3Ph, 1.5HP Inductor Motor Running at Low Speed 1

[sykimk]

Hello,

As a part of startup process, we did motor run but the speed is quite lower than the rated speed. Please refer to the following information.

Rating: 1.5HP, 3PH, 460V, 2.1A, 60Hz, 1760 RPM, SF=1.15
But motor running at 1558 rpm and 1.5Amp. So, I don't think it is overloaded at all. But I don't understand how this motor ran at 1588 rpm (88.5% of rated rpm). Mechanical folks told all flow and pressure are OK while runnig this motor.

Can you please advise why the acutal motor speed is way lower than the rated speed? It this motor's rotor OK with operating at this lower speed than the rated speed?

Thanks for any advices/tips in advance.

Power

 

[LittleInch]

Is voltage 460v?

Is freq 60 Htz?

How are you measuring speed?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jraef]

1760RPM is the UNLOADED slip speed. With load, slip increases, which means the motor slows down.

That said, given that the synchronous speed would be 1800RPM, that would mean you are experiencing almost 9% slip, which is unusually high and would typically mean you are into the Break Down Torque part of the curve, in which case your current would be somewhere around 2x the FLC. I surmise that, as LittleInch is suspecting, there is something else going on here.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

Rated rpm is at rated load, not unloaded.

Either speed is measured wrong or frequency is not 60Hz or motor is not 1760rpm. I suspect the speed measurement is wrong.

 

[che12345]

Dear Mr. sykimk (Electrical)(OP)1 Apr 22 20:14
"...As a part of startup process, we did motor run but the speed is quite lower than the rated speed. ....Rating: 1.5HP, 3PH, 460V, 2.1A, 60Hz, 1760 RPM, SF=1.15...But motor running at 1558 rpm and 1.5Amp. So, I don't think it is overloaded at all. But I don't understand how this motor ran at 1588 rpm (88.5% of rated rpm). Mechanical folks told all flow and pressure are OK ...."
1. I think the rated 1760RPM is the Full-load speed, NOT the no-load speed. Synchronous speed speed is 1800rpm at 60Hz and no-load speed xxrpm are usually NOT stated on the name plate.
2. Check the voltage, frequency and current, pay particular attention on the tachometer (rpm) reading. Counter check the reading on any other motor or use another/calibrated tachometer for accuracy.
3. Check the motor and pump bearings for damage; including any clogs in the pump chamber. In case that the motor is over-sized, it may be possible? that running at 1588rpm the flow and pressure are "OK/acceptable" for the application, without over-loading (running ampere). Attention! Check the pump curve. There must be a significant drop in pressure or flow rate running at 1588rpm instead of 1760rpm.
Che Kuan Yau (Singapore)

 

[sykimk]

Really apprecaite your feedbacks.

For more information, we did this motor run coupled with pump. So, it wasn't no load condition, and the supply voltage and freqency were good at approx. 480V and 60Hz. I was told that tachometer was good condition and used for other mootors as well. I initially thought that motor might be oversized but if so it still doesn't explain why this motor coupled with pump was running at 1588 rpm.

 

[LittleInch]

Can you swap the motor out with another?

Our try another tachometer?

The data just doesn't seem to stack up.

Also try dead heading the pump and measure differential head and compare it to the pump curve. 10% less speed should show up on the differential head of the pump being 80% of what was forecast of the pump is actually going at 1588 rpm.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EngRepair]

Try another tachometer.

 

[crshears]

Might there be some weird confluence of wrong connections and the fan/pump laws?

Positive displacement pump or centrifugal?

If centrifugal, have you tried running the pump un-primed? In my experience, doing this for the minute or so needed to take voltage and current readings should not cause pump damage, although depending on the circumstances it may be necessary to ensure water or other appropriate liquid is applied to the seals for the duration of the test, as well as ensuring proper bearing lubrication.

The readings taken during such a test, when tabulated with the readings from your first test along with another set taken when running the motor uncoupled, should give you three points of comparison to help you figure out what's going on, if your tachometer is mis-calibrated, etc. ; for example, if the tach doesn't read just under 1800 rpm during the uncoupled test, I wouldn't trust it.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LittleInch]

True, we know very little about the set up.

Is the motor going the right way around?

Good shout to try it unloaded from the pump.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

If the motor was coupled to the pump, the only way to come close to an unloaded condition is to run it dry. I suspect the pump is a centrifugal and was run with no flow restriction (no back-pressure). An assumption was made that this is equivalent to unload, which is incorrect. Unrestricted flow is, in fact, an overloaded condition.

 

[LittleInch]

Disconnect the coupling was what I was thinking of...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

yes, uncoupling would be the proper way to unload the pump. But the answer to why the measurements do not look correct is that the pump is not "unloaded".

 

[LittleInch]

Maybe just replace the motor?

It's a 1kW motor FGS.

Maybe it's broken. It's just not worth the trouble to find out what's gone wrong.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[crshears]

"Dry" is what I meant by un-primed. And true enough, it is a pretty teeny motor . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[3DDave]

Mechanical folks told all flow and pressure are OK while runnig this motor.

Leave the motor alone. It's doing whatever it's supposed to be doing. Until it burns up or stops or whatever bad thing might happen, take the opportunity to get the information from the mechanical folks as to what the demand on the motor is so it's possible to ask a more complete question.

 

[LionelHutz]

Is the motor 6-lead? If so, it should be connected in delta for running since you're not indicating dual voltage ratings.

 

[che12345]

Dear Mr. sykimk (Electrical)(OP)2 Apr 22 10:04
"... we did this motor run coupled with pump. So, it wasn't no load condition, and the supply voltage and freqency were good at approx. 480V and 60Hz. I was told that tachometer was good condition and used for other mootors as well. I initially thought that motor might be oversized but if so it still doesn't explain why this motor coupled with pump was running at 1588 rpm...."
1. You had checked and re-checked that the voltage (are balanced?), frequency OK, current (lowered than rated), (balanced on all three phases?) , tachometer OK, bearings on motor and pump are OK and no clog on pump chamber etc., etc....
2. Additional proposal/suggestion for your consideration:
2.1 Inspect the rotor for:
a) any sign of (local over-heating) on any [skewed rotor bars]?
b) any welding cracks/dry connection at the end ring connections?
c) any sign of broken/crack rotor bar?
2.2 Adjust the magnetic centre alignment between the stator and the rotor:
a) by (adding or removing) the [shims on the DE or NDE].
Che Kuan Yau (Singapore)

 

[dArsonval]

Considering the post originated on April Fools day...

Scrolling the OP's previous forum entries dating back to the early 2000's, it does not seem this post is a joke.

"But"....Methods of measurement are highly suspect.

It would be revealing to measure some other piece of equipment nearby using the same indicating devices and arrangements.

The words of Robert Woods come to mind: "At some point in any scientific endeavor it becomes necessary to bring philosophy
into contact with the real world. The tools for doing this are the scientific instruments that can be used to perform critical experiments.
These instruments are tangible hardware, not abstract thought. The theoretician is thus, sooner or later, at the mercy of the instrument maker."

John

 

[jraef]

Hand held tach? Could someone have measured the diameter if what it was held against incorrectly? Being off by just a fraction of an inch would make a big difference in the tach readout.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden