Speed Control Schemes of Three Phase Induction Motor

(i)    Voltage control method

(ii)    Change of number of poles

Control of three phase induction motors using voltage/frequency control

•    A variable frequency supply is considered as the key factor in speed control of induction motors.

•    There are three basic types of variable frequency supply,

   1. Variable frequency motor-alternator set

   2. D.C. link inverter

   3. Cycloconverter


Constant Volt/Hz operation

The emf per phase of an induction motor is given by



 

 The induced emf E is nearly equal to the applied voltage V (neglecting drop in stator impedance).



Then we can write



















  • When the frequency is reduced, the applied voltage also must be reduced proportionally so as to maintain constant flux, and otherwise the core will get saturated resulting in excessive iron loss and magnetizing current
.
  • •     The maximum torque also remains constant under this condition.

  • •     However the voltage is not varied proportionately in the low frequency range to account    for the voltage drop in the winding resistance.

  • •     This type of control (constant V/f) is used for speed control below base frequency (line frequency of 50 Hz)


  • •     As the voltage increases above rated value, when the input frequency goes base frequency, only constant (rated) voltage with variable frequency (frequency control) is used for speed control.

  • •     Under this condition, both fluxes and maximum torque decrease as the frequency is increased

                         




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