Rotor Voltage Control Of Three Phase Induction Motor

In a wound-rotor motor , an external three phase resistances may be connected to its slip rings . the developed torque may be varied by varying the resistance Rx .

This method increase the starting torque while limiting the starting current . This is an inefficient method and there would be imbalances in voltages and currents if the resistances in the rotor circuit  not equal .

 

The wound rotor induction motor is designed to have a low-rotor resistance so that  the running efficiency  is high and the full load slip is low .
The increase in the rotor resistance not affect on the value of the max. torque but increase the slip at max. torque .
The wound-rotor motors are widely used in applications requiring frequent starting and braking with large motor torques .

Because of the availability of rotor winding's for changing the resistance of the rotor , the wound rotor offers greater flexibility for control . However,it increases the cost and needs of maintenance due to slip rings and brushes .
The three-phase resistors may be  replace by three-phase diode rectifier and a DC converter .

Slip power is the portion of the air-gap power , which is not converted into mechanical power.The slip power of the rotor circuit may be returning to the supply by replacing the dc converter and resistances R by three-phase full converter .

By replacing the bridge rectifiers by three three-phase dual converters (or cyclo-converters). The slip PF in either direction is possible .

The static Kramer and scherbius  drives are used in large power pump and blower applications where , limited range of speed control is required.Because the motor is connected directly to the source, the power flow of these drives is generally high .