Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Year: May14, Dec13, May12, May11

1) In this type of control the stator voltage $E_1$ and stator frequency $f_1$ both are changed to vary the speed of the induction motor.

2) The ratio $( E_1 / f_1 )$ is kept constant under all operating condition. Therefore it has an advantage from the stator voltage control that it can produce constant torque ( even equal to $T_{max}$ ) from starting (very low speed) upto base speed as shown in fig

3)

4) Significance of $(E_1 / f_1)$

• Maximum torque over the operating range up to the base speed ( Nb )

• $ ( E_1 / f_1 )$ adjusts to ( $E_1$ rated / $f_1$ rated )

• Below the rated voltage and frequency constant torque region is produced.

• Above the rated frequency , a constant power region is produced( in which the ratio ( E1 / f1 ) decreases ) and torque produce of the motor decreases.

• In this way the motor can be operated in constant torque or constant power region and thus the speed of the induction motor can be varied.

5) Low frequency voltage Boost

• At low frequency the stator voltage versus stator frequency is non-linear.

• The reason is that the stator resistance becomes comparable or even larger than the stator leakage reactance and this reduces the torque at low frequency

• Therefore to overcome the effect of stator resistance and to keep the air gap flux constant , the stator voltage is boosted at lower frequencies.

• This will (boost) the E1 / f1 ratio at low frequency and maintain the airgap flux constant

• However there is a risk of core saturation.

6) Implementation of constant v/f control

7) Operation

• The output of the inverter is a six step waveform. This is to reduce the percentage of harmonics in the inverter output.

• A low frequency of harmonics will reduce the motor heating and torque pulsation.

• Both the output frequency and voltage of the inverter should be variable. In order to vary simultaneously to keep their ratio $(E_1 / f_1)$ constant, a common control voltage ‘Vc ‘ is used to vary both ‘$f_1$’ and ‘α’

• In order to vary the output of a square wave inverter a variable D.C. voltage is required in the input side of the inverter.

• The output frequency of the inverter can be change by changing the rate at which the devices connected in the inverter are switched ON and OFF. The VCO frequency will vary in preparation with the D.C. control voltage to vary inverter output frequency.

• The control voltage Vc is obtained from a (RFG) Ramp Function Generator