3 phase motor overloads

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  #1  
Old 07-16-05, 03:32 PM
blackharley
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3 phase motor overloads

I am working on a 3 phase 460V motor and when I start it phase A overload heater will glow red hot then catch fire, i have never seen anything like it.
The overloads are properly sized and the motor megs out ok.
any suggestions?
 

Last edited by blackharley; 07-19-05 at 08:21 PM.
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  #2  
Old 07-16-05, 03:43 PM
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1.) This is a problem that has occured on previously working motor?

2.) This a problem that has occurred from new installation? Here I am assuming you have not let any smoke out of the motor

My thoughts if this is a problem due to #1 or #2

(if #2 and you didnt follow the wiring instructions on the inside of the wiring motor enclosure or instructions you could have these symptoms but I believe you would most likely burn up the motor)

Does this happen immediately on start up?

Is the circuit protected from overcurrent by fuses or 3 phase breaker?

Is phase A fuse blowing or if overcurrent protection is a breaker is it tripping along with the overload heater burning out?

It has been my experience that if a heater is getting that hot..to the point of catching fire that a phase conductor is shorted to ground or another conductor.

Is this at work? If so I have seen where damage otherwise unknown..ie..fork lift crushing conduit for motor branch circuit causing damage to conductors, creating short.

Also if more than one motors branch circuit conductors are in the conduit and the bad circuits phase A conductor is shorted to them then you will get backfed voltage from those conductors causing burn out the overload.


My best guess:

Phase A of the bad circuit appears to be shorted to ground or another motors conductors in the conduit run. If it was shorted to another phase conductor of its own motor circuit then you should have those corresponding heaters also getting glowing hot.
 

Last edited by Roger; 07-17-05 at 11:37 AM.
  #3  
Old 07-16-05, 03:49 PM
blackharley
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yes, within 3-4 seconds.
 
  #4  
Old 07-16-05, 04:05 PM
blackharley
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My next step is:
a. physically disconnect the motor and try that, just in case.
b. replace the overload block (I don't think thats it, but i will try it)
All phase voltages are normal (all around 277)
 
  #5  
Old 07-16-05, 04:21 PM
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I would do this... I would disconnect at the motor, then I would disconnect T1, T2,T3 on the load side of the starter after the overloads. I would do a continuity check between phase A and ground and the other two phases.

Is there more than one motors conductors involved?
 
  #6  
Old 07-16-05, 04:26 PM
blackharley
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Thanks for the reply
I will physically disconnect the motor (Monday) then megger out the wires form the contactor down to the JB.
The motor did megger out fine the first time though.
Its an existing installation from years back.
The 3-pole C/B trips almost immediately after the O/L heats up, so you are probably correct in that there is a short somewhere downstream of the contactor.
 

Last edited by blackharley; 07-19-05 at 08:20 PM.
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Old 07-16-05, 04:29 PM
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I agree...look for a short and post back Monday...good luck
 
  #8  
Old 07-16-05, 04:40 PM
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Is this conduit run underground where the conduit may have filled with water?
 
  #9  
Old 07-16-05, 04:41 PM
blackharley
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It almost has to be a short, I sort of seen this before but usually in the motor winding themselves, just not to the point of a fire actually starting from it.

Thanks again for the help, I have to get it going Monday so I will post and let you know what happened.
 
  #10  
Old 07-17-05, 06:52 PM
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I would disconnect the motor, use an amp clamp and check for any current leakage.
------------------------------------------------------------
It appears that the motor has a shorted loop in one of the coils.
You can check the resistance of each line, but just one turn shorted to its self will not show up.
a shorted turn can kill the inductance of the coils.

They use an internal growler for checking fields.
an inductance meter should show a problem also.

If you have a variac and an amp clamp.
maybe a 12 volt transformer if you don't have a variac.
You can power up each line with low voltage AC and compare the current readings.
Keep the current down as low as possible for just a reading.

The current will go thru the roof on the coil that has the shorted loop.
be sure to use a low current fuse in series with the variac.

you may need to check all the field lines separated not tied to the three lines.

This test assumes that the loop is shorted at all times not just at 460v.

Danger !
energizing one line can produce voltage on all the other wires.
 
  #11  
Old 07-19-05, 08:19 PM
blackharley
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OK, its turns out that one of the motor winding was shorting out.
There was considerable internal corrosion caused by the location and age of the pump. I installed another motor, including overloads (I found the original ones were oversized) and everything appears to be working ok.
Thanks for the replies




















.
 
  #12  
Old 07-20-05, 12:35 AM
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Glad you came back with the results. Its always good to find out what the problem turned out to be. Looks like GWIZ nailed it.
Gwiz I have only in one instance seen a turn to turn short set an overload on fire. I have seen them get hot...real hot. Most of the time I have seen this occur when the conductors for the branch circuit short from damage of some kind. I have also seen the motor smoke pretty badly from winding short. If it had been single phasing you would have voltage but no current and no shaft movement. Do you see this often....the overload catching fire?
 
  #13  
Old 07-22-05, 12:23 AM
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Originally Posted by Roger
Gwiz Do you see this often....the overload catching fire?
No.
I seen the overloads glow red, had one smoke.
I was able to kill the power before they caught fire.
The motors were shorted, don't know if they were turn to turn shorts.

In the 70's, I would hang around a place that repaired motors, they were using growlers to get an idea on field leakage eventually the motors would get a turn to turn short.
They used it as a life test.
A person would bring in a motor for new bearings, they would tell him it would not last the year, 6 months later the person returned with the motor burned up saying your right! rewind it.
At that time the motor shops would detect the starting of a turn to turn short, the end result was complete burn up.

Now a days the insulation is so good that it is rare to get a turn to turn short.
If the motor gets a lot of corrosion it squeezes the coils together creating a turn to turn short, more on the pump motors.

Depending on the resistance of the short, that can limit the current to a low level and smoke the O/L.
 
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