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power factor correction


By bob g, Section Homebrewed Electricity
Posted on Mon Nov 22nd, 2004 at 11:00:24 PM MST
i know... i know.... but i just can't leave it alone!

some of you will probably remember my long time support for "power factor correction" in stand alone offgrid applications.

granted there is no penalty for poor power factor if you are on the grid in a residential capacity, but all that changes when it comes to "me" being the power company.

this whole thing has been on a back burner in the very back of my mind until this last week, when i went looking at freezers to convert to a refridgerator.

very interesting thing i found, for example

  1. model amana 15 ft chest freezer, current draw running 1.1 to 1.4 amps
  2. frigidaire 7 ft chest freezer, current draw 1.4 amps
  3. westinghouse 5.5 ft chest freezer, current draw 5.0 amps!!!
so why does the new 5.5 ft freezer take more power to run,,, it doesnt have the run capacitor of the two other examples, so.....

it stands to reason that without the capacitor there is an issue of poor power factor with the westinghouse model.

now i need to determine if i can add the needed cap to one without and get an improvment.

i guess it is time to buy a freezer, some cap's and a killawatt meter.

something that runs as much as a freezer or a refridgerator, needs all the reduction in power requirment as is possible, at least in my house.

bob g
power factor correction | 16 comments (16 topical, 0 editorial)

Re: power factor correction (3.00 / 0) (#1)
by commanda (alwynne at unwired dot com dot au) on Tue Nov 23rd, 2004 at 12:57:05 AM MST
(User Info)

BUT; how long does the motor run for? Is it a really high powered motor that chills it fast?

Here in Australia we have a star rating system for whitegoods. The more stars, the less average energy use per annum.

Just looking at the instantaneous current draw while the motor is running doesn't really tell you anything.

If it was a choice between 1.1 amps for 12 hours a day, or 5 amps for 1 hour a day, I know which I'd rather have.

Not saying that adding cap's won't improve the situation, because I honestly don't know the answer to that one.

Amanda



Re: power factor correction (3.00 / 0) (#2)
by Flux on Tue Nov 23rd, 2004 at 01:39:38 AM MST
(User Info)

bob g

See if that fridge has a wattage rating as well as the current draw. If it has a much higher wattage rating than the others, then it has a bigger motor as Commanda suggests.

If there is no starting capacitor it is either hi-tech and probably good, or just plain crude with resistance phase shift and just cheap and cheerful. In these days of concern over energy consumption I hope it hi-tech.

Careful use of power factor correction can be an advantage, but watch it with modified square (sine) wave inverters.

Flux

[ Parent ]



Re: power factor correction (3.00 / 0) (#3)
by Electric Ed on Tue Nov 23rd, 2004 at 05:37:03 AM MST
(User Info) http://www.electric-ed.com

While it is true that power factor correction of a motor will reduce the total CURRENT drawn by the motor, it does not reduce the POWER consumed by that motor.

There will be a small reduction in power consumption resulting from the reduction in voltage drop in the circuit conductors, due to the reduced current flow.

The true power (watts) representing energy converted by the motor is the same with or without PF correction.

Only the out-of-phase (magnetizing) component of the current is reduced by the PF correction.

EE



Re: power factor correction (3.00 / 0) (#4)
by Opera House on Tue Nov 23rd, 2004 at 06:53:32 AM MST
(User Info)

The need for power factor correction has to do with resistance losses. You want the peak current to be at the same time as th peak voltage.  Whatever the application, it will take the same watts.  Watts = voltage X current and if the peak current is later than the peak voltage, the current will have to be higher to get the same watts. The power you save is in the wiring.  


Re: power factor correction (3.00 / 0) (#5)
by laskey on Tue Nov 23rd, 2004 at 08:42:54 AM MST
(User Info)

There is no real need to correct the power factor.

If your home system is leading or lagging, it has no effect on your total generation.  The problem it causes is if you have a watt meter for usage.  A watt meter will measure neither inductive, or reactive power.  The power company wants you to correct the powerfactor so that they can charge you using a relatively cheap wattmeter, and not an expensive real power meter.  

At home you gain nothing from having a powerfactor of 0 degrees.  The device you power will still require the same amount of real power regardless of correction, and the device you use to correct the powerfactor with will use some power in doing so.

Cya,
Chris

[ Parent ]



Re: power factor correction (3.00 / 0) (#12)
by Ungrounded Lightning Rod on Tue Nov 23rd, 2004 at 07:22:53 PM MST
(User Info)

The power company wants you to correct the powerfactor so that they can charge you using a relatively cheap wattmeter, and not an expensive real power meter.

Actually the cheap meters are really good at reading the real power and ignoring the reactive power.

The power company wants you running at a power factor near one because the losses in their wiring and the capacity of their generators is related to the total CURRENT, rather than the total POWER, they supply.

If part of the current is reactive they still have their generating capacity consumed and still have to burn fuel to make up the resistive losses (which are real energy).  But if you're too small a customer to rate two meters (one for real power, one for reactive) you don't pay for the reactive power and its resulting drain on their system.

It's cheaper for the power company to give you a flat rate for your power, assuming you're a "typical" consumer with typical loads and thus a typical fraction of reactive power use, and only measure the real part with a cheap meter.  If your power factor is too low (i.e. you consume more than the percentage of reactive power they expected when they calculated your rate), they're billing you too little.

[ Parent ]



Re: power factor correction (3.00 / 0) (#16)
by laskey on Thu Nov 25th, 2004 at 08:12:38 PM MST
(User Info)

I disagree with you, There is no such thing as a reactive power/inductive power meter.  Those values have to be calculated.  Your power company makes your local industries correct thier power factor to at least .86 to insure that the power company is charging the correct amount using a watt meter.  

Real power is the correct term to used to refer to total power used, not the resistive power used (watts), it's measured in VA (volt amps), and it is a summation each of the three power vectors.

This is how it works, we all believe that volts times amps is the power we use.  That's only true with resistive loads. Where the power factor is 1, when we start adding inductive, and capacitive loads we start changing our power factor.  Now volts times amps = VA or real power.  Say we have a power factor of .8, hang our watt meter on the line and find out we are using 80 watts.  Then we hang a volt meter, and an amp meter on the line, and find that we are at 10 volts, and drawing 10 amp... that's 100 watts?  No, it's 100 VA, but only 80 watts, that leaves 20 VA, that is seen only as an inductive(or reactive)load. A watt meter can only measure the resistive component.

I stuck my kill-a-watt meter on my battery charger , and checked the the PF feed it.  It was .69, lucky me, I'm getting 30% of that charge for free.  The reason power companies don't make home consumers correct their PF, is because they are too small a customer to police.  Now if you work your PF so that the watt meter doesn't move at all, they will show up at your door sooner or later.  

Cya,
Chris

[ Parent ]



Re: power factor correction (3.00 / 0) (#6)
by bob g on Tue Nov 23rd, 2004 at 09:13:47 AM MST
(User Info)

i forgot to post run times

all units have similar run times of between 35 and 40 percent at 70 degree ambient temp.

what i am in reference to is run capacitors and not start capacitors.

all of my inverters, modified sine or rather modified square wave units will start and run capacitor start and capacitor run motors, without problem.

yes pfc is done to cover the magnetizing component, which in some cases is as much as 60 percent of the total power that has to be delivered to the motor before any useful work is done.

i am aware that the power company doesn't charge me for poor power factor in my home, but rather an average with there meter, (cheaper for them).

but the fact remains that if i am the power company, i still maintain that it is in my best interest to incorporate pfc in my system.

the funny thing is pfc is so poorly understood by so many people,including many so called electrical experts,  i have heard everything from...

  1. "just run larger wires to accomodate the line losses caused by poor power factor", problem here is you havent corrected anything but rather backed the problem up to the point of generation.
  2. "you can't use pfc on systems with inverters", which is not the case with fixed frequency inverters such as we use, but rather motor speed control inverters used by industry which regulate the motor speed by varying the frequency to the motor.
  3. and my favorite "why do it when the power company doesnt currently charge the residential user for poor power factor" i love this one because this answer comes on the heals of my stating that i am my "own power company", and as such i have to produce the extra power to provide the magnetizing current.
point is " why produce the added power to magnetize, when pfc can cover a good portion of that extra power?"

i maintain that pfc by means of capacitors are far cheaper and easier than additional fuel for a generator, larger windgenerators, larger inverters, larger pv panels, etc. etc.

if it works for industry, it stands to reason it would work for tha standalone power system, albeit the decimal point moves over a bit.

bob g



Re: power factor correction (3.00 / 0) (#7)
by bob g on Tue Nov 23rd, 2004 at 09:14:16 AM MST
(User Info)

i forgot to post run times

all units have similar run times of between 35 and 40 percent at 70 degree ambient temp.

what i am in reference to is run capacitors and not start capacitors.

all of my inverters, modified sine or rather modified square wave units will start and run capacitor start and capacitor run motors, without problem.

yes pfc is done to cover the magnetizing component, which in some cases is as much as 60 percent of the total power that has to be delivered to the motor before any useful work is done.

i am aware that the power company doesn't charge me for poor power factor in my home, but rather an average with there meter, (cheaper for them).

but the fact remains that if i am the power company, i still maintain that it is in my best interest to incorporate pfc in my system.

the funny thing is pfc is so poorly understood by so many people,including many so called electrical experts,  i have heard everything from...

  1. "just run larger wires to accomodate the line losses caused by poor power factor", problem here is you havent corrected anything but rather backed the problem up to the point of generation.
  2. "you can't use pfc on systems with inverters", which is not the case with fixed frequency inverters such as we use, but rather motor speed control inverters used by industry which regulate the motor speed by varying the frequency to the motor.
  3. and my favorite "why do it when the power company doesnt currently charge the residential user for poor power factor" i love this one because this answer comes on the heals of my stating that i am my "own power company", and as such i have to produce the extra power to provide the magnetizing current.
point is " why produce the added power to magnetize, when pfc can cover a good portion of that extra power?"

i maintain that pfc by means of capacitors are far cheaper and easier than additional fuel for a generator, larger windgenerators, larger inverters, larger pv panels, etc. etc.

if it works for industry, it stands to reason it would work for tha standalone power system, albeit the decimal point moves over a bit.

bob g



Re: power factor correction (3.00 / 0) (#13)
by Ungrounded Lightning Rod on Tue Nov 23rd, 2004 at 07:36:15 PM MST
(User Info)

i maintain that pfc by means of capacitors are far cheaper and easier than additional fuel for a generator, larger windgenerators, larger inverters, larger pv panels, etc. etc.

Assuming your wires are heavy enough that heating them is not
a major fraction of your power usage, a bad power factor doesn't
make any significant difference in your fuel usage, battery
discharge, etc.

What it DOES do is reduce the amount of REAL power you can get
out of your generator, inverter, etc.

In the case of a generator, your generator just runs out-of-phase
(leading) to make it up.  The limit on your generator's output is
the current it produces.  The power pulled from the motor shaft just
provides the real power (which includes the power running your
refrigerator, plus the heating of the generator coils and house wiring.)

Similarly with the energy pulled from the raw DC supply inside your
inverter.  The reactive power pulls energy to magnetize the core of
the generator, then PUTS IT BACK into the raw power supply when that
part of the core is demagnetized again as the cycle moves on.

Where it might bite you is that you might need a bigger inverter
to provide enough current to run the motor.  There are losses in
the converter's guts that are proportional to its maximum capacity,
so a bigger inverter pulls more power just sitting there turned on.

And (as with "copper losses" in a genny) there are also losses that
are (non-linearly) related to the current pulled rather than the
power pulled (i.e. not all of that magnetizing energy gets back into
the raw supply.)  So, like the power company, you do lose SOME real
energy if you have to "generate" reactive power.  Nowhere near as much
as if you needed one VA to provide one VAR.  But it's not zero either.

[ Parent ]



Re: power factor correction (3.00 / 0) (#8)
by bob g on Tue Nov 23rd, 2004 at 09:18:39 AM MST
(User Info)

sorry about the double responce, not sure how it happened or how to delete one of them,

perhaps tomw or someone can remove one

thanks

bob g



Re: power factor correction (3.00 / 0) (#9)
by bob g on Tue Nov 23rd, 2004 at 11:41:26 AM MST
(User Info)

also after reading my followup post it would seem perhaps i have used too broad a brush so to speak, in that my use of the term "electrical experts" might have been construed to include those of you on this board.

i assure you i was not includeing anyone from here, i appreciate all input i get from all sources, particularly those that donate their time and experience on this board.

so if i offended anyone, i apologize.

bob g

[ Parent ]



Re: power factor correction (3.00 / 0) (#10)
by bobn on Tue Nov 23rd, 2004 at 07:07:21 PM MST
(User Info)

 This might be of interest to you Bob.
http://www.ceere.org/iac/assessment%20tool/ARC2400.html

   BobN



Re: power factor correction (3.00 / 0) (#11)
by PowerBox on Tue Nov 23rd, 2004 at 07:17:56 PM MST
(User Info)

It is important that PFC should be considered not only by the Power supply company but to the consumers, It will make the entire system effecient. Most EU requires all offline Power supply and VSD >500W to have PFC. If a power supply company generate 1MW of power and the consumer power factor is only .5, so the amount of power usage is only 500KW, 500KW is not use due to wire losses and lagging/leading of the current with respect to the voltage. Same for Independent power producer if u maintain a good power factor you make use most of your generated power. Help make our electrical system and Transmission effecient to decrease our dependent to the mid east.


Re: power factor correction (3.00 / 0) (#14)
by troy on Wed Nov 24th, 2004 at 03:06:18 AM MST
(User Info)

Whatever you lose due to power factor issues will result (for the most part) in heat in the transmission system (wire).

Aha!  Is your wire getting hot?  Warm?  If not, you're not losing much to PF.  

This is an interesting empirical question and I would love to see before and after data for the amount of real power saved.

Good luck and have fun!

troy



Re: power factor correction (3.00 / 0) (#15)
by bob g on Wed Nov 24th, 2004 at 08:50:06 AM MST
(User Info)

i agree with you on the empirical data thing

i guess someone needs to take a lead on this on, so i will try to find a killawatt meter and do some testing

bob g

[ Parent ]



power factor correction | 16 comments (16 topical, 0 editorial)
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