Page 791 - Basic Electrical Engineering

Figure 10.18 Synchronous motor with squirrel-cage winding for self-starting

Once the rotor attains a speed near to synchronous speed like a three-phase

induction motor, the dc excitation is provided by switching on the field
circuit. The rotor immediately attains synchronous speed and gets locked into
synchronizm. Thus, the two magnetic fields become stationary with respect

to each other and the rotor continues to develop torque. If load is applied on
the rotor shaft, the rotor continues to rotate at synchronous speed, but its axis

will fall back by angle δ. As a result, more current is drawn from the supply
mains. The more is the load applied on the motor, the more will be the angle

of lag, δ. The maximum limit of the angle of lag of the rotor field axis from
the stator rotating field axis is 90°. The electromagnetic power developed, P,

is expressed as

where, V is the terminal voltage;
E is the induced EMF;

X is the synchronous reactance; and
S
δ is the angle between V and E, also called the power angle or torque

angle.