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From: moroney@ramblr.enet.dec.com
Newsgroups: sci.physics,sci.electronics
Subject: Re: Power Factor questions
Message-ID: <20276@shlump.nac.dec.com>
Date: 17 Feb 91 03:14:21 GMT
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Followup-To: sci.physics
Organization: Digital Equipment Corporation
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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