Which of the following statements are correct for repulsion-induction motors?

I. They combine features of repulsion and induction motors.

II. The rotor has both commutator and squirrel-cage windings.

III. Speed decreases sharply with increasing load.

IV. They are used in applications requiring constant speed. 

Explanation:

Repulsion-Induction Motors

Definition: Repulsion-induction motors are a type of electric motor that combines the operating principles of repulsion motors and induction motors. These motors are designed to utilize the advantages of both types to improve performance characteristics, such as starting torque and speed regulation.

Working Principle: Repulsion-induction motors operate based on two key mechanisms:

• Repulsion Motor Action: At startup, the motor operates as a repulsion motor, where the rotor winding connected to a commutator interacts with the stator's magnetic field to provide high starting torque.
• Induction Motor Action: Once the motor reaches a certain speed, the squirrel-cage winding embedded in the rotor begins to dominate, transitioning the motor to operate as an induction motor. This ensures smooth and efficient operation at higher speeds.

Advantages:

• High starting torque due to the repulsion motor mechanism.
• Improved speed regulation at higher speeds, thanks to the induction motor principle.
• Combines the benefits of both repulsion and induction motors.

Disadvantages:

• Complex construction due to the inclusion of both commutator and squirrel-cage windings.
• Higher cost compared to simpler motor types.
• Requires maintenance of the commutator and brushes.

Applications: Repulsion-induction motors are commonly used in applications requiring high starting torque and moderate speed regulation, such as in compressors, pumps, and industrial machinery.

Correct Option Analysis:

The correct option is:

Option 3: Statement I and II only correct.

This option accurately describes the characteristics of repulsion-induction motors:

• Statement I: "They combine features of repulsion and induction motors" is correct because the motor utilizes both repulsion and induction mechanisms to enhance performance.
• Statement II: "The rotor has both commutator and squirrel-cage windings" is correct as the rotor in a repulsion-induction motor is designed to include both types of windings, enabling it to operate using both principles.

Additional Information

To further understand the analysis, let’s evaluate the other statements:

Statement III: "Speed decreases sharply with increasing load."

This statement is incorrect for repulsion-induction motors. These motors are designed to regulate speed effectively, even under varying loads. The induction motor principle ensures that the speed does not decrease sharply with an increase in load, making this statement invalid.

Statement IV: "They are used in applications requiring constant speed."

This statement is also incorrect. While repulsion-induction motors provide good speed regulation, they are not primarily designed for applications requiring constant speed. Instead, they are used in scenarios where high starting torque and moderate speed regulation are needed.

Conclusion:

Repulsion-induction motors are versatile machines that combine the strengths of repulsion and induction motors to deliver high starting torque and stable speed performance. The correct statements (I and II) highlight the defining features of these motors, such as their combined operational principles and rotor design. However, statements III and IV misrepresent the motor's load-speed characteristics and application scope, making them incorrect in the context of repulsion-induction motors.