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submersible pumps on VFD

submersible pumps on VFD

submersible pumps on VFD

(OP)
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.  

RE: submersible pumps on VFD

They probably want galvanic isolation to head off potential corrosion issues from leakage currents real or imaginary...

Keith Cress
kcress - http://www.flaminsystems.com

RE: submersible pumps on VFD

Ask your client.

RE: submersible pumps on VFD

One very real reason is that they may have an RCD installed. The PWM HF capacitive currents is recified in the VFD front end and the RCD thinks it is leakage to ground and trips.

Use either transformer or VFD with sine output, aka sinus drive.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: submersible pumps on VFD

A transformer can provide isolation and also makes a very good sinewave filter.

RE: submersible pumps on VFD

In a lot of deep well submersibles, the motor voltage is stepped up to help reduce voltage drop down the well. If you are going to do that, might as well be an isolation transformer as opposed to an autotransformer.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: submersible pumps on VFD

It might be used to isolate the HF noise generated by the VFD from other transducers that are measuring the level in the bore. Often, the level measuring devices are connected from the bore into ancillary controls and kept away from the VFD. If they are connected to the VFD, they can often 'close the loop' for HF noise in the whole circuit and create additional problems of accuracy.
I have seen this practice a few times.

I also agree with jraef. Boosting the volts the the VFD if the bore depth is long is a means to ensure you have the right volts reaching the motor. Most submersible pumps require you to fit a sinewave filter on the output of the VFD (the motors used often have low insulation quality and need a sinewave/low dvdt) and this output filter will drop the voltage by a few %. This can be critical if the cable is long.
 

RE: submersible pumps on VFD

Yes Patrick. That is also something that I have seen several times. Several municipal waterworks and sewage works in Sweden are using sine inverters for that reason.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: submersible pumps on VFD

(OP)
Thanks for the inputs. I will check with our Mechnical engineer if it is a deep well. Most likely not. If it is not a deep well, I would just add a sinewave filter instead of isolation transformer.  

RE: submersible pumps on VFD

Except that will do noting to prevent the GFI protection from tripping.

In my opinion it doesn't really matter what we think, speculate the need for, or what you think is reasonable.  Your client specified an isolation transformer, so, you should provide one.

Failure to provide one is a good way to inherit all sorts of large legal bills.

Keith Cress
kcress - http://www.flaminsystems.com

RE: submersible pumps on VFD

Yeah, I've never understood all these "my client wants..." threads where somebody wants to argue with the person paying the bills.  If it creates problems for you don't take the job, otherwise just provide what is required.  Why fight it?

RE: submersible pumps on VFD

If it's a blatant waste or wrong it's our duty to try to help the client "get it right" by educating them, but if the client is dead-set or terminally stupid then I do exactly what they ask for.

Keith Cress
kcress - http://www.flaminsystems.com

RE: submersible pumps on VFD

I don't see an isolation transformer as a blatant waste, wrong, nor terminally stupid.  Most of the other 'my client wants..." threads don't fall into those categories either.  I think what it is is that the bidder didn't bother to read the specifications and now wants to recover all the money they left on the table.  Tuff sh!t.

RE: submersible pumps on VFD

We were doing some work on an existing pilot plant for a Steam Assisted Gravity Drainage system (Sag-Dee). There were several pads and each one had a submersible pump, way down the hole. About half had isolation transformers and the rest had filters. I am sure that this client should know which system he wanted to continue with.
Another possible advantage in this instance was that the transformer driven pumps used 3000 Volt motors rather than 600 Volt motors.
This client should also know which Voltage was most advantageous.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: submersible pumps on VFD

davidbeach/itsmoked
I don't think HamidEle is saying the isolation transformer will be left out or is arguing against it, they just want to know the reason for it in this instance.

RE: submersible pumps on VFD

In his 5 Mar 11 1:13 post it sure sounds like he wants to make a substitution.
 

RE: submersible pumps on VFD

davidbeach
It does, my mistake. HamidEle: I suggest you take the advice given...
 

RE: submersible pumps on VFD

(OP)
I did check with one mechanical fellow. It is just slurry submersibale pump,not deep hole pump. I don't think we would need to boost voltage for the motor. The specification from the client is just for that specific application, I believe. We have to make a call whether if it is necesssary to add an isoaltion transformer. HF noise generated by the VFD might be a concern. I will find out more details tomorrow and will discuss here in further detail. Thanks

RE: submersible pumps on VFD

Why are you arguing with the client?

RE: submersible pumps on VFD

I agree with the others. The client specified it so provide it. I know it pisses me off when someone decides what I specify is "wrong" and they do something different. Even more when their thing doesn't work and I get dragged into the project to solve the problem I didn't cause.

RE: submersible pumps on VFD

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.  

RE: submersible pumps on VFD

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.
 

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