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In article <1228@m-net.UUCP> michael@node.UUCP (Michael McClary) writes:
>Therefore it costs the power company to provide this "reactive" power, and
>it charges its large industrial customers a surcharge if they run too far
>out-of-phase.  So the customers buy capacitors to generate their own, keeping
>their bills down and everybody happy.  Sometimes they buy motors which have
>their own power-factor correction capacitors, so the capacitors and the motor
>will be switched on and off together, but usually they just buy a few big
>banks of capacitors and turn them on and off to keep the plant under the
>surcharge limit.
>
>(Most cpacitors that you find with a motor are "starting" capacitors,
> used to provide an out-of-phase current during motor start-up to give
> it extra torque.)

Two corrections to the above:

1.)	Synchronous motors also supply reactive power (ie. look like a
	capacitive load) if their fields are over-excited.  A company
	will often have a synchronous motor driving a load continuously,
	or even doing nothing, just for the sake of having it there to
	supply reactive power.  (The latter case is called a "synchronous
	condenser".)  The amount of reactive power can be increased or
	decreased by adjusting the motor's field current.  This is easy,
	and much simpler than switching capacitors which are only
	available in certain values and require relays and such.  (Field
	over-excitation is how power companies adjust their reactive
	power generation as well.)

2.)	Single-phase induction motors develop zero torque at zero speed,
	where the pulsating field looks like two contrarotating fields
	of equal strength at equal speed.  The starting capacitor and
	winding is used to provide a *substantial* out-of-phase current
	to give the pulsating field a rotating component in one direction.
	Once the motor is started, the rotor tends to latch to the rotating
	component that is moving *slowest* with respect to itself, so once
	it is moving it accelerates to near-synchronous speed in the
	direction that it was pushed.  (You can hand-start an induction
	motor if it has no starting capacitor, in *either* direction.)
	
>	Michael McClary		| SNAIL: 2091 Chalmers, Ann Arbor MI 48104

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