Page 795 - Basic Electrical Engineering

motor. Therefore, OP can be called the normal excitation. I ′ is over
f
excitation and I ″ is under excitation. The graph resembles the letter V of
f
English alphabet, and hence is known as the synchronous motor V-curve. The

magnitude of OC will increase if the load on the motor is increased. Keeping
that higher load constant, if excitation is changed and values of
corresponding armature currents I are plotted, another V-curve will be drawn
a
as shown in Fig. 10.19 (c).

10.15.4 Application of Synchronous Motors

We can state that an over-excited synchronous motor draws leading power
factor current from the mains. The synchronous motor, therefore, when over
excited, in addition to driving some load, will work like a capacitor or

condenser. A capacitor draws leading power factor current. An over-excited
synchronous motor draws leading power factor current from the mains.

An over-excited synchronous motor is also called a synchronous
condenser.

Synchronous motors are used as constant-speed drive motors.
Over-excited synchronous motors are used to improve the power factor of

electrical loads in industries. Generally, the motor is run on load, and by over
excitation the system power factor is also improved.

10.16 REVIEW QUESTIONS

A. Short Answer Type Questions

1. Explain why we use rotating poles and stationary armature in synchronous machines.
2. Distinguish between salient pole and cylindrical pole rotor construction. Why do we use
cylindrical rotors in high-speed turbo generators?
3. What do you mean by pole pitch and coil pitch? What is the relationship between mechanical
degrees and electrical degrees?
4. What is meant by synchronous speed. Establish the relation N = 120f/P, where N is the
s
s
synchronous speed, f is the frequency, and P is the number of poles.
5. For a 50 Hz supply what are the possible synchronous speeds?
6. Draw the cross-sectional view of a salient-pole-type and non-salient pole-type synchronous
machine. Why do we use laminated sheets for the construction of the stator and the rotor?
7. Draw a simple two-pole and four-pole stator winding for a synchronous machine showing the