School Science Lessons
(UNPh32.5)
2024-09-16
Electricity in motor vehicles 1.
Contents
32.5.3.1 Battery care (Experiment)
32.5.5.3.1 Condenser, (capacitor)
32.5.4.0 Electromagnets
32.5.6.0 Generator (dynamo)
32.5.5.0 Ignition system
32.5.1.0 Series and parallel circuits
32.5.2.0 Wring system
32.5.1.0 Series and parallel circuits
32.5.1.01 Series and parallel circuits
32.5.1.3 Current through a bulb
32.5.1.4 Electric power
32.5.1.2 Parallel circuits
32.5.1.1 Series circuits
32.5.2.0 Wiring system
32.2.0 Circuit diagrams, electrical symbols
32.5.2.1 Electromagnetic circuit breaker
32.5.2.2 Fuses
32.5.2.3 Technical and physical diagrams (a) ignition coil (b) fuel gauge
32.5.2.4 Technical and physical diagrams, ignition system
32.5.2.5 Thermal circuit breaker
32.5.2.6 Wires, Lucas cable colour system
32.5.2.71 Wiring system diagrams
32.5.4.0 Electromagnets
32.5.4.1 Cut-out relay
32.5.4.2 Electric bell
32.5.4.4 Voltage regulator
32.5.5.0 Ignition system
32.5.5.1 Motor vehicle ignition system, primary circuit
32.5.5.2 Four cylinder ignition circuit
32.5.7.0 Magneto
32.5.5.5 Primary and secondary circuits
32.5.5.7 Self induction
32.5.5.8 Spark plug gap
32.5.5.9 Spark plug operating temperature, pre ignition
32.5.5.10 Spark plugs
32.5.5.7 Transistorized ignition system
32.5.7.0 Magneto
32.5.5.3 Magneto, permanent magnet magnetos
32.5.5.4 Polar inductor magneto
32.5.5.6 Rotating magnet magneto
32.5.6.0 Generator (dynamo)
See diagram 32.5.6.2A: Alternating current, graphs A.
See diagram 32.5.6.2B: Alternating current, graphs B.
Sulfation
32.5.0 Electrical equipment of motor vehicles
Components:
1. Generator (DC) 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.
32.5.1.01 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.
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: Parallel circuits.
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,
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 6.3.1.4: Electric current, ampere.
See diagram 32.5.1.3: Current through a bulb.
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 × volts, I × 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: Electric power.
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
32,5,2,732.5.2.1 Wiring system diagrams
See diagram 32.5.2.1: Wiring system of automobile.
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.
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.
Table 32.5.2.1
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
distributor 2
 earth
6 spark plugs
. . . ignition S4
 heater, demister

32.5.2.3 Technical and physical diagrams, ignition system
See diagram: Ignition system.
32.5.2.4 Technical and physical diagrams
See diagram 32.5.2.3: Ignition coil.
See diagram 32.5.2.4: 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.
32.5.2.6 Wires, Lucas cable colour system in English cars
Wire, (Commercial).
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.
32.5.2.2 Fuses
See diagram 32.5.2.6: Fuses.
A fuse is a wire or strip of metal mounted in a fitting that 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.
32.5.2.5 Thermal circuit breaker
See diagram 32.5.2.7: Thermal circuit breaker.
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.
32.5.2.1 Electromagnetic circuit breaker
Electromagnetism, (Commercial).
See diagram 32.5.2.8: Electromagnetic circuit breaker.
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.
32.5.3.1 Battery care
Batteries (Commercial).
1. Be careful! Never short-circuit an automotive battery.
Excessive current demand may cause battery acid to boil out.
1.Keep the battery case clean.
Use hot soapy water then make it perfectly dry.
Clean the battery terminals at vehicle service.
2. Remove filler caps and check the electrolyte level each month.
Fill the battery with deionized water or distilled water to just above the battery plates.
Do not overfill.
Mop and dry any spillage of water.
3.The battery must be secure in its cradle.
Vibrations of the battery can damage the battery plates.
Loose or corrodes battery terminals may cause breakdowns.
4.The battery will run down without regular use.
It runs down quicker if use is infrequent or used only for short runs.
To avoid problems make sure that the battery is regularly recharged and tested.
5. If battery is drained, because of leaving lights on revive it with a slow charge.
Under-charging and over-charging will shorten battery life.
6. Poor engine condition will shorten the life of a battery.
7. When working on a battery, do not smoke or have a naked flame nearby, because batteries produce hydrogen gas.
8. Before trying to jump start by pushing the car or using jump start cables, make sure that you understand the electrical design of that car.
Do not try to jump start a car fitted with electronic systems, but get expert assistance.
9. Battery patrol check list:
* No load battery voltage
* Discharge test voltage
* Discharge test gassing
* Physical damage evident
* Electrolyte level
* Terminal security and cleanliness
* Discharge to earth
* Alternator drive tension
* Alternator output
* Age of battery.
32.5.3.2 Lead-acid battery secondary cell
See diagram 32.5.3.2: Lead-acid battery secondary cell.
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.
2. 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.
3.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.
4. 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.
5. 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 a battery
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, 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 gas and oxygen.
This is called gassing.
32.5.3.4 Cell connections of a lead cell battery
See diagram 32.5.3.4 Cell connections of 6 volt batteries
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.
The negative terminal of the battery is usually 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 charged with negative terminal connected to positive terminal of the charger, the battery may be 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 State of charge of a motor vehicle battery
See diagram 32.5.3.5: Battery hydrometer.
There is no direct method to measure a battery's state of charge.
1. Measure its static voltage and compare it to a standardized chart.
This is the least accurate method, but it only involves an inexpensive digital meter.
2. Measure with an amp-hour meter to monitor all power moving in and out of the battery by time, and state of charge is determined by comparing flow rates.
3. Measure the density, specific gravity, of the sulfuric acid electrolyte with a battery hydrometer.
This is the most accurate test, but yet it is only applicable to flooded lead-acid batteries.
Electrolyte density is lower when the battery is discharged and higher as the cells are charged.
The battery's chemical reactions affect the density of the electrolyte at a constant rate that is predictable enough to get a good indication of the cell's state of charge.
4. Maintenance-free batteries supplied to some modern motor vehicles under normal operation conditions will not need water addition within its average battery ife.
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.
5. 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.
6. 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.
7. 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 ondition.
32.5.3.6 Prepare lead-acid battery electrolyte
This solution is sometimes called "accumulator electrolyte".
1. Be careful!
Use safety glasses and nitrile chemical-resistant gloves!
Wear protective clothing!
Follow the recommendations of the manufacturers for filling and initial charging that is usually printed on the battery.
The relative density of sulfuric acid is: fully charged 1.28, half charged, 1.21, discharged 1.15.
Slowly add concentrated sulfuric acid, with stirring, to a strong beaker two thirds full of deionized water or distilled water, until the solution almost boils.
Leave to cool and add more acid until the solution almost boils.
Leave to cool to room temperature.
Adjust the relative density by adding more acid or more water, according to the hydrometer reading.
When the cell is not in use, use a jar with a cover to prevent drying by evaporation.
Use safety glasses and nitrile chemical-resistant gloves!
Slowly add concentrated sulfuric acid, with stirring, to a beaker two thirds full of distilled water, until the solution is almost boils.
Leave to cool and add more acid until the solution almost boils.
After cooling to room temperature, adjust the relative density with more acid or more water, according to a hydrometer reading.
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.
Keep a labelled jar of baking soda solution near by in case of spills on the skin.
2. 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.
BE CAREFUL! REMEMBER: ACID TO WATER, NEVER WATER TO ACID!
3. 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.
Put the water required into the container first then pour the acid slowly into the water while stirring.
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.
4. Sulfation
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 Battery capacity, Ampere-hour (Ah)
1. Battery capacity
Capacity of a battery refers to the quantity of electricity it can deliver.
The ampere-hour (Ah), is the quantity of electricity equivalent to a current of one ampere flowing for one hour.
The milliampere-hour is one-thousandth of an ampere-hour.
(1 Ah = 3 600 coulombs of charge, 1 amp = 1 coulomb per second.)
Capacity in ampere-hours = current in amperes × 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.
2. Ampere-hour (Ah)
Amp-hour capacity for a battery, it is specified at either a given current, given time, or assumed to be rated for a time period, e.g. 8 hours or 10 hours.
Automobile batteries can be rated by measuring the ability to supply sufficient current to operate the starter, or operate the lighting system.
The approximate amp-hour capacities of some common batteries: :
* Typical automotive battery: 70 amp-hours at 3.5 A (secondary cell)
* D-size carbon-zinc battery: 4.5 amp-hours at 100 mA (primary cell)
* Nine volt carbon-zinc battery: 400 milliamp-hours at 8 mA (primary cell).
3. The 10-hour rating. For automobile batteries, 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 × 10 = 50 ampere-hours.
The advantage of the 12 volt automobile battery is that it has twice the watt-hour capacity of the 6 volt battery, 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.
4. Watt hour capacity
Power from 6 volt battery, W = V × I = 6 × 5 = 30 watts.
Energy capacity of 6 volt battery = power × time = watts × hours = 30 × 10 = 300 watt hours.
Power from 12 volt battery, W = V × I = 12 × 5 = 60 watts.
Energy capacity of 12 volt battery = power × time = watts × hours = 60 × 10 = 600 watt hours.
So watt hours = ampere-hours × voltage.
Watt hour capacity = ampere-hour capacity × battery voltage.
22.2.05 Battery storage capacity
The storage capacity is the amount of energy a battery can store.
The storage capacity of a battery is the current that can be delivered × time, so the storage capacity of a battery in watt-hours = ampere-hour capacity × battery voltage.
The storage capacity of a battery will have a maximum value as "maximum deliverable current" or a minimum as "shelf life" in a supermarket.
Battery life / battery longevity may be expressed as Reserve Capacity, (RC), and ampere-hours, Amp Hours (Ah or AH).
Amp hours is the most common unit for battery capacity.
Amp hours = current X time.
Batteries used in photovoltaic systems are rated in Ampere Hours (AH).
So a 100 AH battery can supply 1 amp for 100 hours, or 100 amps for one hour.
Small cells have storage capacity up to 200 milliamperes, nAH.
Large lead-acid batteries have storage capacity more than 100 AH.
If a battery is rated 10 AH at 12 volts DC, Power, P in watts = VI, volts × amps, multiply both sides by t, Watt-hours, Pt = VIt = 12 × 10 = 120 watt-hours.
The Ampere hour rating, Ah, is the current available when discharged evenly over a 20-hour period, the standard time length for rating batteries.
Reserve capacity is the number of minutes that a battery can supply a useful voltage (10.5 volts or more), under a 25 amp discharge rate.
Reserve capacity is used for batteries that run heavy loads.
A battery specification "RC@ 25A = 160 minutes" means that at 80oF (about 27oC), the battery can supply 25 amps of current at a usable voltage for 160 minutes.
Reserve capacity is often a truer test of battery life than amp hours, depending on how the battery is used.
The "shelf life" is the length of time a battery can remain in storage, (not connected to a load), without losing its energy capacity.
However, the metal plates eventually leak and react with each other, even though the battery is not in use.
Maximum deliverable current is the largest current a battery can push through a load without drop in its output voltage.
22.2.06 Deep-cycle battery
A deep-cycle battery should discharge at least 50% of its capacity, for the best life span versus cost.
It may be capable of withstanding repeated substantial discharges of up to 80% capacity.
The depth of discharge of the battery is related to the battery life, i.e. the number of charge and discharge cycles it can perform.
Deep-cycle batteries are used in sweepers, scrubbers, jacks, lifts, electric golf carts, boats, cathodic protection, uninterruptible power supplies for computers, and off-grid solar and wind power systems.
All deep-cycle batteries are classified and rated in amp-hours.
Amp-hours is the term used to describe a standardized rate of discharge measuring current relative to time.
It is calculated by multiplying amps and hours.
The generally accepted rating time period for most manufacturers is 20 hours.
Heavy duty deep-cycle batteries may rate up to 260 AH.
Deep-cycle batteries are generally rated in amp-hours at a C/20 discharge rate, which means 1/20 of the battery capacity for 20 hours until the battery reaches 1.75 volts per cell.
A battery rated at 100 amp-hours will maintain a 5 amp load continuously for 20 hours.
Actual capacity will vary with temperature, the size of the load and the rate of discharge.
Deep-cycle batteries used in UPS and telecommunication applications are rated in reserve capacity, which is the number of minutes the battery will maintain a constant 25A load at 80oF until voltage drops to 1.75 volts per cell.
To provide an approximate conversion to amp-hours, multiply reserve capacity by 0.6.
22.2.07 VRLA sealed battery
A sealed VRLA battery (Valve Regulated Lead Acid), is maintenance free, produces negligible gas when charging, easier to transport, because unlike wet "flooded"' batteries, they are not classified as "hazardous cargo".
The three sub-types are, wet, AGM, Gel.
The sealed wet type is primarily designed for the leisure and marine markets, some of which claim to be able to complete around 400 to 500 discharge cycles (to 80% depth of discharge).
22.2.08 Starting battery, car battery
A starting battery used in most motor cars provides short, high current bursts for cranking the engine.
So it frequently discharges only a small percentage of its capacity.
Engine starting batteries are rated in cold cranking amps, CCA.
Engine starting batteries are designed to provide a heavy surge current of as much as 200 amps for a period of 5-10 seconds.
There is no direct correlation between CCA and amp-hours, because starting batteries are not designed for slow periods of deep discharge.
32.5.3.8 Starting the engine with jumper leads
Automobile Jumper Leads (Commercial).
Precautions before jump starting the engine
Ensure that the ignition switch is in the OFF position.
Turn off the lights and power draining functions to help ease the strain off the donor battery.
Ensure that the donor battery is the same voltage as the flat battery.
Ensure that the two vehicles are not touching in anyway
Make sure that the vehicles are in either Neutral (Manual) or Park (Auto)
Remove the vent caps from non maintenance free batteries.
Be careful! Battery acid is corrosive!
Procedure for jump starting the engine
1. Connect the red coloured jumper lead to the (+) positive terminal of the booster battery and the other end of the red jumper lead to the (+) positive terminal of the flat battery.
2. Connect the black jumper lead to the (-) negative terminal of the booster battery and the other end of the black jumper lead to a good earth point on the disabled vehicle.
The engine block is typically the best place for a good earth point.
3. Start the engine of the disabled vehicle.
4. With the engine speed at idle, disconnect the jumper leads in the reverse order of connection.
The car has now been started and after any caps and covers have been replaced it is ready to drive.
The cause of the flat battery should be identified and rectified as soon as possible.
5. Late model cars are generally more dependent on complex electronics to function and any voltage spike can damage the delicate electronic circuitry.
To avoid this problem, use jumper leads fitted with a "spike guards", (surge protectors).
32.5.4.1 Cut-out relay
See diagram 32.5.4.3: Cut-out system open, generator idling, Cut-out system closed, generator charging, Cut-out system closed, but will soon open, generator slowing.
A cut-out relay is used on all types of DC 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.
So 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.
32.5.4.2 Electric Bell
See diagram 32.5.4.4: Electric bell, electric buzzer.
In the diagram a piece of soft iron A 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.
32.5.4.4 Voltage Regulator
See diagram 32.5.6.5 (a): Simple voltage regulator.
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.
32.5.5.1 Motor vehicle ignition system, primary circuit
Electromagnetism, (Commercial).
See diagram 32.5.5.1: Primary circuit.
1. Ignition system
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.
2. Ignition system components
The components include the following:
* A source of voltage to supply the electrical energy necessary to produce an electric spark, such as a battery, generator, or alternator
* 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.
* 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.
* A device, commonly called an automatic advance unit, for varying the ignition timing to suit the operating conditions of the engine.
* A distributor, in engines having more than one cylinder, to direct the spark to the right spark plug at the correct instant.
* A spark plug for each cylinder.
* A switch to operate the ignition system for starting and stopping the engine.
3. Induced voltage
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.
An electromagnet can 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.
4. Electromagnet winding
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.
32.5.5.5 Primary and secondary circuits
See diagram 32.5.5.2: Secondary circuit.
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.
32.5.5.7 Self induction
See diagram 32.5.5.3: Self induction circuit.
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. 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: Ignition coil.
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 to 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.
32.5.5.2 Four-cylinder ignition circuit
See diagram 32.5.5.4: Four-cylinder ignition circuit.
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.
So 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.
32.5.5.10 Spark plugs
Spark plugs, operating temperature, pre ignition, spark plug gap
See diagram 32.5.5.5: Spark plugs.
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.
32.5.5.9 Spark plug operating temperature
16.6.8.0: Octane (C8H18), Octane number
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.
32.5.5.8 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 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.
32.5.5.3 Permanent magnet magnetos
Motors that 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 that 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.
The necessary high tension output is obtained by opening the primary winding with a contact breaker, when 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 Rotating magnet magneto
See diagram 32.5.5.6: 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, that 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, when 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.
32.5.5.4 Polar inductor magneto
See diagram 32.5.5.6: 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 allows the portions of the magnetic circuit that 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 in the diagram, 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.
32.5.5.11 Transistorized ignition system
See diagram 32.5.5.7: Transistorized ignition system.
A transistor may be compared with an electrical relay in that it is an electrical device that 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 so 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.
33.2.1 Lead accumulator cell
The Lead cell accumulator has EMF about 2 volts and very low internal resistance.
It is a secondary cell.
The terminals are usually marked + (red) and - (black).
Since the internal resistance is very low, great care must be taken to avoid "short circuiting " the cell, i.e. there must always be a resistance of at least 1 ohm in the external circuit connecting the terminals.
1. Use a 250 mL beaker or jar with a cover to prevent drying by evaporation when the cell is not in use.
You need 2 sheets of 40 × 10 cm thin lead foil and 2 lead strips 2 × 14 cm as terminals.
These lead pieces require thorough cleaning by means of wire wool.
Fold the long sheets of lead tightly to the shorter strips so that they make good electrical contact.
The projecting ends will serve as terminals.
A blotting paper B lead c terminals A sandwich is made of alternating strips of lead foil and blotting paper.
When the sandwich is ready it is rolled up quite tightly, secured round the outside with one or two elastic bands, and placed with terminals at the top, in the cup or jar.
Mark one terminal positive, and the other negative.
The roll is covered with a solution of sodium sulfate made by dissolving 40 g of anhydrous sodium sulfate crystals in 200 mL water.
The cell is now ready to charge with electricity.
This can be done with a 6 volt battery charger, or with any low voltage direct current supply giving up to 10 amps.
Connect positive on the charger to positive on the cell.
After only a few minutes charging, the cell will light a 1.5 volt bulb.
Provided that the cell is always connected to the charger in the same way, as described above, the more times it is charged and discharged, the more efficient it becomes.
There will be enough current to make a small 1 volt electric motor spin round.
The cell will remain serviceable for several months if the cover is put on when not in use.
2. Charge a simple lead-acid battery with two electrodes, lead plates, in a sulfuric acid solution for a short time and then discharge through a doorbell.
Charge two lead plates in 30% sulfuric acid and discharge through a flashlight bulb.
3. Internal resistance of batteries, weak and good battery.
Measure similar no load voltage on identical looking batteries and then apply a load to each and show the difference in voltage between a good and weak battery.
History
The contents of this page is based on "Electricity for Motor Mechanics", by the New Zealand Technical Correspondence Institute Government Printer, New Zealand.