A simple circuit diagram of the squirrel cage induction motor is shown in the figure. The stator winding can be either delta-connected or star-connected. The stator winding is connected to the three phase supply terminals. The rotor is represented as shown in the figure and no resistance can be inserted in the rotor circuit.
The figure shows the simple circuit diagram of a slip ring induction motor. The stator can be either delta-connected or star-connected. The rotor is always star connected and any resistance can be inserted in the rotor circuit as shown in the figure.
The external resistance is usually known as starting resistance because it is generally inserted in rotor circuit at the start to limit the heavy rush of current and increase the starting torque.
Three Phase Induction Motor Working Principle
When a three-phase stator winding of an induction motor is connected to the AC supply, current starts flowing in the stator conductors. Due to this current, a rotating magnetic field of constant amplitude and a constant speed is set up in the air gap between stator and rotor.
The rotor winding is still stationary. So rotating magnetic field cuts the motionless rotor conductors and induces an EMF in the rotor winding. Voltage induced in the rotor gives rise to rotor currents.
The direction of the rotor current is such that it will oppose the cause that produces the current (Lenz’s law). And the cause behind producing the rotor current is the ‘relative velocity’ between the rotor and rotating magnetic field.
So the rotor current will flow in such a direction that the rotor will experience a force that accelerates it in the same direction as that of the rotating magnetic field. At the no-load rotor rotates at slightly less speed than Ns.
The speed of the rotor (N) is always less than Ns. Because as soon as N = Ns, the relative velocity between rotor and rotating magnetic field becomes zero, and motor slows down. This happens every time when N tends to become equal to Ns. Therefore, the rotor in an induction motor cannot rotate at the synchronous speed.
When we put a load on the motor, its speed (N) decreases to produce the required torque. The reduction in speed of motor (N) will stop as soon as torque produced by the motor is exactly equal to load torque. The stator winding of induction motors can be either delta connected or star connected.
Causes of Low Power Factor of Induction Motor
When a 3-phase supply is given to a 3-phase wound stator of an induction motor, a revolving field is set up in the stator. This field (flux) is also set up in the air gap between stator and rotor. To set up this air gap flux, induction motor draws magnetizing current from the mains which lag behind the voltage vector by 90o.
The magnitude of this current is quite large because of a high reluctance of air gap between stator and rotor.
The power factor of the induction motor is minimum at no load since this magnetizing current has its dominating effect.
However, the power factor increases with the increase in load on the induction motor and is maximum at full load. Therefore, it is advised to operate the induction motor at the full load.
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