No load, power factor meaurement

[freefallingbody]

Hi,

I have tried measuring no load power of a 20 HP 4 pole motor. The p.f. indicated in the meter ( Power analyser- El Control , Italy make) was very low, 0.15 at no load. I wondered about the accuracy of this measurement. I referred to the calibration certificate of the meter and it gave me the error at 0.3 p.f. to be 4.5% ( meter and the clamp on CT together).

Then a consulted an expert and he said the error at 0.15 p.f. could be even 10%!. He suggested that instead of measuring cos phi at no load, I should try to measure sin phi and the error could be much less. He said, since the phase angle error of the meter ( and CT) should be added to the phase angle measured, it would be accurate if I measure phase angle by measuring sin phi directly. I can observe the reactive power using this meter. I am not sure whether the meter measures the reactive power or just calculates from the active and apparent power.

Making any sense, guys?

regards

Dinesh

 

[Skogsgurra]

Hi,

Considering the low losses when idling and that the magnetizing current often is 50 percent - or more - of full load, I think that 0.15 is a plausible P.F.

A good check is to measure the P.F. of a resistor and a capacitor. They should show 1.00 and 0.00, or very close to. If they do, the instrument is probably OK.

 

[eemotor]

You will most likely have a very low power factor at no load for a 20 HP machine. I suppose an alternate method to check your power factor would be to use two watt meters to measure power input, voltage and current going to the motor. From that data you should be able to calculate the power factor.

 

[Marke]

Hello freefallingbody

Yes, the no load power factor of an induction motor can be as low as 0.15 under virtual open shaft conditions.

There will always be errors at this low power factor due to phase shifts in the measuring components, CTs PTs etc. At a high powerfactor, these errors become very small. the difference between cosine 10 and Cosine 12 is small, but the difference between cosine 80 and cosine 82 is quite large. The phase shift is the same.

The big question is how much of an issue is it and why?
With an induction motor, at this low power factor, the power consumed is small, so errors in your calculations relative to full load, are still small.
Perhaps you can give more detail on your objectives so that we can comment further on the significance of this error and a possible solution.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

Power Factor (PF) is defined as “The ratio of total watts to the total root-mean-square (RMS) volt-amperes”. (IEEE Std-100).

I have a Motor; 20 HP, 1765rpm, 3 Phases, 60 HZ, 460 V, 23.3 FLA, and TEFC, High Efficiency.

Tested no load it has:
V=460, Io = 7.34 Amps, 590 Watts, 1799.6 rpm.

If the VA are calculated, VA = 1.732*460*7.34 = 5848.1

PF = 590 Watts/ 5848.1 VA = 0.10

It could be seen that the no load PF of induction motors is low. Certainly the accuracy of the test equipment is important. We used two watt- meters low power factor, 0.5% accuracy.
Accuracy is the ratio of the error of the indicated value to the true value.
E.g. if the true value is 590 W, the error could be 590 x 0.005=2.95 W

 

[freefallingbody]

Dear Marke & others,

The measurements are for estimating motor efficiency at site, by loss separation method. The uncertainty in efficiency estimated if I use a 10% error meter for no load power, is about 0.8%. If I use a 5% error meter, it is 0.4% and if I use a 1% error meter, uncertainty would be 0.2%. ( an excel sheet was developed to estimate uncertainty as per GUM- ISO)

Since I will be using the estimated efficiency to calculate energy saving by replacing it with a high efficiency motor, an error of 0.8% on efficiency ( say, 85 +/- 0.8) could be very important.

Or, is it?

Regards

Dinesh