A repulsion single-phase induction motor is used when

Question

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Option B: high starting torque is required

Explanation & Concepts:
A repulsion motor is characterized by its high starting torque, often several times its full-load torque, due to the interaction between the stator magnetic field and the induced current in the rotor. It is specifically designed for heavy-load starting applications where high initial torque is a prerequisite for reaching rated speed.

Working Principle: The stator winding is connected directly to a single-phase AC supply, and the rotor acts as a short-circuited winding through brushes. By adjusting the brush axis relative to the stator magnetic field axis, a repulsion force is generated, producing a high starting torque that remains high until the motor reaches significant speed.

Key Formulas: $T_s \propto I_s^2 \sin(2\alpha)$ — where $T_s$ is starting torque and $\alpha$ is the brush shift angle.

Key Points: Starting torque can be as high as 3 to 4 times the full-load torque.; The torque depends heavily on the brush position (angle relative to the stator pole axis).; It behaves similar to a DC series motor in terms of its speed-torque characteristics.

Advantages: Extremely high starting torque; Adjustable speed characteristics; Smooth acceleration

Disadvantages: High cost of commutator and brushes; Requires periodic maintenance of brushes and commutator; Tendency to spark at high loads

Applications: Hoists and cranes; Compressors; Mixing machines; Conveyor systems

Additional Info: The repulsion motor is largely replaced by capacitor-start motors in modern appliances but remains relevant in specific high-torque industrial applications.. Option A is incorrect because repulsion motors are specifically chosen to avoid low torque situations.

Answer: B is correct — Repulsion single-phase induction motors are specifically engineered to provide very high starting torque by adjusting the brush position relative to the stator field.

Exam Tip: Always remember that in repulsion motors, the torque is controlled by the angle $\alpha$. If $\alpha = 0^\circ$ or $90^\circ$, the torque is zero.

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Option A: low starting torque is required

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Option C: high running torque is required

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Option D: low running torque is required

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