[sci.electronics] Power Factor questions

jgk@osc.COM (Joe Keane) (02/15/91)

This subject of reactive power and metering gets rehashed periodically in
`sci.electronics'.  I'll just make a quick summary of what's going on.  Feel
free to correct me if you know what you're talking about.

In article <2110007@hpldsla.sid.hp.com> tonya@hpldsla.sid.hp.com (Tony
Arnerich) writes:
>- There are two orthogonal currents in question, Resistive and Reactive.
>- There is no getting around the Resistive component, for X amount of
>  real work performed by the motor. The Reactive component does nothing
>  for the temperature of my carrots, and has nuisance value only.
>- Both currents add in vector fashion. This increases magnitude of current,
>  and changes phase relationship between voltage and current.

So far so good.

>- My power meter measures the compound current (hypoteneuse of the triangle)
>  and cannot be fooled.

No, it measures the time integral of the product of instantaneous voltage and
current, which is real power.

Of course there are some inaccuracies.  High or low voltage, large reactive
currents, and large harmonic currents may degrade the accuracy.  But over a
wide range of conditions it's pretty darn accurate.)

>- The power company's wires are heated by I^2R and have extra transmission
>  losses because of "bad" power factors.
>- The power company must work to produce any energy that is converted into
>  another form, such as mechanical work, EM radiation, heat. Energy that is
>  merely shuffled about (as in inductors and capacitors) is only lost through
>  conduction losses (I^2R and EM fields around the wire).

Right.

>- My power factor controller provides a local place to shuffle energy to and
>  from my refrigerator. The losses due to power factor are thus confined to
>  the wire between fridge and black box. The power company's transmission
>  lines get to sit that dance out, and will therefore conduct less current
>  on my behalf.

That's the idea.

>They will save energy and bill me less.

Yes and no.  Industrial customers have both real and reactive power meters
(and peak demand meters, but that's beside the point).  They get charged
according to some complicated formula involving these numbers.  For
residential customers, it's not worth it for them to do all this.  They use
only one meter, so you don't have an incentive to improve your power factor.

Actually reducing the current reduces losses in your wiring, and any loss
reduction on your side of the meter saves you money.  But this is a small
change to a small amount, and i think it's not significant for a normal user.

>- The "extra" energy to support the phase shift must still be put into the
>  fridge-blackbox-powercord circuit at the beginning of the motor duty
>  cycle, and that energy is wasted when the motor shuts off. However, it only
>  has to go in once per motor duty cycle, not twice each voltage cycle (120 Hz).

Motor starting tends to draw lots of reactive power, typically many times the
normal power level.  I think your gizmo is doing something about this, but i
don't really know what it's doing.

moroney@ramblr.enet.dec.com (02/17/91)

The way I understand the black box power factor controllers that go on the
fridge is NOT that they try to present a power factor of 1 to the power co,
like the big industrial ones (capacitor banks) mentioned earlier used by big
electric consumers, but the following:

The type of motor used in a refridgerator tends to draw a fairly constant
current regardless of load, as long as the load is within a normal range.
An unloaded motor will be mostly reactive with a low PF, while a loaded motor
will look mostly resistive and have a high PF.

But the motor draws the same current, so has the same I2R losses regardless of
load.  If you reduce the voltage somewhat, you reduce the current, and thus the
resistive heating loss.  The motor also becomes more 'loaded' (has to do the
same work with less voltage/current) so its PF increases.  The little PF
controller boxes sense the PF of the motor, and adjusts the voltage to it so
the current is reduced.  The PF rises indicating the motor is becoming loaded,
negative feedback so the box knows how far it can go.

-Mike Moroney