Starting of Single-Phase Induction Motor(Part 2)

 

Starting of Single-Phase Induction Motor

A single-phase induction motor is not self-starting because the stator produces a pulsating magnetic field. To start the motor, an auxiliary starting method is used to create a rotating magnetic field and produce starting torque.

Common starting methods are:

1. Split Phase (Resistance Start) Motor

2. Capacitor Start Motor

3. Capacitor Start Capacitor Run Motor

4. Shaded Pole Motor

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1. Split Phase Motor (Resistance Start Motor)

Principle

• Two windings are used:

  • Main winding

  • Auxiliary (starting) winding

• The starting winding has high resistance and low reactance.

• This creates a phase difference (about 25°–30°) between currents in the two windings.

• The phase difference produces a rotating magnetic field, generating starting torque.

Diagram

      AC Supply
        │
   ┌────┴────┐
   │         │
 Main      Starting
Winding    Winding
 (Low R)   (High R)
   │         │
   └────Switch┘
        │
       Rotor

Characteristics

• Starting torque: Moderate

• Starting current: High

• Starting winding is disconnected by centrifugal switch when motor reaches about 70–80% of rated speed.

Torque-Speed Characteristic

Torque
  │
  │        *
  │      *
  │    *
  │  *
  │ *
  └────────────────── Speed
        Starting → Rated

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2. Capacitor Start Motor

Principle

• A capacitor is connected in series with the starting winding.

• The capacitor creates a large phase difference (about 80°–90°).

• This produces high starting torque.

Diagram

        AC Supply
           │
     ┌─────┴─────┐
     │           │
   Main       Start
 Winding     Winding
     │           │
     │        Capacitor
     │           │
     └────Switch─┘
           │
          Rotor

Characteristics

• High starting torque

• Used in:

  • Air compressors

  • Refrigerators

  • Pumps

Torque-Speed Curve

Torque
  │
  │           *
  │        *
  │     *
  │  *
  │ *
  └────────────────── Speed

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3. Capacitor Start Capacitor Run Motor

Principle

• Uses two capacitors:

  • Starting capacitor (large value)

  • Running capacitor (small value)

• Starting capacitor is disconnected after starting, but running capacitor remains in circuit.

Diagram

        AC Supply
           │
     ┌─────┴─────┐
     │           │
   Main       Auxiliary
 Winding       Winding
     │           │
     │      Run Capacitor
     │           │
     │     Start Capacitor
     │           │
     └────Switch─┘
           │
          Rotor

Characteristics

• Very high starting torque

• Better power factor

• Smooth and quiet operation

Torque-Speed Curve

Torque
  │
  │            *
  │         *
  │      *
  │   *
  │ *
  └────────────────── Speed

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4. Shaded Pole Induction Motor

Principle

• Each stator pole has a copper shading ring.

• The ring delays magnetic flux in shaded portion.

• This produces a weak rotating magnetic field.

Diagram

        Stator Pole
      ┌───────────┐
      │           │
      │  Rotor    │
      │           │
      └───◯───────┘
         Shading
           Coil

Characteristics

• Very low starting torque

• Low efficiency

• No starting switch required

• Simple and cheap

Applications

• Small fans

• Electric clocks

• Hair dryers

• Toys

Torque-Speed Curve

Torque
  │
  │     *
  │    *
  │   *
  │  *
  │ *
  └────────────────── Speed

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