Characteristics of an open rotor bar 2

[duke709]

Please excuse me if this subject has been covered before,I'm new to this forum.
We recently replace a 10 horsepower induction motor. This motor was used on a conveyor system with numerous stops and starts. It was controlled by a Allen Bradley SMC. The motor would operate flawlessly; then intermittently the inrush current would shoot up and the thermal usage recorded on the SMC would increase rapidly. There was definitely NO mechanical binding.The motor would appear to move slightly as if it was single phasing but the controller only reported overload conditions.Then after about a minute, after the motor cooled, it would operate normally again.
It seemed this would only happen on motor start up but not all the time, nor after a set amount of time.
My question is..is this a symptom of a possible open rotor bar ? Also, the condition could not be demostrated with the load (gear reducer) removed.
Thank you for any response

 

[rlpuck]

hey duke, if these indications are legitimate I would think it could be a rotor problem, perhaps a break in the rotor bar shorting ring, being that this is a small motor it may be worth while to change it out with a new one and see if the problem goes away, check out what the starting conditions are for this motor, how frequently?

 

[Marke]

Hello duke709

I doubt that it is an open bar problem. If you have an open bar, the torque capacity of the motor will drop, and iff you had sufficiently high load on the motor, it will begin to stall and then the current would certainly rise. I have seen a number of motors operating at their rated load with an open bar and they have not exhibited such characteristics. From my experience, if the motor has an open bar, it will show up as a swinging stator current with the frequency of the swing directly proportional to the load. As more loade is applied, the slip will increase so the current swing will be more rapid. The amplitude of the beat can be in the order of 30% of rated current so it is quite pronounced. (It may be hard to see on a digital meter due to the sampling period)
From your description, I would be looking in two places:
1) the stator, possibly an intermittent shorted turn that is causing a problem when the motor heats up, or
2) the starter, an intermittent SCR in the starter will cause this problem and can go away when the SCR cools down again.
Have a look at the output voltage of the starter on an oscilloscope while this is happening, or measure the voltage input to output on each phase while the problem is occuring.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[HarryDampers]

Marke is 100% correct and this type of fault is probably more easily seen if you are using an old analogue Ammeter to display the current .

There will be a noticeable kick in the current reading with every revolution of the machine .

 

[duke709]

Thank you for the responses.
I should have added a few more facts in the first post.The controller was first replaced, nothing changed, thus eliminating any controller problems. The reducer was changed, no improvement, and finally the new motor that we installed was replaced with another new motor... corrected the problem. So we know there is a problem in the first replacement motor. The key to the problem is how intermittantly it occurred. It worked perfectly; then for some reason the motor fails. After the thermal usage temperature dropped, the controller could be reset and the motor worked perfectly again. During all this time, the thermal usage was low, then suddenly it would spike up as the motor appeared to be single phasing, but the controller only recorded overload conditions.
Secondly, how does one check for open rotor problems? I heard using a variac with a small current thru two of the phases and monitoring the current with an amprobe while the rotor is turned by hand will sometimes detect a problem.

 

[electricpete]

You are right, a "single-phase test" can be done to attempt to detect rotor problems. A large swing in measured current as you move the motor is an indication of problems.
Try searching this forum for single phase test, possible with "motor" and/or "rotor" for more detailw.

Intermittent behavior is common in rotor bar problems. In some cases perhaps there are thermal changes causing intermittent contact. If it is in fact intermittent, then the single-phase test will be conclusive if fails (indicates rotor problem) but inconclusive if passes (may be no problem or may be intermittent rotor problem).

If you had the opportunity to run the motor under load (>50% load), broken rotor bar would be indicated by current oscillation, slip higher than expected, pole pass frequency sidebands in current and vib spectra, slow increase in vibs after startup.

 

[Marke]

If it is a thermal problem, and it occurs after a period of time running, then I would expect that it is a stator problem rather than a rotor problem. The rotor is subjected to thermal stress during start rather than during run. If there is a rotor thermal problem, it will tend show up during start.
To isolate thermal problems, put the motor into an oven or kiln and heat it up and then test it. We found an intermittent stator problem that way. Semiconductor fuses kept failing intermittently on a large machine and we could not find the cause. Fortunately, it was at a brick works. We heated the motor in the kiln and there it was, the insulation in the stator broke down. Because we were using semiconductor fuses, the fault energy was limited and the failure was not catastrophic. Normally, the insulation breakdown would have resulted in a major hole in the stator winding.
If your replacement motor is identical, you could swap the rotors and see if the problem moves!
Best regards,

Mark Empson

http://www.lmphotonics.com