Few months ago while testing a Micro master series AC drive (Siemens Make) I faced following problems.

Unit was powered up successfully. There was no motor connected at Output.

As soon as the ON command (Drive Run) was given, Drive tripped with DC Link over voltage fault.

The display value of the dc link as well as the Measured Value with Meter both was rising rapidly with On command and causing trip.

There was no external reasons like Fast Decelerating, Breaking or an over voltage condition at Input.

Subsequently the repair Engineer open the drive and found one support Rod from the Body of drive was touching to the output phase .( Causing Direct short circuit of phase to Earth terminal.)

After removal of this short drive worked very well.

But the questions remain unanswered

• Why didn’t drive trip with short circuit fault.
• Why the dc link should rise if output is shorted to earth.
• What will happen if a running drive’s phase touches earth.

Thanks and Regards,

It didn't trip on short circuit because the short wasn't between phases. Why didn't the drive trip on earth fault? as that is what I would expect it to do.

Cheers

Ground faults in VFDs are usually detected using the residual current method, meaning that if you have current flow in all 3 phases and it does not sum to zero, that means one phase must be shorting to ground. To do this, they use current sensors on the output of the VFD, usually Hall Effect transducers. If you have an internal short to ground ahead of the current sensors AND you have no motor current flowing (because the motor is not hooked up), it will not see the imbalanced current. As far as the sensors go, the sum of 3 zeros is zero, it is therefore satisfied. If the short were to have occurred downstream of the sensors, or the motor leads were connected, the shorted phase would now have more current flowing than the other 2 phases, so it would have tripped.

Basically, the situation you experienced is not supposed to happen, so the protection system was likely unable to see the problem.

Dear Jraef!!!!!!!!
Thank you very Much
You have explain it very Nicely… As you said, the shorting was exactly before the CT.

Now when there was shorting between one o/p phase and earth how doest it caused dc link to rise with on command.
Regards,
VST

hi

did u used dc filter inductor ? i made-up a simple schematic which show what can happen in your situation, but i cannot attach the energy is sored in dc link reactor and the whole inverter working like a boost PSU. of course, is an exceptional situation.

Hello Crriss

You should now be able to attach your circuit.
If you have trouble, PM me.

Best regards,
Mark.

thanx Mark.

is just a simple block schematic, on which i want to represent the current path in this situation.
the current from the output W which is shorted to Earth will follow this path.
the whole drive will act like a boost converter, and the inductance will be the dc link reactor. although this is a very loosley configuration for a boost converter, will be able to increase the DC link voltage. some of the components can be damaged in this working condition, like snubber capacitors from input, power stage, and anything what is connected to DC bus.
please correct me if i'm wrong. this beaviour is valid with dc link reactor.

Dear Criss,
Your analysis is very much correct... Only thing is that there was no DC choke in the circuit.!!!
Only the Line filter was used. And this AC line inductor was acting like a booster.

Thanks.