I have an HVAC condensing unit with a bad 480VAC 3PH 60 HZ motor and the only suitable replacement I can find quickly (less than a 52 week lead time) is a 400VAC 3PH 50 HZ motor. My understanding is that it will work but isn't recommended. Any thoughts or opinions or examples ?
400V 50HZ operate on 480V 60 HZ
- Thread starter td1313
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Tulsa Electrician
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Can you please share why.
Make sure that motor current does not exceed its rating.
NEMA permissible limits for voltage and frequency are exceeded in the proposed application by OP.
synchro
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The Volts/Hz are the same, and so it would be like running it with a VFD in V/Hz mode at 50Hz (but minus the switching noise, etc.).
I believe the torque and current characteristics should be about the same, but with 83% of the HP because of the slower RPM.
I think it's more of a mechanical consideration than an electrical one. The cooling effectiveness of the fan would be reduced, as well as the BTU/hr, for example.
I am afraid it is misleading.
It’s going to run faster on 60HZ, not slower.
zbang
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Then you need to explain why. Please stop making these single sentence answers that say nothing more than "No", they don't help anyone.
Sorry. I react intuitively and wait for much more knowledgeable people here to carry on.
synchro
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Of course. Sorry about that, I hadn't had any coffee today yet.
What I should have said is:
The Volts/Hz are the same, and so it would be like running a 50Hz motor with a VFD in V/Hz mode at 60Hz (but minus the switching noise, etc.).
I believe the torque and current characteristics should be about the same at 60Hz 480V as at 50Hz 400V,
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Tulsa Electrician
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I would still like to here your thoughts.
synchro
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- Chicago, IL
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- EE
synchro
Senior Member
- Location
- Chicago, IL
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- EE
The current rating of a 400VAC 3PH 50 HZ motor should be higher than a 480VAC 3PH 60 HZ motor. And it should run at very close to the same speed as the old motor when it's fed with 60Hz (assuming the same number of poles). Therefore I don't see any obvious problem.
Have you checked whether the 400VAC 3PH 50 HZ motor might also be rated for 60Hz operation?
Have you checked whether the 400VAC 3PH 50 HZ motor might also be rated for 60Hz operation?
The voltage and frequency ratings are nominal. They could vary in the field. So unlike in VFD, constant v/hz cannot be assured and hence the NEMA standard for permissible voltage and frequency limits for a general purpose induction motor is applicable in OP's case.
synchro
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- Chicago, IL
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- EE
I should add: if the two motors have the same HP rating.
zbang
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- Roughly 5346 miles from Earls Court
In most countries the frequency will never vary by more than a few hundredths* of a Hertz, and in many places the voltage won't vary more than +- maybe 5%; it's quite rational to use the nominal voltage & freq as given. Since the OP is in the USA, we can assume that the freq is 60.0 Hz and move on.
480 / 60 = 400 / 50 = 8
The V/Hz ratio is the same.
*yes, it could be more, but the grid has to be wildly out of whack to see a whole Hertz variation. This is discussed in another thread somewhere in the forums.
Didn't I say that already ?
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