Rotor bar melt down 13

[THooper]

Please give your expert opinion on the photos below. This motor failed due to open rotor bars after several years of service in June. The bars on the failure in June 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. If you compare to easa photo below you can see 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 down as easa photo shows. 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?

 

[THooper]

Below is failed motor image.

 

[THooper]

Below is failed rotor image.

 

[roydm]

Many years ago I worked in a large copper mine, they would run the 400 HP crusher motors at 400 HP or beyond 24/7.
If they bogged a crusher and reset the overloads too many times with no cooling off period the copper rotor bars would melt.
I'm not saying that's what caused yours but it might be.

Roy

 

[rasevskii]

TimHarper:

Nice mess you have there. Since the aluminum bars seemed to melt slowly, not explosivly, it looks like many overloads or locked rotor events happened over some long time period. Evidently the overload protection is not working, disabled or wrongly set. Or the motor is being abused by the operators. Better change the protection to something better and see to it that no one tampers with the settings.

What is the application?

rasevskii

 

[electricpete]

What is the speed and horsepower rating of the motor?
What does it drive?
Can you describe typical start for the motor (manual start? is machine operation checked upon start)
You said "it failed again"... in what way did it fail? What did you see the prompted you to remove the motor? Did any protection actuate?
I think it is an a fabricated aluminum rotor, is that correct?
What is the bar shape and depth?

You say the overload works ok. If it's a large (above NEMA) motor, do you have an OEM safe starting current vs time curve and have you compared it to overload protection characteristics? If it is a NEMA / medium motor, what are the overload settings?

What is the purpose of posting the EASA photo? I think you're saying that because your stator winding does not look like that, then there's now way the rotor could be damaged by a stall event (accompanied by presumed inadequate overload protection)? I don't think I would agree with that logic. Start can be much more severe for the rotor than stator because rotor resistance goes up due to deep bar effect. Bars melting during start without stator damage can happen on many motors, especially aluminum rotor.


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(2B)+(2B)' ?

 

[THooper]

The application is a small crusher, called a feeder breaker used in an underground coal mine. Most the information I have thus far is from the Maintenance Coordinator whom says all overload protection is working fine and they have not stalled it out, that they (manual) start it at 7:00am and it runs all shift and they shut it off at 3:30pm. The motor is a dual shaft that one end turns a gear box and the other drives a hydraulic pump. 150hp 445TDs frame, 1785 rpm, 460 vac, Design C, The reason they pulled the motor was ground faulted, the insulation in the stator core area has broke down from the excessive heat of the rotor.

It failed first time in June, I have posted Junes failure.


Attached is the name plate:

 

[THooper]

June rotor fauilure:

 

[THooper]

June stator failure:

 

[THooper]

Rotor slot configuration:

 

[waross]

Have you checked the phase voltages and possibly phase angles? If you have a chronically low voltage on one phase, or a phase angle error, the rotor will overheat. If the motor is mostly lightly loaded the effect may be worse as the rotor may be badly overheated while the phase currents appear to be within safe limits.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

Were the overloads tested? It is not clear to me how anyone could know that the overloads are working properly unless: a) they have been tested or b) they are known to have previously tripped during known motor overloads. If they are known to have previously tripped, what were the circumstances? Could this be a clue as to the reason for failure?

Note that both stators appear to have the same failure mode. That is, the winding tie cords are broken, presumably from sudden, short-time overheating of the winding and stator core. I say short time because in both failures the windings do not have the characteristic black appearance as seen in the EASA photo that would indicate a sustained overload to failure. By the way, did the recent failure include a ground fault in the stator core? If not, you came very close.

It would appear to me that the problem is caused by stalling, multiple repeated starts without adequate cool down time, or multiple short time overloads or partial stalls. Inoperative or improperly sized overloads would contribute to this by not providing proper protection. Another possibility is the use of electronic overloads that are not equipped with a thermal memory function. This would allow repeated immediate resets and/or repeated short time overloads without taking into account the accumulated overload time and the lack of necessary cooling time between events.

Is the crusher loaded or empty during start? Starting when loaded could lead to stalling and/or excessive starting (accelerating) time. Likewise, bogging the crusher motor down with excessive feeding can cause overloading and partial or complete stalling. Either one of these scenarios, if repeated, could lead to this type of failure.

 

[THooper]

We need to verify as you stated on the overload protection. I don’t know what measures were taken that brought them to the conclusion that they are working fine. As far as being loaded and starting under load is very possible, but I am told it is not, that it is started and left running and would see intermittent loading per hour and runs dry after a few minutes of material running through the crusher……These are all great points and I need to do a little more investigating on our end.

 

[electricpete]

Well, you’ve provided a lot of data, that’s good. I haven’t studied it all yet.

One aspect to mention: The rotor bar configuration you posted is tall and thin with a narrow neck just below the top. It’s clearly designed to enhance the deep bar effect. Why designed that way? Because it’s a design C motor. Designed for high starting torque at normal starting current. But it’s not a free lunch. If you compare it to the same rating design B motor, the design C motor will be harder to stall, but physics tells us it will be less tolerant of stalling and repetitive starting. And more likely to be “rotor limited” as opposed to “stator limited” meaning more likely rotor is first item damaged thermally during starting abuse. Which goes again to your question: don’t be surprised to see rotor damaged before stator from starting abuse.

As a medium/NEMA frame motor, the overload settings should be pretty straightforward now that you’ve given the naneplate info. . As far as I know, there is no difference in settings for design C compared to design B (someone correct me if I’m wrong). It might be interesting to know more about your overload protection- manufacturer and settings.

That looks like a concentric wound motor stator. Doesn’t mean anything for this discussion, just mentioning..it’s unusual in the motors I see.

I still haven’t gotten straight in my mind what kind of rotor. The shape looks like it would be cast. You said the rotor was rebarred. How did they do that? They had these odd shaped bars that they inserted into the core, or they did some casting? I have never been involved in repairing a rotor which was originally cast (if that’s what his is).

Stepping back to look at the big picture: There are two sides of the equation: your initial impression I think is that the motor repair shop must have done something wrong. Everyone hear is responding that you should check for things that might be wrong in your installation. Those are very important things to check and also I think the only thing that we can provide input on (how can we know if substandard job was in fact done by rebarrer). The fact that you had a rotor bar problem on the initial motor before rebar suggests there is something with your process/protection challenging the motor. The fact that the rebarred motor didn’t last as long as the motor MIGHT be indicative of something that changed in your process that pushed the first motor over the edge and now pushing the 2nd one as well. Sorry, if that’s all obvious, just talking...


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(2B)+(2B)' ?

 

[electricpete]

If you compare it to the same rating design B motor, the design C motor will be harder to stall, but..

I should correct that: harder to stall at rest (since locked rotor torque is higher), but not harder to stall (since breakdown torque is not necessarily higher).

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(2B)+(2B)' ?

 

[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 it's thermal memory and was password protected. There were no more motor failures.

 

[jraef]

Having worked for soft starter mfrs 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, both of which are mentioned above.

Voltage imbalance: what Waross mentioned is very common on utility supplied plants (portable plants have local DGs so this isn't as prevalent), but he is dead on on it being a likely 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. All he left out is that 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: electricpete mentioned it and LionelHutz hit it dead on, 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.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[gattie]

Motor Aluminium Rotor bars are not a good choice for this application.
Agree with others that an integrating type of impact/overload sensing is required
to reasonably regulate the the choke feed to the crusher.
Operator training monitoring is a big factor - they will always push the limits to get optimum output from any machine.

 

[gattie]

I overcame the rotor problem by installing a Steam Age P&B Golds Motor Protection relay on the Feed process to a gyratory rock Crusher.
A set of Crusher Motor C/Ts were connected to the P&B relay.
The Alarm/Trip contacts were wired to a warning alarm to alert the operator of a possible impending trip.
The alarms were logged.
Rule of the day was:
If Operations bogged the Crusher because of excessive feed – Operators had to dig out the mess Manually!
If Crusher bogged because of Engineering Fault – they had to clear the rocks.
Smooth sailing for Engineering after this!

 

[Marke]

Hello THooper

From the images, it is clear that the rotor has dissipated too much power within a period of time, and probably repeated periods of time.

When the motor is operating at high slip (during start or stall) there is high rotor dissipation. This also reflects as additional stator current and stator heating, however the bulk of the slip loss is in the rotor.
If the rotor has a thermal mass that is less than the load requires, you will slowly overheat the rotor with repeated starts. If the rotor time constant is much too low, then the rotor may fail relatively early. The rotor time constant and the stator time constant are not necessarily coordinated, so, with a short time constant rotor, yoou will get rotor failure without stator failure or damage.

A crusher is generally a high inertia machine and so it needs a long time constant rotor. The motors are typically rated with a maximum locked rotor time or a maximum load inertia. These can vary tremendously between motors and it is important to use a motor that matches the application.

If the rotor thermal mass is adequate for the driven load, but there are repeated starts, then the total rotor dissipation will cause a melt down.

Another cause of excessive rotor heating is negative sequence currents due to voltage imbalance and slip losses due to supply harmonics.
I have experienced local harmonic voltage on the supply above 12% due to the number of VFDs on the supply. This will certainly increase the continuous rotor power dissipation and when this occurs in conjunction with high inertia loads and frequent restarts or stalls, then rotor failure can occur.

I would check the motor thermal rating relative to the load, I would look for overload conditions that cause frequent stalls, I would look for phase imbalance conditions and also check the voltage harmonics.

This could be a) a rotor not suitable for the application, or b) a power quality issue or c) both.

Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd