car tyres)
18.6.6 Catalytic converter in a motor vehicle
38.5.10.2 Coil ignition system in motor vehicles, "points", capacitor across points
38.5.002.4 Coil ignition system in motor vehicles, high voltage at the opening of the contacts
Cresylic acid, cresols, (exhausts from motor cars)
32.5.0.0 Electrical equipment of motor vehicles
12.4.7 Escape from a submerged car
38.5.7 Flashing circuit
3.3.5.4 Graph the speed of two cars
16.1.2.4.3 Helium balloon in a motor vehicle (inertia)
37.39.2 Inversion layers (exhaust gases from motor cars)
32.5.3.0 Lead-acid battery, Motor vehicle battery (battery maintenance)
4.2.5 Necessity of seat belts in a motor car
Neodymium, Nd (electric cars)
16.1.0 Newton's first law of motion, inertia
16.6.8.0 Octane (C8H18), Octane number
10.6.3.1 Penetrating oil, e.g. WD-40, (lubrication)
29.1.03 Permanent magnets (electric cars)
3.8.10 Phenolics (car parts)
3.7.10 Polycarbonates, PC (car parts)
3.7.27 Polypropenes (car parts)
3.7.28 Polypropenonitrile (car parts)
34.5.2.9 Prince Rupert's Drops, tempered glass, toughened glass (car windows)
28.11.4 Projector for filmstrips or slides (from motor car headlamp
32.5.2.7 Thermal circuit breaker
Titanium, Ti (electric cars)
9.2.8 Transmission of compressive energy, (See 4.)
3.6.13 Vinyls, vinyl polymers (See 3. windshields)
32.5.5.0 Motor vehicle generator
Motor vehicle generator (dynamo), charging system
32.5.6.01 Motor vehicle generator (dynamo), charging system
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
32.5.6.6 Current regulators
32.5.6.0 Generator
32.5.6.7 Motor vehicle alternator
32.5.6.8.1 Silicon diode rectifier
32.5.6.4 Motor vehicle generator, Voltage of a generator
32.5.6.4.1 Field magnets
32.5.6.4.2 Self excitation
32.5.6.5 Voltage regulator
32.5.6.8.1 Silicon diode rectifier
See diagram 32.5.6.7b: Alternate current paths through single phase rectifier.
See diagram 32.5.6.7a: Circuit of an alternator.
32.5.6.8.1 Silicon diode rectifier
32.5.6.8.4 Half-wave rectifier
32.5.6.8.2 Full-wave rectifier, single-phase rectifier
32.5.6.8.3 Full wave rectifier, three-phase rectifier
32.5.6.6 Current regulators
32.5.6.6.1 Separate-unit current regulator
32.5.6.6.2 Compensated-voltage control current regulator
32.5.6.6.3 Testing current regulators
32.5.7.1 Motor vehicle starter motor, starters
32.5.7.8 Motor vehicle starter motor, starters
32.5.7.6 Series-wound motor
32.5.7.5 Shunt-wound motor
32.5.7.3 Starter drives
32.5.7.2 Starter motor
32.5.7.4 Starter circuit
32.5.7.7 Starter motor windings
Table 32.5.7.7 Lucas cable colour scheme
32.5.8.0 Motor vehicle lighting systems
32.5.8.01 Motor vehicle lighting systems and equipment
32.5.8.2 Headlamps, side lamps and tail lamps
32.5.8.1 Sources of light, candlepower
23.3.15 Motor vehicle flashing lights
32.5.9.0 Motor vehicle auxiliary units
Motor vehicle auxiliary units, accessories, windscreen wiper, horn, fuel pump, gauges
32.5.9.4 Balancing coil type fuel gauge
32.5.9.3 Electric fuel pump
32.5.9.2 Flashing lamp direction indicators
32.5.9.1 Motor vehicle horn, high frequency horn, wind tone horn
32.5.9.5 Water temperature gauge
32.5.6.01 Motor vehicle generator (dynamo), charging system
The function of the generator:
1. To supply current for the operation of the ignition system, the lighting system, and all other electrical parts, when the engine is running.
2. To replenish the battery after it has supplied current for starting and other purposes, when the engine is at rest.
In an ideal charging system, the battery "floats" across the output terminals of the generator, taking little charging current from the generator and supplying little 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.
32.5.6.0 Generator
See diagram 32.5.6.1: Generator cycle.
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.
1. 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.
2. 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.
3. 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
Alternating current, graph of alternating current generated by an alternator
See diagram 32.5.6.2A: Alternating current, graphs A.
See diagram 32.5.6.2B: Alternating current, graphs B.
1. 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.
2. 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.
3. 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.
4. 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.
5. The loop has continued to rotate so that the maximum number of lines of force are being cut.
The meter indicates maximum current left.
6. 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.
7. The loop has completed one revolution and is back to the starting position shown in 1.
The loop is not cutting the lines of magnetic force, no voltage is generated and the graph line is on the zero line.
8. 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 that flow in opposite directions.
32.5.6.3 Conversion of alternating current to direct current
See diagram 32.5.6.3: 1. Single-loop direct current generator, 2. Point of zero current, 3. Current direction reversed in loop, but unchanged through meter.
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 the diagram.
These conducting pieces, called brushes, form the stationary contacts of what is, in effect, a rotary switching device.
(1) 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.
(2) The loop is not cutting a magnetic field and so no current flows in the circuit.
(3) 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 1.
But the electrical connections between the segments and the brushes are reversed at the time the loop passes through the position 2.
(4) Further rotation of the loop is the same as 2.
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 3. instead of beneath it as indiagram 32.5.6.2B.
32.5.6.4 Voltage of a generator
See diagram 32.5.6.4.0: Two brush generator.
See diagram 32.5.6.4.1: Generator field magnets.
See diagram 32.5.6.4.2: Armature assembly.
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 DC 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, closes the charging circuit, thus permitting the generator to supply current to the external circuit and, under certain conditions to charge the battery.
32.5.6.5 Voltage regulator
See diagram 32.5.6.5: Simple voltage regulator.
Charging circuit with regulator and cut-out
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 that is arranged to open a set of contacts inserted in the field circuit.
The contacts are held closed by a spring that 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.
The diagram 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 an 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 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 that 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.
The diagram 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.
32.5.6.6.1 Separate-unit current regulator
See diagram 32.5.6.5a: Typical three-unit system (voltage regulator, current regulator, cut-out).
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 Compensated-voltage-control current regulator
See diagram 32.5.6.5a: Compensated-voltage-control 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 32.5.6.7c: 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 the following:
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 wave form 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".
32.5.6.8.4 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
See diagram 32.5.6.7c: Two views of an alternator.
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.
32.5.7.8 Motor vehicle starter motor, starters
"Micro Electric Motor", 1.3 V, 10 mA (length > diameter of AU 1 dollar coin), starter motor is used to crank the engine, other electric motors operate windscreen wipers, windscreen washers, and electric fans (toy product).
32.5.7.5 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.
32.5.7.6 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 32.5.7.1: Electric motors.
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.
32.5.7.4 Starter circuit
See diagram 32.5.7.2: Starter circuit.
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: Starter circuit.
7. Armature windings with soldered connections to the commutator,
8. 9. Earthed brushes and earthed connection of the brush leads,
10. Metal casing of the starter mounted on the engine,
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.
32.5.7.7 Starter motor windings
See diagram 32.5.7.2: Starter circuit.
1. Series winding has the current is the same in every part of the circuit, both internal and external.
2. 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.
3. 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 that opposes the voltage of the battery that 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 that 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.
Table 32.5.7.7 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 lamp 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.
32.5.7.3 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.
32.5.8.01 Motor vehicle lighting systems and equipment
See diagram 32.5.8.0: Motor vehicle lighting circuit.
Illumination, headlights: types of bulb filaments, caps, and units, side, tail, and fog lamps, lighting circuit (courtesy light, interior light, roof light) dipper switch (dimmer switch)
The functions of the lighting system of a motor vehicle are to illuminate the path ahead for a sufficient distance to enable the vehicle to be driven safely at night, to enable the vehicle to be seen at night, to provide visible signals.
In the lighting circuit, the light switch controls headlights, park sidelights, and tail-lights.
The dip switch enables the headlights to be changed from dip to full beam, and vice versa, without affecting the park lights or the rear lights.
The stop lights will operate only when the ignition switch is on.
Fuses and other circuit -protecting devices are not usually in the main lighting circuits.
However, the circuits of interior lights may have fuses as there is no driving hazard to be caused by the failure of a fuse.
32.5.8.1 Sources of light, candlepower
See diagram 32.5.8.1: Sources of light.
See diagram 2.0.5: Conic sections, parabola.
See diagram 2.0.6: Parabola equation.
In an electric lamp a short coiled filament of tungsten, enclosed in an evacuated glass envelope, becomes incandescent when current passes through it.
As no oxygen is present the filament will not bum out, but if the temperature is allowed to become too high atoms of tungsten will leave the filament and lodge on the glass, the filament "boils away".
However, the vacuum bulb is used only for low power lamps, such as and tail lamps, where only a small amount of heat is produced.
The more powerful headlamp bulbs are filled with gas at atmospheric pressure with an inert gas, e.g. argon.
The gas pressure prevents evaporation of the filament so a bright light can be produced by a small bulb.
If a 6 volt bulb is fitted in a 12 volt circuit the filament will be destroyed by too much current.
A gas filled bulb produces about 1 candlepower for each 0.7 watt consumed, so a lamp in a 12 volt circuit with a current of 3 amperes, power = 36 watts, would produce about 51 candlepower.
The candlepower of two sources of light may be compared by using a photometer.
If the candlepower of one source is known, that of the other can be easily calculated.
The amount of light falling upon unit area of a surface is called intensity of illumination.
The illumination of a surface by a headlamp beam can be measured with a light meter.
A thin layer of selenium is laid on an iron base plate and the selenium covered with a very thin transparent gold film so that light can easily penetrate to the selenium layer beneath.
Light falling on the selenium generates a small voltage, which is directly proportional to the illumination.
When the circuit is completed through a suitable resistance and a microammeter, the current is also proportional to the illumination.
The light produced by a bulb radiates in all directions.
To obtain maximum illumination ahead of the vehicle the light rays are projected in a parallel beam by placing the bulb in front of a parabolic reflector so that the filament is at the focal point of the reflecting surface.
If the bulb is too far forward, the beam produced is convergent and has a dark shadow in its centre.
If the bulb is too far back, the beam is divergent and gives poor illumination.
Both beams have a smaller range than a parallel beam, and both can produce dazzle.
32.5.8.2 Headlamps, side lamps and tail lamps
See diagram 32.5.8.1: Light meter, Sealed beam headlight unit, Beams, Filaments, Bulb caps.
Every motor vehicle must be fitted with a lighting system, which produces a headlight beam that can be dipped when it is likely to dazzle other drivers.
The headlamp produces two beams, one a driving or high beam and the other a passing or dip beam from double filament bulbs and a changeover switch, called a dip switch.
Early model motor vehicles had detachable bulb in a bayonet lock base, which allowed the bulb to be moved in relation to the reflector so that the light beam could be correctly focussed.
Fixed focus bulbs had bulbs accurately focussed during manufacture and no adjustment is possible when it was fitted to the headlamp.
Sealed beam unit lamps contain the light source, the highball and diploma filaments, the reflector, and the lens assembled into one unit, and sealed from the atmosphere.
The main disadvantage is that when a filament burns out the complete unit must be replaced.
The semisealed beam unit have the improved features of the sealed beam unit, but with a replaceable bulb.
The four headlamp system has two sets of headlamps, one for long range illumination of the road and the other for dipped beams.
Installation may be side by side, or one set above the other.
In the side by side installation the spacing of the head lights must indicates the approximate width of a vehicle as seen at night.
In the twin filament dipping system the main beam filament is set at the focal point of the reflector.
In the four headlamp system the dip filament is at the focal point and the main beam is offset.
The four headlamp system can be supplemented by a long range "pencil" beam for fast night driving.
In a fog, the air contains many water droplets, which can reflect light so that the beam from the headlamp is reflected and the driver sees a white wall of fog.
The brighter the headlamp beam, the greater the reflection.
In fog drivers should switch to low beam.
Special fog lamps may have an amber coloured lens, but the coloured beam does not give more visibility or penetration than a white beam.
If fog lamps are mounted as low as possible and aimed so that the beam does not rise reflection is reduce light can travel along the road under the fog due to thin layer of clear air between the fog and the road.
The halogen lamp has a small glass envelope containing one of the halogens and argon gas.
The gas pressure is higher than in a conventional lamp and operates at a higher temperature giving a brighter light for a given power consumption.
Also the higher gas pressure retards the evaporation of the tungsten filament.
The tail lamp may include a stoplight controlled by a brake operated switch.
The stop / tail lamp is shaped to the contour of the rear wings and has a twin filament bulb, e.g. the stop filament rated at 18 watts and the tail filament 6 watts.
Bulb filaments are made in a variety of shapes.
The bow type filament is found in vacuum type bulbs.
Lamps may have bifocal parallel filaments that are slightly offset from the focal point to provide a main beam and dipped beam of equal intensity.
One beam is reflected slightly upward and the other beam is reflected downward.
The upward beam is corrected to parallel when setting the lamp initially.
Another double filament bulb has a dip filament positioned above the focal point to produce out-of-focus light rays directed from the upper half of the reflector downward.
A front lens concentrates the distorted rays to form a dipped beam.
The main filament is placed at the focal point and produces a parallel driving beam, which is more intense than the dipped beam.
Bulb cap fittings are used to safeguard against fitting the wrong type of bulb into a lamp holder.
Most vehicles use the single-centre-contact (s.c.c.) type for side and number plate lamps and the smaller miniature-centre-contact (m.c.c.) type for instrument panel lighting and warning lights.
32.5.9 Motor vehicle auxiliary units, accessories, wiper, horn, fuel pump, gauges
Windscreen wipers, electric horns, direction indicators, electric fuel pumps and gauges, water temperature gauge, fuel gauge (petrol gauge).
32.5.9.1 Motor vehicle horn
See diagram 32.5.9.1: Horn circuit.
The motor car horn contains an electric motor attached to a corrugated wheel set to strike against similar corrugations on a metallic surface, which vibrates to the horn noise.
In motor horns, the rapid movement of a thin metal sheet, called a diaphragm, disturbs the surrounding air, setting up sound waves, which travel outwards in much the same manner as those from a gong.
An electromagnet, when energized by a current, opens a pair of contacts, which, in turn, open the electrical circuit of the magnet winding.
Thus, so long as a current supply is maintained, the diaphragm is alternately attracted and released by the make-and-break action of the contacts to produces a note 300 to 400 cycles per second.
The high frequency horn produces a higher pitched and more penetrating note by using a tone disc attached to the centre of the diaphragm.
Each impact of the armature plate is transferred to the centre of the tone disc, causing its free outer edges to vibrate at a faster rate to give the high frequency horn its characteristic note.
The wind tone horn has the air is set in motion in the same way, but with the diaphragm attached to a trumpet top produce a musical note, similar to a musical wind instrument.
In twin horn installations, a horn relay is used to reduce voltage drop in the wiring.
Some horns have only one insulated terminal, the electromagnet winding being earthed internally, and it is essential that the horn body and mounting of this type make good electrical contact with the metal structure of the vehicle.
32.5.9.2 Flashing lamp direction indicators
See diagram 32.5.9.2: Flashing lamp direction indicators.
Four light direction-indicating systems, winkers, signal the driver's intention to change direction of travel by a pair of white or amber lights at the front of the vehicle, and a pair of red or amber lights at the rear.
The flashing rate of the lamps is between 60 and 120 flashes per minute.
A flasher unit produce the fluctuations of current to cause the lamps to flash on and off.
A warning light on the instrument panel shows that the flashing lamps are operating.
A turn-indicator switch, usually self-cancelling, directs the current to either right-hand or left-hand lamps.
Four flashing lamps are mounted near each corner of the vehicle.
The flashing lamps may be combined with the side lamps and the stop lamps, but separate bulbs or bulb filaments are used and the circuits become complicated.
One method depends on the linear expansion of a hot wire, a straight wire in tension heated by the passage of an electric current.
Current flows from the battery to terminal B, to armature C, to the hot wire, to the ballast resistance, to contact D, to the iron wire coil around the iron core, to terminal L, to the turn indicator switch, to either the left or the right-hand pair of flashing lamps.
The lamps do not light, because the ballast resistance limits the size of the current.
The current heats the hot wire so it elongates and allows armature C assisted by its spring to move towards the iron core and close the contacts at D.
Now a much larger current bypasses the ballast resistance to flow directly from terminal B to contact D, to the selected pair of flashing lamps, which light.
As the ballast resistance and hot wire have been bypassed, the hot wire cools and contracts, and opens the contacts at D.
The cycle repeats until the turn-indicator switch is returned to the off position.
The hot wire takes some time to heat and to cool so controlling the timing of the flashes.
Until the flashing lamp circuit is completed by the closing of the contacts at D, insufficient current flows to attract armature C towards the iron core.
The current necessary to attract armature C is that required by two flashing lamps.
If one flashing lamp does not function, the warning lamp circuit will carry no current, the warning lamp will not light and so the driver knows that a fault exists.
Each time the hot wire cools and opens the contacts at D, the armature C is also released, causing the warning lamp to flash in unison with the external flashing lamps.
32.5.9.3 Electric fuel pump
See diagram 32.5.9.3: Electric fuel pump.
When the pump is at rest, the outer rocker lies in the outer position and the points are in contact.
The pump diaphragm and its attached armature are at the end of the delivery stroke.
Intake stroke: When the ignition switch is turned on, electric current flows through the coil of the electromagnet to the contact points and to earth.
The electromagnet magnet attracts the armature towards the soft iron core.
The armature pulls the pump diaphragm with it to draw in fuel from the fuel tank through the inlet valve to fill the pumping chamber.
Atmospheric pressure acts on the surface of the fuel in the fuel tank to force the fuel to flow into the pumping chamber.
When the armature has moved almost where it would strike the soft iron core, a "throw over" mechanism separates the points and breaks the circuit.
During the delivery stroke, the armature spring, which was compressed during the intake stroke, pushes the armature and diaphragm back, forcing fuel through the delivery valve to the engine.
As the diaphragm and armature approach the end of the delivery stroke, the throw over mechanism causes the contact points make contact, electric current flows through the coil of the electromagnet and the pumping cycle is repeated.
32.5.9.4 Balancing-coil type fuel gauge
See diagram 32.5.9.4: Balancing-coil fuel gauge.
The tank unit of the gauge is simply a variable resistor with a sliding contact whose position is determined by a float on the surface of the fuel.
It is connected to the dash unit or "heat" unit by a single wire, and the resistor controls the current through the dash unit according to the fuel level in the tank.
A simple instrument for measuring this current with scale calibrated in litres instead of amperes could be used as the dashboard fuel gauge indicator, but the battery voltage varies from 5 to 8 volts in a 6 volt system, and 10 to 16 volts in a 12 volt system so the gauge reading would vary without any alteration in fuel level.
The balancing-coil meter is independent of the battery voltage.
The dash unit has two similar electromagnets, one a control magnet and the other a deflecting magnet.
The magnets influence an iron vane or armature to which the pointer is attached.
Because both windings are equally affected by variations of voltage, the pointer always indicates a true reading.
This arrangement of the coils gives a variable control.
Coil A, that moves the pointer towards the "empty" position, is the control coil.
It is connected directly to the battery through the ignition switch and has two indirect connections to earth, through the other coil and the tank unit respectively.
Coil B, that moves the pointer towards the "full" position, is the deflecting coil.
It is connected directly to earth and has an indirect connection to the battery through coil A.
A resistance shunted across coil A and has the effect of reducing the current in coil A and its magnetic strength.
When the wire leading to the tank unit is disconnected the two coils are in series, a being, of course, shunted by the resistor.
The current in coil B is equal to the sum of the currents in a and the shunt resistor, so that the magnetic effect of coil B is strong and that of coil A weak.
The pointer therefore swings to the "full" position under the strong influence of coil B.
If the gauge reads "full" when the tank wire is disconnected indicates an open circuit fault.
When the tank unit wire is earthed, coil B carries no current, because both its ends are earthed.
The only magnetic action therefore is that resulting from the current through coil A, so that the pointer moves to "empty".
If the gauge reads "empty" when the tank unit is earthed indicates short circuit.
When the tank is half full, and half the tank unit resistance is connected between the coil junction and earth, both coils carry an intermediate current and have equal effect on the pointer, which now reads "half".
The state of the circuit for other scale readings changes gradually as the float is raised or lowered.
32.5.9.5 Water temperature gauge
In a commonly used type of temperature gauge the pointer of the dash unit settles at the "hot" position when the ignition switch is turned off.
When the ignition is switched "on" and the engine is cold, the pointer moves from the "hot" position across the scale to the "cold" position.
The electrical circuit is a simple series one, consisting of two heating elements one in the dash unit and a second in the engine unit.
Each heating element is wound over a bimetallic strip and insulated electrically.
The bimetallic strip in the dash unit is linked to the pointer, which moves as the strip deflects under the influence of heat from the element.
The bimetallic strip in the engine unit has a pair of contacts at one end, which remain in the closed position below certain predetermined temperatures.
When the ignition is switched on, current flows through both heating elements and through the contacts to earth.
With the engine cold, the heat from the engine unit element is rapidly conducted away to the engine metal and to the cold water in the water jacket.
The engine unit therefore remains comparatively cool, and there is little tendency to deflect the bimetallic strip and open the contacts.
The current continues to flow, heating and bending the bimetallic strip in the dash unit and moving the pointer across to "cold".
This occurs at an engine temperature of about 20oC or lower.
At an engine temperature of 110oC the bimetallic strip in the engine unit is hot enough to bend and keep the contacts separated without the assistance of heat from the element.
With the circuit thus broken, the dash unit element remains cold, and the pointer comes to rest at the "hot" end of the scale.
At all engine temperatures between these extremes the engine unit becomes sufficiently hot to cause the bimetallic strip to open the contacts, breaking the circuit and allowing the strip to cool and close the contacts again.
This action is repeated continuously.
In the cooler temperature range the contacts remain longer in the closed position than in the open position.
As the engine temperature rises the open period increases and the closed period decreases.
The ratio of closed to open periods determines the average current, indicated by the distance between the pointer and the "hot" end of the scale, that is, the extent to which the pointer moves towards "cold" before coming to rest.
The action therefore depends on the rate at which the heat is conducted away from the engine unit, which in turn depends on engine temperature.
32.5.10 Motor vehicle circuit testing
Ammeters and voltmeters, ohmmeters, diagnosis of ignition faults, coil testers, regulator adjustment.
32.3.02 Electrical resistance
Constantan
Manganin resistance alloy, 86% Cu, 12% Mn, 2% Ni, low temperature coefficient
See diagram 32.3.0.0 Resistance model.
Roll ball bearings down an inclined bed of nails to simulate current flow in a wire.
1. Resistance, symbol R, unit ohm, (Greek Ω omega, "great O"), resistivity, ρ, rho, specific resistance, resistors
Electrical resistance is the opposition from a component or circuit to the flow of electric current.
The resistance of an object, R, e.g. a wire, measures the potential difference in volts, V, needed for one ampere, A, of electric current to flow through it.
R = V / I.
So 1 ohm = 1 V / A, 1 volt per amp = 1 V / A.
Low resistance materials are called conductors, e.g. silver, copper, aluminium.
High resistance materials are called insulators or dielectrics, e.g. dry air, glass, plastic.
Resistance for ohmic and non-ohmic components is the ratio of potential difference across the component to the current in the component.
Resistivity is the specific electrical or thermal resistance of a substance, usually defined as the resistance of a conductor of unit length and unit cross-sectional area.
Resistivity surveying uses electrodes in the ground to locate something buried that affects the normal resistivity of the ground, e.g. buried treasure!
2. Different materials offer different resistance to the flow of electric current through them and convert electrical energy to heat energy.
Copper, silver and aluminium are examples of good conductors that offer very little resistance.
Glass, wood, and paper are examples of poor conductors, insulators, which offer high resistance to current flow.
The material of the wires in an electric circuit is chosen to keep the electrical resistance as low as possible so that current can flow easily through the conductors.
In an electric circuit, the larger the diameter of the wires, the lower will be their electrical resistance to the flow of current through them.
For alternating current resistance is a component of impedance.
3. The electrical resistance of the conductors depends upon:
(1) the length of the wires,
(2) the diameter of the wires,
(3) the material of the wires, e.g. copper, aluminium,
(4) temperature.
4. For most conductors, e.g. copper, aluminium, iron, resistance increases with temperature.
However, the resistance of carbon decreases as temperature increases, and for some alloys of metals, e.g. Manganin and Constantan,
resistance hardly changes with temperature.
Electric jug elements use 0.3 mm nichrome wire (80% nickel, 20% chromium).
Resistance at room temperature is about 37Ω, but at 100oC, the hot resistance is 38.2 Ω, so about 1.2 Ω higher than at room temperature.
5. Effect of length and thickness on the resistance of a wire.
Resistance is measured with an ohmmeter.
In an electrical circuit, resistance is shown as a zigzag or wriggle line, so a straight line in a circuit diagram shows a wire with zero resistance.
6. Resistivity, ρ, rho), is the specific resistance of a substance
Resistance of a wire, R = ρ (L / A), where L = length, A = area
(L is measured in m and A is measured in m2, so resistivity of a substance is measured in m.)
Nichrome wire has a fairly high resistance, about 10 ohm / m.
Resistivity of electrical insulators: rubber 1 - 100 × 1013, glass 1 - 10, 000 × 109
Resistivity of electrical conductors: lead 22 × 10-8, iron 9.7 × 10-8, gold 2.2 × 10-8, copper 1.7 × 10-8, silver 1.6 × 10-8
Nichrome wire, for heating elements, 80% nickel, 20% chromium, resistance = 13.77 ohm per metre
ρ0 is the specific resistance temperature coefficient at 0oC.
Resistivity depends on the material involved and on the temperature.
The resistivity of pure metals increases linearly with temperature, because a temperature increase causes the lattice ions to vibrate with greater amplitude, increasing the likelihood of electron collisions, and so decreasing the current through the conductor.
Increase in resistivity with temperature: ρT = ρ0 (1 + αΔT )
RT = R0 (1 + αΔT), where RT = conductor resistance at a temperature of ToC,
R0 = conductor resistance at a temperature of 0oC,
α = temperature coefficient of resistivity,
T = temperature change in oC.
7. A fixed resistance is usually a coil of insulated resistance wire in a container.
Mark the value or the resistance on it unless it is an "unknown" for testing students.
8.See diagram 32.3.04: Ohm's law with digital meters and rheostat.
Digital meters measure the current and the voltage in a simple circuit of a battery and resistor.
The rheostat is adjusted so that the meters read the same, differing only by a factor of 1000.
The battery pack contains six 1.5 volt batteries in series.
Change the number of batteries in the circuit OR change the resistance and observe that the meters change proportionally.
The rheostat consists of a long solenoid of resistance wire that can be " tapped " at any part by a sliding contact.
When connected as shown, the current enters at A, flows along the copper or brass bar (negligible resistance) to s, then via the sliding contact to the solenoid, through that part of the solenoid shown in heavy line, and out at A.
The maximum resistance of the rheostat and the maximum current that may safely be passed through it is usually stamped on the instrument, e.g. 5 ohms, 2 amps.
9. Semiconductors that show a drop in resistivity with temperature are used in thermistors, e.g. in the digital fever thermometer.
10. Mercury becomes superconducting, i.e. has no electrical resistance at 4.2 K.
Other substances have been found with higher temperature zero electrical resistance.
History
The contents of this page is based on "Electricity for Motor Mechanics", by the New Zealand Technical Correspondence Institute Government Printer, New Zealand.