Compressor wiring

[wireman]

We were looking at a job the other day that had a compressor wired in an odd way. This may not be an NEC issue since it is part of a manufacturers system, but can anyone tell me what the advantage is? See below:

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480VAC/250 amp compressor. Instead of one size 5 starter, it has two (2) size 4 contactors in parallel connected to one overload block. The compressor is then connected to the load side of the overload block.
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Using list prices, a size 5 starter is approx twice the cost of a size 4, but then you have to figure additional panel space and labor to wire this configuration. Any other ideas on what the advantage may be?

 

[bennie]

Re: Compressor wiring

Is this a dual motor compressor with an alternator switch?

[ April 20, 2004, 10:50 AM: Message edited by: bennie ]

 

[wireman]

Re: Compressor wiring

one motor, no alternator

 

[bennie]

Re: Compressor wiring

Do both starters pull in together? Will one starter be the correct rating, should one fail?

 

[stew]

Re: Compressor wiring

Is it by any chance a 1/2 part winding start arraingement? Do both starters come in at once? AQlso one size 4 is only good for 100 hp 460 v. size 5 200 hp 460. It appears that we are talking about a 200 hp compressor here. hmmm I have seen this arraingement in 1/2 part start systems. The overlaod are wired in series with both coils and the starter coils are further electrically interlocked so that if one coil should fail they both drop out.
?

[ April 20, 2004, 09:48 PM: Message edited by: stew ]

 

[caosesvida]

Re: Compressor wiring

does the compressor have unloaders for variable loads? I have seen this on ac compressors, the compressor can have 2-4-6 cylinders loaded for different capacities. But both contactors energize at the same time if I remember right.

 

[wireman]

Re: Compressor wiring

*******
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does the compressor have unloaders for variable loads?
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I do not have the answer to any more specifics about the unit, as it is supplied by others and we would just be connecting to it. I looked at the prints briefly, and the main thing that stood out was the two contactors in parallel with one overload block. Sorry.

 

[bennie]

Re: Compressor wiring

Please investigate this further. I think there is many who would like to know why the two starters.

There must be a reason. I've never heard of two starters in parallel.

 

[steve66]

Re: Compressor wiring

I wonder if it could be a reversing starter, but I can't imagine any reason why anyone would want to reverse a compressor motor.

Steve

 

[marissa2]

Re: Compressor wiring

Could it have been a part start at one time.
Lou

 

[shaun]

Re: Compressor wiring

Yeah, wireman investigate this a little further! I would like to find out why also

 

[stew]

Re: Compressor wiring

part start was my original thought also. look for a time delay relay in the control circuit.That would be you tip off to a part start arraingement.

[ April 21, 2004, 02:33 PM: Message edited by: stew ]