unexplained 10% high amps 2

[Tmoose]

- 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

 

[dpc]

The first obvious question is how the motor amps are being measured. I assume that measurement error has somehow been eliminated as possibility?

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

http://www.cvoes.com

 

[rasevskii]

for Tmoose-

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

 

[waross]

You mentioned the possibility that the rotor has been turned. The air gap is critical and ANY amount of turning that a mechanical guy thinks a rotor needs is too much. Those of us old enough to remember DC sometimes saw this when we sent a DC rotor to the machine shop to have the commutator turned. Occasionally a well meaning machinist would "Clean up" the rotor as well as turning the commutator. Kiss the rotor goodbye.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

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.

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.

You mentioned the possibility that the rotor has been turned. The air gap is critical and ANY amount of turning that a mechanical guy thinks a rotor needs is too much. Those of us old enough to remember DC sometimes saw this when we sent a DC rotor to the machine shop to have the commutator turned. Occasionally a well meaning machinist would "Clean up" the rotor as well as turning the commutator. Kiss the rotor goodbye.
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????...

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

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

Hi electricpete;
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

 

[edison123]

Increases air-gap due to rotor core machining would definitely result in increased no-load current. What is the compative power factor of all the motors under the same load ?

Muthu

http://www.edison.co.in

 

[Tmoose]

thanks all.

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.

 

[electricpete]

3 - How much extra air gap (turned undersized rotor OD?) would it take to cause a motor to draw 10% higher 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).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

"weekly" LOL. Should've been "weakly"





=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

I don't think of an eccentric rotor as having much of an effect on power,

Eccentric rotor causes small increase in reactive power requirement, but the effect would be small unless the eccentricity gets very large.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

On second thought, I would say change in reactive power with eccentricity would be negligible under most circumstances.

Change in reactive power with airgap as I estimated above.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rbulsara]

It is amazing to see lots of theories being thrown around where there is not an iota of information that suggests that there is anything wrong with the motor!

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

 

[waross]

Hi Rafiq;
It was the original poster who asked;

3 - How much extra air gap (turned undersized rotor OD?) would it take to cause a motor to draw 10% higher amps?

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

 

[rbulsara]

Everything is ASSUMED to be equal but one or more of the ASSUMPTIONS are ERRONEOUS.

There lies the problem, if there ever is in this case.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

It is amazing to see lots of theories being thrown around where there is not an iota of information that suggests that there is anything wrong with the motor!

I’m amazed at posts like this. I’ll point out a few things:
#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.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rbulsara]

Identical.....more conjectures.



Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

What is your contribution to this thread?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rbulsara]

pete:
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

 

[electricpete]

Why assume anything when there is no evidence?

I have stated my opinion the odds are the cause is mechanical/mill related. However that cannot be the end of the story if we want to answer the original poster’s questions. So to proceed with an answer, we need to use the word ‘assume” to compartmentalize the motor discussion from the mill discussion. In general, that will also serve to insulate the subsequent discussion from criticism on the basis of mill-related causes.

dpc already posted what needs to be done, even to determine where the problem is.

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).

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.

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.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.