Difference between Squirrel Cage and Wound Rotor

Contrasting Squirrel Cage and Wound Rotor Induction Motors

Both squirrel cage and wound rotor induction motors are types of 3-phase AC induction motors. They share a similar stator construction but differ significantly in their rotor design, impacting their starting characteristics, control capabilities, and overall performance.

Stator: Shared Construction

The sources describe the stator of a 3-phase induction motor as a stationary component consisting of:

• Steel Frame: Encloses the core and provides mechanical support.
• Laminated Core: Made of thin silicon steel laminations to reduce hysteresis and eddy current losses.
• Slots: Evenly spaced on the inner periphery of the core to accommodate the stator windings.
• 3-Phase Winding: Wound for a specific number of poles to determine the synchronous speed.

Both squirrel cage and wound rotor induction motors share this stator construction. The stator winding, when connected to a 3-phase power supply, produces a rotating magnetic field that is essential for the operation of both motor types.

Rotor: Key Distinctions

The rotor is where the primary difference lies between squirrel cage and wound rotor induction motors.

1. Squirrel Cage Rotor:

• Construction:

  • Laminated cylindrical core with parallel slots on its outer periphery.
  • Each slot contains a copper or aluminium bar.
  • All bars are permanently short-circuited at both ends by end rings, forming a cage-like structure.

• Characteristics:

  • Simple and robust construction: Enables operation in harsh environments.
  • Low starting torque: Limited control over starting characteristics due to the permanently short-circuited rotor.
  • High efficiency and good speed regulation: Under running conditions.
  • Lower cost: Compared to wound rotor motors.

2. Wound Rotor:

• Construction:

  • Laminated cylindrical core with slots containing a 3-phase winding similar to the stator winding.
  • Rotor winding is usually star-connected.
  • Open ends of the rotor winding are connected to three insulated slip rings mounted on the rotor shaft.
  • Carbon brushes rest on the slip rings, providing external connections to a 3-phase star-connected rheostat.

• Characteristics:

  • High starting torque: Achieved by adding external resistance to the rotor circuit through the rheostat.
  • Smooth acceleration under heavy loads: Due to controlled starting torque.
  • Adjustable speed: Possible by varying the rotor resistance.
  • Higher cost: Compared to squirrel cage motors.
  • Requires more maintenance: Due to the presence of slip rings and brushes.

Working Principle: Similar but with Control Differences

Both squirrel cage and wound rotor motors operate based on the principle of electromagnetic induction, as detailed in the sources and previous conversation turns. The rotating magnetic field from the stator induces currents in the rotor conductors, producing torque and causing rotation.

However, the presence of the external rheostat in the wound rotor motor allows for greater control over the rotor current and, consequently, the motor’s performance characteristics:

• Starting: The external resistance in the rotor circuit of a wound rotor motor increases the starting torque while reducing the starting current. This is not possible in squirrel cage motors, resulting in lower starting torque and higher starting current.
• Speed Control: The speed of a wound rotor motor can be adjusted by varying the external rotor resistance. Squirrel cage motor speed control is limited to changing the number of stator poles, typically allowing only two or four speeds.

Applications: Based on Performance Requirements

The choice between squirrel cage and wound rotor motors depends on the specific application requirements:

• Squirrel Cage Motors: Preferred for applications where high starting torque is not essential and constant speed is desired. Examples include pumps, fans, compressors, and conveyors.
• Wound Rotor Motors: Suitable for applications demanding high starting torque, smooth acceleration under heavy loads, or adjustable speed. Examples include cranes, hoists, and elevators.

Summary:

Feature	Squirrel Cage Motor	Wound Rotor Motor
Rotor Construction	Simple, robust	Complex, with slip rings and brushes
Starting Torque	Low	High
Starting Current	High	Low
Speed Control	Limited	Adjustable
Cost	Lower	Higher
Maintenance	Low	Higher
Applications	Constant speed, moderate starting torque	High starting torque, adjustable speed