From: John Popelish
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Subject: Re: Motor Run Capacitor Selection Question
Date: Fri, 29 Nov 2002 18:59:17 GMT
NNTP-Posting-Date: Fri, 29 Nov 2002 13:59:17 EST
John Ely wrote:
> If a motor nameplate specifies a certain value of
> capacitance for a motor run capacitor, will a
> larger value of capacity work just as well or
> will that cause a loss of performance?
The value of capacitance is, in effect tuning the effective inductance
of the auxiliary winding (which varies with load) to be near resonance
to get a current phase with respect to applied voltage that is about
90 degrees ahead of the phase of the current in the main winding.
This phase relationship is what produces torque most efficiently.
The motor run capacitor is chosen to give the optimum current phase
shift between the main and auxiliary windings, to produce the most
efficient torque. This optimum value varies a bit with motor load, so
if you are not running the motor at name plate load rating, then a
slightly smaller capacitor may produce the most efficient torque. If
you are overloading the motor, then a larger cap would produce more
torque, but, of course, the motor can be expected to overheat,
eventually. If the load is not above nameplate, then a larger
capacitor will just increase the auxiliary winding current (and
possibly slightly reduce the main winding current), with the risk of
overheating the auxiliary winding. That said, a small increase in the
capacitance is probably not much of a risk, because the tolerance on
such caps is often pretty wide, anyway, so the motors are designed to
handle some variation.
I think the best way to optimize the running capacitance is to connect
the motor to a watt meter, while it has its normal load connected, and
find the cap that minimizes the total power consumption. Then verify
the motor still has adequate starting torque (if you have reduced the
size of the cap below specified).