SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=332610
Please click through to the above URL source to read the entire discussion of this rotor bar melt down problem. Below is a snippet of the solution.
QUESTION
This motor failed due to open rotor bars after several years of service. The bars on the failure were at the end ring connection point and appeared to be from fatigue. The rotor was re-barred by a rotor re-bar facility, after about four months of service it failed again. The rebuild shop is saying failure is due to a locked rotor stalled condition. The overload protection in place that did not shut it down and is working fine. The windings in the failed motor did not see a lot of high current, in my opinion no signs of an overload before rotor melt downs. I feel that the rotor failed first due to possible casting flaws in re bar or possible resistance of bars not correct from re bar?
REPLIES
LionelHutz
The operators never admit to doing anything wrong. We had a site where our equipment was being blamed for killing their motors every few months. We changed the overload from the simple electronic model that could be reset by power cycling to a better unit that retained its thermal memory and was password protected. There were no more motor failures.
jraef
Having worked for soft starter manufacturer for 15 years and owning a panel shop/integrator specializing in the aggregate industry for 5, I have built, commissioned, trouble shot and repaired a lot of crusher control systems. Broken rotor bars, although rare, have almost always been caused by one of two things in my experience:
Voltage imbalance: Utility supplied plants (portable plants have local DGs so this isn’t as prevalent) is likely the cause if that is how your plant is fed. Could be caused by a lack of attention paid to load balancing from single phase loads, a bad transformer, even as mundane as a bad connection somewhere. The damaging effects of voltage imbalance are focused almost specifically on the rotor heating effects. The imbalance creates negative sequence current, which itself creates a counter rotating torque in the rotor that “fights” the normal torque. The result is extra heating in the rotor bars, even though the highest phase current may not even be above the pickup point of the thermal overload curve. So your simple style overload relay will NOT trip because the current is still “normal”, but your rotor over heats and that leads to separation at the rotor bar end ring junctions. A solid state overload relay with phase current imbalance protection will prevent that. Make sure it is phase CURRENT imbalance, not just phase VOLTAGE imbalance, because the current imbalance can be caused by more than just imbalanced voltage.
Excessive duty cycling: The operators NEVER abuse the equipment, just ask them, they’ll tell you so. But install something that monitors or prevents it, and it will either stop or you will get complaints about the new inability to “use it like we used to” that will uncover the truth. A common one I came across was on primary jaw crushers where after an unexpected power loss, the jaw would have rocks left in it that required the operator to get out of the cab, unhinge the jaw to let them drop out, then remove them from the under-jaw conveyor to the screen because they would damage the screen. But rather than do all that work, they will “jog” the crusher motor forward to where it stops (locked rotor), then release it and let the belt pressure rock it back to the opposite apex, then repeat until the rocks in the jaw crack and fall through. The motor of course is not only exceeding the rotor thermal limits from the excessive starts, it is doing so AT LOCKED ROTOR, the worst possible way. I’m not saying that’s exactly what is happening here, just that this sort of abuse is NEVER admitted to up front. Again, a high end solid state overload relay that has features like “minimum time between starts” and “maximum starts per hour” protection will prevent this kind of abuse and often expose it because either the motor damage stops, or someone complains about the new features “interfering with production”, at which time you have found the culprit.
Side issue; on two occasions I have discovered broken rotor bars in advance of disabling motor damage because soft starters tend to drag out the effects long enough to hear them (you can see it in a current waveform spectrum analysis by the way, there are several white papers on this available on the Internet), whereas across-the-line starting tends to happen so fast that normal machine noise drowns it out. In both cases the owners did not believe me (or did not want to deal with it) and kept running. The eventual result was winding failure shortly thereafter. I mention this because it’s entirely likely that the broken rotor bar preceded the winding failure, you just didn’t know it had happened.