Overheated electric motor

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  #1  
Old 09-07-06, 06:32 PM
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Red face Overheated electric motor

I wanted to test the limits of my new generator so I hooked it up to my air compressor. The compressor motor is a 240v/15 running amp draw. The generator is 7k watts running/12K starting... theoretically everything should have worked. I saw the generator struggle somewhat when the compressor kicked on but kept it on and went inside to see how the compressor was doing - it ran but the motor was smoking! By the time I went back out and cut off the juice, the motor had made enough steam to fill the basement. The thermal overload was never triggered, how come?? Anyway, I let it cool for quite a while and ran it on regular household 240. It worked but towards the end on the cycle started puffing out some smoke again. I wanted to shoot myself for ever getting the bright idea. The main things I want to know at this point is: is the motor about to die soon and is it safe to even try running it again? Anything I can do myself to check it? Thanks for any help.
 
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  #2  
Old 09-07-06, 08:26 PM
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I'd check the compressor amperage with an Amprobe. You say "steam" but did you smell frying insulation? How old is the compressor? Did you check the oil (if applicable)? Any sign of problem before?
 
  #3  
Old 09-08-06, 04:15 AM
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Red face

Sounds like you may have fried the start/run capacitor.
 
  #4  
Old 09-08-06, 06:13 AM
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Originally Posted by ray2047
I'd check the compressor amperage with an Amprobe. You say "steam" but did you smell frying insulation? How old is the compressor? Did you check the oil (if applicable)? Any sign of problem before?
Yes, I smelled frying insulation. The motor is about 7 years old and always ran fine before. The smoke seemed to be coming from the inside, where the copper windings are.
 
  #5  
Old 09-08-06, 06:21 AM
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Sounds like you fried the motor. It could have been worse, you could have damaged the generator.
I don't understand why it happened. Does the generator actually put out 240 volts? Sounds like the motor "drew down" the voltage and caused the motor to overheat.
The thermal overload in the motor should have tripped.
steve
 
  #6  
Old 09-08-06, 06:26 AM
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Generators (except expoensive ones, usually larger in capacity) do not put out clean voltage. For this reason it is not recommended that you run sensitive electronics on them, such as televisions and computers, unless you run the power through a UPS or other device that cleans up the power.

However, a motor typically does not care how clean the power is.

I suspect that the startup draw was more than the generator could handle, and that it caused the power to go way out of whack and damaged the compressor motor.

Most generators (smaller ones anyway) cannot handle a single large current draw item, like a well pump. Perhaps your compressor fits that category.
 
  #7  
Old 09-08-06, 07:02 AM
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thanks, guys

Bummer. I got the damn thing with the idea of running my well pump (among other things) in an emergency. Granted, the well pump motor does not draw nearly as much as the compressor motor (I think it's 4 amps on 220v), but that is one thing I don't want to take a chance with because it's 300 feet deep in the ground.
If it's true what they say about the starting wattage of a motor sometimes being 5 times the running one, then I could see why this happened - if the generator had to put out 75 amps on start-up, the voltage must have dropped like crazy and never stabilized. If I ever get the nerve to run my well pump with it, I'll make sure every other breaker is off.
 
  #8  
Old 09-08-06, 08:51 AM
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For maximum protection of the motor, consider a starter, rated in horspower. The advantage is that you can connect reliable over-load devices with a amp-rating that is 125% of the full-load motor current. Possibly the existing O-L devices are defective, or a design or rating that does provide the necessary protection.

I suggest that you first measure the full-load motor current with the motor operating on "Normal" power. Next, with the motor operating on "Standby" power, measure both the motor-current and the voltage impressed on the leads to the motor, to determine that the generated voltage is neither to low or to high, and that the motor-current is not an excessive value.

There is an internal "voltage-drop" across the internal generator windings which increases as the generator output- current increases. This would tend to reduce the "output" voltage of the generator.
 
  #9  
Old 09-08-06, 03:32 PM
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And as you bog down the generator, the frequency drops as well, which makes the impedance of the motor less, which tends to draw more current, which bogs down the generator more........
 
  #10  
Old 09-08-06, 04:57 PM
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So what can I do to see if I still have a worthy motor? It ran after the incident with a little smoke, maybe it isn't junk yet.. I can run it with load or no load.
 
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Old 09-08-06, 05:04 PM
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If it works, it works.

If you can, take a current draw. As long as it is not WAY beyond nameplate rating, you should be fine. There really isn't anything you can do at this point with it anyway.

Like my dad told me, "if it ain't broke, you don't need to fix it" So , as long as it runs and has enough power to do what you need it to, don't worry about it until it melts down.

I can only think of a very few motors that actually started on fire when the went into meltdown. Most simply just die.
 
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Old 09-08-06, 07:14 PM
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Originally Posted by nap
I can only think of a very few motors that actually started on fire when the went into meltdown. Most simply just die.
Thanks, that's actually what I wanted to hear. Since the motor is unattented, I was afraid of starting a fire. I'll just check it with a multimeter for the first few cycles - if it draws too many amps I'll toss it. I won't feel too bad since I know the thermal overload don't work
 
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Old 09-08-06, 08:21 PM
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You can also add external thermal protection as PATTBAA posted earlier as well.
 
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Old 09-08-06, 09:24 PM
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Originally Posted by markiz37
I wanted to test the limits of my new generator so I hooked it up to my air compressor. The compressor motor is a 240v/15 running amp draw. The generator is 7k watts running/12K starting... theoretically everything should have worked. I saw the generator struggle somewhat when the compressor kicked on but kept it on and went inside to see how the compressor was doing - it ran but the motor was smoking! By the time I went back out and cut off the juice, the motor had made enough steam to fill the basement. The thermal overload was never triggered, how come?? Anyway, I let it cool for quite a while and ran it on regular household 240. It worked but towards the end on the cycle started puffing out some smoke again. I wanted to shoot myself for ever getting the bright idea. The main things I want to know at this point is: is the motor about to die soon and is it safe to even try running it again? Anything I can do myself to check it? Thanks for any help.
No harm meant..(to mod)
Humor not allowed!

This is how our great scholars became great scholars.
Trial and error. School aint cheap!
(is that better!?)
 
  #15  
Old 09-09-06, 08:04 AM
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Remember rules 1 & 2 when working on anything electrical:
Rule #1...Don't get Killed
Rule #2...Don't let the smoke out.
steve
 
  #16  
Old 09-09-06, 08:10 AM
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"Rule #2...Don't let the smoke out."

Hey, don't tell them that. I get paid good money to put the smoke back in. What are you trying to do? Put me out of work??
 
  #17  
Old 09-09-06, 08:17 AM
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To QUOTE nap " "Rule #2...Don't let the smoke out."

Hey, don't tell them that. I get paid good money to put the smoke back in. What are you trying to do? Put me out of work??"

If you know how to put the smoke back in I will pay for the info. I always have to go buy new stuff with factory installed smoke.
 

Last edited by jwhite; 09-09-06 at 08:29 AM.
  #18  
Old 09-11-06, 06:20 AM
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update

The motor's toasted. It was drawing close to 30 amps (double the plate number) with no load on it. Smoking enough to be on the ALA hit list. Thanks for the help, fellas, appreciate all the info and the ribs (well deserved). Anyone know where to shop for a new motor?
 
  #19  
Old 09-11-06, 08:11 AM
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Time to investigate the overload protection issue. A motor-starter with the correct Overload setting may have saved you the cost of the motor.

Also, the rating of the overcurrent device ( fuse or circuit-breaker ) in the panel.If the fuse/CB rating was excessive, and the motor-current was the "locked-rotor" current because the motor failed to start, and the fuse/CB did not open the circuit with locked-rotor current flowing, the result is motor burn-out.
 
  #20  
Old 09-11-06, 09:41 AM
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Motors hate undervoltage. It overheats the windings. Which in turn breaks down the winding insulation. That's the smoke you described, varnish insulation. Once the insulation goes, the windings will short out.

When dropping a fairly substantial load onto a generator, the voltage dips. If the nameplate FLA (full load amps) is truly 15A, I'm guessing your compressor is approximately 2.5 HP. Starting amperage for a motor, or "locked rotor" amps, can be 300% or more of FLA.

At 7 kw your generator is probably rated at around 30 amps. That compressor motor of yours could be drawing up to 45 amps during the initial startup. That's why your generator groaned when you switched on the compressor.

The nameplate on a compressor, and most other motorized equipment, will often have a "Max OCD" or "MOP" number listed on it. That's max overcurrent protection device, a fuse or breaker. You should never exceed this size, or you can risk damage or fire. If you do not see this rating on your motor nameplate, the NEC specifies that you multiply motor FLA x 2.5, then you may round up to the next standard breaker size, but you are permitted to go down to the next lower size.

Hope that helps. ~ Juice
 
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