Page 118 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

With this method, four different speeds can be obtained

1 When only motor A works, corresponding speed = N = 120f / P
s1 1
2 When only motor B works, corresponding speed = N = 120f / P 2
s2
3 If cumulative cascading is done, speed of the set = N = 120f / (P + P )
1
2
4 If differential cascading is done, speed of the set = N = 120f (P - P )
1 2
c By injecting EMF in rotor circuit: In this method, speed of induction motor is controlled by injecting a voltage
in rotor circuit. It is necessary that voltage (emf) being injected must have same frequency as of slip frequency.
However, there is no restriction to the phase of injected emf. If we inject emf which is in opposite phase with
the rotor induced emf, rotor resistance will be increased. If we inject emf which is in phase with rotor induced
emf, rotor resistance will decrease. Thus, by changing the phase of injected emf, speed can be controlled. The
main advantage of this method is a wide range of speed control (above normal as well as below normal) can
be achieved. The emf can be injected by various methods such as Kramer system, Scherbius system etc.
Single phase motors & special type of motors

LObjectives: At the end of this lesson you shall be able to:
• explain briefly the types of AC single phase motors
• explain the necessity and methods of split phasing the single phase to obtain a rotating
• explain the principle & construction, operation characterists & application of split phase motor, capacitor start
- permanent capacitor motors
• shaded phase motor, capacitor start - permanent capacitor motor.

Single phase motors
Introduction: Single phase motors are working in single phase only. When stator is supplied by single phase
AC supply, the field produced by it changes in magnitude and direction sinusoidally. Such an alternating field
is equivalent to two fields of equal magnitude rotating in opposite direction at equal speed. These are in the
fractional horse power sizes widely used in air crafts, power tools etc.

When fed from a single phase supply its stator winding produces a flux which is only alternating. This alternating
flux acting on a stationary squirrel cage rotor cannot produce rotation. That is why a single phase motor is not
self-starting.
Classification
1 Split phase motor.
a Capacitor start induction run
b Capacitor start capacitor run.

c Resistor start induction run.
d Induction start induction run.
e Permanent capacitor motor.
f Shaded pole motor.
2 Commutator motor
a Series motor
b Repulsion motor
3 Unexcited synchronous motor.
a Reluctance motor
1 Plain repulsion
2 Reluctance start induction run
3 Reluctance induction motor
b Hysteresis motor

105

CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75