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Discussion Starter · #1 ·
I saw Jza's thread and it reminded me that I wanted to post this.

I've been using a Fluke 1507 for a few years now, absolutely love it, takes the mystery out of motors with tripping breakers and overloads. Just the other day though I had one that's making me question my troubleshooting technique, or lack thereof?

I tested a 150HP motor at 1000v, 0.0 megaohms on all 3 leads. I pulled out my Fluke T+Pro and I had continuity between the motor case and the motor leads. Definitely bad. The customer pulled the motor and sent it off to the rewind shop.

A couple days later the motor rewind shop called and said they had it running in their shop just fine????? I never heard if they megged it though.

Just today, I had a submersible 90HP pump tripping the breaker, it was reading 15.5 megaohms. We're going to pull the pump tomorrow just to make sure the impeller is free before we unhook it. I tell my customers that anything below 50 megaohms is time to start looking for a spare or plan on a rewind soon.

I haven't tried the ohm test myself, I'd like to have one of those Fluke's with the LoZ option to try it though.

Is there anything else a guy in the field can do to reliably confirm a bad motor?

Thanks for the help.
 

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A couple days later the motor rewind shop called and said they had it running in their shop just fine????? I never heard if they megged it though.
My biggest fear. Most of our clients fully trust us, but a bad call on a large motor could be our last.
 

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I once had a dedicated motor shop not do insulation resistance test on a faulted motor. I have absolutely no idea why the hell they wouldn't test that, but apparently it happens.

The other thing is if you had a winding lead fault, just moving the motor around may have been enough to clear it. Trust your test equipment. Running ≠ Fixed.
 

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Discussion Starter · #4 ·
My biggest fear. Most of our clients fully trust us, but a bad call on a large motor could be our last.
Fortunately, it was a long time customer and he laughed about it when he told us what the rewind shop said.

I once had a dedicated motor shop not do insulation resistance test on a faulted motor. I have absolutely no idea why the hell they wouldn't test that, but apparently it happens.

The other thing is if you had a winding lead fault, just moving the motor around may have been enough to clear it. Trust your test equipment. Running ≠ Fixed.
I mentioned the same thing to my boss. Just because it may be running right now doesn't mean it still isn't going to blow in an hour or a day.
 

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Fortunately, it was a long time customer and he laughed about it when he told us what the rewind shop said.
For sure, it really depends on the customer. We're on somewhat thin ice with one of our clients. I diagnosed and helped them remove the presumably bad motor on Tuesday. Found out today that the motor shop wrote it off and we were installing a new one tomorrow. I'll sleep better tonight.

In an ideal world I would like to start handling sending the motor off for rewind myself. But this client has motors in some crazy awkward places, so usually I'm not even around by the time they get around to taking it down.
 

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I have had motors meg less than 5 ohms resistance and end up running another two years. It was a 250 hp motor, I called the motor shop that we use for our rewinding, and they weren't real concerned with the low meg reading. They told me that condensation can reduce the meg reading down to zero but the motor will be fine. When I get a motor call, I check my voltage, amps, and temp. We have flir thermal camera in the vans now. I will image the motor to check for bad bearings. I have had motors run fine in the summer, but trip the overloads in the winter. Come to find out they were using the wrong grease to grease the machine. The cold temps caused the grease to stiffen enough to trip the overloads. A lot of the time the motor has been changed but the overloads were not. I replace the overloads for the right motor fla, and that cured the problem.
 

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I've had the same experience. Condemned a motor and motor shop said it runs fine. Turned out one of the leads, where it came through the motor case into the peckerhead was scuffed and it was manipulated in a different position during shipping back. Good thing I was able to talk to the motor shop on the phone and convince them to check it over thoroughly, or I might have gotten the motor back in the same shape.

On a related note, I once installed a freshly rebuilt motor, only to have it immediately fault. Shame on me for not testing it. Turned out, the peckerhead was held on by 4 bolts. Motor shop replaced one of the bolts with a slightly longer one and ran it straight into the winding head of the stator.
 

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I have had motors meg less than 5 ohms resistance and end up running another two years. It was a 250 hp motor, I called the motor shop that we use for our rewinding, and they weren't real concerned with the low meg reading. They told me that condensation can reduce the meg reading down to zero but the motor will be fine. When I get a motor call, I check my voltage, amps, and temp. We have flir thermal camera in the vans now. I will image the motor to check for bad bearings. I have had motors run fine in the summer, but trip the overloads in the winter. Come to find out they were using the wrong grease to grease the machine. The cold temps caused the grease to stiffen enough to trip the overloads. A lot of the time the motor has been changed but the overloads were not. I replace the overloads for the right motor fla, and that cured the problem.
Having a considerable amount of experience working with motors, that was one of the first things I checked. The last place I worked, the electricians, didn't give a sh*t. A few motors were being replaced, way to often. I found some starters that had overload heaters that were rated for twice the FLA, of the motor being used. :(
 

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Now, one of the old guys!
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I usually do a quick 1000 volt test, 1-2 minutes then run the 500 volt test for 15 min. One of the old guys I worked with taught me that, and it seems to be able to pick out the motors that are questionable. If it is a motor on a VFD the spikes can be upwards of 1000 volts. If it a VFD driven motor I will do the 1000 volt test longer, 5 min. Just this morning I replaced a 100hp blower motor that seemed to read ok on the short time test but as time went by, at the 10 min mark, the insulation broke down more and more until I felt it was bad. It was on a VFD.
 

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Depending on the environment, the old "sniff test" can weed out many failing motors before you even break out the test equipment. The peckerhead, as soon as the cover is removed, seems to hold that distinctive stink pretty well. :thumbsup:
 
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Supco 500's will show red lights for bad motors. And something even better it says ''Bad'' next to the red lights. You should toss those cheap Flukes in the trash can where they belong and purchase a highly reliable and modern high end meter such as a Supco 500.......
 
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Supco 500's will show red lights for bad motors. And something even better it says ''Bad'' next to the red lights. You should toss those cheap Flukes in the trash can where they belong and purchase a highly reliable and modern high end meter such as a Supco 500.......
You are saying Fluke is junk? Seriously?? What the hell is a supca and why do you need lights and sirens to interpret readings.
 

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sparkywannabee said:
You are saying Fluke is junk? Seriously?? What the hell is a supca and why do you need lights and sirens to interpret readings.
I think he was being facetious.
 

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I used to rely on the sniff test. If it smells burned then it probably is. But with these new Chinese motors we get anymore they smell like they are burning up when they are new. Don't know why but I have had a couple running for a year before the burned smell goes away.
 
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