Single-Phase Induction Motor: Theory, Design, and Starters

1. Constructional Details of a Single-Phase Induction Motor (5 Marks)

A single-phase induction motor is a widely used AC motor in domestic and commercial appliances. It works on the principle of electromagnetic induction and has a simple and rugged construction similar to a three-phase squirrel cage induction motor.

Main Parts:

1. Stator:

   • The stator is a stationary part made of laminated steel to reduce eddy current losses.

   • A single-phase winding is placed in slots distributed around the stator core.

   • An auxiliary winding is also provided for starting, placed 90° electrical apart from the main winding.

   • A centrifugal switch is used to disconnect the auxiliary winding once the motor reaches about 75–80% of its rated speed.

2. Rotor:

   • A squirrel-cage rotor is used.

   • It consists of aluminum or copper bars short-circuited by end rings.

   • No external electrical connection is required with the rotor.

3. Frame and Shaft:

   • The entire motor is enclosed in a cast iron or aluminum frame for protection.

   • The rotor is mounted on a steel shaft with bearings to reduce friction during rotation.

   • Thus, a single-phase induction motor consists of a stator with two windings and a squirrel cage rotor, where the auxiliary winding provides initial torque, making the motor 

Principle of Operation (2 Marks)

• When a single-phase AC supply is applied to the stator winding, it produces a pulsating magnetic field.

• According to Double Revolving Field Theory, this pulsating field is equivalent to two rotating magnetic fields of equal magnitude but opposite direction, rotating at synchronous speed.

• 

Why It Is Not Self-Starting (2 Marks)

• The stator field does not rotate, it only vibrates back and forth.

• Due to equal and opposite torques (T1T_1T1​ and T2T_2T2​), there is no net starting torque.

• The rotor experiences no directional force to start rotating.

• If the rotor is given a small push, one torque dominates and the motor accelerates in that direction.

1) Statement of Theory (1 Mark)

According to the Double Revolving Field Theory, the pulsating magnetic field produced by a single-phase AC supply can be resolved into two equal rotating fields, each of half the magnitude of the original field.

• One field rotates clockwise.

• The other rotates anticlockwise.

• Both rotate at synchronous speed (Ns).

2) Torque Production (2 Marks)

• Each revolving field induces EMF in the rotor.

• This EMF causes rotor currents since rotor bars are short-circuited.

• Interaction of these currents with the rotating flux produces torque.

Let:

• T1T_1T1​ = torque due to forward field (CW)

• T2T_2T2​ = torque due to backward field (ACW)

• At standstill, slip for both fields = 1

• If rotor is given a push:

  • Slip for forward field ↓

  • Slip for backward field ↑

  • T1>T2T_1 > T_2T1​>T2​ → motor starts rotating.

Q2. Equivalent Circuit of Single-Phase Induction Motor (5 Marks)

1) Introduction (1 Mark)

The equivalent circuit of a single-phase induction motor is similar to a transformer with its rotor short-circuited. The stator acts as primary, rotor as secondary.

2) Components (2 Marks)

• As slip decreases, R2′/sR₂'/sR2′​/s increases → more power converted to mechanical form.

3) Working (1.5 Marks)

• Supply voltage V₁ is applied to stator.

• Current splits into magnetizing branch (Rc, Xm) and rotor branch (R₂'/s + jX₂').

• As motor speeds up, slip ↓, power is transferred to mechanical form.

• Circuit shows electrical losses + mechanical conversion.

4) Conclusion (0.5 Mark)

This equivalent circuit helps analyze performance, power losses, and efficiency of the motor.

📝 Tip: Use PDF Page 28–29 diagram in your notes for full ma

Q3. Need for Starting Mechanism in Single-Phase Induction Motor (5 Marks)

1) Why Starter Is Needed (2 Marks)

• At start, slip = 1 → high induced EMF in rotor.

• Low rotor impedance → very high starting current (4 to 10 × full-load).

• Causes:

  • Voltage drop

  • Heating

  • Poor power factor

• Starter limits current and ensures safe acceleration.

2) Why It’s Not Self-Starting (2 Marks)

• Stator field is pulsating, not rotating.

• Produces two opposite torques (T₁ and T₂).

T1=T2  ⟹  Net Torque=0T_1 = T_2 \implies Net\ Torque = 0T1​=T2​⟹NetTorque=0

• No directional push → rotor doesn’t start on its own.

3) Conclusion (0.5 Mark)

Starter is essential to:

1. Create initial phase difference

2. Limit starting current

3. Enable smooth starting of the motor

Q4. Resistance Split-Phase Starting Method (5 Marks)

1) Introduction (1 Mark)

• Uses a high-resistance auxiliary winding placed 90° apart from the main winding.

• Phase difference between currents produces starting torque.

2) Construction (1 Mark)

• Main winding: thick wire (low R, high X).

• Auxiliary winding: thin wire (high R).

• Centrifugal switch disconnects aux winding after starting.

3) Working Principle (2 Marks)

• Main current lags supply.

• Aux current has less lag → phase difference of 25°–30°.

• Rotating field produced → starting torque.

• Aux winding disconnected after reaching 75% speed.

4) Applications (0.5 Mark)

• Fans, blowers, washing machines, grinders.

5) Conclusion (0.5 Mark)

Simple, low-cost starting method for small motors.

Q5. Capacitor Split-Phase Method (5 Marks)

1) Principle (2 Marks)

• Capacitor in series with auxiliary winding creates large phase shift (~80°).

• Strong rotating field → high starting torque.

• Capacitor removed or retained after start, depending on motor type.

2) Types (2 Marks)

📌 Q6. Shaded-Pole Induction Motor (5 Marks)

1) Construction (2 Marks)

• Salient stator poles with a copper shading ring around one part.

• Only main winding present.

• Rotor: squirrel-cage type.

• Shading coil covers one-third of pole face.

2) Working (2 Marks)

• Flux in unshaded part induces current in shading coil.

• Lagging shaded flux creates phase difference.

• Sweeping field gives small starting torque.

• Simple, rugged design but low torque.

3) Limitation & Applications (0.5 + 0.5 Marks)

• Low starting torque, low efficiency.

• Used in:

  • Table fans

⚙ 2) Direct-On-Line (DOL) Starter

Construction:

• Contactor

• Start (ON) and Stop (OFF) push buttons

• No-volt coil (NVC) or no-volt release

• Overload relay

Working Principle:

• When ON is pressed, NVC energizes → main contacts close → motor gets full line voltage.

• A holding contact keeps the circuit latched.

• If overload occurs → relay trips → NVC de-energizes → motor disconnects.

• OFF button breaks the circuit manually.

⚡ 3) Rotor Resistance 

Construction:

• External resistances connected in rotor (for slip-ring motors) or in series with stator (single-phase).

• Contactors used to insert or cut resistance step by step.

Working Principle:

• At starting, maximum resistance is inserted → limits current and increases starting torque.

• As motor speeds up, resistance is reduced step by step.

• At rated speed, all resistances are bypassed.

• DOL Starter: Simple and cheap, used for small motors.

• Resistance Starter: Used when controlled starting and high torque are required.

📌 Flow Map:
High Start Current → Starter Needed →
DOL: Full Voltage | Cheap | Up to 5 HP
Resistance: Extra Resistance | Smooth Start | High Torque

1) Introduction (1 Mark)

Single-phase induction motors are widely used because:

• They run on single-phase AC supply.

• Simple, low-cost, and require less maintenance.

• Ideal for fractional horsepower applications in homes, offices, and light industries.

(a) Domestic Applications (PDF Page 39)

• Fans: Ceiling, exhaust, and table fans.

• Pumps: Water pumps, centrifugal pumps.

• Home Appliances: Refrigerators, air conditioners, washing machines, mixers.

(b) Commercial / Office Applications

• Office Equipment: Vending machines, conveyors, blowers, photocopiers, packaging machines.

• Reliable for continuous operation.

(c) Industrial / Workshop Applications

• Power Tools: Drills, small grinders, saws, vacuum cleaners.

• Servomotors: Used for automation, control, and positioning.

⚡ 3) Suitability of Each Type (1 Mark)