[Prev][Next][Index][Thread]
Re: PFC Caps (why to not use motor "start" caps) (fwd)
---------- Forwarded message ----------
Date: Fri, 19 Dec 1997 14:42:01 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: PFC Caps (why to not use motor "start" caps) (fwd)
>
> Russ,
>
> PFC caps are just AC capacitors like are used for motor start capacitors
on AC
> motors. I guess I used the plural (caps) because I always end up using a
> couple in parallel to get the right values.
I wouldn't use motor "start" caps, because they are not designed for 100%
duty cycle. Get motor "run" capacitors, which are designed for 100% duty
cycle. Even with a run cap, you should check the current rating. A typical
PSC motor (which uses the run cap) only puts about 10% of its power through
the auxiliary winding.
Taking a 50 uF for example, the impedance at 60 Hz would be =1/(377*50E-6)
ohms or around 53 Ohms. At 110Volts thats a little less than 2 Amps through
the cap, so make sure it is rated for it. I seem to recall that these units
dissipate something less than 1%, so the 200 (or so) VARs the cap is
providing would work out to 2 Watts dissipation, which seems reasonable for
a device that is a few inches long and an inch in diameter.
Ultimately, try the cap out, and if gets hot, get a bigger (physically)
one. The motor start caps are smaller for the same voltage and capacitance
as the motor run caps.