Question

Differentiate between squirrel cage and wound rotor motor

A major difference between the wound rotor motor and the squirrel cage rotor is the conductors of the wound rotor consist of wound coils instead of bars.

These coils are connected through slip rings and brushes to external variable resistors. The rotating magnetic field induces a voltage in the rotor windings.

Increasing the resistance of the rotor windings causes less current flow in the rotor windings, decreasing speed.

Decreasing the resistance allows more current flow, speeding the motor up.

Induction motors use two types of rotors:

A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings.

A wound rotor has a three-phase, double-layer, distributed winding. It is wound for as many poles as the stator.

The three phases are wired internally and the other ends are connected to slip-rings mounted on a shaft with brushes resting on them.

Relationship load, speed and torque 

At start: high current and low “pull-up” torque 

At start: high current and low “pull-up” torque 

At 80% of full speed: highest “pull-out” torque and current drops 

At full speed: torque and stator current are zero

Wound rotor motor drives use a specially constructed motor to accomplish speed control. The motor rotor is constructed with windings that are lifted out of the motor through slip rings on the motor shaft. These windings are connected to a controller, which places variable resistors in series with the windings.

The torque performance of the motor can be controlled using these variable resistors.

Wound rotor motors are most common in the range of 300 HP and above.