Starting Power Factor of Air Conditioner Motors

[osueepe]

I work for an electric utility in Oklahoma. Our Design Standards department is trying to tell me that a residential air conditioner has a starting power factor of .35 lag. I find this hard to believe. This results in almost four volts of flicker drop in a 25 kVA transformer. Does anyone know the typical starting power factor for an air conditioner motor? Is the starting power factor for a reciprocating compressor motor different than the pf of a scroll compressor motor? If anyone knows where I can find this type of information I will be glad to go look it up. I searched ARI (American Refrigeration Institue) but had no luck finding anything pertinate. Thanks.

 

[redtrumpet]

Wasn't this question posted earlier in another forum?

Have you checked with motor manufacturers? They should have this data.

The information is 37 years old, but FWIW my 1965 copy of the Westinghouse Distribution Systems book lists exactly the information you are looking for in Tables 4 and 5 of Chapter 2.

 

[electricpete]

0.35 starting power factor is not at all unreasonable for an induction motor in general.

In fact, the IEEE Red Book identifies a typical starting power factor of 0.2.

Look at the equivalent circuit. The only significant resistive component is R2/s. When s is high that component becomes less important and inductive elements dominate.

 

[osueepe]

Thanks for your help. I found my answer in a paper published by IEEE. Author: Charles W.Williams, Jr., P.E.
Title: The effects of distribution system parameters on air conditioning motor startup flicker. Year: 1999?

Charlie took actual field measurements of voltage and power factor for both piston (reciprocating) and scroll compressor motors. Table 1 of the paper shows that most air conditioners have as low as 0.70 and as high as .95 starting power factor. I'll take actual field measurements any day.


Thanks again for your help.

 

[electricpete]

The motors that he was testing must have included a starting capacitor, which will dramatically increase the starting power factor.

I don't know for sure but I would guess there are likely some split phase motors used in residential air conditioning which don't use starting caps and will have much lower power factor in the 0.2-0.3 range.

 

[stevenal]

Seems like a strange spec to be looking at to find flicker voltage. Locked rotor current is what you want. It is easily available from manufacturers, and typical values are tabulated in the NEC.

 

[electricpete]

Hi Steve - Remember the transformer is primarily inductive. If you put an inductive starting current through it you get a larger voltage drop then if you put the same magnitude resistive current through it. (I'm sure you knew that already.)

 

[stevenal]

Pete,

You're right, I knew that. I just don't bother with it. Worst case is when the impedance of the starting motor (which is also primarily inductive) is exactly in phase with the source impedance. Make that assumption, and impedances add up numerically making life much simpler.

If that approximation doesn't suite, the T&D Reference Book has a different one: 6X FLA at 35% pf.

Did you get my email the other day?