Page 763 - Basic Electrical Engineering

10.8 INDUCED EMF IN A SYNCHRONOUS MACHINE

In synchronous machines, the armature winding is made on the stator. The
rotor consists of magnetic poles excited by dc field current. The rotor poles

are rotated by a prime mover, may be a steam turbine or a water turbine, as
the case may be. The poles when rotating, as shown in Figure 10.6, will

induce EMF in the armature coils because the magnetic lines will cut the coil
sides. The EMF induced in phase R –R , Y –Y , and B –B will be identical
2
1
2
1
2
1
but will have a time phase difference of 120°. Phase difference of 120°
degrees corresponds to the time taken by the rotor to rotate by 120 electrical
degrees. The generated voltages in the R, Y, and B phases can be expressed
as

Figure 10.6 EMF is induced in the stator winding due to rotation of poles

e = E sin ωt
m
R
e = E sin (ωt − 120°)
m
Y
e = E sin (ωt − 240°)
B
m
The three EMFs induced in the three-phase windings will be displaced in

time phase by 120°. They can be represented by three phasors of equal
magnitude but displaced by 120°.
We will now derive the equation of the induced EMF in each of the phases

of a synchronous machine.

10.8.1 EMF Equation