Power Factor
Power Factor
(OP)
I'm a mechanical engineer trying to measure power input to a 150kW motor. The power factor of the plant to which the motor is also connected is known to be low at present (without the motor running). The motor is probably a major proportion of plant load when operating. The nameplate PF for the motor is 0.91 at full load. If I measure current and voltage at the motor terminals can I assume the PF to be the motor nameplate value or is it likely to be influenced significantly by the plant power factor and if so to what extent.





RE: Power Factor
RE: Power Factor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power Factor
PF is also influenced by voltage. High voltage reduces power factor.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Power Factor
The motor is 2pole, 50Hz, 1000V (special application voltage). I do have theoretical PF and efficiency curves for the motor although just manufacturers data - not tests on that particular machine.
Question arose because we did have a rented power meter but it gave an odd power factor readout of 0.556 at near full load and a correspondingly low reading of power. I was confident about load within about 10% and the measured amps confirmed the high load (assuming theoretical PF). Electrician argued that PF was affected by plant PF but I didn't think so and you have confirmed that there is no linkage.
They tried different connections but despite consulting the manual did not succeed in getting correct readings. I think it is likely that power meter is OK. Any ideas what might be wrong? There is a 9:1 ratio transformer in the system to reduce voltage to 110V within instrument limits. This ratio is programmed into the power meter which shows correct line voltage.
RE: Power Factor
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Power Factor
RE: Power Factor
You are not using a VFD on this motor??
Best regards,
Mark.
Mark Empson
L M Photonics Ltd
RE: Power Factor
If reading is at full motor load, then the reading is suspect as was stated above. The level of suspicion heightens because you rented a meter and are doing something you're not familiar with -> error likely situation.
Can you describe your measurement setup?
Can you describe all the measured paramerers: preferably three voltages, three currents and three angles or power factotrs.
In addition to wye/delta, it is possible to create errors by swapping the polarity of an input to a 3-phase power analyser. Flipping a clamp-on changes the polarity of a current input.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Power Factor
We assume it's induction, of course synchronous would be a different story (power factor depends on excitation).
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Power Factor
Good observation about sensor reversal / polarity inversion. Should have thought have that myself...
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Power Factor
I am some distance from the plant so I cannot readily check all the details at present and the meter may have already been returned. However I will pursue the issue of flipping the clamps if they still can.
The way that cos phi is being interpreted by the meter seems to be the problem but it is difficult to figure what combination of phase angle difference would produce this effect.
Thanks again to all - very informative.
RE: Power Factor
Let's say your real power factor was 90%
=> theta = arccos(0.9) = 25 degrees.
Now let's say you have a 90 degree phase error. That can be introduced by a wye delta voltage transformer. Then the phase angle looks like 55 degrees. Power factor looks like cos(55 deg) = 0.57
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Power Factor
=====================================
Eng-tips forums: The best place on the web for engineering discussions.