DC and AC Generators and AC Induction Motors - Learn

DC and AC Generators and AC Induction Motors – Learn

Generators

A generator is a device that transforms mechanical kinetic energy into electrical energy. The structure of a generator is very similar to that of a motor, however, the function is basically the opposite:

A generator converts mechanical energy into electrical energy via electromagnetic induction.
A motor converts electrical energy into mechanical energy via motor effect.

In a generator, the relative motion between a coil and a magnetic field induces an emf in the coil. In small generators the coil is rotated in a stationary magnetic field but in larger applications like power stations, the coils are stationary and the magnet which is usually an electromagnet rotates inside them. For the purposes of developing our understanding of generators, we will consider a rotating coil inside a stationary magnetic field.

Operation of a Generator

As a coil rotates in a magnetic field, the magnitude of the magnetic flux passing through the area of the coil changes. This changing magnetic flux produces a changing emf across the ends of the wire. The induced emf in the coil is equal in magnitude to the rate at which the magnetic flux through the coil is changing with time.

The diagram below illustrates a coil rotating in a magnetic field and the amount of flux through the coil (yellow) and the induced emf (green). Observe how the induced emf follows the law above:

Important points to understand:

It is the rate of change of flux that determines the emf, not the magnitude of flux passing through the coil
The magnitude of the induced emf varies. It is proportional to the rate of change of flux passing through the coil

DC Generators

A DC generator has a similar structure to a DC motor. A motor connects the brushes to the terminals of a power supply but a generator connects the brushes to terminals which supply an emf to an external circuit.

DC generators produce an emf that varies with time, but keep the current flowing in the same direction. A DC generator consists of a split ring commutator that connects the rotating coil to the terminals. The purpose of the commutator is to reverse the direction of the current in the generator every 180° – this supplies the external circuit with a current flowing in the one direction – DC current. The diagram below illustrates the varying current that a DC generator produces.

The output from a DC generator can be made more consistent by including more coils on the armature.

AC Generators

AC generators have a varying emf induced across the ends of the coil. The shape of a graph of the emf versus time has the same shape as a sine graph. The emf across the ends of a coil rotating at a constant rate in a magnetic field produces an alternating current (AC). When connected to an external circuit, this will supply an alternating current to that circuit. Alternating current is the method for delivering large scale power around the world due to its ability to greatly reduce the power loss through the use of step-up and step-down transformers. An AC generator connects the coil to the external circuit or distribution system by the use of slip rings commutator. Slip rings rotate with the coil. Brushes make contact with the slip rings and transfer the current to the terminals of the generator, which in turn connect to an external circuit and a load.

The diagram below illustrates the varying current that an AC generator produces.

The output from an AC generator can be made more consistent by including more coils on the armature. This is known as 3-phase AC and is generated at power stations for distribution.

Determining Polarity

It is important to be able to understand how to determine the direction of the induced current in a generator at any instant and also the polarity of the terminals. *Note: The direction and hence the polarity can only be determined at any instant in an AC generator as these factors are constantly changing as the coil rotates.

Consider the diagram below:

The coil is experiencing a mechanical force that rotates it clockwise
Lenz’s law states that a current will be induced that opposes the motion that caused it
Using the right hand palm rule and considering side LK (fingers indicating the direction of the magnetic field from north to south, palm indicating the direction of the opposing force downward – thumb will indicate the direction of the conventional current going from L to K)

Negative charges travel in the opposite direction and accumulate on terminal A

Terminal A provides the source of electrons for an external circuit and is therefore the negative terminal
Terminal B accepts the electrons as they travel around an external circuit and is therefore the positive terminal

*Note: It is common for students to conclude that terminal B is negative as the electrons travel from B to A through the generator. However, the purpose of the generator is to produce an emf and supply an external circuit – the movement of charge in the external circuit is a better way of remembering and determining polarity.

AC Induction Motors

An AC induction motor consists of a stator which provides the external magnetic field in which the rotor rotates. The rotor of the AC induction motor comprises a series of conductors (metal bars) and the rotor rotates about the axis of the motor’s shaft.

The rotor of the AC induction motor is commonly known as a squirrel cage rotor. It consists of a number of conducting bars made of either aluminium or copper. These are attached to two rings at either end of the bars. The end rings ‘short-circuit’ the bars and allow a current to flow from one side to the other of the cage.

An AC induction motor works by producing a rotating magnetic field. The stator of an AC induction motor is made up of pairs of electromagnets. The AC current runs through opposing coils, creating a magnetic field. AC current reverses its polarity at a rate of 50 Hz, so the coils are energised in pairs, producing a magnetic field that rotates around the outside of the motor. As the magnetic field from the stator is rotating, producing a changing magnetic field, an electric current is induced in the conductors of the rotor. This is due to Faraday’s law. The induced electric current produces its own magnetic field. According to Lenz’s law, that magnetic field works to oppose the original changing magnetic field causing the rotor to move in the same direction as the changing magnetic field produced by the stator.

There are many advantages to AC induction motors, one of which is they only have one moving part: the rotor. DC motors have more parts that will wear out and need replacing. A disadvantage to simple AC induction motors is that their speed is fixed at the rate of the AC source (50 Hz).