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Repairing electric motor. troubleshooting and resistance in windings.

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fastline

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I am working on a proprietary 1ph, 1.5-2.0HP cap st/run motor. I was there when it went down and now need to repair. We would install another motor but this one was made with a special case to fit. What luck!!

Anyway, I am not new to motors but this one has beaten me. Motor developed a vibration when running after a long period of running. Shut down and pulled motor thinking bearings went bad. Bearings were fine but found run cap to be shorted and swollen up. Replaced. Motor will not start. Pulls approx FLA (no FLA in name plate). I jumped the potential relay hoping to confirm the start winding was coming in. Nothing. Start cap was tested fine. I even tried to rope start by getting motor up to about 300-400rpm and threw the switch. It just growls.

We are now looking to rewind it since custom fitting a new motor will NOT be fun. I tested resistance in the windings and values do not gel with ohms law and want to confirm. I am estimating about 350ft of 24ga wire for the start winding and 350ft of 17ga for the main windings.

The start winding shows 2.8ohms, both mains in series shows .8ohms or .4ohms per coil separately. I do not feel my wiring estimates are that far off. I have counted the turns and estimated loop lengths. I am curious where I could be seeing problems here? Nothing is leaking to ground.
 
Got a brand, model and some frame tag numbers to work with?
 
Very limited. The only data is 16.8/8.4A at 120/240V. PF, eff, etc is unknown but guessing it is a 1.5-2.0HP motor and seems only to be designed for int duty. Pretty wimpy build.
 
Shouldnt be too hard to rewind if its a 2/4 pole.
Sounds as though its single phasing, as in only one of the windings is working.
If it has start and run caps then I assume theres a centrifugal switch to switch over, have you checked this out, you could use a lathe to check it if you have access to one.
If it just growls then it looks like only one of the windings is pulling current, you need both on a permanent cap motor.
Have you tried new caps, cheapo's are not all that reliable in a demanding application like this.
 
This is a cap start, cap run motor. The run cap was confirmed shorted and was replaced. Start cap is testing fine. I even jumpered the potential relay to ensure it was latched for a start up. I also tried the rope start trick where the motor was spinning at 200-400rpm when I hit power. It still just growled. It is pulling about 60A. The sound from it is obviously not right but could just the be sound it makes in locked rotor.

I really just want to confirm the windings are bad but I am honestly having a hard time pointing the finger. I want to think that the start winding was damaged with the run cap shorted but it still had close to 3ohms resistance which seems reasonable.
 
Think your right a rewind is required.
 
Is there a frame size code on it?
 
Supposedly special built motors still tend to follow the standard frame designation codes.

A picture and a brand and model of the saw would be helpful at this point.
 
Supposedly special built motors still tend to follow the standard frame designation codes.

A picture and a brand and model of the saw would be helpful at this point.


I would gather as much info as I could from the motor , and contact the manufacturer.

B T W , does it have a centrifugal switch ?
 
the run cap shorted could have overheated the start winding to the point the start winding is shorted to the run winding... this will cause other shorts elsewhere.
5 ohms per 1000 feet for 17 awg. 350 feet should be about 2 ohms, so 2 ohms in series for 240 and .5 ohms in parallel for 120vac.

why your motor pulls FLA when it should pull 4-6 times that under locked rotor is a mystery..
 
Entirely my bad! I meant LRA for current. It is pulling approx 58A with an FLA of 8.5A. I agree that the mains seems to test ok with a DMM but the start winding resistance seemed low. I think spec for the wire should have been around 8-9 and it ws about 2.8.

I too agree the shorted run cap caused a problem. We are looking to order a megger just to confirm the findings.
 
I think we got it! I decided that the start winding had failed but the mains were still testing good so I very surgically gutted the windings and removed thee start winding from the stator. With only a slight spin with a string, the motor fired right up and pulled proper amps. We are now working to get the 10 dollar spool of wire required to rebuild the start winding. I can't say this was a financially smart move but did some learning. Not out of the woods just yet though. Since there is zero shot of getting a replacement motor or fitting anything else in it, this was about the only option.
 
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I hope you counted your start winding turns! Motors can be a bit fussy about having too many or not enough.

How do you get three start windings? There should either be two or four depending on if it is a two pole or a four pole motor.
 
Typo...."the start windings". Yes, we calculated 356ft of wire, and the strand count per slot is stepped so 20 in the inner slot all the way to 50 on the outside. I think we have her covered if we can get the windings back in without damage. That old 40yo motor did not want to play ball...
 
Well done if you can rewind just the start winding, sure saves on copper.
 
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