Current Harmonic Distortion, XFMR, VFD & Motor

[KatyDave]

Hi, I have a problem.

I have a pump house at the end of a electric utility line out in the sticks. The utility drops the 14.4 kV single phase (only) to 240 v for distribution. We have a 50 kVA Xfmr to step up the voltage to 480 V. This is feed to a 75 HP Mitsubishi VFD which drives a 40 HP three phase pump.

Whenever the motor is operated at 60 Hz, the Xfmr blows its 200 amp fuses. We had the utility look at the power, and with their crap and the crap from the VFD converting single phase to three phase they measured current THD to be 24% RMS with 65% spikes (egad!), for voltage the values were 6.5% and 11%, and for the phase balance, etc it seems okay. Note that at 60 Hz the pump operates below FLA of 53.5 amp and at 460 V RMS.

I figure the transformer is too small because the drive is supposed to be over 93% efficient (1P to 3P) so the power to the VFD is about 45 kVA. With that distortion the dissipation within the Xfmr can drive the total load to over 50 kVA. So I want to put a 75 kVA in to handle the excessive current distortion. Is this thinking correct, or am I missing something? Thanks.....

 

[Skogsgurra]

I would never even think of running a 75 HP drive from a single phase supply - even less so a 240 V supply. Putting a transformer to get 480 V might seem to be the right thing to do. But, given that a VFD is somewhat unpredictable with regard to current waveforms drawn from a weak supply, you can easily get something similar to DC saturation for a split period. And that is sufficient to create problems.

Also, the kVA rating shall be ample compared to the kW rating of the motor. Especially when the current THD is high. I doubt if even a 75 kVA transformer is enough. But I have no experience with such high loads on single phase mains. So I leave that open.

Also, a fuse may not be the best protection. Fuse the transformer line for short-circuit protection (usually possible to use a higher rated fuse) and protect the transformer with a thermal protection. If code allows (not sure what part of the world you are in) you may even skip the transformer (primary) protection and rely entirely on protection in the secondary (like the one built into the VFD). As long as it protects the lower rated motor, it will also protect the transformer.

If you are lucky, that may be a better solution.

Gunnar Englund

http://www.gke.org

 

[wareagle]

If I read you post correctly the utility is feeding you 240
volt single phase. What size is the utility transformer?
At 240 volts 50 kva = 208 amps. You could raise the fuse size to a larger fuse. Have you tried to get an amp reading
on the 240 volt side of the transformer? Does the fuse
take a while to blow at 60 Hz or is it instantaneous?

 

[waross]

Quickest easiest firstest. Use dual element or time delay fuses.
Time delay fuses will withstand the short transient currents. If dual element fuses don't do the trick, put in a 400 amp switch and go to the code maximum on the fuses for the 50 KVA transformer before changing out the transformer.
Another option is to limit the maximum speed of the motor and let it run a little longer. I understand that this option is not always possible.
You said that the motor blows it's fuses at 60 Hz. Can you run above 60 Hz.? If so you can probably set a forbidden frequency zone around 60 hz.
What are your transformer voltages? If you have 240:480 there is nowhere to go.
If you have 240;240/480 or 120/240:240/480 you can reconnect it as a 100 KVA auto transformer.
There is quite a bit you can do before buying a new transformer.
respectfully

 

[dewone]

The current on the transformer should be no greater than 104 Amps on the 480 side. So starting would require a slow blow dual element fuse to get started. Even though the drive should be limiting the current back on the 480 side with ramping the a/v ratio some what. Check the drive protection parameters, and change the fuse. Also check the input current to the drive what it requires it should be on the nameplate. Maybe we are under sized. A 75 HP motor with 6 poles like I'm assuming you have with the 40 HP runs 93.1 Amps so we could be close to the transformer capcity