Starting Power Factor of Air Conditioners

[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.

 

[buzzp]

I cant help you here but if you get no results post this question to the electric motors and controls thread. Someone will know there. Good luck.

 

[jwerthman]

The 0.35 starting power factor doesn't sound at all surprising. The starting pf of larger motors is typically 0.15-0.20. On starting, and induction motor looks similar to a transformer with a shorted secondary winding, consequently its impedance is mostly inductance. When calculating the starting voltage drop, keep in mind that the motor also appears as a constant impedance during starting rather than a constant kVA during running conditions. This means that as the voltage is depressed during starting the starting current is also reduced, and hence the voltage drop. It is a common mistake to assume that the starting current is constant even though the voltage is reduced. This is an easy first calculation, but will many times give overly conservative results. The best way to look at it during starting is as a voltage divider consisting of the transformer Z, cable Z, and motor starting Z all in series.

 

[dpc]

I agree with jwerthman that the .35 sounds high, if anything. We typically assume 0.2 for starting induction motors across the line.

 

[osueepe]

Yes, I understand that starting an induction motor results in a low starting power factor. However, I thought that residential air conditioners have a starting capacitor that reduces the amount of current draw on start up and raises the starting power factor to something higher than .35?

Thanks for replying.

 

[jwerthman]

You are correct that most, or all, of these units utilize capacitor-start motors. But, the capacitor is there primarily to provide a phase shift in the winding fluxes to create a starting torque. The power factor is also improved in the process, but I'm not sure how much. My guess is not much, but I could be wrong.

 

[MarkAthay]

I'm not atl all surprised that a 25 kVA transformer would have problems starting a central air unit. That's one reason why many utilities have stopped stocking 25 kVA transformers and started stocking 37.5 kVA transformers as their minimum size. I know of one utility that has almost standardized on 75 kVA pad mounted transformers because they live in a very hot area and so many people are using large AC units. They've found it almost impossible to load up the units and still maintain adequate voltage support to adjacent customers during motor starts.

It doesn't matter whether it's a reciprocating compressor or a scroll compressor, you're starting an electric motor and not the compressor.

You didn't say what size unit this was, but try running the calculations for a 5 ton unit, not pretty.

Mark in Utah

 

[Cowdog]

At one time I had a really good reference book that was published by GE in the 1950's. It said the starting PF for small motors was .7, and it was referring to motors up to at least 5hp. Unfortunately, I don't have that book anymore to double check my memory.

Concerning your flicker of 4 volts, I have measured 5.5 volts of flicker(120V base) at a customer's service panel, served from a 15kVA transformer with 200' of 4/0AL underground service wire. This is about 4 1/2% flicker. A study done many years ago(1960's ?), found that most people don't find the flicker of incadescent lights objectionable until it exceeds 4.2%, this is at 4 motor starts per hour, which is typical for air conditioners.

 

[Cowdog]

I forgot to mention in my previous post that the motor flicker I measured was a 3hp motor on a residential air conditioner.