Power Factor 1

[fredt]

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.

 

[davidbeach]

Not influenced by plant power factor one iota. Probably won't find it to be nameplate power factor either, that happens for exactly one operating condition, load, voltage, etc. Use a meter that can measure power factor.

 

[waross]

We may be able to give a little guidance if you include the rest of the nameplate data. We have PF, need rated voltage and full load current.

Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

As said above; the .91 PF occurs only at rated power (i.e. fully loaded motor) and rated voltage. At partly loaded motor, PF goes down significantly. Idling can put you to .2 or less.
PF is also influenced by voltage. High voltage reduces power factor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[fredt]

Thanks for all your responses.

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.

 

[ScottyUK]

Without getting in to too much technical detail, there are basically two ways of connecting a three-phase circuit: star (also known as 'wye'), and delta. If your meter is expecting the instrument transformers to be connected in (say) delta and yours are actually connected in star then your power factor readings and very possibly the numerical readings will be incorrect. Most modern meters are configurable to accomodate several variations in connections but if you pick the wrong one then the readings will be wrong.


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If we learn from our mistakes I'm getting a great education!

 

[edison123]

Post the motor nameplate; voltages, currents & pf as shown by the meter and CT&PT details.

 

[Marke]

Just done a quick run through on calculations to see if I can see an obvious mis-connection, but nothing seems to make sense with the numbers quoted.
You are not using a VFD on this motor??
Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[electricpete]

The reading pf = 0.556 was on the motor alone?
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.

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[electricpete]

Also - what is it that is "special" about this motor?
We assume it's induction, of course synchronous would be a different story (power factor depends on excitation).

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[ScottyUK]

ePete,

Good observation about sensor reversal / polarity inversion. Should have thought have that myself...


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If we learn from our mistakes I'm getting a great education!

 

[fredt]

There isn't anything special about the motor, it is a conventional induction type, my reference was about the voltage which is a bit unusual. No VFD it is DOL start. It is a delta connection.

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.

 

[electricpete]

but it is difficult to figure what combination of phase angle difference would produce this effect.

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


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[electricpete]

Whoops. Here is a correction in bold:

Let's say your real power factor was 90%
=> theta = arccos(0.9) = 25 degrees.

Now let's say you have a 30 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

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