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ElectricalMachine
PrevNext

The repulsion-start induction-run motor is used because of

A

Low cost

B

Good Power factor

C

High efficiency

D

High starting torque

Correct Answer

тЪЩя╕П TE тАв Technical Concept & PrincipleElectricalMachine
Option D

High starting torque

Quick Summary:

A repulsion-start induction-run motor is a single-phase motor that starts as a repulsion motor and runs as an induction motor. It is specifically used in applications requiring high starting torque, typically around 350% to 450% of full-load torque, combined with a relatively low starting current.

тЪЩя╕ПTETechnical SolutionConcept & Principle
ЁЯТб Explanation

A repulsion-start induction-run motor is a single-phase motor that starts as a repulsion motor and runs as an induction motor. It is specifically used in applications requiring high starting torque, typically around 350% to 450% of full-load torque, combined with a relatively low starting current.

ЁЯФв Key Formulas

TsтЙИ3.5┬аto┬а4.5├ЧTflT_s \approx 3.5 \text{ to } 4.5 \times T_{fl}TsтАЛтЙИ3.5┬аto┬а4.5├ЧTflтАЛ тАФ starting torque relative to full-load torque

IsтЙИ2.5┬аto┬а3├ЧIflI_s \approx 2.5 \text{ to } 3 \times I_{fl}IsтАЛтЙИ2.5┬аto┬а3├ЧIflтАЛ тАФ starting current relative to full-load current

тЪЩя╕П Working Principle

During starting, the short-circuited brushes positioned on the commutator produce a high electromagnetic torque through repulsion action between the main field and rotor field. As the motor accelerates to approximately 75% of full-load speed, a centrifugal mechanism short-circuits all the commutator segments together, converting the rotor winding into a cage-like structure so it operates as a standard induction motor with stable running characteristics.

ЁЯУМ Key Points
  • тЦ╕

    Combines the extremely high starting torque of a repulsion motor with the constant-speed characteristics of an induction motor.

  • тЦ╕

    The centrifugal switch short-circuits the commutator segments when speed reaches about 75% of synchronous speed.

  • тЦ╕

    In some designs, the brushes are also lifted off the commutator when short-circuited to reduce wear and noise.

тЬЕ Advantages
  • тЦ╕

    Extremely high starting torque (up to 450% of full-load torque)

  • тЦ╕

    Low starting current compared to high starting torque developed

  • тЦ╕

    Fairly constant speed under varying load conditions during running mode

тЭМ Disadvantages / Limitations
  • тЦ╕

    High initial cost due to commutator and brush mechanism construction

  • тЦ╕

    Requires frequent maintenance of brushes and commutator

  • тЦ╕

    Higher mechanical noise during starting operation

ЁЯЫая╕П Applications / Uses
  • тЦ╕

    Compressors and refrigeration units

  • тЦ╕

    Large air conditioners

  • тЦ╕

    High-inertia loads such as hoists, pumps, and machine tools

ЁЯУД Additional Information
  • тЦ╕

    Option A is incorrect because repulsion-start motors are expensive due to complex commutator assemblies.

  • тЦ╕

    Option B is incorrect because its power factor is moderate, comparable to conventional single-phase induction motors.

  • тЦ╕

    Option C is incorrect because running efficiency is similar to standard induction motors, not significantly superior.

ЁЯУК Diagram / Illustration
Repulsion-Start Induction-Run Motor TransitionRepulsionStart ModeCentrifugalSwitch (~75% Ns)InductionRun ModeHigh Starting Torque (350тАУ450%) тЖТ Constant Speed Operation
тЬЕ

D is correct тАФ The repulsion-start induction-run motor provides an exceptionally high starting torque (350%тАУ450% of full load torque) with low starting current.

Core Concepts Used
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Repulsion Motor Principle Centrifugal Mechanism Single-Phase Induction Motor Starting Methods
ЁЯТб EXAM TIP

For competitive exams, remember that Repulsion-Start Induction-Run motors deliver the highest starting torque among single-phase induction motors (higher than Capacitor-Start motors).

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