submersible pumps on VFD 1

[HamidEle]

Our client specification calls for an isolation transformer on LV VFD output side. This VFD is for the submersible pump application. Don't understand what the reason behind it. Any input would be appreciated.

 

[itsmoked]

Amen brother!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rhatcher]

I would guess that the reason for the isolation transformer is isolation of ground faults downstream of the VFD in the submersible pump or the pump cables. As others have mentioned, only the client can tell you why they specified the isolation transformer. The client's reason may be something that makes sense to us or it may not be.

In the 'make sense' category:

For a residual current ground fault detection scheme as used on many VFDs, the ground fault is detected because the three phase output current supplied to the motor does not sum to zero. Specifically, all of the current leaving the VFD through the three phase motor leads is not returning to the VFD through the same leads (Kirkoff's Current Law).

The assumption is that the 'missing' current is returning to the VFD through a different path than the three phase motor leads. That path is assumed to be an unintentional ground circuit from a faulted motor winding back through the (grounded) wye connection of the secondary side of the transformer supplying the VFD.

If you have a residual ground fault detection device of any type in the circuit between the grounded motor and the power supply (in this case VFD), then the it will trip.

Application of an isolation transformer (delta-wye) between the residual ground fault relay (drive ground fault circuitry) and the faulted motor will isolate the fault from the ground fault relay and prevent a trip.

Because Hamidele has not provided information abot what this pump is used for I cannot judge if the ground fault isolation is a good thing or not.

 

[LionelHutz]

Another reason not yet suggested is the use of "cabtire" type motor cable, which is like a rubber extension cord. That stuff is useless in preventing any RF radiation from the drive output. It will typically interfere with lower frequency radio, such as RFID tagging (I've seen this on a farm) or AM radio.