# Speed Control of the Given 3-phase Squirrel Cage Induction Motor using the VVVF Drive

Experiment No.: 6

Experiment Name:

Speed Control of the Given 3-phase Squirrel Cage Induction Motor using the VVVF Drive

Objective:

1. To control the speed of the 3-phase squirrel cage induction motor using the VVVF drive
2. To plot line voltage (VL) vs. speed (N) characteristics
3. To plot frequency (f) vs. speed (N) characteristics

Theory:

VVVF stands for Variable Voltage Variable Frequency. VVVF Speed Control method is widely used method for Induction Motor.

As we know that synchronous speed of machine is given as

Ns (rpm) = 120f/P …………………………………(1) where f is frequency and P is number of pole.

Thus if we can change the frequency f then it is possible to change the speed of induction motor. Now frequency of power supply can easily be varied using power electronics devices like inverter. The inverter converts DC power into AC power and feeds to induction motor. Inverter output may be either constant voltage variable frequency or variable voltage variable frequency.  Which one to choose?

Suppose inverter output is constant voltage but variable frequency. Thus we can write V = Constant but f = Variable.

The relationship between voltage V and frequency f is well known and can be written as

V = 4.44fNØ ……………………………….(2)

where N is number of turns per phase and Ø is resultant air gap flux.

Suppose we want to reduce the speed of induction motor. For this, obviously we will have to reduce the frequency f while keeping V constant as per (1).  But from (2),

Ø = V / (4.44fN) …………………………..(3)

Air gap flux Ø will increase which may cause machine core to saturate which is not desirable. Thus the concept of speed control with constant voltage variable frequency cannot be adopted.

But from (3), it is possible to achieve constant flux Ø by maintaining (V/f) constant. This allows us to change voltage and frequency simultaneously to have speed control while maintaining constant air gap flux. This is the basic concept behind VVVF speed control of induction motor.

“In VVVF speed control, motor stator supply as well as frequency is varied such that ratio (V/f) is constant.”

Block Diagram of VVVF Drive:

Circuit Diagram:

Observation Table:

Apparatus Used:

Graph:

Remarks:

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