What are the advantages of wound rotor over a squirrel cage rotor for induction motors

Wound rotors are used in applications where high starting torque is required.  External resistances may be added to these rotors via slip rings shaft.  These resistances serve to increase the starting torque and ensure smooth starts.  

However, these rotors are more expensive than induction motors.  In the wound rotor, the rotor windings are insulated to the ground.  The slip rings and the brushes also require maintenance.

The starting current drawn by a wound rotor machine is lesser than that that of a squirrel cage motor.

The wound rotor is designed to have the same number of poles as the stator winding of the motor.  The windings are designed to with stand high mechanical forces as these motors are used for high-torque applications. 

Wound Rotors are used for applications which require soft-starts and adjustable speeds

Squirrel cage rotors are the most common type of rotors found in induction motors.  These rotors are simple to construct, robust and relatively inexpensive. 

They are particularly suited for low inertia loads.  Their easy construction enables lower rotor weight and lesser centirfugal force and windage losses.

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