VSD and circuit breaker

[Murlinor]

Hi all.

If one has the following scenarion:

Supply network->VSD->transformer->cable->motor. Is it possible to utilise the VSD as a protection against short circuits? Or should one have the following solution:

Supply network->VSD->transformer->Circuit breaker->cable->motor

Help is appreciated.

 

[Mobius44]

No, the overcurrent protection should be in between the supply and the VFD, never between the VFD and the motor. Ref your VFD's installation manual for details.

Please note if you interrupt the wiring between the motor and VFD you can damage the VFD.

VFD's typically shut themselves down if they detect the motor wiring is open or shorted, but it's not a code-approved overcurrent device like a breaker.

VFD's also typically provide overload protection to protect motors from long periods of slightly (typically 125-140%) elevated current, but again this doesn't replace breakers.

 

[Murlinor]

Thanks for the reply. So it should be as follows:

Supply network feed->CB->VSD->transformer->Disconnector/earthing switch->cable->motor?

 

[pwrengrds]

If you close in the disconnect with the drive energized, you would be buying a new drive as mentioned above.

What voltages are we dealing with anyway? I am not sure how a drive would work with a transformer.

Typically I have seen just a safety disconnect at the motor, the drive providing the protection for the cable and motor. (and in this case the TX)

 

[jraef]

Most VFDs provide adequate short circuit protection for the load, but not the wires feeding the VFD from the line, so your second iteration is more correct. But I too am a little concerned about the transformer issue, that is not typical and I don't know how that affects the short circuit protection scheme normally attributed to the VFD. This is not a typical installation (transformer on the output) so the only people who can adequately address this would be the VFD manufacturer in my opinion.


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[ozmosis]

Usually it is:
Transformer->CB->VSD->Cable->Motor

The CB will protect 'upstream'. i.e. the transformer and the VSD should protect itself if the motor or motor cable encountered a short circuit.

 

[Murlinor]

Thanks for the reply, friends.

The client has chosen this system. It confuses me too.

Supply network feed->CB->VSD->transformer->Disconnector/earthing switch->cable->motor

I have heard that the VSD should not be used as a protection of the cable, so how will this protection scheme be?

Could you also explain why the VSD will be damaged, if one closes the disconnector while the VSD is energized?