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230 volt motor #56754 09/29/05 05:19 PM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
Customer has a 230 volt single phase 5hp air compressor. It does not list 208 on the nameplate. They never check with us when they buy equipment to see what voltage they should get, just "save a lot of money" when they buy the cheapest one.

The service is 480/277 3 phase. There is a 208/120 volt panel next to it. The unit will sit 20 ft from the panel. I still think I need to install a 7.5 KVA buck-boost to get the 230 volts it calls for on the nameplate. The 208-230 volt motors I have seen many times must be 3 phase, right? Do I need the transformer?

[This message has been edited by poorboy (edited 09-29-2005).]

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Re: 230 volt motor #56755 09/29/05 07:41 PM
Joined: Jul 2004
Posts: 101
jraef Offline
Most NEMA design motors are rated for -10% of nameplate voltage, so you should be OK applying 208V to that motor. But if the line drops too much lower than that, i.e. 200V, you are pushing it. You will also have a loss in starting torque and higher current draw when running, but that may or may not be a problem. You can't really tell until you hook it up and try.

The thing I would do is hook it up as is, and if it starts successfully, run the compressor continuously for 10 or 15 minutes by opening the bleed valve, and monitor the motor current. If it climbs up into the service factor amps, let the customer know that this will shorten the motor's life. Chances are that the motor was a little oversized to begin with, so it may be just fine.

Re: 230 volt motor #56756 09/29/05 08:11 PM
Joined: Mar 2001
Posts: 2,056
Redsy Offline
Sizing buck-boost xfmrs is different than sizing conventional xfmrs.
You probably only need a 1 kva buck-boost.
Look into it, first!

[This message has been edited by Redsy (edited 09-29-2005).]

Re: 230 volt motor #56757 09/29/05 10:07 PM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
I love this can ask counter guys or inside salesmen questions like this all day and rarely can they help. Thanks for the info, you guys.

Re: 230 volt motor #56758 09/29/05 11:11 PM
Joined: Feb 2005
Posts: 219
sierra electrician Offline
I had this very problem just a couple of weeks ago. The customer actually installed it (the air-compressor) and called me to fix it. The Utilty power was infact running low at 198-203V due to peak summer usage. The Buck/Boost Xfmr is the only true fix for this scenario.


Re: 230 volt motor #56759 10/07/05 06:26 AM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
Haven't hooked up the compressor yet but will be soon, will report on whether it needed transformer.

In the meantime, I was trying to work thru a similar installation with a guy who had a 240/120 to 32/16 volt transformer and was trying to hook it up to boost 208 to 230. The diagrams that were in the instructions did show this, and we made it work, but I am amazed at how little exposure I have had to this sort of thing and how unable I would be to pick out a transformer off the shelf to to this. I would have simply seen the secondary voltage listed on the label and said you have the wrong one, you can only make 32 volts.

Any suggestions on a primer for guys like me on transformers? Think I saw that in a thread here recently. Inside sales guys at the supply house rarely can help with this stuff. They often know less than me.

Re: 230 volt motor #56760 10/07/05 07:13 AM
Joined: Mar 2001
Posts: 2,056
Redsy Offline

I know what you mean.
Mostly they can be connected as an isolation xfmr for lower voltages for such things as landscape lighting, or connected as an autotransformer to boost (or buck) voltages for installations like yours.
They can be confusing at first. Especially the kva rating required.


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