Power Factor Correction (PFC) for solar power system

telljf · October 2011

I have read a lot of discussions on other forums on whether Power Factor Correction Devices will or will not reduce the electricity bill. They argue that power supply companies charge for KWhr and not KVA and hence no savings other than some minor improvement from the reduced heat loss due to lower current. But there is one thing that people on either sides of the argument agrees, and that is that these devices reduces the current flowing through the lines. That is where my question comes.

I plan to place one of these devices purchased on ebay at the point where the inverter supply enters the home supply line. What I would expect to see is that the AC current at the output of the inverter drops when the PFC device is connected. Now my question is will this result in an equivalent drop in the DC current drawn by the inverter from the batteries. Since inverters are rated in KVA, PFC should save the AH discharged by the battery and thereby increase the backup time. Mine is a pure sine wave inverter. The batteries are charged by solar panels.

Any thoughts on that? I will post the results once I get the device and make some measurements.

Fullpower · October 2011

Re: Power Factor Correction (PFC) for solar power system

What makes you think that power factor correction will save you money?
How could the addition of MORE Ac devices lower the amp draw from the batteries?
MORE devices = MORE power.

Cariboocoot · October 2011

Re: Power Factor Correction (PFC) for solar power system

There have been discussions here before about whole-house PF correction systems.
They absolutely can not work.
PF is individual to a particular device, such as a refrigerator. It may have a PF of 0.85, but that is only relevant when the refrigerator is running. Extended to include everything in the household, this means the "whole-house Power Factor" will vary depending on what happens to be in use at any given moment. Simply applying a big capacitor to the utility feed (which is about all any of these things are) can not correct for this moment by moment change as there is no way to vary the capacitance according to need. They are nothing more than a scam.

Here's one thread on them: http://forum.solar-electric.com/showthread.php?t=6782

Fullpower is right when he says they won't save you money: your meter rolls up Watts, not Volt Amps. Industries have to pay for VA, but you don't so even if it did fix your household PF it wouldn't alter the bill one dime.

Inverters are indeed rated in kVA. Adding a capacitor to the output won't alter anything there either. Power Factor is basically the load demand being out of synch with the supply ability: when the sine wave is producing its maximum Voltage (and therefor has the most power available) is not the same as when the load is drawing the most current. This difference is the Power Factor. You can shift the time in which the power is drawn to bring it more in line with the supply (which would require individual capacitance tuned to the particular device), but you can't erase the power need.

The best you can hope to gain by adding such capacitance to any type of induction load (there's essentially no effect with resistance loads) is to ease the surge demand on the inverter, potentially allowing the use of a smaller inverter to run the load. One clever Canadian here (not me) has managed to get a refrigerator to run from a 300 Watt inverter by careful choice of components whereas normally it would take a 1kW (or more) unit to do the same job.

BB. · October 2011

Re: Power Factor Correction (PFC) for solar power system

There are different types of wave forms too... Out of phase (Voltage vs Amperage sine waves) waveforms for inductive (or capacitive loads). And non-linear (aka spiky) current wave forms (typically from the diode/rectifier input to a power supply charging a capacitor (most of the current is right near the +/- peaks of the AC voltage wave form.

The linear (inductive) phase issues can be "fixed" very nicely with a capacitor (usually turned on only when the motor is running).

The non-linear (computer power supplies, AC Battery Chargers, etc.) current wave forms (and square wave profiles from MSW inverters) cannot be fixed with a capacitor (or other types of filters).

From the thread Marc posted---One person (poster SteveK) measured (with lab equipment) around a 1.85% savings in DC current by using a motor run capacitor on an induction motor powered by a good TSW (true sine wave) inverter... That will be roughly a ~1.85% savings in power from the battery system.

For the most part, "bad power factor" is of little consequence to the average home.

For people running off grid with batteries, inverters, and small generators--If you can find the "Power Factor Corrected" version of an appliance--it can be worth while.

For off grid systems and gensets--Typically you want to run them closer to their rated load instead of lightly loaded (a 5kW genset running an 800 watt load will use much more full than a 1,600 watt genset running that same 800 watt load). Bad power factor means higher current (in the AC side of a system)--so you need larger transformers, generator windings, etc. to support bad power factor/high current from those loads...

However, in terms of energy usage--there is little to be saved directly (inverter input current/power; or explicit increase in genset fuel consumption).

Here is a really neat thread started by SteveK on trying to find the best battery charger to mate with a Honda eu2000i 1,600 watt genset. Turns out, Power Factor plays a very large role in the choice of an AC Battery Charger to be run off of a small genset:

Question about battery charger selection with EU2000 generator

In the end, those Power Save 1200 type units twist the basic engineering facts into such pretzel logic that the companies become scammers for 98% of the customer base those products are aimed at.

-Bill

mike95490 · October 2011

Re: Power Factor Correction (PFC) for solar power system

telljf wrote: »

.....
Any thoughts on that? I will post the results once I get the device and make some measurements.

You will need an "automatic switching" corrector, otherwise, you are just swapping I leads V, for V leads I .

http://www.pfc-engineering.com/pfc_automatic_capacitors.html

telljf · October 2011

Re: Power Factor Correction (PFC) for solar power system

Thanks for the comments. My argument is if PFC is good for the power company that supplies power to our homes (they demand that atleast from industrial customers), for an offgrid solar power system, where I produce energy myself, shouldn't I get some benefits from PFC?

Fullpower · October 2011

Power Factor Correction is wasted effort

simple answer:
NO.
For normal residential loads, power factor is insignificant.
IF you are running freight elevators, air compressors, or continuous duty induction motors, capacitive PF correction would properly be SIZED and connected at each particular load or device, to apply some random bit of "correction" to an ac panel is absurd, and will (probably) not result in ANY power or dollar savings.

telljf · October 2011

Re: Power Factor Correction (PFC) for solar power system

Is there anyone here who has actually done some test measurements before and after PFC for an off grid power system?

As I promised I will test it. Keeping the AC load the same, if the current drawn from the battery drops after introducing PFC device in the circuit, then I will say that the money spend on the PFC device is worth it. Currently I measure the PF at the output of the inverter changing between 0.4 to 1 depending on the type of load. So there is scope of improvement.

Cariboocoot · October 2011

Re: Power Factor Correction (PFC) for solar power system

You may have missed some of the clues in the posts about what power factor is and how it is corrected.
In summary, there is no such thing as a power factor correction device which can be applied to any source of power and provide improvement. Power factor and its correction is individual to loads. You can improve it somewhat by applying the right correction to an individual load, but it only changes the Amperage draw curve against the supply cycle so that they are more in synch.

There is no need to test any "whole house power correction device" because there is no way that such a thing can work. That's physics, not sales hype.

BB. · October 2011

Re: Power Factor Correction (PFC) for solar power system

Yes, see Marc's link. Less than 2% savings in battery power when power factor correcting an induction motor on a tsw inverter.

Do not run a capacitor on an msw inverter. It will over heat the capacitor because of the square wave from the inverter.

-Bill

telljf · October 2011

Re: Power Factor Correction (PFC) for solar power system

Cariboocoot, on your comment "In summary, there is no such thing as a power factor correction device which can be applied to any source of power and provide improvement. Power factor and its correction is individual to loads. "

I came across this link:
http://www.lmphotonics.com/pwrfact.htm

It goes into the technology behind PFC. They say "Capacitors connected at each starter and controlled by each starter is known as "Static Power Factor Correction" while capacitors connected at a distribution board and controlled independently from the individual starters is known as "Bulk Power Factor Correction".

They also say "Static PFC must not be used when the motor is controlled by a variable speed drive or inverter. The connection of capacitors to the output of an inverter can cause serious damage to the inverter and the capacitors due to the high frequency switched voltage on the output of the inverters."

Cariboocoot · October 2011

Re: Power Factor Correction (PFC) for solar power system

You can find web sites that say the world is flat and the moon is made of cheese.
That doesn't make it so.

techntrek · October 2011

Re: Power Factor Correction (PFC) for solar power system

There are two exceptions. One is the device mike linked to on the last page. It is designed to monitor the current power factor of the connected loads - by measuring the kva and the kw and dividing the two to get the PF. Then switching in various amounts of capacitance to bring the PF close to 1.

The other is a double-conversion UPS. It has a PF-corrected power supply which in turn runs an inverter, so no matter what the PF of the AC loads the PF is corrected by the time it reaches the AC source. But even modern units only have overall efficiencies around 92% so you would actually use more energy from the utility to use one.

PF correction will only reduce the heating within the AC wires, which if they are sized correctly (or oversized) the reduction will be minimal. It won't reduce the wattage drawn from the batteries on the DC side (except by reducing the heating on the AC side).

RCinFLA · October 2011

Re: Power Factor Correction (PFC) for solar power system

There are so many different items effecting the consequences of attempting power factor correction on a battery powered system it is hard to cover them all.

Never on a modified sinewave inverter. MOSFET's will overheat because comp cap forces a higher Vds when their current is at max.

Rectifier/filter cap power supplies with short factional cycle current peaks will not be corrected by parallel cap.

Refridge isn't worth it.

If you get too close to 100% PF compensation on an inductive load you can create ringing resonances that can damage things like motor caps and inverter.

Some of the so called PF correction devices are triac 'dimmers' that reduce the motor's current by reducing its effective run voltage. More sophisticated units do a high freq PWM to reduce the motor voltage. The amount of adjustment depends on motor mechanical loading.

As to effect of poor PF on a sinewave inverter, it is basically the inverter loss at the VA of the load. Additional battery power is only the additional inverter efficiency loss, not VA of load.

I have a 3.5 ton central air conditioner that I occasionally run from two series stacked Xantrex SWplus 5548 inverters. Its unmodified compressor PF is about 0.89, drawing 14.95 amps run current at 240 vac. With an 80 uF comp cap across compressor the run current drops to 13.65 amps. That is 3590 VA versus 3275 watts comp'd. Air Handler draws another 300 watts for blower motor.

My battery current running the Air Cond. went from 76.1 amps to 75.7 amps. This is basically the inverter efficiency going from about 94.0% to 94.5%. 20 watts less heating of the inverter. Whoopy!

jim4000 · September 2017

I have used an 80 MFD motor cap I got out of the junk yard and caused a 30 APROX percent decrease in the motor start current of an inverter run well pump.The cap was in the start winding only and was switched out of the circuit by the motor once it reached the 3600 RPM. Current was pegged on my 60 amp clamp on meter but after the mod the current was down to around 20 - 30 amps for that short start time hope this info helps. Remember that the cap was NOT left in circuit all the time

BB. · September 2017

One of our "retired" posters has done a lot of work to allow his refrigerator/freezers to run on a 300 Watt MorningStar 12 VDC inverter (experimenting with different motor start capacitors, and voltage controlled relay and auto-transformer to reduce starting voltage):

http://forum.solar-electric.com/discussion/11992/appliances-on-a-suresine-300

It can be done in some cases--But takes a bit of work and the right parts to make it happen.

-Bill

Power Factor Correction (PFC) for solar power system

Comments

Categories