Powerflex 700 series B problem

We are using Powerflex 700 VFD to drive the starting motor of gas turbine. Reference speed signal to VFD comes from AB control logic PLC. When we give a start command to the turbine On crank mode PLC give a ref. speed of 36 Hz to VFD but VFD rotates the motor upto 12 Hz in 80 sec after that turbine stopped at fail to crank alarm. During running the out put voltage of VFD fluctuate. After reset of alarm we immediately give the second start command to turbine. This time the motor rotated upto 20 Hz in 80 sec after 80 sec turbine stopped on fail to crank alarm. During the second attempt the out put voltage and frequency increase smoothly upto 15 Hz with out any fluctuation above 15 Hz out put V/f again fluctuate. The parameter of VFD are same as when it was running normally. Kindly guide where is problem.

Is the vfd speed controls on Ethernet or analogue ?

Allen bradley control net card 20-comm-C (series B) is installed on VFD for communication between PLC and VFD.

The parameter 238 fault config1 the share pin is disable. if the load on the motor is high what will be effect of overload on VFD drive.

It could be that there is some reason why the VFD is going into current limit mode. If that feature is enabled, the way a VFD controls current is to reduce speed, overriding the commanded speed. If speed fluctuates, voltage fluctuates because they go together. The way to test this theory is to locate that parameter that describes the action for current limiting and change it from reducing speed to faulting. Then try again and if it faults, you know that was the issue.

As to why it needs to limit current, it could be a mechanical issue or it could be a failing winding or motor lead conductor, failing phase to phase (because it would trip on ground fault otherwise).

Or, the drive/motor is simply too small for the load required to spin up the turbine.

The same motor was working accurately before this problem.