2 replies to this topic

[subrao]

I am somewhat mixed up with use of VFD drives in case of double cage motors.

1. In case of some VFDs we are required to feed equivalent circuit parameters of the motor.
In case of a double cage motor rotor resistance and reactance are different at starting and
running. These values are usually given at slip =1 and at slip =0.
The question is which values to feed?
To my mind it appears that because with VFD, slip =1 condition is not encountered we should
consider values at slip =0. Am I correct?

2. A double cage motor is used to have high rotor resistance at start and low rotor resistance at
running.This incidentally gives more rotor reactance at running than at start giving some what
lower power factor. When we use VFD, slip =1 is never encountered as frequency is adjusted as
per speed. So why to go in for a double cage motor for VFD drives ? It may unnecessarily
reduce power factor of motor. Is my analysis on right track?

[marke]

Hello subrao

The double cage motor is designed to give a higher starting torque with low slip. It is in effect a combination of two designs, a design A rotor with a low resistance winding giving good running characteristics, and a design B rotor which gives good starting characteristics.

When variable Frequency drives are used, the motor is never operated under high slip conditions, so there is no advantage in having a high resistance winding in addition to a low resistance winding. The high slip winding will in fact increase the losses in the motor as it will dissipate energy due to harmonics present in the output waveform.

The best motor to use on a VFD has a low resistance winding and a poor performance under high slip conditions.

Best regards,

[mariomaggi]

subrao,

The question is which values to feed?
To my mind it appears that because with VFD, slip =1 condition is not encountered we should
consider values at slip =0. Am I correct?

Yes, slip=0

Regards
Mario