QS Study

Generation of three-phase emf

In a three – phase a.c. generator three coils are fastened rigidly together and displaced from each other by 1200. It is made to rotate about a fixed axis in a uniform magnetic field. Each coil is provided with a separate set of slip rings and brushes.

An emf is induced in each of the coils with a phase difference of 120o. Three coils a1 a2, b1 b2 and c1 c2 are mounted on the same axis but displaced from each other by 1200, and the coils rotate in the anticlockwise direction in a magnetic field (Fig: a).


Fig; a Section of 3 phase ac generator

When the coil a1 a2 is in position AB, emf induced in this coil is zero and starts increasing in the positive direction. At the same instant the coil b1b2 is 120o behind coil a1a2, so that emf induced in this coil is approaching its maximum negative value and the coil c1 c2 is 2400 behind the coil a1 a2, so the emf induced in this coil has passed its positive maximum value and is decreasing.


Fig: b Three phase emf

Thus the emfs induced in all the three coils are equal in magnitude and of same frequency. The emfs induced in the three coils are;

ea1a2 = E0 sin ωt

eb1b2 = E0 sin (ωt – 2π/3)

ec1c2 = E0 sin (ωt – 4π/3)

The emfs induced and phase difference in the three coils a1 a1, b1 b1 and c1 c1 are shown in Fig: b & Fig:c.


Fig: c Angular displacement between the armature