Hi folks,

I just wanted to let you know that in an AC current circuit, the law P = UxI is not always true. Well, not always *exactly* true

There are 3 "types" of power:

- Active power

- Reactive power

- Apparent power

This is because in an AC circuit, capacitibr and inductive load make the current lag or be in advance compared to the volts. If the current is not synchronized with the voltage, electricity is not used efficiently.

This is measured by the Power Factor. Generally, it is considered to be good when it is between 0.95 and 1 (you can see it on the specs on the transformer: it's 0.99 for this one).

In the case of this transformer, and generally in all the circuits we deal with (alarm systems, access control, ...), this is no big deal because the equipment we use all have a power factor very near to 1.

But if you deal with motors, a lot (I mean a huge amount) of fluorescent lamps, or big industrial machines, there can be a problem and corrective measures may need to be taken.

If you look at recent PC power supplies, sometimes you can read "active PFC" which means that they have a circuit to correct the power factor (an active circuit i.e. not only an inductor or a capacitor, but a full electronic circuit that is as efficient as possible)

For more info, Instead of reexplkaining something badly when it's done correctly somewhere else, I prefer to redirect you to the following pages rather than continuing the explanation myself:

http://www.energy-in-motion.com/PFC.html (explained informally)

http://en.wikipedia.org/wiki/Power_factor (explained with the maths)

uski

Did you know magnetic chokes are being phased out by the EU now?

PF correction on flourescents will soon be a thing of the past, motors will still be a PF problem, though having said that most have a PF correction capacitor(s) anyhow?

Then again single phase units use caps to put the current out of phase don't they?

Ah the old college course did some good, I remember all the useless stuff!

PF correction on flourescents will soon be a thing of the past, motors will still be a PF problem, though having said that most have a PF correction capacitor(s) anyhow?

That's right. I just wanted to enlarge the subject of the discussion to something more theorical

Ah the old college course did some good, I remember all the useless stuff!

Useless, for sure, unless you have a PF problem

uski