Capacitor load balancing

Factual Questions
1

Any EEs out there? The local news radio station has been advertising heavily this company. The ads don’t mention exactly what is installed but play up “recycling energy you’ve already paid for” and how the energy savings will “pay for itself” in about three months. The site just about pegged my B S meter.

What little I know tells me that the capacitor is only effective on circuits with motors. This isn’t so bad here in Arizona; almost everyone uses heat pumps for heating and cooling and that unit is probably the single biggest electical consumer in the home. Are the benefits that great? If so, why hasn’t someone thought of this before?

2

Residential customers don’t pay a penalty for power factor imbalance. In industry where there are a lot of inductive loads from motors, ballasts, etc. there is a penalty to be paid for having too low a power factor. Basically the capacitors form a kind of tank circuit with the inductive loads to bring the power factor back towards unity. Some commercial installations would benefit from capacitor banks but not all. It is true that you would waste less power but a residential customer won’t see any savings on their power bill from installing capacitors. They may offer some surge protection possibly. They say “why pay for energy you don’t use” but you don’t pay for that power right now anyway. We have had a thread recently (in the past year) on this topic.

3

A few recent threads: here and here that may interest you.

4

Residential Watt-hour meters do not respond to reactive power, thus the “power you have already paid for” part of the claim is bogus.

If you have large reactive loads, (motors) placing power factor correcting capacitors at_the_load will reduce (by a small amount) the real (metered) power lost in the wire supplying those loads. In this case the power saved is never paid for, so doesn’t fit the claim.

Placing the power factor correction near the service entrance will NOT do this, but will reduce the utilities distribution losses, and if you are an industrial user may save you from a penalty sur-charge assessed to customers with low power factors. Such companies are often also paying a peak demand based sur-charge, so the savings can be worthwhile.

5

Power is made up of watts and vars (var = volt-amp reactive, in case you were wondering).

Watts are simple. If you have a 40 watt light bulb, it takes those 40 watts of electrical energy and converts them into light and heat. Simple. The more watts you use, the more electricity you use. I’m sure everyone gets the concept.

Vars are a bit harder to explain but let me give it a shot here.

Take a coil of wire. If you run electricity through it, a magnetic field is formed, storing some of the electricity. Remove the current and the magnetic field collapses, releasing all of that stored energy as electrical current again. The coil of wire doesn’t really do anything with this energy, it just stores it and releases it.

Your home electricity is a sine wave with a frequency of 60 Hz (50 in some parts of the world). For coils of wire (aka inductors) this means that for part of the AC cycle they are storing up energy and for part of the AC cycle they are releasing energy. But the inductors never do anything with the energy other than store and release it, so it’s kinda “wasted” energy. We engineers also call it “imaginary” power (“real” power is the watts part). The thing is, the generator supplying your power has to send out extra current to charge up the inductor, even though that energy just gets released later. It makes your generator work harder without actually accomplishing any extra work. Basically. the generator has to supply the vars that charge up the inductors.

There’s also a thing called a capacitor. In its simplest form, its just two plates of metal next to each other. Capacitors also store energy, but instead of storing it in a magnetic field like inductors do, capacitors store it in an electric field. If you hate physics and are completely staring into space after that last sentence, the important thing is that if you have a sine wave (like your home electricity), capacitors and inductors work kinda opposite of each other. During the time when the inductors are charging, capacitors are discharging, and vice-versa.

Residential loads tend to be slightly inductive, due mostly to motors in things like hair dryers, vacuum cleaners, washing machines, etc. Because of this, the power company has to supply extra current to handle the extra vars. But, since capacitors and inductors kinda work opposite each other, instead of using bigger generators, the power company can just add capacitors onto the lines. So, that’s what they do. Back at the substation they have big honking capacitors that they switch on and off of the line. If you get it perfectly balanced, then what happens is that the capacitors discharge and supply the energy while the inductors are charging, then later in the cycle when the inductors discharge the energy gets stored in the capacitors. You end up with the “imaginary” power just bouncing back and forth between the inductors and the capacitors, so the generator only has to supply the “real” power you use, the watts. The power company therefore tries very hard to balance out the inductive loads of houses with capacitors, because this makes things most efficient for them.

This “kvar” unit does essentially the same thing (you’ll notice they even use “var” in the name of their little gizmo). So what’s in a kvar? Just some capacitors. This will help to eliminate the vars and make power transmission into your house more efficient.

But, here’s the important thing. The power company doesn’t charge you for the vars.

The power company only charges you for the watts. So, this thing is going to save you money by eliminating something you don’t pay for anyway.

You may have noticed a slight flaw in their plan. They plan on saving you money by making you not pay for something that you aren’t paying for anyway. In other words, they are going to take something that costs you ZERO and make you pay less for it.

So, just to be clear: It is not possible for these devices to save you money.

The way we measure how efficient your power is being delivered to you is called “power factor”. A power factor of 1 means all you have is watts. A power factor of 0 means all you have is vars. Like I said above, it’s most efficient if you get all of the vars to balance out, so you want your power factor to be as close to 1 as possible.

You’ll also see devices marketed as “power factor correction devices” or some such. They are the exactly the same thing is this kvar piece of crap. Instead of using the word “var” they are using the word “power factor” but we’re still talking about exactly the same thing.

The old fashioned spinning wheel type electrical meter doesn’t measure or record vars. The newer fancy shmancy meters with the LCD displays and such do measure the vars, but the power company still doesn’t charge you for them. The reason the new meters record them is so the power company can tell if you are doing something weird that has your vars way out of whack. If your power factor is really low (which you’d have to be doing something very abnormal) then the power company may come along and tell you to do something about it, but they won’t charge you for the vars.

Quite frankly, the power company doesn’t care about your vars because they planned on the cost of the capacitor banks at the substation when they figured out how much to charge you for electricity. Residential loads are fairly predictable, so as long as you don’t do something strange it’s all just business as usual for the power company.

If you are a business or a manufacturing plant, though, things are different. In your contract with the power company, they will flat out tell you up front that you need to keep your power factor above a certain level (typically something like 0.7) or they’ll charge you extra for the vars. If you are a manufacturing plant running big honking motors, you’ll pay through the nose for the extra vars. So, businesses and manufacturing plants will often install power factor correction devices. They are not these little kvar pieces of crap, but they are essentially the same thing, only much bigger.

So, if you are running a couple of 40,000 hp motors in your back yard you may need to look into power factor correction devices, but if you are a typical home owner you are wasting your money.

The guys that sell these things are a bunch of sneaky bastards too. They usually have a current meter on the device, which only shows the magnitude of the current running through it. The meter doesn’t tell you how much of that current is “real” and how much of it is “imaginary”. When you switch on the device, the magnitude of the current drops because you are balancing out the vars. The drop isn’t usually dramatic (a few percent or so, maybe 10 percent tops) so the fact that you aren’t saving anything really isn’t dramatically obvious on your electric bill.

6

Ditto Kevbo. There is a small loss in the wires because of the current circulating between reactive loads and wherever in the system this reaction is countered.

Motors are the main cause of this. When a motor is working hard is is less reactive, but if it is spinning without much effort (when you are not pushing any wood through your table saw) this is maximized.

Having the right capacitance, which might need to be adjusted situationally, right there next to each motor, will certainly save the real power lost due to wires conducting extra current over some distance.

This is a separate issue from the power company’s metering policy. Those wire losses will turn into real heat you could measure with a real thermometer. Probably not much of it, though.

Now, as to whether you could save money by installing capacitors in a number of places - that is not a sure thing at all. I bet it would take a damn long time to pay for the cost of installing capacitors. Throwing extra insulation around in the attic is probably a much better investment.

7

Suspicions confirmed, then. I’ll write the station and see what happens – nothing, I’d bet.

8

My failing eyes may have missed a link, but if there isn’t, the Straight Dope on capacitors and energy vampires.

9

So Cecil figures these “correction” devices can save you up to 2% on your electric bill. That’s pretty close to the % I’ve read elsewhere from sources not trying to sell these things. A superficial search shows a price per unit on the “KVAR EC” of $650, how many months of 2% savings do you need to break even on that?

I’ve seen a number of websites selling different versions of this scam. I think the first one I saw was the old XPower Energy Saver, KUTV in Utah did a consumer report on the XPower, here’s a youtube link. It’s amazing what some people will try to sell.

10

geek
Nice discription of power factor and vars. I have tried to explain it to others and only ended up with blank looks. Again nice.
Just a greasy old snipe

11

How on Earth did Cecil get that 2 percent savings figure? The only possible savings is from the reduced I[sup]2[/sup]R losses from inductive loads, for the current flowing from the load to the power meter. That seems implausible for a residential home.

12

Here is a link that calculates your savings from resistive losses.

The upshot is that you are saving nothing.

One thing is confusing to me though. The makers of these devices claim that they correct the power factor. This seems at least understandable with an inductive load even if it has no practical benefit. But what if your load is purely resistive? Wouldn’t the “correction” then move the voltage and current back out of sync (just in the opposite direction — lead vs. lag)? Wouldn’t you then be creating the same problem you are trying to fix? How large are the capacitors in these things? How big would they have to be to get the reactive resistance down to zero at 60Hz?

To see what nasty pieces of work the salesmen for these things can be take a look at the comments at this link. If someone wanted to go fight ignorance in a foreign land then that thread would seem to be a good place start.

BTW, I have run into one of these salesmen locally and he is as mad as a hatter and every bit as unpleasant as “Angelo” in the thread I linked. I don’t guess that says anything about his product but for some reason these devices seem to attract that sort of individual.

13

I don’t know enough about these particular units but I can tell you that in an industrial setting if the controller mistakenly leaves too many capacitors online it will negatively affect the power factor as you guess. The controller in our plant pulls sets of capacitors on and offline as needed; I doubt these little units have such a capability but I’ve never actually seen one in use.

14

I’da won the bet. And the ads are still on. Credibility of endorsements by the talent: zero.

15

Load balancing capacitors offer smoothing to the power supply when changes in load causes strong fluctuations in current.

It is very important to have because without it, damage could occur to the circuit or it may function abnormally.

Here is a link below to read a little more about it:

The part about load balancing or smoothing capacitors appears toward the end of the article.

16

I was under the impression that fluorescent lighting presents an inductive load that screws up the power factor - and that large, industrial or office building settings with lots and lots of fluorescent lighting sometimes require PF correction. Can someone confirm?

17

It’s not impossible or unheard of to need “bulk” PF correction on lighting circuits, but almost all commercial-grade ballasts now incorporate noise filtering and PF correction internally, and it’s financially advantageous to upgrade the ballasts, rather than to apply external PF correction measures.

One payback figure I see tossed around a lot is that the expense of re-ballasting all of the fluorescents and converting from T-12 to T-8 tubes in an office building can be usually be recovered in under three years. This comes from the combined effects of newer tubes putting out more light per watt, and less load on the air conditioning, etc.

18

You are correct.

If the fluorescent lights use simple inductive ballasts then they do present an inductive load. Some inductive ballasts include a power factor correction capacitor just for this reason, and electronic ballasts often don’t present enough of an inductive load to be a problem. So basically it depends on what kind of ballasts they use as to whether or not they need to worry about pf correction.

And unlike residential service, business and industrial services usually get dinged quite a bit on their electric bill for the power factor, so they definitely have a financial incentive to correct their power factor if it’s far enough off.

19

Well done, sir. This is what makes this board invaluable.

20

There are cases where power factor correction can be important for residential users; take your standard electronics with switchmode power supplies (or even just regular transformer based power supplies since both use a rectifier to charge a capacitor), which typically have rather low power factors, sometimes less than 0.5, and more significantly, draw current in a highly nonlinear manner (see last waveform in first chart), which can easily cause problems with noise and even distort the AC waveform. The European Union requires that all such power supplies of 75 w or more have PFC for this reason. Low power factor can also require larger circuit breakers; a standard 15 amp 120 v outlet can supply up to 1800 w (recommended to derate to more like 1500 w for continuous use) resistively but possibly less than 1000 w when power factor is considered (motors tend to be better than this, but equipment is increasingly going to electronically controlled motors which have a rectifier to convert AC to DC which then goes to a set of transistor switches to drive the motor).