Which of the following motor is self-starting?

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ALP CBT 2 Electrician Official Paper (Held On: 23 Jan 2019 Shift 3)
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  1. Single-phase induction motor
  2. Low horse power motor
  3. Three-phase induction motor
  4. Variable speed motor

Answer (Detailed Solution Below)

Option 3 : Three-phase induction motor
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Detailed Solution

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  • Three-phase induction motor is self-starting, because winding displacement is 120 degrees for each phase and supply also has 120 phase shift for 3-phase.
  • It results in a unidirectional rotating magnetic field is developed in air gap which causes 3-phase induction motor to self-start.
  • The single-phase induction motor is non self-starting in nature.
  • When single phase AC supply is given to stator winding of single-phase motor, it produces alternating flux i.e. which alternates along one space axis only.
  • It is not synchronously revolving (or rotating) flux, thus it cannot produce any rotation. Hence single phase induction motor is not self-starting.
  • To overcome this problem and to make the motor self-starting, it is temporarily converted into two-phase motor during starting.
  • For this purpose, the stator of a single phase motor is provided with extra winding known as starting winding in addition to the main winding. These two winding are placed across the single phase supply.
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