Page 430 - Basic Electrical Engineering

which produce the magnetic field. Direct current is supplied to the field
windings so that the field magnets are excited. When the rotor is rotated by a
turbine, the magnetic flux are cut by the stator coils RR′, YY′, and BB′ in a

sequence. The emfs induced in these coils are sinusoidal in nature because of
the nature of flux distribution. The voltage induced in the three-phase

windings will be identical in nature but they will be displaced in time-phase
by 120° as has been shown. The order in which the phase voltages attain their

maximum value or peak value, V is called the phase sequence. If the rotor
m
rotates in the clockwise direction, voltages in the phases will be induced in

the sequence, V , V , V . If the rotor poles rotate in the opposite direction
Y
B
R
the phase sequence of the induced voltage will change from RYB to RBY.

Figure 4.4 (a) Three-phase two-pole generator; (b) three-phase voltages induced in the windings