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 V 1 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.

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