Adding a VFD (Variable Frequency Drive) to a motor running at a fixed speed and continuing to run the motor at that speed, does not make the motor efficiency improve except where:
The motor is very underloaded and a reduction in voltage will reduce the flux in the iron and thereby reduce the iron losses in the motor, BUT
- The reduction in voltage applied to the motor will :
- reduce the magnetising current
- reduce the flux in the iron and therebye reduce the inductive current flowing in the motor
- Increase the slip in the motor, thereby increasing the slip losses
- increase the work component of the current flowing in the motor
- Reducing the flux in the stator can result in an element of energy saving BUT
- Reduced Flux ==> reduced Iron loss
- Reduced Flux ==> increase slip loss
- Reduced Flux with an open shaft motor ==> reduced line current and reduced copper loss
- Reduced Flux in the motor with load current equal to or greater than the magnetising current of the motor ==> increased motor current and increased copper loss.
- At line speed, the potential to save energy by using a VFD is limited by the true losses due to the use of an induction motor. With many modern machines, the efficiency is is typically quite high and until the load is very low and the actual iron loss is commonly less than a few percent of the full load of the induction motor.
- The iron loss is independent of the load, but there will only be a net gain when the load is very low, BUT the addition of a VFD to control the motor can reduce the iron loss by a small amount, but there is an immediate loss of two to three percent due to the the additional losses inherent in the additional losses in the from the use of a VFD.