A generator is a rotating machine that converts mechanical energy into electrical energy. Alternating-current (AC) generators are called alternators, and direct-current (DC) generators, are referred to as dynamos. Generators make use of a phenomenon discovered by the British chemist and physicist Michael Faraday. In general terms, when an electrical conductor is moved in the vicinity of a magnet, a voltage is created, or induced, in the conductor. If the ends of the conductor are connected in any kind of closed electrical circuit, this induced voltage (electrical pressure) causes a current to flow in the circuit. The result is that mechanical energy has been converted into electrical energy.
For a more detailed explanation it is necessary to consider the principles of electromagnetism. In an electrical circuit it is voltage that causes the current to flow. The voltage, or electromotive force (emf), is the cause, and current the effect. By analogy, in a magnetic circuit, the driving "pressure" is called the magnetomotive force (mmf). This is the cause and magnetic flux is the result, or effect. Between the north and south poles of a magnet can be envisaged a set of flux lines spreading out from the magnet—the closer these lines are together, the greater the flux density. Flux density is determined by the mmf of the magnet and the permeability of the surrounding medium.
The emf induced in an electrical conductor moving in a magnetic field is determined by the rate at which the conductor "cuts" the lines of flux. The induced emf is therefore related to the speed of the conductor and the flux density. It is also related to the length of the conductor.
A simple alternator
When a closed rectangular loop of wire is mounted on a rotating axis (the rotor) and rotated between the north and south poles of a horseshoe magnet (the stator), the following occurs. When the two sides of the loop parallel to the axis of the rotor (these are the conductors) form a line between the north and south poles, the rate at which these two conductors cut the lines of flux is at a maximum. The induced emf in the conductors is therefore also at a maximum and the current flowing around the loop at its largest value.
When the rotor has turned through 90 degrees, the instantaneous direction of motion of the conductors is along the lines of flux. No lines of flux are therefore cut, no emf is induced in them, and the loop current is zero.
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CAR DYNAMO |
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This typical car dynamo provides DC power through the arrangement of brushes and a commutator. The windings of the revolving armature cut across the flux of the magnetic field provided by the field windings and induces an electric current in the armature windings that can then be used to power the electrical components of the car. The fins on the back of the pulley act as a cooling fan, blowing air through the dynamo to cool the windings.
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When it has rotated by a further 90 degrees, the rate of flux cutting is again at a maximum with maximum emf and loop current. The loop is now, however, upside down compared with its position 180 degrees ago, and the induced emf and current are at a maximum in the opposite direction. A further 90-degree rotation and the current is again zero. By breaking the loop at one end near the axis and connecting the ends to two slip rings on the shaft, this alternating emf can be tapped using brushes touching the rings to drive a circuit.
Most large-scale alternators do not use permanent magnets because they give no degree of control of the magnetic field generating the voltage. Instead, the field is provided by electromagnets consisting of coils of wire fed with direct current (DC). The amount of power required by the field-producing magnets is of the order of around 20 megawatts. The power generated in a large power plant alternator, however, may be 550 megawatts. It would be impractical to collect so much power through slip rings and brushes, and accordingly, alternators are built so that the electromagnets are made to rotate and the coils that receive the induced voltage remain stationary. The DC current for the electromagnets is fed via slip rings and brushes.
