high motor load

[upm]

gents,
i am going through a buzzle here. two identical 3 kw pumps, motor for one of them is pulling 95% FLC and cannot even throttle the discharrge block valve? we inspected motor, pump bearing, impeller and even seal. do not know what else can i do?motor no load test is 30% of FLC and vibration levels are ok. i am only questioning the volute geomatry for that pump which will effect the hydrolics? if they are pumping water and no change in the process.
pls. advice

 

[electricpete]

Just to recap:

30% FLC with this motor running solo. (that was in your 1st post... later you said not checked?).
95% FLC with this pump at SOH.
60% FLC with other pump at SOH.
Curernt balanced all three phases.
Voltage checked good (?). Bearings good. No indication of misalignment.

Forget my comment about checking power factor. Air gap problemw ould affect solo current.

Lots of good comments by others above.

One possibility - Oscillating current is one symptom of a rotor bar problem. Check to see if the frequency of oscillation is pole-pass frequency (slip speed times number of poles). Also look for pole-pass frequency sidebands around 1x in vib spectrum (may require high resolution). Just a possibility.

 

[dadfap]

d23,

you gotta admit, that's onle lousy pump curve! The GC4100 is much better

(For those of you who are not aware, d23 & I work for competing companies)

All the best

 

[CB2]

ump,

If your pump is "centerline discharge" type you can ignor my post.

If the discharge nozzel is to either side of centerline, the pump is probably available in a CW or CCW assembly. The point is you cannot mix the impellers and volutes, they are dedicated to a specific rotation direction. If you had a CW impeller in a CCW volute it would be similar or worse than running a pump in the wrong direction.
OK?

 

[upm]

cb2
thanks for the comment
upm

 

[buzzp]

The fluctuating motor current at fixed valve location is likely pointing to fluid problems: viscosity changes, cavitation, etc.
I have seen motors pull near FLC pumping into a dead head condition. I would be looking at a fluid problem. assuming all are powered from the same circuit (same electrical parameters).

 

[upm]

Buzzp,
What other checks do you suggest? system NPSH is adequate and the other pump is running fine from mechanical/performance point of view.
I think i have checked for everthing and am looking at others to go.
appreciate your advice
upm

 

[electricpete]

Some ways to check for rotor bar problem:

Measure running speed with a handheld strobotach.
Slip speed = Syncrounous speed - running speed
Pole pass frequency = #poles x slip speed.

If the current is peaking at twice pole pass frequency, the problem is likely broken rotor bars.

Also if you have spectrum analysis capabilities, look for pole pass frequency sidebands around running speed in vibration spectrum and around line frequency in current spectrum.

 

[edconde2455]

Ed(mechanical)

Gents,

We have deep well pump(3 stages) model grundfos @ 110 gpm, 2dia discharge line, 65M tdh & 11.4 amp. FLC. Triffing once a day, the contrl panel indication are overload current & dry run.
What is the probable cause of the stated pump. Pls advise.

 

[electricpete]

Ed - I think it would be pretty confusing to discuss symtpoms from two different problems in the same thread. Can you start another thread?

 

[buzzp]

I think electricpete has some good advise on rotor bar problems. I have no experience with this but sounds reasonable.

Are both pumps getting fluid from the same source?

Does the motor trip on OL if allowed to pump anyway? What is the current when the valve is fully open, is it normal? Maybe I missed this info from a previous post.

 

[poppeye]

This happened to me during operational testing on a 500hp Baldor motor long coupled to an end suction pump. Except for one thing, during testing at full load the motor RPM decreased dramatically. 75%! The motor was no load tested prior to installation with satisfactory results but would not operate under full load conditions. During inspection of the motor a full core test was conducted and found that core insullation was the cause for the motor malfunction.
Have your motor checked under full load at a reputable repair center and ensure the core is checked also.

 

[upm]

Poppeye,
is there a procedure for such testing? i am not aware of this type of teseting, what facility (shop) is it required for such testing. FYI at our industry, there is no motor manufacturer?
upm

 

[poppeye]

UPM,
In Cleveland there are two motor repair facilites that can test the core, Monarch, and Redmond Waltz. They can also perform a complete core test for insulation breakdown and perform bench full load testing. Ingersoll Rand recommends that motor repairs on motors over 50hp, full core tests and load testing be performed prior to reinstalation.

 

[upm]

Gents,
to update you on the status of this problem. after a hard work trying to convence electrical group to send the motor to a specialized work shop, the problem was solved. during the motor inspection, it was found that the rotor was bowed and unbalanced (was not shown in the vibration spec) and grease was found on the windings. bearings were overgreased . after repair, the motor load is about 76 to 90 % running smooth , load at SOH is normal and the unit behaves much better.
it is a leason learned that we need to do detailed inspection even for such a smaller motor.however, this smal motor was about to trip the plant as it it pumping from a compressor knock out drum
upm

 

[d23]

upm:

Thanks for taking the time to reply!

D23

 

[electricpete]

Yes, always great to have the feedback.

Now we have to ask what did we learn. Have you drawn any conclusions on the exact cause?

There were two problems... excessive grease and bowed rotor. Which one caused the symptoms?

Which would cause excessive current and oscillating current? I would say the oscillating current could come from the bowed rotor due to pole pass frequency oscillations associated with varying air gap. Hard to think how excessive friction from grease would cause oscillation.

An interesting thing was that the current at no-load was 30% and apparently not oscillating (?). I think most of us assumed 30% was normal, but do you have any info on actual no-load current? One would think that regardless of the cause (either excessive friction from grease or varying magnetizing current from rotating airgap assymetry), it would show up at no-load.

 

[upm]

electricpete
our electrical group is on top of this invistigation.as you said and as i was expecteing earlier, something will be related to the air gap and pole pass freq. we will include measuring this feq. as long as the rotor bars freq. to catch such a failure (if possible).overgrease can kill somtimes specially with high viscus grease (higher temp. )and maint. practice is to re-grease whenever there is a problem
upm