A repulsion induction motor is a specialized type of single-phase alternating current (AC) electric motor that cleverly combines the superior starting characteristics of a repulsion motor with the stable running performance of an induction motor. This ingenious design allows it to deliver exceptionally high starting torque while maintaining constant speed during normal operation.
How Repulsion Induction Motors Work
These motors are essentially hybrid designs, transitioning their operational principle as they accelerate. They typically feature a stator winding (like an induction motor) and a unique rotor with a commutator and brushes (similar to a repulsion motor), along with a centrifugal mechanism.
1. Starting Mode (Repulsion Principle)
- High Starting Torque: When power is initially applied, the motor operates on the repulsion principle. The alternating current in the stator winding induces a current in the rotor windings via the commutator and brushes.
- Commutator and Brushes: The brushes are short-circuited and positioned to create a strong magnetic field in the rotor that repels the stator's magnetic field, generating a very high starting torque. This is particularly useful for applications requiring significant force to begin rotation.
- Historical Context: The underlying repulsion motor principle was historically valued for its powerful start-up, even being used as a traction motor for early electric trains, such as the SR Class CP and SR Class SL electric multiple units, due to its ability to generate high torque from a standstill.
2. Running Mode (Induction Principle)
- Transition to Induction: As the motor accelerates and reaches a predetermined speed (typically around 75-80% of synchronous speed), a built-in centrifugal switch mechanism activates.
- Short-Circuiting the Commutator: This switch causes a ring to short-circuit all the commutator segments together, effectively converting the rotor winding into a squirrel-cage type rotor, similar to that found in an induction motor. Simultaneously, the brushes may be lifted from the commutator to reduce wear and friction.
- Stable Operation: Once converted, the motor operates on the induction principle, providing stable speed regulation, good efficiency, and quieter operation compared to a continuously running repulsion motor.
Key Features and Advantages
- High Starting Torque: This is the primary advantage, making them suitable for loads that are difficult to start, such as compressors, pumps, and refrigerators, where single-phase power is the only option.
- Single-Phase Operation: They can run directly from a standard single-phase AC supply, eliminating the need for a three-phase supply or external starting capacitors for high-torque applications.
- Good Speed Regulation: Once in induction mode, they maintain a relatively constant speed under varying loads.
Historical Context and Decline
Repulsion induction motors gained prominence in the early 20th century, especially before the widespread adoption and improvement of capacitor-start induction motors. They provided a robust solution for single-phase applications demanding high starting torque.
However, despite their advantages, their complex construction involving commutators, brushes, and centrifugal switches led to:
- Higher Manufacturing Costs: More components and intricate assembly.
- Increased Maintenance: Brushes and commutators are wear items that require periodic inspection and replacement.
- Noise and Sparks: Commutator-brush systems can generate more noise and electrical arcing than induction motors.
Due to these factors, and the development of more cost-effective and reliable single-phase induction motors (like capacitor-start and capacitor-run types) that offer comparable performance without the complexity, repulsion induction motors have largely been superseded in modern applications.
Operating Modes of a Repulsion Induction Motor
| Characteristic | Starting Mode (Repulsion) | Running Mode (Induction) |
|---|---|---|
| Rotor Action | Commutator and brushes are active; rotor acts like an armature | Commutator segments are short-circuited; rotor acts like a squirrel cage |
| Primary Benefit | Extremely High Starting Torque | Constant Speed, Good Efficiency, and Quiet Operation |
| Mechanism | Field interaction between stator and rotor via brushes | Centrifugal switch activates at speed, short-circuiting rotor |
| Torque/Speed | High torque at low speed | Moderate torque, stable speed |
For more general information on electric motors, you can explore resources on electric motor types or delve into AC motor principles.
