DC motor risers break
DC motor risers break
(OP)
Two 500 V 1500 kW DC motors for a reversing rolling mill have had catastrophic failures in that the risers have broken and (as it is assumed) caused a collector round-flash. In both cases it happened after around three months of operation and there is no sign of arcing or other electrical damage to the breaks. They look like fatigue breaks.
There are some speculations as to the reason for the breaks. I do not disclose what the thoughts are, because we would like some unbiased and fresh thoughts. What are the more common reasons for a case like this?
There are some speculations as to the reason for the breaks. I do not disclose what the thoughts are, because we would like some unbiased and fresh thoughts. What are the more common reasons for a case like this?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.





RE: DC motor risers break
Breaks in risers have been documented to occur where they (connective-ly) mount, and not necessarily where they flex.
Attached photo of stripped armature showing commutator risers for forum reference.
Are the breaks near the commutator, or up near the winding connection?
John
RE: DC motor risers break
Is there any chance of conductor movement.
Heat expansion? Do the conductors follow a zig zag or curved path to the risers to allow for heat expansion.
Do the conductors go straight to the risers so that heat expansion will cause pressure on the risers?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
They have to induce some radial airflow as the rotor rotates.
... which is periodically frustrated by the brush ends passing nearby.
A siren-like noise, and fatigue to the risers, would result.
Do you have fatigue life data for the riser alloy?
The fatigue would go faster if the excitation is close to a natural frequency for the riser elements as beams.
Mike Halloran
Pembroke Pines, FL, USA
RE: DC motor risers break
RE: DC motor risers break
Mike's thinking is new input. There is some doubt as to annealing after cold-working the risers.
Bill, there are curved sections to take care of thermal expansion.
Breaks are close to middle of risers. There are supporting "ropes" with cured impregnation keeping the risers from heavy vibration. A few of those ropes have broken.
The motors came directly from the manufacturer and are brand new.
As I said, I will take pictures Monday. Thank you for the comments.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
With two almost new motors failing, can we consider that the risers may have been damaged during the manufacturing process? Is it possible that the risers were cracked and waiting to fail even before the motors were put into service?
" A few of those ropes have broken."
Let's consider if this is cause or effect.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
I found a picture that I magnified so that the Brush, brush holder and risers can be seen. The structure is definitely "Siren like".
See:
Furthermore, the breaks are in the curved region running close to brush holders:
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
RE: DC motor risers break
I wonder, are the drive ends of the windings restrained so that most of the thermal expansion must be absorbed by those curves?
There is differential movement between the windings and the core during warm-up. The heat is generated in the conductors and then conducted to the core.
I once had an exciter that developed shorted windings due to differential movement. The rotor tested good when it was cold. When the rotor was heated in an oven there was no differential movement and the short would not develop.
The shop was reluctant to rewind a rotor that appeared to be good.
We ran the machine until the short developed and then immediately did a brush null test. That is, we excited the field with AC and measured the voltage across the brushes. The varying voltage as the machine was rotated proved a winding fault.
The point is that heating and thermal expansion as the motor warms up is not a simple process. There is probably differential movement and depending on the construction of the rotor the curves may not be enough to absorb the cycling movement.
You mentioned that the manufacturer may have problems with other motors.
I would expect that the time to failure would correlate with the number of starts more than with the number of hours run if this is a heat expansion issue.
Extreme load-unload cycles may skew the results somewhat.
If you are able to contact the person concerned at the manufacturer they may be able to confirm a correlation, even a loose correlation, with the number of starts. Yes, I know.- It may not be possible to talk to anyone with first hand knowledge.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
Bill: It is a reversing mill. So there are continuous starts/stops and starts/stops in reverse direction. The speed is ramped up/down and the thyristor rectifier is current limited, so there are no extra high currents during operation. And, yes, the movement is restrained and expansion is absorbed by the curved parts.
I tend to follow Mikes tip. There are sound recordings where a whining sound that comes and goes. Much like a siren sound. And there is a steady 300 Hz (50 Hz grid, hence 6x50 Hz ripple) sound in the background.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
It appears the surge rope laced around the risers shifted from its possible initial midpoint location
(perhaps by centrifugal force) leaving the middle portion of the risers inadequately braced.
The blocking shown closer to the commutator in the first photograph would have contributed to increased
rigidity had it been located more centered along the riser length where flexing is prone to occur.
John
RE: DC motor risers break
Possibly earlier designs had a larger curve and so did not experience the stresses that the current motors endure.
But, I am not discounting Mikes tip either. A ball park estimate of expansion may rule out my suggestion.
And, looking at design and construction issues: A change in the shape of the risers from the original design would probably change the natural frequency also.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
Or they were cold bent instead of annealing and then bending?
Muthu
www.edison.co.in
RE: DC motor risers break
Muthu
www.edison.co.in
RE: DC motor risers break
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
We have checked the motor in a winder's shop and ran it through the whole speed range. The resonances are there, but we have a problem seeing exactly what is happening. There are several reasons for that - the stroboscope we had available is rather weak, and it wasn't synchronized with with the rotor. There is also very little space between risers and brush holders. We are trying to find a better strobe, synch it with the rotor and find a good mirror so that we can see better what is happening behind the brush holder and stabilizing ring.
My question is: Do you have any references, pictures or recordings that can help us explain the phenomenon to the motor manufacturer? They are sceptic and not willing to accept our explanation without any hard evidence.
We need to finalize the report by Tuesday and it would be extremely helpful to have your input Friday or Monday - if at all possible.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
It also occurs that if the VFD is not completely stopping the motor before electrically reversing it, there could be large currents induced by said 'plugging' of the motor. I did that with a 1/15 HP ac gearmotor, and blew a 15A breaker right out of the breaker box. I did not photograph the remains of the breaker, which broke into little pieces on hitting the floor.
Maybe you can find a high speed video camera, which may capture flexure even without being synchronized.
For a small motor, you could rig a dove prism to rotate at ~half the motor speed, and look at the assembly along the motor axis with a telescope. The illumination would not have to be strobed, but your motor may be too big and too enclosed for any such rig to be practical.
Maybe you could rig up some soft plastic, maybe fibers like PlastiGage, adjacent the riser bars, and examine any witness marks left from flexure of the riser bars.
Mike Halloran
Pembroke Pines, FL, USA
RE: DC motor risers break
I was more looking for a documented case. Also, this is a DC machine so I think that mentioning a VFD was a slip of mind. And the high torque scenario (bridge failing to commute during braking) is not applicable. Operation is smooth and an occasional bridge short circuit would not produce fatigue breaks. There was no tripping and no blown fuses until the failure occurred.
The PlastiGage or similar technique is something to ponder. Will think more along that line.
We have been turning this problem inside out and tried to find a plausible explanation. There are torsional and flex resonances, but they are damped with a Q somewhere around 5 or less (estimate from impulse test) and they also do not produce forces that act in the plane where the risers are.
Your suggestion is the only plausible one, so far. But we need to either show the effect in a convincing way or find a case where it has been documented.
Anyhow, we now have a Shimpo DT 31 strobe that is rather powerful (xenon, not LED) and can easily be synchronized to the rotor and we will run a test on Monday. The synchronization will allow us to get a steady picture while changing speed slowly across the whole working range. A high speed camera is available at the nearby ABB HV breaker lab, but we haven't taken that step - yet. That would involve bean counters and would complicate matters more than we wish.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
Check this link http://iccinternational.com/new-blog-1/ and attached file. I hope help you.
RE: DC motor risers break
The curved risers. The bend, or shape of them is not uniform. This observation aside, I spent some time searching to find an unrelated photo
of a D.C. motor utilizing a curved shape riser in its design and have pursued an attempt at learning what purpose it would even serve.
And have so far come up empty.
If someone out here has an excellent picture showing a new product with a curved shape open arrangement riser, please post it.
The other troubling aspect of that second photo is that rope. It just doesn't look like something a manufacturer proud of their product would utilize.
It looks loose. It looks like an "after thought", and it just doesn't appear to be quality engineering for the size motor it is stated to be.
It's convenient for the motor user to point a finger at poor material and workmanship, yet the risers have physically broke.
Well then, the next place to consider is the application of the motor. (Which is typically the finger pointed by the manufacturer).
What, if anything was overlooked in selecting such a motor, and how it's controlled?
Staying "tuned" on this one. Keep us posted on what you find.
John
RE: DC motor risers break
Yes, I can agree that the "neatness" leaves something to desire. But, apart from that, it is the reduced longevity that is of concern. The two motors are identical and they have run around 3 months (yes no more than 90 days) before failure. Both failures are riser fatigue breaks in the curved region and both failures occurred after three months of operation. So, the mechanism seems to be consistently the same. Even failure progress seems to be identical.
The riser bends are there for thermal expansion. It is a standard feature in larger DC motors (these are 1500 kW) and I think that your experience is more in the FHP or even smaller motors where thermal expansion is a lesser problem. Or are there manufacturers where straight risers are standard for motors this size? I see straight risers in DC motors up to a few hundred kW, but in our neck of the woods, the bend is standard for larger ones.
The uneven look may be a result of the vibrations. I never saw these machines in virgin state. So I cannot say what they looked like before the failure. Shall try and get pics from that time.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
Keith Cress
kcress - http://www.flaminsystems.com
RE: DC motor risers break
They are all from the 80's and GEC production. We did not had a similar experience with deformities and fractures of risers.
The rope is common for fastening end of stator coils in AC motor and I think it should do a good job in this motor.
Might be riser material is problem like Muthu mention..
Good luck
RE: DC motor risers break
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
Muthu
www.edison.co.in
RE: DC motor risers break
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
If the siren or vibration effect is the cause then a hardness comparison between new material and failed material may be interesting.
I think to be valuable, samples must be tested from new material and failed material.
Keep us posted.
I have found that when you are asking instead of helping, the problem is extremely challenging and interesting.
Thanks for sharing with us.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
MotorSpeed*NumberOfRisers
… or an interger multiple thereof.
That's one thing to watch for as you do your investigation if you haven't already thought of it (I'll bet you have).
=====================================
(2B)+(2B)' ?
RE: DC motor risers break
Tests with a stroboscope didn't reveal much. Too bleak light. I have now put super bright LEDs (COB 2024) on a heat sink and protect them with some shrink tubing. See pictures below (dark):
And light:
I can set pulse width from around 20 us to 500 us and pick up the trig signal from the shaft. So, we should get a stationary view at all speeds. The ventilation fans are 7.5 + 7.5 kW and the air flow is said to be very high - I think around 1200 m3/minute. Hard to believe. But that is the number I have heard. So Mike's thinking seems to be plausible.
Results will be available in around one week. Stay tuned!
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
That would be new info (to us reading this thread).
Would certainly tend to support the theory. (Although as you know not 100% conclusive, since that same frequency could also represent rate of brush passing over commutator segments, or possibly an electromagnetic slot-related frequency).
Good case study and look forward to hearing more.
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(2B)+(2B)' ?
RE: DC motor risers break
The vertical scale is 6 dB/division, so the 3370 is quite powerful. Only around 4 dB down from the 300 Hz (6x50 Hz) and that is a lot of energy. Switching the fans off momentarily (we are running with no load) reduces that level a lot. So it is almost certain that it comes from air flow. Someone used the expression "air cavitation" which, incorrect as it is, still describes the situation rather well.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
=====================================
(2B)+(2B)' ?
RE: DC motor risers break
RE: DC motor risers break
Obvious, but didn't think of it. The machine is at the Imtech shop in Örebro and a "special arrangement" makes it possible to do a test across all speeds Tuesday next week. There will be several people from parties involved present at the test.
If anyone feels like joining us, just shout - I think that it can be arranged. On top of that, management offers something that doesn't exist: Free Lunch!
I think that we shall start with the plucking. Very good. Somewhat ashamed that I didn't think of it myself.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
RE: DC motor risers break
RE: DC motor risers break
We had another go at the risers today. Armed with a very intense strobe light that was synchronized to the motor shaft using a NeB magnet and a reed relay mounted on a magnet stand, we had high hopes to be able to see variations in the distance between the risers. The strobe pulses were 50 microseconds (a standard Xenon strobe has around 20 us) and the reed element closes in less than 100 us with an estimated maximum spread +/-20 us. The resulting uncertainty in riser position from frame to frame is between 0.3 and 0.5 mm tangential. There seems to be some vibrations in the magnet stand and they come and go at certain speeds. A more stable stand was not available, but those frames where the influence from the stand is obvious have been deleted. The results are shown in pictures below (snap number plus an F indicating fan on and a number giving approximate RPM).
Comments invited.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
Keith Cress
kcress - http://www.flaminsystems.com
RE: DC motor risers break
RE: DC motor risers break
controldude - the risers I have seen are always of two single pieces so that they than wrap around the armature conductors for soldering/brazing.
Muthu
www.edison.co.in
RE: DC motor risers break
Mike Halloran
Pembroke Pines, FL, USA
RE: DC motor risers break
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
1) Whoa yeah that looks like a lot of flexing in the images for copper, which annealled or not work hardens quickly. Visually issolating just a single laminated pair based on the indications on the edge... you can see that the apex of the curve is moving radially in and out, and the gap between the laminations is not remaining consistent, so those puppies are moving.
2) My experience is with auto shredders, 2500hp to 9000hp. Cardan Shaft Drivelines for these chaos machines can last for a decade, or they can last for 10 days, even given the same maximum torque rating. It is all about the natural frequency of the ASSEMBLY in this case, not just the individual component.
3) What you are seeing on a bench mounted motor may be dramatically amplified in the installed state due to the assembly natural frequency.
4) We use wound rotor motors of various designs up to 9khp at 450rpm (16 pole, 60zh)and EMD and GE DC motors 2500hp down to 1000hp, and periodically piggybacked EMD 2500hp motors for 4000 to 5000 hp applications. In all cases, the most dramatic failures of mechanical parts of the motors involve inadequate attention to the torsional vibration of the assembly.
5) I'm suggesting, for giggles, a bump test of the installed motor.
RE: DC motor risers break
In our case, attached to the other end of the drive shaft is a machine, the rotor of which is a stacked up assembly of plates, shrunk to the shaft, and then through bolted with 3" dia tie rods with 300 tons of tension on each. It is a complex assembly. Determining the spring rate for this creature is complex as well and using "rules of thumb" is insufficient.
The method of coupling, the degree of interference between components of differing diameters, all quite complex and easy to have very broad errors due to the ranges of tollerances of manufacture.
Bump test on the other hand shows the real world, not the stacked up errors of hopes and dreams.
Changing from one motor design to another... different inertias, different shaft spring rates, these things obviously will affect spring rate of the assembly.
We've (our motor supplier) have done strain gage analysis on problem assemblies to save our asses when motor problems occurred after the customer changed drive shafts, where we've found dramatic torque reversals due to torsional natural frequency convergences. Very nasty stuff there.
Talk is cheap. I pulled off company refferences to protect the innocent and attached an example.
RE: DC motor risers break
RE: DC motor risers break
They've been approaching us for our abusive application, and I've spoken with their refference, a mill maintenance manager claiming very good performance and reliability. If you want to know more contact me directly and I can connect you to the right guy.
If they are doing it, likely others are as well.
RE: DC motor risers break
You have a more challenging task for a variety of reasons. I guess variable frequency strobe was not bright enough or precise enough.
I'm not sure exactly what results are expected by non-sync strobing while varying speed. Away from resonant you don't expect to see much. The only points of interest are near resonance and since the flashes are not sync'd with anything, the images obtained will vary somewhat randomly within their range of motion. If you only get a few snaps passing through resonance, you may not catch it near an extreme range of its motion.
If stuck with single speed non-sync'd strobe, I think I would run the machine at a speed that I think excites a resonant frequency and take lots of shots at that single speed in hopes of catching it at various extremes of motion. I guess you could figure that speed out as the speed where the wandering peaks in audio spectrum or highest. Or else estimate riser resonant frequency and run at a speed which is a fraction 1/Nrisers of that frequency.
Just thinking out loud....
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(2B)+(2B)' ?
RE: DC motor risers break
Not that such a difference is very big, but the possibility that there is a difference can lead to unnecessary discussions and by using a synchronized light with an estimated maximum uncertainty of 0.3 to 0.5 mm (ten to twenty mils), one can say for sure that we are looking at the same risers in all the snaps shown. And, since the variation between the pictures is, as Mike, Keith and WJ point out, quite big the reason for the breaks is flexure in the risers themselves and not any thermal or shear/stretching forces from resonances. Torsional and flexional resonances have been proposed, but there is no possibility that a complex body like a rotor with plates, insulation and a low Q can resonate in such a manner that forces concentrate in the bend of the risers. Not even if running constantly at a speed where resonances occur. (Refresher: it is a reversing mill where speed is all but constant and risers broke in two machines after around three months of operation)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
I think however that the voice of experience is where to start first, and the most relevant shared experiences so far involve the "siren" effect.
Alternating vortex building and vortex shedding create tremendous oscilating forces in every endeavor in which they occur.
Still, due dilligence should be performed. When the consequences are high it is best to consider all possible contributing factors.
RE: DC motor risers break
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: DC motor risers break
I mention the resonance of the entire assembly - of the motor connected to the rest of the machine, because it appeared to be missing in the conversation.
The curved elements that are failing have, no doubt, a substantially lower natural frequency than the rotor.
When the inertia of this rotor however is connected, commonly in rolling mills by long cardan shafts and often at relatively extreme operating angles, the motor rotor mass on one end and the mill roll on the other.... things can get pretty springy in a bad way.
And if someone accidentally puts the drive shaft together wrong... well things get exiting quickly.
Yep...one more thing to check. If Cardan Shafts are used, be sure that they are correctly aligned.
When the splines are slid together it is critical that they align properly.
I've seen so much intelectual equity dying and retiring that nothing surprised me, and nothing can be assumed to be correctly assembled.
RE: DC motor risers break
RE: DC motor risers break
The pictures were taken when running in a winder shop test bench with motor mechanically disconnected and with no load. The cardan installation and other possible mechanical problems have been ruled out in an early stage of the investigation that took place several months ago.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
Was A weighting applied to the sound spectrum?
Are the "brush holder rings" continuous complete circles, and stationary?
Are there any risers with cracks/indications that have not progressed to complete breaks?
If a concern is riser-pass causing a siren effect, there may be ways to "soften" the pulse.
- Perhaps a non conductive disk could be added to the row of risers. Like the spoke guard on a Derailleur bicycle.
https://i.ytimg.com/vi/o9oVJUplaQI/maxresdefault.j...
Pumps are sometimes modified to reduce the strength of blade pass pulsations that are causing noise or vibration.
http://www.mcnallyinstitute.com/12-html/12-06.html
Machining a full size impeller OD even a few percent to increase the gap to the cutwater can help a lot.
Sometimes just the vane OD is reduced. Or, the vanes can be cut on a taper.
Can the brush holder be shifted to increase the spacing?.
Perhaps the profile of the leading edge of the brush holder can modified to create a "softer" pulse. Or maybe adding epoxy blocks to create a rounded, or angled profile.
Like the upper edges of these motorcycle engine exhaust ports on t5he attached file.
RE: DC motor risers break
Did the OEM weigh in yet? After all, they are still under warranty?
Muthu
www.edison.co.in
RE: DC motor risers break
Tmoose. The machines are no longer available. The papers you refer to are quite interesting and show that seemingly non-relevant details can have an effect. The air streams are, in comparison to the derailleur, enormous. It is actually difficult to stand on one's feet in front of the exhaust from the commutator side when the cover has been removed.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor risers break
Understood. It will be interesting to see how this problem is fixed.
Muthu
www.edison.co.in
RE: DC motor risers break
I mentioned that we have a lot of electric motors with a similar form of risers .Yesterday we had inspection of several DC motors and I send a picture how does it look this part on them.
Good luck!
RE: DC motor risers break
I may be missing something that has been reported already.Have you done a test without the fan running, to isolate the cause to air flow?
I am sure that you have, but it will be nice to have that information stated.
Apologies if I have missed this information in previous posts.
Bill
--------------------
"Why not the best?"
Jimmy Carter