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

 

[desertfox]

Cheers electricpete

A star for you.

desertfox

 

[electricpete]

Lest I be corrected, in my post above "similar load" on pumps simply means these are similar-impeller pumps operating nominally in parallel. There pump condition (wear ring wear), branch piping differences etc can causes differences in actual pump brake horsepower requirement.

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[electricpete]

One more clarification: When I said "variability", I really meant "range": (max-min)/average

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[LionelHutz]

A motor running with a slightly higher current then other similar systems yet not overloaded? Personally, I don't really see the problem either. Notice I said other similar systems and not identical systems?

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.

 

[burnt2x]

"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 thoroughly yourself. You'd be amazed just how poorly some people troubleshoot."

You nailed it and it's very true! I've learned something basic today, thanks a lot Mr. Hutz!

 

[waross]

And remember "Bullshoot Baffles Brains". I have lost track of the times that a customer has given me untrue answers to specific questions. Sometimes it is better to not ask, just find the problem, and don't believe anything you are told.


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

 

[iainuts]

Hi Tmoose, Are all the hammer mills identical? In particular, check to see if the rotational inertia (esp: flywheels) are the same. A high amp reading could be due to the motor slowing slightly if power for operation increases suddenly. I had a similar situation on a reciprocating pump that had twice the current draw that it was supposed to. After adding inertia to the system, current draw was cut in half and is now operating per theoretical calculations. Note the power didn't change, but the peak amps were reduced. When electric motor speed is forced to fluctuate by a few percent due to sudden loads being imposed on it, peak current can go way up.

 

[7533]

Electricpete,
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

 

[LionelHutz]

Jan - The motor Z typically is not stable. The motor keeps doing the same work which requires a certain input power. It will then require more current to keep running at the same power level as the voltage is lowered.

 

[7533]

Lionel
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

 

[LionelHutz]

The motor is doing a certain amount of work. To do that work requires a certain mechanical output power from the motor and a certain electrical input power to the motor. P ~ V x I. If V goes down then I must go up. Otherwise, the motor would not be able to continue doing the work.

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.

 

[7533]

Lionel,

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

 

[raok]

If it's drawing 10% more current than the others, it is NOT identical!
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

 

[roydm]

Here's my input.
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

 

[burnt2x]

If I may backtrack a bit, these are hammer mills guys! These are motors driving an equipment with a set of hammer arms with hammer heads at the tips! I think raok could be correct to say the motors are not driving "identical equipment" in the correct sense of the word! Hammer heads could differ a lot in weight due to wear and improper installation!

 

[edison123]

At this point, the OP has left us high and dry. In case he is still reading it, decouple both the motors and compare their no-load currents and voltages so that the load factor does not come into play. It should take just a couple of hours to do this.

Muthu

http://www.edison.co.in