MCQs on Electrical Drives

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01․ Which speed control method preferred for constant torque drive?
Field control.
Armature voltage control.
Mechanical loading system.
None of above.

We know that the speeds up to the rated speed can be achieved by controlling armature voltage.Thus, speeds beyond rated speed are never achieved by armature voltage control. For this purpose, field current is reduced. Reduction of field flux results in less produced torque. However, power remains same. Hence, the field control is called as constant power control. Reduction of armature flux results in less produced power. However, torque remains same. Hence, the armature voltage control is called as constant torque control.

02․ While operating on variable frequency supplies, the AC motor requires variable voltage in order to
protect the insulation.
avoid effect of saturation.
improve the capabilities of the inverter.
protect thyristor from dV / dt.

The induction motor characteristics require the applied voltage to be proportionally adjusted by the drive whenever the frequency is changed. For example, if a motor is designed to operate at 440 Volts at 50 Hz, then the applied voltage must be reduced to 230 Volts when the frequency is reduced to 25 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (440/50 = 8.8 in this case). If not, then due to the saturation in the core, the machine can get the magnetization soon and still if we run the motor, then motor can be damaged.