Increasing current requirements with age 1

[Steventyj]

I am asking the general question of what would cause the current requirements of a motor at full speed with no load to increase after being in use for a length of time.

Below are more details specific to our problem, although not necessary for the question.

We have a number of specialty 2HP motors.
"MOTOR ELECTRICAL 2 HP 1710 RPM 575V 2.6A CLASS F CONTINUOUS DUTY SF 1.15 TEFC" 80 - 81% eff at 100% load.

The problem is that when some of these motors are removed to have new bearings installed etc.. we can not get their no load amperage requirements low enough to put them back in service. 1.2 -1.3 amps is what we would like, but it is as high as 1.58 amps on some of these used motors. At that level of current draw they would likely trip the monitoring protection when installed in their field application. We have done some experimentation relating to bearings but that has not helped so far.


Thanks,

Steven

 

[IRstuff]

You didn't say what the current draw was before the bearings are changed.

TTFN

 

[Steventyj]

I believe the current draw was also high before the bearings were changed. The issue is more that the motors current requirements are increasing with age and it appears to be related to something other than bearings.

 

[electricpete]

No load current is primarily inductive magnetizing current.

Things that would affect inductive magnetizing current to increase with age:
- deterioration of core laminations over time, possibly due to heat, vibration, sustained operation at high voltage
- change in airgap geometry due to bearing replacement. If initially symmetric airgap becomes assymetric, the magnetizing current can increase.
- increase in supply voltage over time

 

[Steventyj]

These motors due operate in enviroments of high heat. In order to test if it is a core lamination deterioration problem would it suffice to do a core loss test on a new and used motor of the same model?

The problem seems to be independent of bearings and so although we have also thought of the air gap possibility we have so far dismissed it.

The supply voltage should be stable.

 

[electricpete]

Now that I stop and think about it.... since the magnetizing current is inductive, it would take a very large resistive component (from core loss) to substantially increase it. Seems not too likely. But your proposed comparison core loss test would definitely rule it out.

That leaves air gap as said before. Certainly you would normally expect to see increased vibration in that case also, but who knows.

How about shorted turns? I guess I have not often heard of a motor running for a long time with shorted turns, but it's easy to check with a resistance comparsion.

Here is one other thought. No-load current often is substantially imbalanced between phases. If you havn't checked all three phases go back and recheck... maybe one is lower and that is the one you recorded before?

 

[electricpete]

Also you mentioned that your voltage is stable. Have you measured it directly at the motor terminals (if can be safely done?). That would help to rule out possibly high-resistance problem in a contactor or termination.

 

[Steventyj]

Here is some more information:

Most of the motors are around 1.2A - 1.3A when they are new.

They are typically 1.42A - 1.54A after being rebuilt. At least one is as high as 2.2A.

Those numbers are the averages of all three phases.

It is important that the current draw does not get too high because otherwise it will trip protection settings.

We are looking into this because we have many of these motors and they are expensive for their size.

If we do an investigation into one of the motors that is drawing higher currents, what tests should we do?

Meggar
PI
Balance
VA

Anything else?

Thanks
Steven

 

[CB2]

If you have a new motor available, have you tried swaping a new rotor with a old stator and so forth. I am checking some resources about the rotor, don't rule it out.

 

[electricpete]

check winding resistance to look for shorted turn (maybe that's what you meant by balance)

 

[jbartos]

Suggestion: It may also be due to the bearing exchange, namely, how it is done. Have you tried a different motor repair shop?

 

[GusD]

Hi Steventyj

You mentioned that when had the motors overhauled\,perhaps new bearings as well,you experienced higher current draws.From 1.2 Amps to a 1.58 after repairs.

Did the load come down after a few hours running?
New bearings,with fresh packed grease, could draw a slightly higher load for a couple of hours.It should normalize afterwards.

Are the bearings the same type as the old ones?
Old motors with new bearings and proper housing fits should not cause higher loads.
If motors were rewound ,you have to make sure that the
shop did not use excess heat to burn the old windings.
Some shops use way too high a temp.to be able to remove them.If the laminations are overheated you could have a large drop in motor efficiency.I personally have seen a motor in the gas oven with the lamination core cherry red.
This motor could have gone to the waste basket.


GusD

 

[Steventyj]

The motors are not rewound, if they were we would have core loss tests to check that the core laminations were not damaged.

The motors are currently being repaired in house. We are continuing different things with the bearings and have a new type on order but the feeling of our experienced motor repair electrician is that it is not the bearings because of the variety of things he has tried and the range of current draw required by the used motors.

We will try swapping rotors and performing a PI.

 

[stardelta]

Have the motors been reassembled correctly. i.e with the rotor and core packs directly in line with each other. Even a slight difference can increase current demand, and its particularly noticable on smaller machines.

 

[UKpete]

As stated in the postings above, the no-load current consists of a power loss component and a magnetizing component, modelled by a resistor and an inductance respectively, in parallel across the line in each phase.

If the increased no-load current is due to a power loss (increased core loss, winding loss, bearing friction, windage) this will give an increase in the real component of current, whereas an increase in the magnetizing reactance (mis-aligned rotor) will give an increase in the imaginary component of the current.

So if it is practical to measure the phase difference between the no-load current and the voltage, eg using a scope or a power analyser, comparing a good motor with a bad one you may be able to narrow it down a little. I can't promise it will be easy to spot the difference though; the magnetizing component usually dominates.

On the subject of winding tests, a surge test might be useful at spotting differences between the phases and shorted turns. Unfortunately surge testers aren't cheap but a good rewind shop should have one.
For example see:

http://www.bakerinst.com/NewWebSite/Products.htm

 

[induction]

As stardelta has said when you assemble them are your parts marked. On some small motors the stator looks centered but it is not it may be a .25 of a inch to one side. Look at your rotor with one endbell on see if it lines up lamanation to lamanation.

 

[jbartos]

Suggestions to Steventyj (Electrical) Apr 16, 2003 marked ///\\I am asking the general question of what would cause the current requirements of a motor at full speed with no load to increase after being in use for a length of time.
///Leakages implying insulation deterioration, surface resistance deterioration, winding displacements, etc.; mechanical deterioration, interturn shorts, etc.\\Below are more details specific to our problem, although not necessary for the question.

We have a number of specialty 2HP motors.
"MOTOR ELECTRICAL 2 HP 1710 RPM 575V 2.6A CLASS F CONTINUOUS DUTY SF 1.15 TEFC" 80 - 81% eff at 100% load.

The problem is that when some of these motors are removed to have new bearings installed etc. we can not get their no load amperage requirements low enough to put them back in service. 1.2 -1.3 amps is what we would like, but it is as high as 1.58 amps on some of these used motors. At that level of current draw they would likely trip the monitoring protection when installed in their field application. We have done some experimentation relating to bearings but that has not helped so far.
///In addition to what has already been posted, the motor winding insulation deterioration increases in time; especially, if the motor experiences the higher temperature. That is why there are so much of paid attention to the motor operating temperature, motor ambient temperature, motor storing temperature, etc.\\