Hi,
Power factor values of AC loads are not the same for all members of a device family. For example an induction motor's power factor is 0.9(fully loaded). However, an another induction motor's can be 0.8 (fully loaded). The exact PF value can only be determined by the device's manual or by measurement. But for rough calculations an average value (for ex. 0.8) can be used. On the other hand, the PF of resistive AC loads (like heaters, incandescent (hot flament) lamps, electrolysis pots etc.) is equal to 1.
In addition, the PF of most of AC devices (including transformers) vary by their loading rate. (for example an induction motor's PF varies between 0.6-0.85 when it's load increases %40 to %100 respectively).

Thanks for the reply!!....I understand how PF varies but was looking for rule-of-thumb values I once seen published for quick ballpark calculations.

David Baird
dbaird@gemcity.com

EET degree.
Journeyman Electrician.

Hi,
Here some values for induction motors. First value is rated power, second is PS, third is efficiency (%) and the last is power factor.
1.1-1.5-77-0.82
1.5-2-78-0.83
2.2-3-81-0.84
3-4-81-0.84
4-5.5-82-0.84
5.5-7.5-83-0.84
7.5-10-86-0.85
11-15-87-0.86
15-20-87-0.86
18.5-25-88-0.86
22-30-89-0.87
30-40-90-0.87
37-50-90-0.87
45-60-91-0.88
55-75-91-0.88
75-100-91-0.88
90-125-92-0.88
110-160-92-0.88
132-180-92-0.88
160-220-93-0.88
200-270-93-0.88
250-340-93-0.88
315-430-93-0.88
355-483-95-0.89
400-545-96-0.89
that's all for now.

Azmi Demirel
azzmi@elk.itu.edu.tr
Welcome
I am handsome but married :-)

Something worth remembering about Power Factor.
Power factor is defined as the ration between reactive and apparent power (kvar / kVA).
This is not 'always' the same as the angle between voltage and current. Whenever harmonic currents and voltages are present, how do you define the angle between voltage and current ? For which harmonic ?
Do you use it only for the Fundamental, and what about the harmonics then ...
This problem is source of a lot of discussion right now.
What do you guys think about it ?

Hi,
I think there is a mistype about the definition of the power factor. The active power is equal to PF times apparent power.
You are right, when the harmonic components get into the account, the calculation of the power factor gets more difficult.

Such as non sinusoidal cases, as you pointed out, the angle between the fundamental components of the  current and voltage waveforms are generally used. This approximation results accurate enough to use in any power factor compensation capacitor or wiring calculations.

But if you want to obtain the exact PF value for a non sinusoidal system, i think you can make a few true RMS measurements (for active and apparent powers) or if the waveforms are defined, you can also make some calculations (average value for i(t).v(t) for a period).  

Regards

Azmi Demirel
azzmi@elk.itu.edu.tr
Welcome
I am handsome but married :-)

I think Azzmi was correct in saying that using only the fundamental component of the nonsinusoidal waveform is fairly accurate. The amplitude of most harmonics is usually much smaller than that of the fundamental frequency. For instance, in a square wave, the next frequency following the fundamental f is 3f, but the amplitude is 1/3 that of f. I believe this greatly reduces the contribution of the phase angle between the voltage and current of the 3f harmonic (and successive harmonics) to overall power factor.

Have find out how to compute total power factor of installation yet? it is easier to improve existing power factor correction needed, than, estimating power factor from new installaiton.
Here is what i do with my project. I gather all the data on the motors installed and average out the power factor given at 50% full load. the total power factor is then computed into the kVA from the total connected kW. if you go to www.lifasa.com, there is info on the common power factor used for equipment like lights (sodium, mercury, flourescent, etc). but this would not be the final figure yet. there are many factors else involved such as installation method (cohabitation of differential induced current of cables), switching effect, well being of instllation such as loose connection, etc. however, this is too tedious to compute. the best way to do this is thru stastitical method, i.e., take similiar installation to benchmark. like in my case i benchmark the power factor calculation based on the production capacity/total connected load,etc ratio.
this might not be the most intelligent way, but a smart way.
hope this helps.