generator

Generator

AC Generator

Construction of an ac generator

It consists of a rectangular coil of insulated copper wire. This coil is placed in a magnetic field between the poles of a strong permanent horseshoe magnet. This magnet is known as the field magnet. In actual practice, a large number of turns of the insulated copper wire are suitably wound on an iron core called armature.

AC Generator
AC Generator a
AC Generator
AC Generator b

The two free ends of the coil are connected to the two slip rings R1 and R2. Current generated in the coil is taken out through the two carbon brushes B1 and B2 pressed lightly against the slip rings.

Working of an AC generator

Let the coil ABCD be initially in the horizontal position, and is rotated in the anticlockwise direction. When the coil is rotated anticlockwise, the arm AB moves downwards and the arm CD moves upwards. The coil during this movement cuts the magnetic lines of force, and produces induced current in the coil.

According to the Fleming’s right–hand  rule, during this downward motion of the arm AB, the induced current flows from B to A in the arm AB, and from D to C in the arm CD. The current so produced is taken out through the two slip rings, and  the carbon brushes.

After half the rotation (after rotating through 180º), the arms of the coil interchange their position; the arm AB becomes the right arm and the arm CD becomes the left arm. Then, the arm CD starts moving downwards and the arm AB upwards. During this half–rotation, the induced current flows from C to D in the arm CD, and from A to B in the arm AB. The two slip rings also rotate with the coil. As a result, their polarities (+ and – poles) keep changing at every half–rotation.

The current which changes its polarity after regular intervals of time is called alternating current (AC). So, this electric generator produces alternating current (AC).

Frequency of the alternating current

Alternating current (AC) so produced has a definite frequency. This frequency is equal to half the number of times the polarity changes in one second. In our country, the frequency of the alternating current supplied by the power generation units is 50 cycles per second (or Hz). This means, the alternating current (AC) produced in our country changes polarity 100times in one second.

DC generator

DC generator stands for direct current electric generator. A DC generator produces direct current. A DC generator is also called DC dynamo. A simple DC generator is shown in fig.

DC Generator
DC Generator

Construction of  a DC generator

A simple DC generator consists of a coil of insulated copper wire. The coil is placed between the two poles of a strong horseshoe magnet. In actual practice, a large number of turns of the insulated copper wire are wound on a soft iron core.

The two ends of the coil are connected to the two halves of a split ring (R1, R2 called commutator). Two carbon brushes press against the two half–rings lightly. The current is taken out through the brushes B1 and B2.

Working of a DC generator

Let the coil ABCD be initially in the horizontal position, and be rotated anticlockwise. When the coil is rotated anticlockwise, the arm AB moves downwards and the arm CD moves upwards.

The coil during this motion cuts the magnetic lines of force and an induced current is produced in the coil. According to Fleming’s right hand rule, during the downward motion of the arm AB, the induced current flows from B to A in the arm AB, and from D to C in the arm CD. The current so produced is taken out through the two half–split rings, and the carbon brushes.

After half the rotation (rotation through 180º), the arms of the coil interchanged their positions; the arm AB comes to right and the arm CD to the left. Then the arm CD starts moving downwards, and the arm AB upwards. During this half–rotation, the induced current flows from C to D in the arm CD, and from A to B in the arm AB.

The two half–split rings (R1 and R2) rotate with the coil and touch the two carbon brushed (B1, B2) one by one. As a result, each carbon brush continues to have the same polarity, (+ or –). The brush B2 always remains positive (+) terminal, and the brush B1 remains negative (–) terminal. The current so produced is called direct current (DC).

Difference Between AC and DC Generator

The basic design of AC and DC generators is similar. The two generators differ only in the design of slip rings at the ends of the coil wire. An AC generator use two full rings called slip rings, one at each end of the coil wire, while an DC generator has two half–rings (called split rings) of a commutator.