Question

Poly-phase Induction Motors - Comparison of a Squirrel cage and Phase wound induction motor

Answer 1

Poly-phase induction motors are widely used in various industrial and commercial applications due to their simplicity, reliability, and robustness. Two common types of poly-phase induction motors are squirrel cage induction motors and wound rotor (phase wound) induction motors. Let's compare these two types based on various characteristics:

1. Construction:

Squirrel Cage Motor: The rotor of a squirrel cage motor consists of a cylindrical core made of laminations with evenly spaced bars or conductors that resemble a squirrel cage. These bars are short-circuited at both ends by end rings.

Wound Rotor Motor: The rotor of a wound rotor motor has a similar laminated core, but instead of bars, it contains a winding with multiple coil turns. The winding is connected to external slip rings, which allows external resistors to be connected for controlling motor performance.

2. Starting Torque:

Squirrel Cage Motor: Squirrel cage motors typically have lower starting torque compared to wound rotor motors. This can limit their use in applications where high starting torque is required.

Wound Rotor Motor: Wound rotor motors offer higher starting torque due to the external resistor control. The resistance can be adjusted to provide higher torque during starting, making them suitable for applications with heavy starting loads.

3. Starting Current:

Squirrel Cage Motor: These motors tend to have higher starting current, which can cause voltage drops and affect the power distribution system during motor startup.

Wound Rotor Motor: Wound rotor motors can be started with reduced starting current by initially introducing higher resistance in the rotor circuit and gradually decreasing it as the motor accelerates.

4. Speed Control:

Squirrel Cage Motor: Squirrel cage motors have limited speed control options. Speed control is generally achieved through external means, such as variable frequency drives (VFDs), which alter the input frequency to the motor.

Wound Rotor Motor: Wound rotor motors offer more flexible speed control due to the ability to control the rotor resistance. By changing the resistance in the rotor circuit, the torque-speed characteristics of the motor can be adjusted, allowing for a wider range of speed control.

5. Maintenance:

Squirrel Cage Motor: Squirrel cage motors are known for their low maintenance requirements due to their simple construction without slip rings and brushes.

Wound Rotor Motor: Wound rotor motors require more maintenance due to the presence of slip rings and brushes, which can wear out over time and need replacement.

6. Efficiency:

Squirrel Cage Motor: Squirrel cage motors generally have higher efficiency due to their simpler construction and absence of slip rings and brushes.

Wound Rotor Motor: Wound rotor motors might have slightly lower efficiency due to the additional losses introduced by slip rings and brushes.

In summary, squirrel cage induction motors are preferred for applications where simplicity, low maintenance, and cost-effectiveness are important. On the other hand, wound rotor induction motors are suitable for applications requiring high starting torque, adjustable speed control, and where moderate maintenance is acceptable. The choice between the two types depends on the specific requirements of the application and the trade-offs between starting torque, speed control, and maintenance considerations.