Vector control tripping on over current 2

[als01]

Hi,

We have ABB 3.3 KV SAMI MEGA STAR vector control VFD with tacho feedback. VFD is tripping on over current. We checked for any mechanical overload or abnormal process conditions, but they seem to be alright. During the inspection of the tachometer we found the coupling between motor shaft and the tacho is slightly loose. We would like to know whether the loss of tacho feedback would result in motor over current. Any advise on this matter will be very help full. Thanks...

 

[DickDV]

Yes, it could. As the coupling slips, the tach output would suddenly drop causing a large speed error in the speed regulator. The regulator would respond by immediately increasing motor torque to speed up which, depending on the speed loop tuning, could cause an overcurrent fault.

If the above is correct, clearly the tach coupling needs to be fixed but, in addition, it might be worth looking at the drive setup to see whether the speed loop tuning would tolerate setting max torque down a bit to avoid such faulting. There is little gained by having the speed regulator tuned to generate faults as a course of normal regulator operation.

 

[JBinCA]

DickDV is right, an error in the tach signal could cause an overcurrent fault. His explanation is also correct, but might be too complicated if you are not familiar with motor dynamics and VFD regulation.

The drive tries to make reference speed (how fast you want it to go) equal feedback speed (the signal from the tach). When this is the case, the current it sends to the motor is constant. When this is not the case, it changes the current sent to the motor (more to speed up, less to slow down).

Since the tach was loose, it could "fool" the drive into thinking it needed to send a LOT of current to the motor to speed up fast, which could cause an over current trip. A lot of drives have a current limit setting that will prevent this, but it sounds like yours either doesn't have it or it is not set up correctly.

You should start with fixing the tach and retesting.