SAMPLE PROBLEM:

The number of stator poles produced in the rotating magnetic field of an 3phase induction motor having 3 slots per pole per phase is ________.
a) 12 b) 2 c)6 d) 8

Solution: P = 2n
= 2(3)
= 6

= equivalent reactance due to the power develop by the rotor
S= slip
Rs-1= rotor resistance referred to the stator
X2-1= stator reactance referred to the stator
Induction motor = rotating inductor transformer
IT= stator current/ph
¬IR = rotor current/ph
I2= component of I1 that will supply ¬IR
IN=no load current
I= magnetizing current
r2= rotor resistance/ph
r1= stator resistance/ph
X1= stator leakage reactance/ ph
X2= stator leakage reactance/ ph
Xn= magnetizing reactance

AC MOTORS

Induction motors (1 & 3)
 Is one in which the motor field is induced by currents flowing through the stator. The rotor circuit has no connection whatsoever to supply the circuit. The power factor of this motor is lagging and operate with a lagging pf.

General types of 3 and 1 INDUCED MOTOR

a) Squirrel cage rotor type = its rotor is self contained and resembles a bird cage. The rotor conductors are permanently short circuited by end rings.

Speed characteristic: nearly constant or adjustable speed
Torque characteristic: in general it has medium starting torque
Uses: blower, compressor, pump, prime mover of generators

b) Wound rotor = its rotor has insulated coil of wire and resembles a DC armature. The rotor conductors are connected to slip rings and shorted through branches.

Speed characteristic: variable speed
Torque characteristic: high starting torque
Uses: hoist, crane, escalator, conveyor, elevator, etc.





CIRCUIT DIAGRAM

IL=line current EL=line voltage EΦ=phase voltage IΦ=phase current ENΦ=No load voltage per phase

Rf=field winding resistance If=field winding current Po=power output Pf=power factor