Unit III Induction Motor Drives

Topic: Voltage / frequency control

Voltage/Frequency Control

• The speed control of 3φ IM mostly prepares this control method.

• The increase in the supply fz increases the motor speed and also reduces the max

torque of the motor.

• But the increase in voltage results in an increase in the max torque of the motor.

• If we combine these two features we can achieve a control design by which the

speed increases and torque is kept the same. This is known as V/f control.

• Consider the reference voltage V1 and frequency f1.

• If we increases the frequency of the supply to f2(new) while keeping the voltage

v1 unchanged.

– The motor speed increase and Tm decreases.

– The load larger in this case is higher than the max torque provided by the

motor

– Thus, no steady state operating point can be achieved.

• Now let us keep the supply frequency to the new value at f2,

– But increase the magnitude of the voltage, here max torque is increases

– The motor operates at point (2) and a new steady state point is achieved.

• The change in v/f is a powerful method for speed control of IM.

• There are several variations where the v/f ratio is also adjusted to provide a

special operating performance.

• The most common method is the fixed v/f ratio.• Tm should be constant by keeping fixed v/f ratio

– The relation between max torque and v/f is,Tmax = K (v/f)2

• From this equation, – (v/f) is maintained constant, Tmax also remains constant and air gap flux is

also constant.– Therefore motor operates in constant torque mode.

•The low fz operation at a constant v/f is not satisfactory particularly at very low

frequency.

•Disadvantages/ effects of very low frequency

1. Starting torque and breaking torque considerably decreases and no -

load current increases.

2. The air gap flux cannot be maintained constant at low fz maintaining at

constant v/f ratio

3. Therefore we can maintain the v/f ratio constant, the supply voltage

should be increasing.

• The v/f control method,

– One must be careful not to increase the voltage magnitude beyond the rating

of the motor.

– The increased voltage can cause instant damage to the insulations of the

motor winding.

– Normally the voltage should be kept below 110% of the rated value.

• The main features are,

– The max torque should be constant.

– The starting current is also constant.

Thank you