When an electrical current passes through a wire, a small magnetic field is produced around the wire.
If the wire is wound into a coil, the magnetic fields combine to create a stronger and larger magnetic field, with a north and a south pole, just like a permanent magnet.
Turn the current flow off and the magnetic field collapses and disappears. Therefore, by passing electrical current through a conductor coil a magnetic field is produced, and turning it off causes the magnetic field to collapse. This is the operational theory behind an electromagnet.
The reverse is true as well. If a conductor is passed through a magnetic field, or a magnetic field is passed over a conductor, then a current flow is induced into that conductor. This is referred to as electromagnetic induction.
Electromagnets are constructed by winding a conductor wire, many thousands of times, around a soft iron or metal core and passing current through the coil. The strength of the magnetic field produced is determined by the number of turns, or coils, and the value of the current flow through the conductor. The core uniformly aligns the magnetic fields, which strengthens the magnetic effect. The size of the electromagnet is determined by its application. A fuel injector will contain a small electromagnetic coil using very fine wire, whereas a starter motor will use heavier wire in larger coils.
Many components use electromagnetism to operate, so if a component has an electrical connection and movement is created by or within the component, then that component will use an electromagnet. Devices such as relays, solenoids and motors, use electromagnets to create movement. Ignition coils and transformers use electromagnetic induction to raise or lower voltage outputs.
A relay is a device that uses an electromagnet to operate an electrical switch. When a small current is applied to the relay coil, a magnetic field is produced. The magnetic field causes a set of contact points to close and complete an electrical circuit.
Relays are used in circuits with small, low current switches to control circuits that carry high current flow. Most electrical components found in a motor vehicle are controlled by relays. ECU's use relays to control components such as fuel pumps or headlights. Both these circuits carry large electrical loads.
Relays are classified as Normally Open (N.O.) or Normally Closed (N.C.). N.O. relays are open circuited when de-energised and closed by the electromagnetic action. N.C. relays are closed when de-energised and are open circuited when the electromagnet is energised.
Fuel injectors and starter motor solenoids are two of the many solenoid-type components used in a motor vehicle. The operation of a solenoid is similar to a relay, but where a relay uses a magnetic field to close an electric circuit, a solenoid uses a magnetic field to create lateral movement. The metal core, used by the electromagnet to strengthen the magnetic field, is now referred to as an armature. It is positioned so that it is partially outside the electromagnetic coil and free to move in and out. When the coil is energised, the magnetic field draws the armature into the centre of the coil. If the armature is attached to a lever or plunger, it will be forced to move as well. Stopping current flow causes the electromagnet to de-energise and a spring pushes the armature out again.
Another device that uses a solenoid action is a vehicle horn. When the armature is drawn in by the electromagnetic coil, it opens a set of electrical contacts so that current flow through the coil is stopped. This causes the armature to move out again, closing the contacts drawing the armature back in. This process happens at very high speeds. The vibration caused by the rapid movement is transferred to a diaphragm and the familiar horn sound is produced.
Solenoids use magnetic fields to create lateral movement; electric motors use magnetic fields to create rotary movement. Motors consist of two main components: the armature and the field. Both the armature and the field contain electromagnets. The interaction between the stationary field coils and the moveable armature coil causes the armature to rotate. The design and orientation of the electromagnetic coils ensures the armature continues to turn. The armature is connected to the electrical supply by a set of carbon brushes that contact a commutator, which is a component part of the armature. The brushes allow the electrical connection even when the armature is turning. Various design characteristics of electric motors speed and torque characteristics appropriate to their application.
Ignition Coils and Transformers
Ignition coils and transformers both operate under the operating principles of electromagnetic induction. They use electromagnetism to produce electricity, rather than movement.
An ignition coil can be described as a step-up transformer. That is because the output of 60 kV (or more) is higher than the input, nominally 12 V. Ignition coils are sometimes referred to as auto-transformers, as some designs connect both the primary and secondary coils at a common point.
Two coils are used - one coil, referred to as a primary coil, is wound around a second, or secondary coil. The primary coil will have 200 or 300 turns of light gauge wire while the secondary will have approximately 30,000 to 60,000 turns of very fine wire. When current is passed through the primary coil, the magnetic field builds. When the current is turned off, the magnetic field collapses and influences the secondary coil by its moving (collapsing) magnetic field. A current flow is induced into the secondary coil that has an EMF that is many times greater than the EMF in the primary coil. This is delivered to the spark plug, which ignites the air/fuel mixture in the engine.
The transformer action causes heat to be produced. In the past, the internal coils were immersed in cooling oil allowing the heat to be conducted to the case. Modern ignition coils do not use oil. They are usually constructed using a heat conducting hard resin and are cooled by their location on a heat sink, or by passing air.
Step-down transformers operate under the same operating principles. The only difference is that the secondary coil has fewer turns than the primary, providing a lower induced output.