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Tool Talk Discussion Forum

Electric motor question

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TnT

11-03-2007 09:08:06




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I bought a monster Air Compressor from government salvage. The only problem was it has a three phase motor. I have found a single phase motor replacement but I have a couple questions. The original motor is 7.5 HP 3 phase. It has a 213T frame and ran at 1765 RPM. The replacement motor is 7.4 HP single phase, 1740 RPM and 215T frame. Is the replacement frame compatible?




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steveormary

11-03-2007 18:41:31




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:08:06  
TnT

buickand deere is right on. You do nees a professional licensed electrician to install your wiring. And have it inspected.

steveormary



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buickanddeere

11-03-2007 11:13:40




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:08:06  
Tnt

No offense but this is time to call in a licensed professional. It would be a shame to burn down the shop or electrocute yourself.



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DickelDawg

11-03-2007 09:16:50




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:08:06  
NEMA motor frame numbers cover a variety of dimension specifications. Basically a 213T is not the same as a 215T BUT..... the important specifications to your particular application may be compatible. You will have one major problem though! The 3-phase motor has MUCH more starting torque than the single-phase motor. You probably will not be able to roll the compressor from a standing start without tinkering with the pulley ratio.

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TnT

11-03-2007 09:35:25




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 Re: Electric motor question in reply to DickelDawg, 11-03-2007 09:16:50  
Do I need a larger or smaller motor pully? This is a three belt unit.



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markiz41

11-03-2007 13:33:29




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:35:25  
Just go with a little smaller one. Try to keep the pump rpm and cut-off pressure to a minimum that'll still meet your requirements. You'll be fine.



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steveormary

11-03-2007 09:56:01




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:35:25  
TnT;

Why not install a phase converter and keep the original 3phase motor. A 7.5 hp single phase motor on 230 volts will draw 40 amps. On 230 volt 3phase it will draw 22 amps. My info is from an old motor chart and I use it here just to show the difference between the 2 motors.

steveormary



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MarkB_MI

11-03-2007 11:40:27




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 Re: Electric motor question in reply to steveormary, 11-03-2007 09:56:01  
This has been discussed before. Phase converters work fine for machines with low starting torque: drill presses, lathes, etc. An air compressor requires very high starting torque.

Also, if you run a three-phase motor on a phase converter, you still need about the same current as an equivalent single-phase motor would require.

A big single phase, capacitor start/capacitor run motor is needed for this application.

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TnT

11-03-2007 14:41:06




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 Re: Electric motor question in reply to MarkB_MI, 11-03-2007 11:40:27  
ATWOODS has one here locally. The trouble is the highly skilled and motived employee's at ATWOODS can not answer any questions concerning this motor.



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TnT

11-03-2007 09:56:50




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 Re: Electric motor question in reply to steveormary, 11-03-2007 09:56:01  
That's the first I have heard of a phase converter. Where would I find out more?



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Slowpoke

11-13-2007 11:33:23




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:56:50  
eBay sellers sometimes will have information on converters, and you can get an idea on prices. Or Google "phase converters".



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steveormary

11-03-2007 10:43:06




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 Re: Electric motor question in reply to TnT, 11-03-2007 09:56:50  
TnT

Not sure where you could find one. An electric motor repair shop might be a good place to start. Or an elctric contractor. Are you going to be doing your own wiring? You will need someone with knowledge of the local elect. code to help you with this.

steveormary



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Bob

11-03-2007 10:59:45




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 Practical Machinist Magazine... in reply to steveormary, 11-03-2007 10:43:06  
Practical Machinist Magazine has a whole Forum devoted to rotaty and staic phase converters.



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36 coupe

11-04-2007 04:44:24




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 Re: Practical Machinist Magazine... in reply to Bob, 11-03-2007 10:59:45  
When I was selling books I sold a 3.00 booklet that gave info on using 3 phase motors.A friend got a big band saw with a 3 phase motor.With the booklet and parts he aready had on hand he ran the saw on single phase.You can also use a 3 phase motor to generate 3 phase power for other motors.3 phase motors lack a start winding and switch.



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