Normally single phase induction motor has cage rotor and a single phase distributed stator windings. However, this motor does not develop andy starting torque which considers flux. When the stator of the motor is connected to the supply, it will create a pulsating mmf which will produce as pulsation flux along the axis of the stator of the motor. THis pulsation flush x also induced a current by transformer action in the rotor. The rotor current which produces a pulsation flux or which act along the same axis. As that angle between this two flux is zero there will be no torque developed.
A pulsating field is equivalent to two rotating fields of half magnitude. But normally these fields rotates at same synchronous speed in the opposite directions.
Bothe of these rotating mmf will produce induction motor torque which they are in opposite direction which considers a single rotating magnetomotive field (mmf) which produce induction motor torque they are in opposite direction which considers a single rotating mmf which create a starting torque in a three-phase induction motor. Both of these rotating MMF produce induction motor torque which they are in opposite direction.
Consider a single rotation mmf create a starting torque in the three-phase induction motor. At a stand still, the two torques are equal and one is forward direction and other is the backward direction. So the resulting torque is zero (starting torque).
At any other speed, the two torque are unequal and there is a resultant torque which keeps the motor rotation.
Classification of the single phase induction motors.
Single phase induction motors are normally known by different names. The names are given based on the methods which used to produce the phase difference between the current in the main winding and auxiliary windings.
a) Split phase Motor (Resistive Start)
The auxiliary windings have a higher resistance to reactance ratio. It is also obtained by using thinner wire for auxiliary windings.
The Split phase Motor is permissible because the auxiliary winding is in the circuit which only during startup. When the motor reaches about 75% it synchronous speed the centrifugal switch cut it out.
The motor has low to moderate starting torque. The starting torque can be increased by melting a series resistance in the auxiliary windings.
b) Capacitor Start Motor
Higher the starting torque can obtain if a capacitor is connected with the auxiliary windings. It also increases phase angle between the winding currents. Since the capacitor is additional cost normally this motor is expensive rather than other types of motors. Typical capacitors values for half horsepower motor is 300 uf . AC electrolytic type can sued for this type of motor. As the capacitor is in the circuit only during the startup the electrolytic type can be used. The high starting torque is the out of the starting torque.
c) Capacitor Run Motor
The capacitor in the auxiliary winding and the circuit is not cut off after the starting and it is left in the circuit in all the time.
Normally this motor decreases the cost because of the simplified construction feature. The power factor torque pulsation and the efficiency are also improved because the motor works as the two-phase motor. Normally this capacitor runs motor runs fast compared with other types of single phase induction motors. As the capacitor is in the circuit all the time it cannot increase the capacitance value of the circuit.
d) Capacitor Start-Capacitor Run Motor
Normally two capacitors on in this circuit and one capacitor is used for a start the motor while the other is used to run the motor. Normally it optimized the starting and running performances. The starting capacitor is normally contained larger values electrolytic type of motor. The running capacitor is smaller than the starting capacitor.
e) Shaded Pole Motors
Those shaded pole motors got the salient pole construction. The shaded band consisting of a short-circuited copper which is used on one portion of each pole. The short-circuited coil is known as shading coil. The result is that the current induced in the shading band causes the flux in the shaded pole portion to lag the flux in the unshaded portion.
The flux of the shaded portion is reached its maximum after unshaded portion flux reaches its maximum. This normally creates a progressive shift of flux which the result is similar to rotating field. The last expensive motor in the fractional horsepower motor built for low horsepower rating.
