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#189450 10/08/09 05:29 PM
Joined: Oct 2000
Posts: 5,392
sparky Offline OP
why these things aren't quite the magic widget they make themselves out to be....

someone here (forgive my poor memory & search engine skills) had a good argument for these caps being okay for industry, but not so safe for residential



thanks in advance
~Steve aka sparky

Joined: Jul 2004
Posts: 787
Because residential users are not billed based on their reactive load, only their actual power consumption. Correcting their power factor might help the power company but it won't help the residental consumer.

Joined: Mar 2005
Posts: 1,803
Because no Poco that I know of penalises domestic customers for the phase shift caused by running electric motors in the home. That's why we have a grid - it self-balances by averages. Large industrial and office users are encouraged to balance out their shift by arranging balancing of loads from their motors and fluorescent lighting by phase shift penalties. Thus this device is useless in the home, as already discussed here several times: you only pay for watt you use!

Wood work but can't!
Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Like a vampire, you kill it and kill it, and it just keeps coming out of it's grave at the next full moon!

OK, here's the story, in very simple terms, that explains both the science and the silliness behind these magic widgets:

Let's start with AC theory. How is AC different from DC? Well, AC varies in voltage over time. Since the voltage varies, so does the current drawn by any load. That's the first brick in our wall of theory.

Remember, where you have electricity, you also have a magnetic field. And vice-versa. We make current flow, in fact, by moving a magnet past a wire. Hoe many magnets, how often, is what give AC it's particular 'frequency.' As a corillary to this, a second magnetic field near a circuit can be used to either impede ot assist the flow of current. That's brick #2.

So ... when you have a wire coiled tightly together, the magnetic field of one section of the wire affects the magnetic field of the section of wire next to it. When you have this wire in a nice tight coil, this interaction gives the overall magnetic field a 'spin,' since - remember, this is AC - the macnetic field changes over time, and it takes time for that change to move from one end of the wire to the other. Brick #3

A motor, transformer, ballast, relay, solenoid - in short, anything with a coil of wire in it - has this interacting magnetic field. This magnetic field, as well as the induced magnetic fields of any stators or secondary windings, act to impede the flow of current. Brick #4

Now ... how is this impedence different from simple resistance? Again, we're talking about AC. The equation for resistance (volts divided by amps) only works (using simple math) when you can measure the volts and the amps AT THE SAME TIME. The magnetic field interference has the effect -you can see this on an oscilliscope - of introducing a delay in the amp curve as compared to the volt curve. So, when you measure the volts and amps together, you are measuring the current drawn at a different time, different voltage than the voltage you're measuring at that instant.
Brick #5

The effect of this delay is that you are measuring more watts than you are actually using. The electric meter runs 'fast.' This is the mortar that holds the sales pitch together. This artificially high use is described by the term 'power factor.'

Now ... for the magic widget: Remember I told you you could either impede ot assist the magnetic field? Well, there's a component that does just that: the capacitor. A very, very eccentric man named Steinmetz figured this out - and also was able to use capacitors to counter the impedence of the coils. The 'power factor correcting capacitor' was born. (It's worth reading a bit about this guy).

OK, so the thing works. Why do I denounce it as a fraud? Two reasons.
First, they can only correct something that is created by coils of wires. Motors, fluorescent ballasts, transformers, etc. Very little of our household loads are able to introduce a power factor into our useage. They're correcting a problem that is negligible.

Second, most motors, ballasts, etc., already have power factor correcting capacitors built into them! You're paying for something you already have.

But, you say, isn't 'more' of a good thing 'better?' Remember the time relationship I mentioned? Too much correction, and you're again making the amp curve out of time with the volt curve - and thus, you're now getting high meter readings again.

Lastly .... there's a type of math that accounts for things that change over time. It's called 'calculus.' If you look at more involved forms of Ohm's law, you will find a calculus expression in there. If you use calculus, you calculations work again - even for AC smile

Joined: Apr 2002
Posts: 7,333
Likes: 7
Similar devices show up on resi service upgrade jobs by one EC; don't know, nor care to hear the sales pitch.


Nice explanation job!

Joined: May 2005
Posts: 984
Likes: 1
If I may add, they also don't affect your bill if your POCO doesn't incorporate power factor into the rate structure.

Joined: Nov 2002
Posts: 794
Likes: 2
The kilowatthour meter only measures real power consumed, and ignores the reactive power. So you won't save anything on the electric bill with the capacitors. The residential electric rate probably assumes some reactive power factor anyway, but they don't measure individual customers for it.

Also notice that the motor in that demo on youtube appears to have no load on it, so the real power amperage is a lot lower than if the motor was driving something.

Oh, if you have a circuit breaker that trips out, reducing the power factor of a motor on that circuit with a capacitor may be enough to get the current down so the breaker doesn't trip anymore, but such a branch circuit shouldn't be so heavily loaded anyway.

there's a type of math that accounts for things that change over time. It's called 'calculus.'
I enjoyed calculus so much, I took it twice back in college... :-)

Joined: Oct 2000
Posts: 5,392
sparky Offline OP
interesting responses, thanks

seems some entity has to call me up with these "marvelous devices unknown to man, be first in your trade area" etc etc magic widgets a few times a year here.

but the fact is, it's not doing anything more than simply altering eli the ice man....

i would, should people ask, tell them their $$$ might be better spent on a grid tie inverter that could harvest for net metering, instead of this voodoo


Joined: Jun 2004
Posts: 1,273
The economics behind measuring reactive power is due to the impact it has on conductor size -- with particular emphasis upon the heavy industrial user with lots of motor loads.

Reactive power demand is met by increasing the conductor size -- not the prime mover nor the heat rate at the power plant.

Since reactive loads ( motor loads, ballasts ) are such a trivial fraction of most residential services it has never paid for the utilities to meter it. Instead, a 80% power factor, lagging, is built right into the NEC service load calculations.

So selling capacitance/reactive correction is snake-oil-in-a-can.

Joined: Oct 2000
Posts: 5,392
sparky Offline OP
well i think i'd get along a whole lot better with most of these salesmen if they were within arms reach....~S~

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