In message , Tim
writes
Hi,

Nick wrote:

Could someone explain power factor for me please (and I guess a few
others) ?

Power factor = 1 means that voltages and current are in phase
Power factor = 0 means they are 90 deg out of phase.
In all cases, PF = cos(V/A phase angle)

Try this for a fuller description:

http://en.wikipedia.org/wiki/Electric_power

Which is all true for linear loads (be that resistive, inductive or
capacitive).
However the input to CFLs is not linear. Its diodes into a capacitor
(followed by a high frequency inverter to drive the lamp).
A diode capacitor input arrangement only draws current from the mains to
"top up" the capacitor near mains voltage peak, i.e. the current only
flows for 2-3ms of every 10ms half cycle, and has a higher rms
(root-mean-square) value than you might expect.

PF can be defined as real power divided by apparent power,
where apparent power is Vrms * Irms.
Remember Irms is higher for non-linear loads because the current is
narrow and peaky (i.e. the squared term is much bigger before you
average (mean) it then square-root it). Therefore you get low
power-factors.

Real / Apparent power works just as well for linear loads, where as
cos(phase angle) doesn't work for non-linear loads.


The reason being is that I have a lot of CFLs and so the rated power
consumption is somewhat less than if these were ordinary filament bulbs.

However the other day I plugged in a few to my gizmo that tells power
being drawn in watts and also power factor.

The power factor was around 60% on the CFLs.

True, and typical of diode-cap inputs.

I believe the elec company doesn't like devices with a poor (low?) power
factor but I don't know why.

Because it means that while you are paying for a certain amount of energy
used (in phase), there are disproportionatly larger currents flowing in the
suppliers cabling, which means that if everyone ran with a PF much 1, the
supplier would need to beef up their distribution (= more dosh for no more
sales). There might be a further impact on the generating stations too, but
the engineering of that is beyond me.

The same applies to the non-linear loading version of PF, more rms
current flowing, bigger cables/transformers, more losses, but the energy
co's aren't selling any more power.

The other concern is how does the electricity meter measure power drawn by
a
device with a low power factor ? - does it over or under read ?


The same non-linear, high rms input current, factors apply to (small)
switch mode power supplies. Depending upon equipment class, larger
switch mode power units (75W for most) must have some form of power
factor correction, either simple passive to slug off the current input
peak, or clever electronics to force a near-sinusoidal input current.
Good designs will often achieve a PF of 0.99.



--
steve