Operating pumps with low pf

[heber]

I am looking for reference material on how running a pump at low power factor (around .6) will affect the lifecycle of the pump motor. Any suggestions would be appreciated.

Thanks.

 

[electricpete]

Assuming we talking syncronous motor?

Stator current will increase as as pf decreases, causing increased heating of stator. At pf=0.6, current will be 1/(0.6) ~1.7 times as high as the same mechanical load at pf=1.0 Stator I^2R heating would increase by 1.7^2. That sounds pretty high... someone check my math.

On large generators there may be a concern for core end-iron heating due to axial flux component during very low pf operation. Not sure if it applies to motors.

 

[dpc]

The power factor is not the concern - it's the amps drawn by the motor and the subsequent heating. As long as you are within the capability curve of the motor (if we're talking synchronous), operating at 0.6 pf is OK.

Temperature monitoring in the motor windings is always a good idea.

 

[jbartos]

Suggestion: Some motors are designed to have the maximum efficiency at lower then rated HP and power factor.
However, any pump motor may be oversized, which leads to lower power factor and lower efficiency. The life-cycle of the oversized motor shall be longer than the fully loaded motor.

 

[jOmega]

What type of pump :

[li]Centrifugal ...?[/li]

or

[li]Positive Displacement ?[/li]


What operating speed ?

What BHP ?


jO

 

[uno]

A good explanation from usmotors , considering a larger picture of any system , at the following site:

http://www.usmotors.com/Products/ProFacts/1-115.htm

 

[heber]

The motors are actually A-C induction motors coupled to centrifugal pumps for use in a water distribution system.
They are rated at 400HP, 1185rpm, 4160 Volts. The pump station is sized for future demand so the motors are oversized. With current demand they are running at around 300Bhp and 78% efficiency.
Thanks.

 

[electricpete]

The power factor of an induction motor cannot really be changed (although it may vary slightly with load and voltage).

You can increase the power factor drawn from the system by adding caps in parallel with the motor I suppose you could get it down to 0.6 by adding some type of inductor in parallel, but I have no idea why that would be done.

What changes to the system are you contemplating?
What is it you are trying to accomplish?

 

[electricpete]

78% percent efficiency sounds low for motor alone. How did you come up with that number?

 

[heber]

This is more of a hypothetical question...the pumps have power factor correcting capacitors. However, the power factor correcting capacitors were removed to troubleshoot a source of harmonic distortion in the system.
So my question was if I ran these pumps without pf correcting capacitors(which is not an option) could I expect that the increased load current and subsequent I^2R losses cause decreased life of the pump motor.
Thanks.

 

[edison123]

heber,

Adding a pf correction capacitor to the motor, does not reduce the motor current. It only reduces the current in lines that supply the motor and the capacitor by way vectorial addition (or subtraction). However, if you have a poor voltage regulation at your motor end (i.e. a heavy voltage drop at motor terminals) without the pf cap, then p.f. cap may help in raising the voltage at motor terminals and reduce the motor current for a given load.

 

[Marke]

Hello heber
Submersible pumps typically have a low power factor. This does not affect the lifetime of the pump or motor. Adding power factor correction will reduce the current flowing in the supply, but will not change the current into the motor or the power factor of the motor itself. If there is significant voltage drop caused by running the motor, then adding correction could increase the terminal voltage of the motor a little and cause a resultant reduction in current. This would not generally be of any significance in affecting the life of the motor.
The addition of power factor correction can however, cause higher voltage surges in some installation, due to supply resonance. This can be a problem.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[edison123]

heber,

Also, you are running the motor at only 3/4th of rated HP. This would mean lower copper loss (about 50%) with resultant lower winding temp. So, I see no cause to worry about motor life at present. Whether your utility will permit such a low p.f. is another matter.

 

[electricpete]

I'd like to take a shot at summarizing what others have already said:

There are two things we need to distinguish: effect of voltage and effect of power factor correction capacitors.

1 - Voltage can have an effect on the motor. Low voltage as edison points out low voltage will increase heating when running. Low voltage can also cause starting problems and possibly stall.

2 - Power factor correction caps are only one of many factors that can affect your motor voltage. Other than the effect on voltage, the removal of caps cannot have any effect on the motor.

 

[heber]

Thanks for clearing up my misconceptions about pf correcting capacitors. I was also assuming that the pf of an underloaded induction motor will lower greatly. Thanks again you all have been a great help.