unexplained 10% high amps
unexplained 10% high amps
(OP)
- A customer has 4 of our hammer mills. All verified to the best of their ability as having identical feed rates, internal condition, etc, etc.
- 3 draw 60 amps
- 1 draws about 68 amps, (not over nameplate) and has ever since being installed 5 or 6 years ago.
Recently the motor on #4 was overhauled just to remove motor condition as a factor for the high amps, since they want to make changes to the line that will work the mills harder, and #4 has no margin presently.
The "report" came back fine from the motor shop, but it still draws 68-69 amps in service.
1 - Is there a motor condition would make a "good" motor draw almost 10% more amps than an identical motor?
2 - Could that condition be detectable with vibration analysis, or some other on line test?
3 - How much extra air gap (turned undersized rotor OD?) would it take to cause a motor to draw 10% higher amps?
thanks,
Dan T
- 3 draw 60 amps
- 1 draws about 68 amps, (not over nameplate) and has ever since being installed 5 or 6 years ago.
Recently the motor on #4 was overhauled just to remove motor condition as a factor for the high amps, since they want to make changes to the line that will work the mills harder, and #4 has no margin presently.
The "report" came back fine from the motor shop, but it still draws 68-69 amps in service.
1 - Is there a motor condition would make a "good" motor draw almost 10% more amps than an identical motor?
2 - Could that condition be detectable with vibration analysis, or some other on line test?
3 - How much extra air gap (turned undersized rotor OD?) would it take to cause a motor to draw 10% higher amps?
thanks,
Dan T





RE: unexplained 10% high amps
If it's worth the trouble, the motor could be swapped with another one of the other motors to verify that the variable is the motor and not the mechanical side.
Are the motor currents balanced?
David Castor
www.cvoes.com
RE: unexplained 10% high amps
An idea would be /possibly impossible/ to uncouple the motor from the load and measure the amps. Also the voltage. Then do the same with one of the other motors using the same instruments. Easy to see if it is a motor situation, or the load.
If the motors are absolutely identical and running off the same busbar, that would be a good test.
regards, rasevskii
RE: unexplained 10% high amps
Machinists seldom get to play with AC rotors as they did with DC rotors so this effect is very rare now, however if the rotor has been turned, do you have a spare rotor????...
Another possibility is that the "Identical" loads are not identical for some reason. If this is belt drive, a pulley may be a little off size. If this is the first or last mill in the line-up, the transport system may be classifying the material so that one mill gets a slightly different mix of fines and coarse. One mill may be running at a slightly different product level.
Bill
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"Why not the best?"
Jimmy Carter
RE: unexplained 10% high amps
Is the voltage different? Lower voltage at machine terminals results in higher current.
** Is the measured power factor different? That can be a big clue to see whether you have an increase in reactive power (lower power factor) or increase in real power. Increase in reactive power could be the airgap you mentioned... either larger or more likely just not centered. Increase in reactive power could also occur on sleeve bearing machine if rotor iron is not centered on stator iron. Increase in reactive power could also result from replacing magnetic wedges with nonmagnetic wedges during rewind (that happened to us). Higher real power would point toward the mill or very much less likely some type of rub within the motor or increased losses (such as core losses due to core burnout)... again not at all likely that you will have 10% increase in motor loading due to motor losses without noticing other dramatic evidence.
Check the machine speed with high resolution. If slip is higher in proportion to current, it tends to confirm increased real power / mechanical load (in absence of rotor bar problem and assuming motors are identical).
May as well do a vibration survey to see if it points to any anomalies... I know you are well familiar with that.
Be careful how you measure current... especially if the load is oscillating vs sinusoidal, there are a number of ways to characterize it. Interesting case study in the thread "low power factor on recip pump". In that case I believe the inherent load torque variation frequency was not too far from the electromechanical torsional resonant frequency. The result was periodic reversal of real power flow.
Oscillographic recording of current(s) and ideally voltage might provide clues in this case as it did in that case.
I'm not sure I understand your point. If it is that turning of the SCIM rotor is not to be taken lightly, I agree. If it is that turning of a SCIM rotor is always bad or always fatal, I disagree. We have done it on one of our large machines to remedy slight out of round condition on a slow speed motor with elevated 1x vibration. Resulted in a more uniform airgap, perhaps a few mils larger on average. Although there is attempt to perform the machining in a manner that
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RE: unexplained 10% high amps
I was referring to rotor turning that is done because a machinist thinks it would look better "Cleaned up", not turning planned by an electrical engineer to correct a fault.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: unexplained 10% high amps
Muthu
www.edison.co.in
RE: unexplained 10% high amps
The customer has asked for help since they have made a best effort on their own. I'm making a list of things to "check" and how to check them.
There is one feeder per mill. The feeders are designed to deliver controlled (and "equal") amounts of coal as they serve as one of the "gas pedals" for a power plant.
I am kind of focused on undersize rotor ( and its
unknown-to-me vibration characteristics) because I'm pretty sure a larger gap reduces the motor efficiency.
I don't think of an eccentric rotor as having much of an effect on power, even though its vibration characteristics are fairly well known.
RE: unexplained 10% high amps
Here's my swag
Let's say initial current is 0.9 at pf = 0.8.
I real = 0.72, I reactive = 0.54, I total = sqrt(Ireal^2+Ireactive^2) = 0.9
To increase current by ~ 10% to 1.0, we need to increase Ireactive to 0.7, so that
I real = 0.72, I reactive = 0.7, I total = sqrt(Ireal^2+Ireactive^2) = 1
Reactive current needs to increase to 0.7/0.54 = 1.3 of it's initial value.
Roughly half of reactive power consumed at full load is associated with Lm and half with leakage reactances.
Increase in reactive power consumed in magnetizing reactance would be proportional to airgap dimension g.
Increase in reactive power consumed in leakage reactance would increase weekly with airgap dimension... (endwinding leakage reactance does not increase, slot leakage reactance increases slightly). My guess is reactive power in leakage reactance increases as g^0.2.
So we need 0.5(g+g^0.2) = 1.3
g~1.5
So my guess is that airgap dimension would have to increase by ~50% in order to increase total current by 10% near full load. (I assumed this is near full load... you didn't tell us exactly).
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RE: unexplained 10% high amps
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RE: unexplained 10% high amps
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RE: unexplained 10% high amps
Change in reactive power with airgap as I estimated above.
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RE: unexplained 10% high amps
OP has to get past what dpc suggested in the very first response to the original question. Combined with the question about the voltage and is the motor is fed from the same source?
Rafiq Bulsara
http://www.srengineersct.com
RE: unexplained 10% high amps
It was the original poster who asked;
I see this type of question (not normally asked) as a red flag that someone may have a suspicion that the rotor may have been turned.
When I get a statement that "Everything is equal but one draws more current" my response is that "It is obvius that everything is not equal, but the phrasing of the original statement adds an order of magnitude to the difficulty of determining what is different.
Everything is ASSUMED to be equal but one or more of the ASSUMPTIONS are ERRONEOUS.
One of many things that may be unequal is the air gap, and the OP has asked specifically about the air gap.
Personally I suspect that the machine is working harder for some reason. Probably an instrumentation miscalibration on the feed control.
The air gap will not change on its own. The rotor has to be intentionally turned in a lathe. Turning a rotor is not a normal procedure but when the OP specifically asks about air gap, the red flag goes up.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: unexplained 10% high amps
There lies the problem, if there ever is in this case.
Rafiq Bulsara
http://www.srengineersct.com
RE: unexplained 10% high amps
#1 – My very first sentence in this thread:"Odds are the cause is mechanical to do with the mill. But we'll assume here it's not and focus on the other stuff. "
#2 – The original poster asked a question about possible cause related to the motor: "Is there a motor condition would make a "good" motor draw almost 10% more amps than an identical motor?"
#3 – The original poster asked a specific quantitative question about relationiship between air gap and current.
"How much extra air gap (turned undersized rotor OD?) would it take to cause a motor to draw 10% higher amps?"
I can understand you wanted to refocus the thread. That's fine. But your criticism of others' posts is out in left field.
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RE: unexplained 10% high amps
Rafiq Bulsara
http://www.srengineersct.com
RE: unexplained 10% high amps
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RE: unexplained 10% high amps
What do you want it to be? Why assume anything when there is no evidence?
dpc already posted what needs to be done, even to determine where the problem is.
There is no problem that I see from what is posted.
Even you were to assume, do you really think chances of two hammer mills, material, rate of feed, etc and feeds being identical is greater than two motors being identical?
Having knowledge and putting it to a good use are two different things. Conjectures and talking about possibilities based on assumptions are useless.
Rafiq Bulsara
http://www.srengineersct.com
RE: unexplained 10% high amps
It should be clear from my comment about the mill that I would support swapping motors. However I am not on-site. I do not know if Production can support taking out two motors at a time for a swap or how painful it would be. Even if this were happening on my own site and swap was determined feasible, I would talk through the other options and possibilities to develop a reasonable troubleshooting plan before embarking on labor-intensive and disruptive troubleshooting like swapping two motors. There may be some easy measurements that will shed some light.... I have suggested voltage... it would be embarassing to swap motors and find out it was a voltage problem. I have suggested slip ... if slip of this motor is not higher than the others, I may not be in a hurry to do that swap. I have suggested power factor... if power factor is higher than the others I would not be in a hurry to swap. Maybe there is consideration of inspecting the airgap.... how likely is it an airgap problem? I believe we can better judge the probability given an estimate that it takes 50% increase in airgap to create this increase in current (makes the probability of airgap as a cause less likely imo, but not ruled out).
I for one leave allowance for the fact that I cannot with 100% certainty determine the single most appropriate troubleshooting plan for a situation based on limited information provided over the internet for a machine and facility that I have never even seen. And even if I were tempted to try such a feat, I personally would want to talk through all the options first. And if I were presenting a recommendation to the responsible person who is closer to the situation than me, I would certainly answer his direct questions rather than ignoring them based on my own preconceptions. You apparently have a different style than me and would be comfortable with all of the above. Good for you!.... State your case and make your recommendation, but don't make it your business to suppress others' comments.
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RE: unexplained 10% high amps
Are you interested to discuss what you have done to rule out a difference in loading from the mill? Others may have suggestions on that. My suggestions included measurement of slip which is very easy to find from vibration, along with other listed electrical measurements... I don't know anything about mills.
What voltage and horsepower is the motor (just trying to get a mental picture).
Is there a drive involved? I'm assuming no, but wanted to check.
Was there in fact some machining done on the rotor that you heard about, or just came up with that as a possible cause on your own?
One other tool in the toolbox during a site visit might be infrared camera. It MIGHT point out something of interest on the motor or the mill, but that is somewhat a shot in the dark.
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RE: unexplained 10% high amps
Muthu
www.edison.co.in
RE: unexplained 10% high amps
Please don't be impatient with me. None of "our" requested tests have been done because I'm still assembling one (hopefully) comprehensive and thoughtful (with all your help) checklist of non-invasive real time online tests.
First on my list is, and was, Voltage and amperage balance of all 3 phases on at least one "good" and the "bad" motor. If it is determined that the bad mill's motor is being underfed, then I expect that will be addressed first.
Not long ago, On another thread, about another project, more than one responder suggested that my wish to measure voltage and current balance was the sign of a reckless unsafe cowboy.
As EPete alluded, shutting down mills and swappin' motors is NOT trivial. By most any measure big bucks are involved. Big outages are planned a year or more in advance. People have lost their jobs when poor or unlucky decisions have resulted in delayed completion of outages. Not much harm in getting as much info as possible while they remain online. Call it stacking the deck.
Perhaps the test report for the recently overhauled-out-of-desperation motor will include details of measurements of rotor gap and eccentricity. I can only hope. For me, measuring the gap is the only way I'll be sure it is the not the one undersized rotor made by Westinghouse ( or whoever) that year.
The professionals at a power plant have spent several years conscientously working to understand why 1 of 4 mills continues to consistently draw about 10% more amps than the other 3. The list of what they have tried is reportedly long and includes Mechanical inspections, calibration of feeders, and lots else. I do not (yet) have details of which, if any, components they have swapped already. I intend to ask.
Among the checks WE are going to do (sitting a thousand miles away at a computer) is Using coal characteristics, fuel flows, coal fineness, tons of steam produced, and other stuff to predict the power we think the mills >>should<< be using. Maybe 3 mills have problems that are causing them to draw 10% too few amps? (just kidding)
The physicist Richard Feynman in one of his books tells a story that as a young man amazed an older relative. The relative had a broken watch or clock or something. Young Feynman checked the watch for symptoms, then paced back and forth for a while, head down, looking at the floor, just thinking. Then he did something simple to the watch, which immediately started working. Thereafter the relative would often proudly tell the tale that Feynman fixed the watch "just by walking around."
RE: unexplained 10% high amps
Sometimes, it is as easy as that.
There has been a lot of focus on air gaps in this thread. I have a problem accepting that someone started the commissioning with taking a rotor out of a motor and turning it. The commissioning was only five or six years ago. Surely such an uncommon action must be remebered by someone? Perhaps also noted in the commissioning report?
Checking slip is what I would do first thing. But I would not calculate slip on an assumed 60 (or 50) Hz, but on actual grid frequency when doing the (also accurate) speed measurement. Small deviations in grid frequency mean a lot to calculated slip.
If there is a key or something else that could be measured with a proximity switch and a recorder, then I would do that. And record grid frequency (cycles) simultaneously. Then evaluate as many cycles as I think needed to find out real slip. You can actually see the pulses from the key 'slipping' behind the grid cycles.
If possible, I would also include power, using current clamps, VTs and a power transducer with analogue output. And I would do that measurement on all four motors.
Such measurements are extremely valuable when one sits down to discuss what is going on in the plant.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: unexplained 10% high amps
I had four identical pumps on a cooling skid. One ran hot. The rotors had cast aluminum squirrel cages. The hot motor had an internal void in the squirrel cage casting. A replacement rotor fixed the problem.
Bill
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"Why not the best?"
Jimmy Carter
RE: unexplained 10% high amps
I think of it as having a slip ring rotor. More resistance increases slip, but not current drawn.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: unexplained 10% high amps
I am intrigued as to why 10% more current is being investigated so intensely, is it really that significant?
what would be the normal tolerance on no load current for two identical motors?
I can see that variations in what the motor drives would give rise to additional current, ie friction, inertia, changes in mechanical load etc and these in my opinion can never be identical across four different mills.
All the mechanical components will have been manufactured to a tolerance and therefore each build will inherit different masses, which in turn will result in different friction, inertia etc.
Just curious as to why the additional current is so critical.
Although I do understand if they want to work the mills harder, then they can't overload the motors, but I would of thought there was more than 10% margin in the motors to start with.
desertfox
RE: unexplained 10% high amps
RE: unexplained 10% high amps
yesterday I think you said -
"OP has to get past what dpc suggested in the very first response to the original question. Combined with the QUESTION ABOUT THE VOLTAGE and is the motor is fed from the same source?"
I have not asked this customer to check their (4000) volts/amps yet, but you may recall last month, a customer declined to perform those tests on a 460 V motor.
http:/
The voltage test request will be the first item on my list, but frankly I am anticipating encountering a certain amount of (non-electrical) reluctance
RE: unexplained 10% high amps
Ah, that is interesting.
This tells me that even you do not have the first hand information. Not only that, there is not enough second hand information available to do any meaningful analysis. On top of that the customer appears non-cooperative.
Per your statement, the motor is not even overloaded. The 10% high current is relative to other supposedly identical machines means nothing. A motor draws what it needs to draw. You first need to eliminate (or confirm)the load as the cause. Decoupled motor test, as Edison123 suggested, would be the least you need to do.
I worked in cement and chemical plants, in my past life and have dealt with many real and perceived problems, like this. (I can almost picture the non-technical reason behind this perceived problem, but that is a different subject.)
You really need to get the first hand information, talk to the customer and involve an electrical engineer to tackle this issue, if it really exists.
I will share one of my experiences, in a similar 'troubleshooting' of a 40HP air compressor motor and starter, some other time.
Rafiq Bulsara
http://www.srengineersct.com
RE: unexplained 10% high amps
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: unexplained 10% high amps
In the instance that I mentioned the indication of a problem was a hot running motor. The motor shop diagnosed a faulty squirrel cage. A replacement rotor was supplied under warranty and the problem was solved.
If all the rotor bars have a higher resistance, we can expect a higher slip. If one or two rotor bars have a higher resistance the other bars are working harder (ie: drawing more current).
Consider a low slip motor coupled with a higher slip motor. The lower slip motor will "Hog" the load and run hotter. Similarly, one or more higher resistance squirrel cage bars will be taking less than their share of the load, and the I2R in the other bars will result in more heat in the good bars.
Bill
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"Why not the best?"
Jimmy Carter
RE: unexplained 10% high amps
rasevskii
RE: unexplained 10% high amps
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RE: unexplained 10% high amps
It seems that these are 4160V motors, having reread the posts. Obviously then out of the question to do tests with clamp on meters. What sort of metering is actually provided then?
You are not telling us the size of these motors, or what type. They could be synchronous for all we know. Or wound rotor. Therefore other considerations.
Much more info needed by all here, as Rafiq has said.
rasevskii
RE: unexplained 10% high amps
I have seen two motors with rotor bar failures in my (working) life. That makes the frequency something like one each 25th year. So, it is not a very common thing.
OTOH, I have seen many motors with a supposed rotor bar failure. But it has almost always been something else.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: unexplained 10% high amps
Page 3 of 11 shows figure 3: [Calculated] "Effect of broken bars on steady state performance". The horizonttal axis is broken bars per pole. The vertical axis is full load slip and % change in full load current (two curves). You can see that neither slip nor current is very useful as a predictor until the number of broken bars is huge. Figure 7 on top left of the next page shows that the change in torque characteristic from a broken bar is more evident during start/acceleration than at full load.
Tangent to a tangent:
1 - Apparently, the current actually decreases for small number of broken bars (!?)... I would have liked to see an explanation for that.
2 - By the way, the document is a GE publication "GER".... ot was at one time available for free from GE - no copyright listed.
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RE: unexplained 10% high amps
Rafiq Bulsara
http://www.srengineersct.com
RE: unexplained 10% high amps
http://www.scribd.com/doc/24157736/Motor-Standards
Now another question if the motors had type test certs would that show that it drew more current when compared with the other motor type tests.
Reading the posts now I take it 10% difference is not significant under normal circumstances.
desertfox
RE: unexplained 10% high amps
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RE: unexplained 10% high amps
A star for you.
desertfox
RE: unexplained 10% high amps
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RE: unexplained 10% high amps
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RE: unexplained 10% high amps
On another note, I read the comment about others on site have spent 2 years troubleshooting the problem. That makes me recall a similar problem I visited a while ago. I had to cut short my vacation to go to a site. The customer claimed they had replaced parts and fully checked the connection of every wire in the system. They just knew there was a major problem because everything simple had checked out and they had spent 2 days working on the problem. I had to wait for them since they had some other troubleshooting ideas they wanted to try. Once it was my turn, I pulled out the ohmmeter and began to ohm out some of the wiring. The problem was fixed about 5 minutes later. I knew some re-wiring had been done and it had not been tested after.
So, moral of the story. Make a list of things to investigate and put them in order with the most likely first and then check everything throughly yourself. You'd be amazed just how poorly some people troubleshoot.
RE: unexplained 10% high amps
RE: unexplained 10% high amps
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: unexplained 10% high amps
RE: unexplained 10% high amps
In your reply from 24 Feb 10 13:49 you've mentioned the fact that:
Lower voltage at machine terminals results in higher current.
It's a dumb question from me and my collegues as well, but we had this discussion a couple of days ago, and we couldn't make it clear why your statement, mentioned above, should be true.
It is a fact that Z is a stable factor, and since Ohm's law tells us that U = I*Z, you can see that if U decrease, I also decrease and vice versa.
Is it possible for you to explain why your statement is correct?
Thanks in advange,
Jan
RE: unexplained 10% high amps
RE: unexplained 10% high amps
How is it not possible to have a stable Z?
We thought the Z should be stable.
The only thing we could think of is that, when we have a constant torque the current decrease when the voltage decrease. When the load will be changed to keep a optimum in the process, then you can say the current will increase.
If we think the wrong way, please try to explain what is wrong in our thinking.
Maybe this should be a item for a new thread.
Thanks,
Jan
RE: unexplained 10% high amps
Of course, there is only a certain range of voltages where this will hold true, but it's generally the case when going +10% on the motor rated input voltage.
The Z will change because the slip changes with voltage.
RE: unexplained 10% high amps
You've convinced me with your reply. You've clearified enough.
Thanks a lot for your help, now there is a possibility for me to win our discussion.
Thanks a lot,
Jan
RE: unexplained 10% high amps
Even very minor differences in the materials used to manufacture 2 supposed identical machines can result in significant operational differences.
In practice it is very difficult to perfectly match 2 individual machines built to the same spec.
10% though does seem rather high. Engineering tolerances should keep 'exact' items within a maximum of 1-2% of each other
RE: unexplained 10% high amps
Is it possible that the rotor is not sitting in the stator properly.
I once worked in a mine in Papua New Guinea where there were two lines of crushers an exact mirror immage. The terminal boxes on one row were on the opposite side of motor(crazy I know) so you couldn't move a motor from one line to the other. We tried swapping the end bells and rotor on a spare so that the terminal box was on the correct side, unfortunatly this put the rotor slightly out of alignment with the stator (not fully inside).
Just a thought.
Like some of you I am always dubious when told two pieces of equipment are identical, I have spent too much time investigating electrical problems when it turns out to be mechanical.
Roy
RE: unexplained 10% high amps
RE: unexplained 10% high amps
Muthu
www.edison.co.in