Principle of Operation for DC Generator

PRINCIPLE OF OPERATION

Let us consider a single turn rectangular coil ABCD rotating about its own axis (Fig.2.6) in a magnetic filed provided by either permanent magnet or electromagnets. The two ends of the coil are joined by two slip rings 'a' and 'b' which are insulated from each other and from the central shaft. Two collecting brushes made up of carbon or copper press against the slip-rings. Their function is to collect the current induced in the coil and to convey it to the external load resistance R. The rotating coil is called as armature and the magnetic as field magnets.

Assume the coil is rotated in an anti clock wise direction.

Let 'l' is the length and 'b' is the breadth of the coil in meters. When the coil sides AB and CD are moving parallel to the magnetic field, the flux lines are not cut and no emf is induced in the coil. At this position assume angle of rotation 'Ɵ' as zero.

This vertical position of the coil is the starting position. According to faraday's law, the emf induced is proportional to the rate of change of flux linkages.

Where

e = - N dϕ/dt …………(1)

N - number of turns

Φ - flux

t - time

If N = 1,

e = - dϕ/dt

Initially, the coil is moving parallel to the flux lines, no flux line is cut. So

dϕ/dt = 0, and e = 0

After 't' seconds, the coil have rotated through an angle 'ωt' radians in the anticlockwise direction. The flux linking with the coil is

B l b cos ωt.

e = - d/dt (B l b cos ωt)

= - B l b ω( - sin ωt) = B l b ωsin ωt

e = E_m sin ωt ……....(2)

Where

E_m = B l b ω

E_m = maximum value of induced emf

When Ɵ = 90°, the coil sides are moving at right angles to the flux lines. The flux

lines are cut at the maximum rate and the emf induced is maximum.

When Ɵ = 180°, the coil sides are moving parallel to flux lines (AB and CD have exchanges positions) and the emf induced is zero.

When Ɵ = 270°, the coil sides again move at right angles to the flux lines but their position reversed. The induced emf is maximum in the opposite directs.

When Ɵ = 360°, the coil sides again move parallel to the magnetic field and the emf induced is zero. The coil has now come back to the starting point.

If the rotation of the coil is continued the changes in the emf are again repeated. The changes in voltage 'e', with respect to the angle 'Ɵ' can be plotted as shown in fig.2.7.

The emf changes from instant to instant and becomes alternatively positive and negative. This emf is called as an alternating emf.

The induced emf in the coil can be increased by

i) Increasing the flux density (B)

ii) Increasing the angular velocity (ω)

In commercial generators a large number of coils are used and they are housed in the armature.

The current flowing in the external resistance to a DC generator is made unidirectional by replacing the slip rings by a split rings as shown in fig 2.8.

The ring is split into two equal segments P and Q and the segments are insulated from each other and also from the shaft.

The coil side AB is always attached to the segment P and CD to Q. It is shown in fig.2.9. The brushes B₁ and B₂ touch these segments and to collect the current. During the first half revolution, current flows along ABLMCD through brush B₁(Positive) and into B₂ (Negative).

After half a cycle AB and CD have exchanged positions along with the segments P and Q and current flows through DCLMBA. B1 is in contact with for each half revolution the positions of segments P and Q also reverse shown in fig.2.10.

The current in the load is always unidirectional is shown in fig.3.11.

In a generator, the split rings are called commutator.