Speed Control of Three-Phase Induction Motor

 

Speed Control of Three-Phase Induction Motor

The speed of an induction motor is given by:

$$N_s = \frac{120f}{P}$$

Where:

• $N_s$ = Synchronous speed (rpm)

• $f$ = Supply frequency (Hz)

• $P$ = Number of poles

Thus, the speed can be controlled by changing:

• Supply voltage

• Frequency

• Number of poles

• Rotor resistance (for slip-ring motor)

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1. Stator Voltage Control (Squirrel Cage Motor)

Principle

The speed of an induction motor is proportional to the square of stator voltage.
By reducing the stator voltage, the torque decreases, causing speed to drop.

Method

• Supply voltage is controlled using:

  • Autotransformer

  • AC voltage controller

  • Thyristor controller

Characteristics

• Simple method

• Used for small motors

• Suitable for fan and pump loads

Applications

• Fans

• Blowers

• Small pumps

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2. Pole Changing Method (Squirrel Cage Motor)

Principle

From the speed equation:

$$N_s = \frac{120f}{P}$$

If the number of poles changes, the synchronous speed changes.

Method

• Stator winding connections are changed to obtain different pole numbers.

Example:

Poles	Speed (rpm)
2	3000
4	1500
6	1000

Features

• Provides discrete speeds

• Efficient and simple

Applications

• Cranes

• Elevators

• Machine tools

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3. Rotor Resistance Control (Slip Ring Motor)

Principle

External resistance is inserted in the rotor circuit through slip rings.

• Increasing rotor resistance → Increase slip → Decrease speed

Diagram

   Stator
  ┌─────────┐
  │         │
  │  Rotor  │
  │         │
  └─────────┘
      │ │ │
   Slip Rings
      │ │ │
   External
   Resistance

Characteristics

• Smooth speed control

• High starting torque

• Used only in slip ring motors

Disadvantages

• Power loss in resistors

• Low efficiency

Applications

• Cranes

• Hoists

• Elevators

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4. Variable Voltage Variable Frequency (VVVF) Control

Principle

Speed depends on frequency.

$$N_s = \frac{120f}{P}$$

By changing frequency and voltage simultaneously, speed can be controlled smoothly.

Method

Uses VFD (Variable Frequency Drive).

Steps:

1. AC supply → Rectifier (AC to DC)

2. DC → Inverter (DC to variable AC)

Block Diagram

 AC Supply
     │
  Rectifier
     │
     DC
     │
  Inverter
     │
 Variable Voltage
 Variable Frequency
     │
   Motor

Advantages

• Wide range of speed control

• High efficiency

• Energy saving

• Smooth acceleration

Applications

• Electric vehicles

• Elevators

• Industrial drives

• CNC machines

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