Please Help with Transformer feeding one motor!!!

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jjmodoc1

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High Point, NC
I am a licensed Electrician from North Carolina. BUT......I have never had to do any calcs with transformers or transformer OCPD. I need to do a design build without ever having stepped foot in the facility. All i have is a little info on the area in question. There is a 480v 3ph 225 amp panel on the wall in this area. Oddly enough there is nothing in the existing panel! The customer has an air compressor that needs to be set and wired. the air compressor specs are 50HP 208v 3ph 170fla. My thoughts, calculations, ideas, are as follows: PRIMARY SIDE: removing the existing 480v panel and in it's place i will put a 480v 3ph 200amp fusible disconnect with 175amp fuses feeding a 112.5 kva transformer. SECONDARY SIDE: off the transformer feeding a 208v 225amp 3ph disconnect with 225amp fuses feeding the air compressor.

Does all this seem right? I based OCPD on both sides at 125%. If anyone can help I would really appreciate it.
 
In my opinion your plan will work.

That said the compressor is likely capable of being changed to 480 by some simple connection changes. I would look into this first.
 
I've already called Ingersol Rand. It's a rotary screw air compressor. No multi-tap. Will only work on 208v. That was the first thing I did was call them. Thought I would get off easy but.....nope.


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I would just get a 175A breaker to go into that panel, why toss it?

And there is no such thing as a "225A disconnect", it will end up being a 400A disconnect and 225A fuses. other than that, looks good to me too.

Seems like such a waste though. Can't you just change out the motor? Seems like a 50HP 460V motor would cost less than a 112.5kVA xfmr and a 400A disconnect and fuses and big ass cables etc.
 
Thanks everyone for responding. I've been doing more research and now I'm finding that a 50HP motor running at 170FLA calculates to roughly 61kva so.... I would need a 75kva transformer. Upsizing to 112.5kva would be way overkill, no? And about a $5k difference on purchase price.


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Actually a 208 rated compressor will have a 200 volt rated motor, which is the NEMA nominal motor rating for a 208 supply.

so with a FLA of 170 amps@200v you will have 58888VA or 58.9 KVA. which is just over 70a @ 480v which at 125% is 88.5 amps so you could size the 480v disco at 125 amps if you wanted to (200 amp disco with 125 amp fuses), or leave the panel in place so it can feed other loads since you have extra supply, just put a 125 amp 3-pole breaker in it to feed the transformer, a 75 kva is the next size up for the transformer then just install a 200 amp disco for the compressor, you shouldn't have any problems with this, I think the only time you would have a problem with inrush on the transformer is if the compressor was trying to start at the same time you energized the transformer, to prevent this after a power failure is to just put a short time delay in the compressor controls if it will start automatically after a power failure.

The reason the motor doesn't have windings that can be changed for different voltages like most motors do is the 200 volt rating, this would be a problem trying to rewire it for a 460 volt rating which is the NEMA rating for a motor on a 480 volt supply.
 
Is it that difficult to provide additioal end taps on all of the motor windings rather than just winding seperate coils that could placed only in series or parallel?
Or is there just not enough demand to justify the added complexity?

Tapatalk!
 
Thanks everyone for responding. I've been doing more research and now I'm finding that a 50HP motor running at 170FLA calculates to roughly 61kva so.... I would need a 75kva transformer. Upsizing to 112.5kva would be way overkill, no? And about a $5k difference on purchase price.


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The problem with cutting the transformer close is no head room for start up currents.
 
Thanks everyone for responding. I've been doing more research and now I'm finding that a 50HP motor running at 170FLA calculates to roughly 61kva so.... I would need a 75kva transformer. Upsizing to 112.5kva would be way overkill, no? And about a $5k difference on purchase price.
The problem with cutting the transformer close is no head room for start up currents.

The general rule-of-thumb is to have the transformer kVA sized at 2.5-3X the HP to avoid more than a 5% voltage drop on start up. One transformer feeding one motor, you were likely fine on 112.5kVA because although you will see more voltage drop, the motor will just take a little longer to accelerate (like an Autotransformer starter). But at 75kVA you start running a risk of stalling it. Motor starting torque drops at the square of the voltage drop so if you drop the voltage to 70% because the transformer saturates, the motor torque drops to 49% of rated. Might work, might not
 
The reason the motor doesn't have windings that can be changed for different voltages like most motors do is the 200 volt rating, this would be a problem trying to rewire it for a 460 volt rating which is the NEMA rating for a motor on a 480 volt supply.

This suggests yet another option for the OP: have a motor shop rewind the motor for 480V. A motor shop might also be able to re-connect coils that have not been brought out as external terminals, to get a more useful voltage (eg. 400V plus a buck boost).

Is it that difficult to provide additioal end taps on all of the motor windings rather than just winding seperate coils that could placed only in series or parallel?
Or is there just not enough demand to justify the added complexity?

Tapatalk!

I don't think that I've ever seen a motor design where individual coils were 'tapped' to leave turns unused (the way that a transformer would have taps for different voltages). I think that you would have to have taps in _each_ coil or in each coil group, not simply in each phase.

The place where such taps would be useful is on motors intended for operation on 208V or 240V supply, and that situation is usually handled by having motors designed to operate over the range of 200-230V.

-Jon
 
I would just get a 175A breaker to go into that panel, why toss it?

And there is no such thing as a "225A disconnect", it will end up being a 400A disconnect and 225A fuses. other than that, looks good to me too.

Seems like such a waste though. Can't you just change out the motor? Seems like a 50HP 460V motor would cost less than a 112.5kVA xfmr and a 400A disconnect and fuses and big ass cables etc.

I agree. Change the motor. it will be more cost effective and better all around.
 
They already have the compressor. Being shipped from another facility. It cost 25k so a new one is pretty much a no go.


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You are going to spend a lot of that 25k just to make this one work, either change the motor, sell or trade the compressor and get one that will run on 480 volts and possibly save a little in the long run.
 
In my experiance it would be rare that an existing 225 amp panel would be able to accept a 175 amp breaker.

The panel would have to have an unused subfeed postion.
I would agree, however I mentioned it because ...

...There is a 480v 3ph 225 amp panel on the wall in this area. Oddly enough there is nothing in the existing panel! ...
 
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