Understanding Repulsion Motor Starting and Running

The question asks about the operating principle of a repulsion motor during both its starting and running phases. Let's delve into how this type of motor functions.

What is a Repulsion Motor?

A repulsion motor is a type of single-phase AC motor. It has a stator winding, similar to a standard AC motor, connected to the AC supply. However, its rotor is different. The rotor has a DC armature winding connected to a commutator and short-circuited brushes positioned along a specific axis.

How a Repulsion Motor Starts and Runs

The operation of a repulsion motor relies on the principle of electromagnetic repulsion. When the stator winding is energized by the AC supply, it produces a magnetic field. This changing magnetic field induces current in the rotor windings through transformer action. Due to the position of the short-circuited brushes, the induced currents in the rotor create a magnetic field that interacts with the stator field in a way that produces a torque by repulsion.

Key points about its operation:

• The stator magnetic field induces currents in the rotor.
• The brush position is crucial; it's typically shifted from the stator field axis.
• The interaction between the stator field and the rotor field (created by induced currents) results in a repulsive force.
• This repulsive force creates the starting torque, causing the rotor to rotate.
• The motor continues to run based on this same repulsion principle as long as the stator is energized and the brushes are correctly positioned.

Therefore, the motor starts and continues to run specifically because of the repulsion between the magnetic fields generated by the stator and the rotor induced currents, governed by the brush position.

Analyzing the Options

Let's look at why the other options are incorrect:

• Split-phase motor: A split-phase motor uses a main winding and a starting winding with a phase difference (created by resistance) to produce a rotating magnetic field for starting. It runs primarily on the main winding after the start winding is disconnected. This is a different principle than repulsion.
• Capacitor-start motor: A capacitor-start motor is similar to a split-phase motor but uses a capacitor in series with the starting winding to create a larger phase shift and better starting torque. It also disconnects the start winding after starting. This is not how a repulsion motor works.
• Compound motor: A compound motor is a type of DC motor that uses both series and shunt field windings. The question is about an AC motor (repulsion motor), so this option is irrelevant.

Based on the operating principle, the repulsion motor starts and continues to run as a repulsion motor, utilizing the interaction of the stator field and the rotor field created by induced currents and brush placement.

The correct answer is that the repulsion motor starts and runs as a repulsion motor.

Revision Table: Repulsion Motor Operation

Additional Information: Repulsion Motor Characteristics

• Repulsion motors are known for their high starting torque.
• The direction of rotation can be reversed by shifting the brush position.
• Pure repulsion motors are not very common now; they are often combined with induction motor principles (e.g., repulsion-start induction-run motor).
• In a pure repulsion motor, the speed can vary significantly with load, similar to a series motor.