Unmarked single phase motor - is there a 480VAC 1Ph motor

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Old 09-15-20, 11:48 AM
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Unmarked single phase motor - is there a 480VAC 1Ph motor

Hi all, I have acquired a huge air compressor. The seller told me that the motor is single phase and it seems to have the correct number of wires. He also said that it was 7-1/2 HP. and it look big enough. It has no labels or markings whatsoever on the motor. The pump is marked Atlas Copco. but the seller said that he was told by the previous owner that the original motor failed and that this was a replacement.

I hooked 220VAC 1P to it and it began to run very slowly and labored then a wisp of smoke came out so I immediately pulled the disconnect.

The motor connections are similar to a dual voltage motor and the stator coils seem to be in series. (like a 120/240 motor wired for 240).

Is there such an animal as a 240/480 single phase motor?

I don't think I burned it up, at least I hope not.

Photos attached



 
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Old 09-15-20, 01:06 PM
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I would run that motor to a motor shop. there, they will figure it out, plus do a HI-POT on the windings.

Do you have enough AMP @ 240 to run that? In the 40 Amp circuit capacity area.
 
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Old 09-15-20, 03:18 PM
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Thanks for the reply telecom guy, unfortunately the budget won't support a trip to the motor shop. The whole unit only cost $50. Actually, budgetary issues are why I'm found lurking in the hallowed grounds of a DIY forum till all hours. My local pawn shop just happens to have a megger for another $50. I'm sure I'll need to test insulation soon, but for right now, I'd just like to see if I can rewire this peckerhead for 220VAC. ( it's not really a peckerhead, just an ordinary side terminal with a missing cover plate) I just wanted to sound like a real sparky. Thanks, Jim

This is what $50. worth of scrap metal looks like. You
 
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Old 09-15-20, 05:53 PM
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Wow..... that's quite an air compressor....and it's going to be loud. Needs some hp to run it.
If that was a 7.5hp motor it would definitely draw well over 20A on 240v.
Typical rule of thumb is 7amps per HP.

Is there such an animal as a 240/480 single phase motor?
No.

I'm wondering about the connection bar in the middle. That sure looks like a 3phase motor.
 
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Old 09-15-20, 06:52 PM
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I did find a dual voltage motor on Amazon that was 240/480. I may just rewire the stators to put them in parallel instead of series. I should be able to do it correctly by using the formulas for series and parallel resistances. The risk is in possibly getting them polarized incorrectly as far as the relative magnetic fields are concerned.

If all else fails, bright copper wire is bringing $2.62/Lb. My 14" chop saw will make quick work out of that frame
 
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Old 09-15-20, 07:05 PM
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You will find there are likely only 2-3lbs of copper in that motor.
 
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Old 09-15-20, 07:35 PM
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Three phase motors usually have a lot more leads and connectors than that. They are usually wound so that they can run from a Delta or Y - 3 Phase transformer. The various permutations make a wide variety of voltages.
 

Last edited by okrobie; 09-15-20 at 09:29 PM. Reason: typo
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Old 09-15-20, 07:41 PM
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Tolyn, I chopped up a big motor one time and took about 25# of copper to the scrap yard. (Excuse me. Of course I meant to say "Recycling Center")
 
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Old 09-16-20, 04:41 AM
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If the motor is single phase, you should be able to see the centrifugal switch on one end of the motor, usually the end where the wire connections are made. If there isn't one, it is not a single phase. Single phase motors require a starting capacitor connected at the windings. I don't see one in the photo, but it may be hidden or removed before you purchased it. Too bad you didn't see it run before purchasing.
 
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Old 09-16-20, 05:49 PM
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Hi, I opened the housing today and it does indeed have a centrifugal switch, but no capacitor. I think I remember something about Inductive start.

The wires coming from the field windings were labeled 1,2,3,4

I didn't have time to Ohm them out, but I'll do that tomorrow and try to put the field coils in parallel.

Thanks for the support, Jim
 
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Old 09-17-20, 06:53 PM
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I ohmed out the windings and was able to identify the two field windings and the starting coil with centrifugal switch. I put the two field windings in parallel and the start circuit across the input voltage (240VAC). I then closed the disconnect and the motor began to rotate, seemingly without effort and just by observation seemed to be a correct speed - BUT - there was a very loud humming sound coming from the motor. It seemed very unhealthy so I shut it down. The belts are not yet installed so the results as reported are strictly motor related.

Any ideas?

Thanks, Jim
 
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Old 09-18-20, 12:49 PM
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One of the things that has occurred to me is the possibility that the centrifugal switch is not opening up to allow the field windings to take over. Ifr growling sound that I'm hearing. that were the case, the field and start windings would be fighting one another and that could cause the hum or growling that I'm hearing.

Any thoughts or comments would be gratefully appreciated.
 
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Old 09-18-20, 01:21 PM
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You should be able to hear the centrifugal switch snap open.
Do you still have the end cap off ? You could visually watch it.

If the start winding stays engaged for more than a few seconds you'll get smoke and the motor will quickly get hot.
 
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Old 09-18-20, 03:50 PM
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Thanks PJ, I didn't hear the centrifugal switch and there was no smoke or overheating.

I'm thinking that the next thing I should try is to disconnect the starting circuit and see what happens if I start it by hand.

I'll report back how it goes.
 
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Old 09-18-20, 03:57 PM
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Start it normally and CAREFULLY open the start wiring.
 
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Old 09-20-20, 12:36 AM
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Well, PJ, I liked your idea. When I disconnected the start winding, I decided to try starting the motor without the start winding connected. Guess what...the motor started and ran just like yesterday, including the loud humming sound.

What can I conclude from that?
 
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Old 09-20-20, 06:04 AM
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You really need to get a ammeter on the winding to check line current. Loud hum could be sign of shorted turns and lots of current, or something is loose but normal current. In the first case, torque will be low and a rewind is called.
 
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Old 09-20-20, 06:21 PM
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Hi telecom guy, I think you are right on target. I put my Klein clamp meter on the line and it was reading close to 60A. which is way too high. I didn't get a chance to check the torque, but I have a feeling that it is weak. I'm on good terms at my local junk yard. I think I'll paw around in their motor bin to see if they have a good candidate for rebuild. I watched a few YouTubes this morning and I think I can do it. I have an uncle who got filthy rich by rewinding motors after WWII. If I'm going to do it, I'd rather work with a more modern frame. The one on the compressor now could be WWII vintage itself. I would like input on the advisability of rebuilding the one I have. The megger at the pawn shop was purchased by the time I went for it. There are several $50 new meggers on Amazon, but none of them has a hand crank. I'Ve always wanted one of those.
 

Last edited by okrobie; 09-20-20 at 06:25 PM. Reason: to expand my rambling.
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Old 09-21-20, 05:45 PM
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I went to the junkyard today. Slim Pickins Not many single phase. I may have to rebuild the old motor after all.
 
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Old 09-21-20, 08:44 PM
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It would be nice if I got some feedback. Seems like I'm talking to myself.
 
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Old 09-22-20, 08:01 AM
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We luv to give feedback. However, lots of facts not yet known to us.

Is the compressor in good shape? Pistons move, not rusted solid, oil looks good, etc etc.
Before I would rebuilt or replace the motor, I'd put a smaller motor on there and try to build pressure. I'd think only a 1/3HP motor could do that. Spin that thing before you get carried away with heavy motors....
 
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Old 09-23-20, 09:17 AM
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Thank you telecom guy, that's wise to have a reality check.

The compressor seems to be in good shape. If you turn it by hand, it goes, GLUG, GLUG, and if you put your finger over the outlet port, it builds pressure very quickly. If I had any doubts, I would use your suggestion of running it with a small motor. I've got a few 1 and 2 Hp. 56 Frame laying around I don't happen to have a 5/8" pulley handy. If you still have doubts, I'll find a pulley at the junk yard and jury rig it up.

Regards, Jim
 
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