A.C. GeneratorCopy

Learning Objectives

• Distinguish between d.c. and a.c.
• Describe and explain a rotating-coil generator and the use of slip rings
• Sketch a graph of voltage output against time for a simple a.c. generator
• Relate the position of the generator coil to the peaks and zeros of the voltage output

Contents to learn

A.C. Generator

The diagram below shows a simple A.C. generator , which provides the current to a small bulb. The coil is made up of insulated copper wire and is rotated by turning the shaft. The slip rings are fixed to the coil and rotate with it. The brushes are two contacts which rub against the slip rings and keep the coil connected to the outside part of the circuit. They are usually made up of carbon, that is why they are called carbon brushes.

When the coil is rotated, it cuts magnetic field lines, so an e.m.f. is induced and electric current flows. As the coil rotates, each side move continuously upward and downward, in side the magnetic field. So the electric current flows backward and forward i.e. A.C. (Alternating Current). Below graph shows the output of the A.C. generator.

The graph shows, how the current varies when the coil rotates. It is a maximum when the coil is cutting the magnetic lines of forces at the maximum rate. It is zero when the coil is vertical and no magnetic lines of forces are cut by it.

The magnitude of induced electric current increases by increasing the following factors.

• Increasing the number of turns in the coil
• Increasing the area of the coil
• Using a stronger magnet
• Rotating the coil faster

In A. C generator, the slip rings run on carbon brushes to maintain a constant connection between the rotating coil and the external circuit. It means that as the induced emf changes polarity with every half-turn of the coil, the voltage in the external circuit varies like a sine wave and the current alternates the direction.

Difference between slip rings and split rings

In A. C generator, the slip rings run on carbon brushes to maintain a constant connection between the rotating coil and the external circuit. It means that as the induced emf changes polarity with every half-turn of the coil, the voltage in the external circuit varies like a sine wave and the current alternates the direction.

Slip rings are used to supply A.C. voltage where ever we need, while split rings are used to get pulsating voltage generally in case of D.C. motor.

Note: The split ring commutator reverses the flow of current every half turn. If it will not happen, the torque generated by the current’s movement through the magnetic field simply hold the motor in place, which is opposite of what we want to get. Reversing the current, with a small period of time allows the armature to keep spinning in the correct direction.