VFD rotor resistance value

[Gman1977]

Hi, there is lots of info regarding the value of actual rotor resistance when using vector control, eg, the change of value due to temperature changes. I am looking to find some info regarding what is the effect if the VFD rotor resistance parameter setting is incorrect. For example, if the rotor resistance calculated value in the VFD setting is higher than the actual motor value, what is the effect to the VFD output/motor performance, or, if the VFD rotor resistance calculated value is less than the actual motor value what is the effect to the VFD output/motor performance. What effect do we expect to see by manually adjusting the rotor resistance parameter higher or lower when trying to tune a VFD for maximum performance? Thanks in advance.

 

[Drivesrock]

Hi Gman1977

The effect of wrong Rotor resistance (Rr) in the motor model in the drive particularly in flux vector control will be to impair the accuracy of torque control and in extreme cases can cause the drive to trip. A drive can calculate what current it needs in the rotor to create the magnetic field from the tracked Rr. This generated magnetic field is what 'chases-after' the magnetic field in the stator. If the real Rr is lower then there will be higher rotor current (bigger field) and if the real Rr is higher there will be lower rotor current (weaker field). So for dynamic applications - say where rapid accelerations or decelerations are required the performance of the controlled equipment will suffer - speed regulation - ie the 'chasing-after' is impaired. A flying shear would be a critical application for needing an accurate motor model with correct Rr. But I have also seen ship generators enter in to a kind of resonance due to the main propulsion drives' instability because of wrong Rr and consequently badly tuned speed loop trying to compensate.

Therefore, having the initial cold Rr correct from auto-tuning or manually adjusting is required. The effect will be to improve your torque control and hence equipment performance. If Rr is incorrect and without load you may see that the no-load current supplied by the drive is not close to the expected 30% approx value of motor FLC. With load and the motor may not be able to reach rated speed because it has reached current limit (which it shouldn't if the load is less than the motor nominal rating).
Some drives will have internal tracking parameters available for applying gains and some even allow an external source to calculate and overwrite the drive's own calculated value via serial or other network connection.