Power Factor Correction

Brianellul

Hi All

What are your views on Power Factor Correction devices? I have looked on youtube for demos however most are commercial and won't be realistics. Do they actually work?

Regards
Brian

Cariboocoot

Re: Power Factor Correction

Do they actually work? Yes, but then again no. Mostly no.
There are different ones, you see. Putting a properly sized capacity on a motor can improve its power factor. Putting a "whole house" capacitor on your breaker box doesn't do anything but make your wallet lighter. There are units designed for whole-house systems that do work, but they aren't the simple, cheap things hawked on shyster web sites. They have to continually re-evaluate the power stance of the house and adjust accordingly.

Now for most people it wouldn't matter anyway as the utility doesn't charge for "real" (PF corrected) power only "apparent" power. Thus you save nothing. Commercial installs that get charged for "real" power do benefit from them.

On an off-grid system getting the right capacitor on any motors you may have will make running them less taxing on the inverter, but over-all it's the same amount of power coming from the batteries.

So mostly there isn't much advantage to them.

BB.

Re: Power Factor Correction

As Marc says, residential meters only charge for actual energy used and PF does not matter. For commercial, they pay for poor power factor so setting up motor run capacitors and such on large motors makes sense.

Also, PF correct capacitors only work with inductive loads. It does not work with "non-linear" loads (CFL, florescent lighting, computer/electronic power supplies, etc.). Non-linear loads take current in a "spikey" current wave form and capacitors do not do much to modify the current wave form/change power factor.

Poor PF is still important for home and off grid wiring. Poor power factor devices (motors ~ -0.67, many CFL's at 0.5) cause "high" current flow, and wiring, transformers, inverters, generators both (for small systems) are usually rated to carry the same Watts or Volts*Amp amount of "energy"...

For example. Say you have a 120 watts of CFL lightning for an off grid system. And those 120 VAC CFL bulbs are rated at a PF of 0.5 (poor).

Power = Watts = Volts * amps * PF = 120 Watts
Amps = Watts/(volts*PF) = 120 watts / (120 watts * 0.5 PF) = 2 amps
VA = Volts * amps = 120 volts * 2 amps = 240 VA
Power = Volts * amps * PF = 120 VAC * 2 Amps * 0.5 PF = 120 Watts

So, for the above system, even though it is a 120 watts of power, and what the DC battery supplies, you would need a 240 Watt (or VA) rated inverter (or larger, and wiring, and larger fuse, etc.) to supply the 120 watts of CFL lighting (and a larger inverter to supply lighting+other loads).

-Bill

techntrek

Re: Power Factor Correction

One device that does work to correct a whole house close to 1 PF, including non-inductive loads, is a double-conversion (online) UPS. However, you don't get something for nothing, as usual. You will pay more in the end because there are losses in the AC to DC to AC conversions, in the power supply and inverter. Expect about a 7% loss. Since this translates into extra watts needed to run everything, you pay more even though you corrected the PF.

As others stated above, correcting PF for most residential installations won't gain you anything. There will be exceptions with horrible PFs that may - may - save enough to make it worth it because of additional heating in the wires (heating = additional watts used in the wiring itself), but it is unlikely.

RandomJoe

Re: Power Factor Correction

A hospital I used to do a lot of HVAC controls work in had some outfit come in and put PF correction devices on all their large motors - air handlers, elevators, so forth. Apparently the elevators were where they saw the most marked improvement, always wondered why that was - never have seen in detail how an elevator operates! The newer chillers we had installed shipped with the PF units from the factory.

They said they saw a measurable difference on the utilities, but I don't know how much. I figure it can't be *that* much of an improvement, or everyone would be doing it - and that's the only place I've seen do so in over 16 years.

One possible reason - they still had old tech on the air handlers, inlet guide vanes to control duct pressure, so the motors were straight across the line and full speed. Most other places of that size I've worked in have gone to variable speed drives so of course are doing the AC-DC-AC conversion by design. Maybe the hospital's elevators were an older design as well...

BB.

Re: Power Factor Correction

PF is worse when motors are lightly loaded (empty elevators--which is probably almost all the time except during "rush hour").

Air handlers and other equipment probably operate closer to rated power--so PF will be better.

Also, fixed loads means that you can put the "optimum" fixed capacitor bank on the motor/plant. If you have highly variable load, variable capacitance bank may be needed to get "optimum" PFC (power factor correction).

-Bill

BB.

Re: Power Factor Correction

From what I understand, the utility will charge commercial users for peak kVAR use for 15 minutes out of the month or year...

If you have a lot of motors operating at 0.67 and you can correct to 0.95 PF, then you are looking at saving:

1/0.67 =1.49x increase in kVAR over watts used
1/0.95 =1.05 increase in kVAR over watts used

So, on the motor side of the plant, you can save around 44% of the "demand" part of bill (worst case). The "demand" part of the bills, for many commercial users is about 1/2 of the bill.

And then there is the watts used--Which I think is converted into kVAR used by multiplying 1/PF against the kWH reading on the meter. Which is about the other 1/2 of the "typical" commercial power bill.

So, overall, I could see upwards of 30-40% savings if an old plant with no PFC was upgraded to PFC.

Perhaps somebody here has some real experience vs my back of the envelope calculations... I know that for many decades the oil refineries in our area have been charged for poor Power Factor--And I am sure they have been addressing this issue for decades (if not more).

-Bill

niel

Re: Power Factor Correction

random joe,
i think it is possible they don't know about the fix for power factor and are among the typical of the general population when it comes to electricity, stupid. let's face it, they are business men and women and not electricians and like many people they just sit back eating, up complaining about, and paying for the power when there are measures they can take to conserve that power. once a smart business person realizes there is a fix for pf, they would institute it somehow.