VFD Troubleshooting

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If run Variable Frequency Drive average frequency below 20 Hz. The VFD shows tripping of Motor High Temperature. But greater than 25 Hz no such problem occurs.
Most variable frequency drives determine input phase loss fault by comparing ripple content in DC bus section and comparing it with set values (fault response will be based on setting done in VFD like trip level and time duration) . Whenever one phase is isolated, the VFD will function as 4pulse instead of 6 pulse rectifier and will have high ripples in DC content and more capacitor bank loading.
Connect the VFD to a motor, such as a three phase induction motor. The #1 motor is mechanically coupled to an identical three phase motor via a pully/ belt drive, #2 motor. The power connection for motor #2 is the mains, 60 hz supply.. If the VFD frequency speed is such that the slip is negative, i.e. greater than the line frequency, the #2 motor acts as a generator, and will supply energy to the mains.
From my experience it is probably due to some mechanical problems eg, bearing, or damaged mechanical parts of mechanism driven by motor causing stall , which then sense by variable frequency drive and resulting over-current causing mv breaker to sound an over-current alarm.
We are developing a 1.8MW medium voltage drive. Inverter o/p voltage is 3.3kv and stepped up to 6.6kv using a transformer. Now the issue is, we are able to operate VFD directly with motor. When we connect transformer, We are getting vcesat error from the gate drive. By reducing v/f ratio, we are able to energize the transformer now. But when we connect transformer along with motor, it is tripping though we have gone for lowest frequency and lowest v/f ratio.
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