Aux 2 Unit 5 3 Phase induction motor Q3 Answer

  1. Describe, with the aid of a sketch, a variable frequency drive for speed control of a three phase motor.(10)

Variable Frequency Drive (VFD) for Speed Control of a 3-Phase Motor

A VFD is an electronic system that varies both the frequency and voltage supplied to an induction motor in order to control its speed while maintaining a constant V/f ratio for stable torque.

Operation (explained step by step):

  1. Rectifier stage
    • The incoming AC supply (3-phase, fixed frequency) is converted to DC using a diode or thyristor rectifier.
    • Produces a pulsating DC voltage.
  2. DC link (filter)
    • The DC output from the rectifier is smoothed using capacitors and inductors, creating a stable DC supply.
    • Provides energy storage and reduces harmonics.
  3. Inverter stage
    • The DC is then converted back into AC of variable frequency and voltage using power semiconductor switches (IGBTs or MOSFETs).
    • Pulse Width Modulation (PWM) is used to control the output.
  4. Motor supply
    • The inverter output is fed to the 3-phase motor stator.
    • By varying the frequency, the synchronous speed of the motor can be increased or decreased.
    • The voltage is adjusted proportionally to frequency to maintain torque.

Advantages of VFD speed control:

  • Smooth and stepless speed variation.
  • High starting torque with reduced inrush current.
  • Energy savings at reduced loads.
  • Protection features (overload, overvoltage, undervoltage, thermal).

Sketch (block diagram):

   3-Phase AC Supply
           │
           ▼
   ┌─────────────┐
   │  Rectifier  │   (AC → DC)
   └─────┬───────┘
         │
         ▼
   ┌─────────────┐
   │   DC Link   │   (Filter: capacitors/inductors)
   └─────┬───────┘
         │
         ▼
   ┌─────────────┐
   │   Inverter  │   (DC → Variable AC, PWM)
   └─────┬───────┘
         │
         ▼
   3-Phase Induction Motor