«Auto−Electric Basic Technology − Part 2 Table of Contents Auto−Electric Basic Technology − Part 2 1. Fundamentals of magnetism Effects of ...»
Auto−Electric Basic Technology − Part 2
Table of Contents
Auto−Electric Basic Technology − Part 2
1. Fundamentals of magnetism
Effects of electric current
2. Function of electromagnetic components
Kinds and function of relays
Further electromagnetic components
3. Starting systems
Principles of electric motors
Starter motor principles
Practical starter motors
Inertia drive starter motor
Single−stage sliding gear starter motor
Pre−engaged starter motor
Faultfinding and repair of starter motors
4. Charging systems
Regulator types for DC−generators
Permanent magnet generators
Principle of voltage regulation
Types of regulators
System inspection and checks
HARTMUT ARLITT/GERMAN DEVELOPMENT SERVICE IN NAMIBIA
INSTRUCTOR AT THE RUNDU VOCATIONAL TRAINING CENTRE
BY USING PARTS OF:
1. Automotive Encyclopedia/Fundamental Principles, Operation, construction, Service and Repair 1995 Edition; South Holland, Illinois; The Goodheart−Willcox Company, Inc.
2. Different information material of the BOSCH company Germany
3. Different teaching material from the Deutsche Gesellschaft fuer Technische Zusammenarbeit (GTZ) GmbH
4. Own material, scripts and circuit diagrams MAY 1999
Lehr− und Lernmittel, Informationen, Beratung Educational Aids Literature, Consulting Moyens didactiques, Informations, Service−conseil Material didáctico, Informaciones, Asesoría
1. Fundamentals of magnetism Effects of electric current
There are four effects of electricity that a flow of current will produce:
1. Heating effect: The friction created by the movement of the electrons causes a generation of heat in the material known as resistive heating. The resistive heating is made use of in elements of electric stoves etc.
2. Lighting effect: By passing of an electric current through thin wires of metal with a high melting point (i. e. wolfram, tungsten) those wires heat up so strongly that they begin to glow.
In this state they serve as a source of light. The higher the temperature, the greater the light yields. In bulbs the wires are placed in a vacuum or in a protective gas. So the wire cannot oxidise. A current flow trough gas can also be used to produce light as a result of the collisions between the charged gas particles (fluorescent tubes)
3. Chemical effect: The passage of a current can split up the molecules in liquids and solids, a process known as electrolysis. This forms the basis to produce an e.m.f. (electromotive force or voltage) by a battery.
4. Magnetic effect: The current flow in a conductor or coil sets a magnetic field flux or force around it which is more pronounced when an iron core is present. The strength of that force (symbol: F) depends upon the value of the current and the number of turns on the coil/winding.
Magnetism is, like electricity as well, still a mystery. We know many laws governing its behaviour and have applied it in the automotive field but no one knows what magnetism really is.
The effects of magnetism were first discovered when it was found that pieces of iron core and also other pieces of iron on several places of the earth attract each other.
In the automotive needs of magnetism there are used permanent magnets as well as electromagnets, depending what we do want to reach with that different types of magnets.
Magnetism Magnetism we do find in nature mostly as permanent magnets. Magnets attract iron filings (and nickel & cobalt). In the vicinity of the magnet there is a magnetic field with a definite direction, which is strongest at the ends of the magnet.
See below the field lines of permanent magnet bar:
If a bar magnet is positioned on a pointed pivot so that it can move freely, it aligns it self in a north−south direction. The pole−pointing north (of the geographical pole of the earth) is the North Pole of the magnet; the opposite is the South Pole.
The field lines are imaginary plots, which indicate the direction of the magnetic force. In the space surrounding it (air or non−magnetic material) they run from the north to the South Pole. Inside the magnet, they run from the South Pole to the North Pole. The field lines are therefore closed (continuous).
If a magnet gets divided or broken, in every part will be obtained a north and a South Pole.
If a conductor is wound into a coil, the magnetic field lines are concentrated together inside this coil. And there inside the field lines of the individual turn of the coil are added together.
If a conductor is formed into a loop the lines of force on the outside of the loop spread out into space.
Lines on the inside of the loop are confined and crowded together. This increases the density of lines of force in that area and a much greater effect is produced with the same amount of current.
The total number of coil field lines refers to the magnetic force depending on the current intensity and the number of windings. That means, too, the same magnetic effect can be reached by − a low current strength and a large number of windings or by − a high current strength and a small number of windings.
By increasing the number of loops, the magnetic field will be greatly increased.
By winding the loops or a coil on a soft iron core, the field is further intensified. That means the magnetism of this electromagnet increases. On that way there is a possibility to build different electromagnets by using different strength of the magnetic fields.
This kind of magnetic pattern is obtained i.e. in coils of the solenoid of starter motors, of the ignition coil etc.
Magnetic conductivity The conductivity of air for the lines of force is defined by Kelvin. “It is the ease with which lines of force may be established in any medium as compared with a vacuum”.
When a soft iron core is inserted in a coil to form a true electromagnet, the lines of force will be increased several hundred times. That means there are more lines of force created. Field coils in starter motors, regulator−windings on iron−cores and ignition coils are using all the same principle.
A solenoid is a tubular coil of wire. It is designed to produce a magnetic field. Mostly the solenoid includes an iron core that is free to move in the tubular coil. The movement of such kind of an iron core is used to operate in the case of mechanical work i.e. as a switch. If a solenoid is used to close/open the contact points of an electrical switch then is it called an electromagnetic switch.
NOTE: The South Pole of the iron core is adjacent to the North Pole of the coil. The polarity of a movable iron core is induced by the lines of force from the coil. The poles of the coil and the core are in opposite polarity and so there is an attraction that draws the movable core into the centre of the coil whenever current flows through the coil.
2. Function of electromagnetic components
Out of the last chapter we do know already that in the automotive field is given a far use of electromagnetic components. In this chapter, you get not the information to all the possible components. The function of such parts/components like starter motors included starter solenoid or like generators and alternators included the types of electromagnetic regulators will be handled in the following chapters.
Kinds and function of relays
Now let us follow up three different kinds of relays:
The main parts of a relay are the winding, the iron core, the contact points and the terminals.
1. Normally open relay As you can see in the sketch below the contact points of this relay are open as long as the electromagnet is not energized.
How works such kind of a relay?
Between the terminal numbers “85” and “86” there is placed the electromagnet (winding & soft iron core). This part of the relay we may name as steering circuit. That means, if we connect one of these terminals to the ground and the other one to the positive a current flow through the winding and build up a magnetic field. The strength of the built up magnetic field develops higher by the soft iron core and is in the end strong enough to pull the movable arm with one contact point downwards by making contact with the other contact point. This is possible because the magnetic power is now stronger as the mechanical power of the spring what holds the movable arm normally in the top. When the contact points are closed the main circuit is closed as well.
Now the current from the side of the terminal “30” flows over the contact points to the terminal “87” and from there to the connected consumer.
Note: if there is not a current flow through the steering circuit and the contact points are not closed anymore then the main circuit opens again and so the connected consumer is switched off.
2. Normally closed relay The contact points of the relay are closed as long as the electromagnet isn’t energized.
As you can see in the sketch above by the situation of closed contact points the so named main circuit of the relay between the terminals “30” and “87” is closed as well. That means when the electromagnet of this kind of relay isn’t energised the consumer connected to the terminal “87” works.
First by energising of the electromagnet (steering circuit) and so pulling down the movable arm the main circuit get switched off and the consumer doesn’t work anymore.
3. Change over relay By using this third type of relay there is always one contact point (“87” or “88”) in closed position with the terminal “30”. By the example of the sketch below the contact point of the terminal “30” is in connection with the upper contact “88” as long as the steering circuit isn’t energised. If the electromagnet is switched on the connection changes over to make contact between “30” and lower the contact point (in the example of the sketch it is the terminal “87”).
Such a relay can be used for example to connect spot light and fog light over it to change automatically in connection with the dim light/bright light.
Why there is a need to use relays?
The reason to use in several cases a relay is given by a higher current flow in some circuits like for double horn, spotlight, fog light or for the headlight.
But practically you will find always again i. e. headlight circuits in motor vehicles without using a relay.
It is to realise that this depends on the kind of installation for some circuits in a motor vehicle include the use of different parts like switches.
By using a relay the needed stronger current follows up a shorter way (main circuit over the relay). For the steering circuit to the relay (by a small amount of the steering current) is it possible to use a much thinner cable. By looking to the safety aspect shorter ways of the high current flow can be created and about this is to think as well by the installation through the metal body of a car. The use of original installed switches is a further point by a later installation of a circuit and this is an economically aspect. For example a change from a single horn to a double horn circuit can lead to a problem by using the original horn button. If the contact points of that button are to weak/to soft the points can fast be damaged and on the other hand installing another horn button can be more expensive as the connection of a relay.
Final remark to the relays:
There is never a possibility to use inside a relay a permanent magnet. It have always to be an electromagnet (by the use of a winding and an iron core) to ensure the relay can be switched on and/or off.
Further electromagnetic components Up to now there were handled relays only but we do have much more other electromagnetic components in motor vehicles in use.
Here let’s mention only some of the other electromagnetic components because they get their necessary attention by following up that several topics in the later modules.
It is a special kind of a relay used in connection with the starter motor by protection of the ignition lock and as link between the battery and the starter motor.
This types of electromagnetic components are used for the regulation of the voltage/current output of generators and alternators to ensure by speeding up of the engine that the voltage/current increases not to high and damages other electrical components.
The regulator is a very important component to ensure that the recharging process goes on properly and the battery remains always in good condition
The flasher unit is used in the indicator circuit and is responsible to let the switched on indicator lamps (left or right side) and if necessary all the indicator lamps by the use of the hazard unit flash.
Responsible to build up the high−tension voltage needed for developing of strong sparks for the ignition system.