The movement of electrons from one place to another. What is electricity? The movement of electrons from one place to another.

Electrical terms. List 4 terms used to measure electrical activity.

Electrical terms. Volts. (Pressure) Voltage (V) is the pressure forcing energy to move through a circuit and is equivalent to water pressure.

Electrical terms. Amperes (Current Flow) Current (A) is a rate of flow of energy through a circuit and is measured in amps. it is like the amount of water flowing through the pipe

Electrical terms. Ohms (Resistance) Ohms (Ω) are a measure of resistance to current flow through a circuit and are equivalent to the water pipe size or diameter.

Electrical terms. Watts. (Power,) Work done. Watts is the measurement of work done. Electrical power is measured in watts. In an electrical system power is equal to the voltage multiplied by the current Headlight bulbs are rated in watts

Conductors and insulators Factors affecting the resistance of a conductor. Length of the conductor. Cross sectional area The material the conductor is made of The temperature of the conductor

Factors affecting the resistance of a conductor.

What is the main conductor used in motor vehicles? COPPER Reasons why copper is preferred, Very low resistance to electron flow. Malleability. Describes a material that means it can be easily shaped. Ductility. The property of a material that allows it to be drawn into wires.

The main insulator used is PVC because of its, High resistance to electron flow. Resistance to water, fuel, oil and other contaminants.

Earth return List 2 reasons why the vehicles chassis is used as the earth return circuit. Cheaper, less wire is required Lighter, less wire means less weight

Insulated earth return List vehicles where the vehicle frame could not be used as the earth return. Vehicles with composite bodies Fuel and chemical tankers

Cable sizes. The quoted size of a cable refers to the number of strands and the wire diameter. Example. 9/0.3 = 9 strands of wire, each being 0.3mm diameter. As a guide, a 0.3mm strand of wire will carry approximately 0.5 amps of current.

List 2 reasons why it is important to select the correctly rated cable for each application. To ensure enough power is supplied to the consumer with minimal losses Over rating of the cable will increase vehicle weight and cost.

Fuses a fuse contains a metal strip which is designed to overheat and melt when subjected to a specified excessive level of current flow, breaking the circuit and stopping the excessive current flow from potentially damaging the more valuable components

circuit breakers. Circuit breakers are not destroyed by excess current. A bimetallic strip heats up and bends, opening a set of contacts and breaking the circuit. Circuit breakers can be either manual or automatic reset types depending on the type of circuit they are protecting. Electric windows, window heaters

Batteries.

Batteries - Cell Construction Each plate consists of a lattice type grid of lead-antimony. Into this is pressed the active material. This is a lead oxide paste electrically formed into: Lead peroxide (positive and brown in colour) and Spongy lead (negative and grey in colour) When fully charged.

At this point it is worth noting that; The loss of sulphur from the electrolyte to the plates during the discharge process decreases the density of the electrolyte. This feature enables the state of battery charge to be assessed by using a hydrometer.

RELATIVE DENSITY The relative density goes up as the battery is charged. This is because sulphuric acid is released from the plates and this mixes with the electrolyte, thus increasing relative density. CONDITION RELATIVE DENSITY BATTERY VOLTAGE Cell fully charged 1.270-1.290 12.7 Cell 70% charged 1.230-1.250 12.4 Cell discharged 1.110-1.130 11.9 Distilled water as a relative density of 1.000

Battery rating. Reserve capacity Means the time in minutes a battery will supply 25A at 25°C, to a final voltage 1.75v per cell. This gives an indication of how long a battery could run the car if the charging system was not working. (44 Ah batteries will have a reserve capacity of approx 60 mins).

Battery rating. Ampere-hour rating The amount of energy measured in AMP hours from a fully charged battery down to a nominal voltage of 10.8v. A 20 hour time period is used, e.g. a 50 Ah battery should deliver 2.5 amps for 20 hours before the terminal voltage falls to 10.8 volts.

The battery internal resistance

The starter motor converts electrical energy to mechanical energy What is the engine speed required during cranking to start an engine? A minimum of 100rpm.

There are 3 types of starter motors used on modern motor vehicles: 1.       inertia drive 2.     pre engaged 3.     reduction drive

Inertia starter

Starter action

Describe the 2 actions that the solenoid completes during starting. 1 The plunger movement engages the drive pinion with the ring gear 2 Closes a set of heavy duty contacts, allowing a large current to flow from the battery to the starter.

Solenoid wiring. The starter solenoid consists of 2 sets of windings closing windings, used to pull the plunger into the closed position holding windings, used to maintain the plunger in the closed position.

Over run clutch This clutch uses a system of rollers and a cam. As the starter motor turns (armature), the springs force the rollers down the cam face in the clutch housing, this action causes the rollers to lock onto the pinion collar and the pinion rotates with the armature. As soon as the engine starts, its rotational speed instantly exceeds the speed of the armature. At this instant the over-running clutch breaks the connection between the pinion and the armature shaft by allowing the rollers to move up the cam against the spring. This action prevents over-speeding of the armature.

Permanent magnet starter operation Permanent magnet starters increase output while at the same time the unit is lighter and more compact. The permanent magnets are used in place of the field coils. When the driver actuates the key, the starter solenoid operates the same as in the pre engaged starter. As the heavy contacts close, power flows through the armature windings only which create a magnetic field and it acts against the permanent magnetic field to rotate the armature. Points to remember about permanent magnet starters. Six-pole permanent-magnet is used. The planetary reduction gears have ratios between 3.38: 1 and 5.67: 1. The gears are made of glass-fibre reinforced polyamide plastic

Size comparison

Alternators

The rotor

The rotor The rotor has two clawed shaped pole pieces that are mounted on the rotor shaft. The windings of the rotor field coil fit between the poles. The shaft also carries two slip rings for power to reach the rotor field coil. When the field coil is energised, one end of it becomes a North pole and the other a South pole. The rotor can reach speeds up to 12000rpm. Two small brushes rest on the slip rings which allow the rotor to receive current

The rotor

The stator

The stator The stator has three-phase windings. The three separate windings are assembled into slots in a soft iron frame. Each winding has the same number of loops to produce a uniform voltage. The windings are connected so that the voltage produced in all the loops is combined. The windings can be connected in two different ways. These are known as star and delta.

Star and delta windings

Stator/Rotor relationship

Alternators produce an AC in each set of windings. The component that changes this into the required DC voltage is the rectifier.

The rectifier Six diodes (three pairs) are needed to rectify the current the stator windings.

Rotor Field Excitation The rotor requires voltage to produce a magnetic field. Engine stopped, ignition on, battery voltage is applied to rotor, via charge warning lamp. Lamp is on and current flows through rotor winding. Engine running, alternator speed increases, voltage output also increases. Alternator voltage > battery voltage = lamp out, battery being charged = alternator current flowing through rotor, via field diodes.

The diagram below shows a simple potential divider circuit

Good luck with your exam.