Short Answer:
Squirrel cage and wound rotor induction motors are two types of three-phase induction motors that differ mainly in their rotor construction, starting methods, and speed control features. A squirrel cage motor has a simple, closed rotor with permanently shorted bars, while a wound rotor motor has a winding similar to the stator and uses slip rings to connect external resistors for control.
Squirrel cage motors are low-cost, rugged, and maintenance-free but offer limited control. Wound rotor motors, though more complex and costly, provide high starting torque and better speed control, making them suitable for heavy-duty applications.
Detailed Explanation:
Differences between squirrel cage and wound rotor induction motors
Induction motors are widely used in industries because of their reliability and simplicity. Based on the type of rotor used, induction motors are classified as squirrel cage and wound rotor (slip ring) motors. Both operate on the principle of electromagnetic induction but serve different purposes due to their construction and features.
Let’s understand their major differences clearly:
- Rotor Construction:
- Squirrel Cage Rotor:
The rotor consists of solid bars of aluminum or copper placed in parallel slots and short-circuited at both ends by end rings. It resembles a squirrel cage, hence the name. This rotor is robust, simple, and does not need external connections. - Wound Rotor:
The rotor is made of insulated windings similar to the stator windings. These windings are connected in a star configuration and end at slip rings mounted on the rotor shaft. Brushes make contact with the slip rings and connect to external resistors during starting.
- Starting Method:
- Squirrel Cage Motor:
This motor is self-starting and usually started directly on line (DOL) or using star-delta or autotransformer starters. It offers low starting torque. - Wound Rotor Motor:
Uses external resistance connected through slip rings to control starting current and improve starting torque. As the motor accelerates, resistance is gradually reduced.
- Speed Control:
- Squirrel Cage Motor:
Speed control is limited and requires expensive methods like variable frequency drives (VFD). These motors generally run at a nearly constant speed. - Wound Rotor Motor:
Speed can be easily controlled by varying the external rotor resistance. It provides a wider range of speed variation under load.
- Maintenance and Cost:
- Squirrel Cage Motor:
Very low maintenance since there are no brushes or slip rings. Also, it is cheaper and more commonly used in standard industrial applications. - Wound Rotor Motor:
Requires regular maintenance due to brushes and slip rings. The initial cost and complexity are higher, but it is useful in heavy-duty operations.
- Applications:
- Squirrel Cage Motor:
Used in fans, pumps, compressors, conveyors, and other fixed-speed applications. - Wound Rotor Motor:
Used in cranes, hoists, elevators, and machines requiring high starting torque and speed control.
Conclusion:
Squirrel cage and wound rotor induction motors differ in rotor design, starting methods, and control features. Squirrel cage motors are simple, reliable, and widely used for general purposes. Wound rotor motors are more complex but offer better torque and speed control, making them suitable for heavy load applications. Understanding these differences helps in choosing the right motor for specific operational needs.