>
> The AC motor is running at a power factor of 0.6 which is rather low.
> I was wondering if this was typical for a 3/4 HP motor. I calculate
> that I would need 170 mfd of non polarized capacitors rated at 8 amps
> to bring the power factor to 0.9
Small inexpensive motors often have terrible PF. Mostly because they're
not used in applications that are PF sensitive: a large consumer getting
charged for reactive power (or low PF) is probably going to have bigger
motors (which are usually better) and/or have electronic drives these days.
The other thing is that the motor's speed/torque characteristics will
affect the PF, in that how the winding is done affects both. The mfr
may have wanted a particular curve, and the PF came out that way. Maybe
they're adding the excess inductance to limit the stall current, for
instance. (i.e. a motor with a PF of 0.5 will have a locked rotor
current of around twice the full load current, as opposed to 7 times for
the usual big motor).
Is there a thermal cutout on the motor? Does it say "impedance
protected" on it?
>
> The reason I would like to bring the power factor closer to unity is
> to reduce the line current. Anyone have any experiences with this.
>
The tesla coil community fools with PF all the time, because they often
use paralleled neon sign transformers (lots of inductance) and want to
run off the wall socket, so PF correction helps get more watts for the amp.
HOWEVER, for motors with varying loads, you want to be careful. You're
hear the phrase "trouble with voltage stability when overcompensated"..
What this means, in more familiar to RF people, is that you can get huge
voltages from resonant rise. Consider the motor as a L and an R in
series, where the R is the sum of the winding resistance and the
equivalent of the mechanical load. Now, put a C in the circuit to
improve the PF. By definition, at PF=1, the reactance of the C and the
reactance of the L are equal (and opposite sign). But hey, that's what
you need for a resonant LC, and at the line frequency, no less. If
there's enough R there, then the resonance is low Q, and the voltage
doesn't rise very much. But if the equivalent R is low (i.e. lightly
loaded motor), you've got a recipe for insulation breakdown.
Hence the advice to make sure you never over correct for PF. It's not
that a leading PF (over capacitive) is bad.. it works just as well for
current limiting as the more common lagging PF. It's that in a varying
load system, you'll pass through that resonant condition, and that's
when the problems arise.
> Denis ve6aq with Ve6fi
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