In the manual's parts list, the cap is labelled "Capacitor, Motor
Start."
Barry
On 29 Nov 2004 Jim Lux wrote:
> At 04:31 PM 11/29/2004 -0600, Bill Aycock wrote:
>
> >Barry- I think the Cap is really a phase control in the reversing
> >circuit, not a starting Cap.. The value may be more critical than for
> >starting. Check the manual. Bill
> >
> >At 06:49 PM 11/29/2004 +0000, Barry wrote:
> >
> >>Tnx for all the responses about my stuck rotor. Of course, today,
> >>it is working just fine! However, I thought I'd replace the motor
> >>starting cap anyway, as it is 14 years old.
> >>
> >>The original cap is rated 130-156 mfd. Is the value critical? I
> >>have a motor starting cap here that's 88-108 mfd. Is that close
> >>emough? Tnx, Barry W2UP-- Barry Kutner, W2UP Internet:
> >>w2up@mindspring.com Newtown, PA Frankford Radio
> >>Club
> >>
>
> Those kinds of values (especially as a range) usually indicate a motor
> start capacitor. How big is the motor?
>
> A motor run capacitor (used in things like PSC motors or reversible AC
> motors) will typically be somewhat smaller, and a single discrete
> value.
>
> As to the effect of the wrong value..
>
> Mostly, it changes the starting or running torque. A smaller
> capacitor has a larger impedance, so less current flows through the
> winding connected to the capacitor. Less current means less torque.
>
>
> For a motor start application, it's probably not a big deal, since
> capacitor start motors are usually used in applications where huge
> starting torques aren't required (think about running a fan or a
> washing machine). All the capacitor (and the start winding) have to
> do is get the rotor started turning the right direction, and then, the
> usual induction motor mechanisms take effect. Induction motors are
> designed with various torque speed curves (identified by letters like
> A or D), but basically, once it's turning at all, the primary (non
> starting) winding will dominate the effects, since the current is MUCH
> higher in it. Consider that at 60 Hz, 100 uF is about 26 ohms
> reactance (about 4-5 amps)
>
> If it's a run capacitor, a too small capacitor has two effects: It
> reduces the current through the winding with the cap; it makes the
> phase shift between the two windings different thatn what it's
> designed to be. Both effects generally cause a reduction in the
> torque (at a given speed).
>
>
>
> The real difference between run and start capacitors is the duty
> cycle. A start cap is designed for a low duty cycle (current flows
> through it for a few seconds, then it's disconnected). A run cap is
> designed for current always flowing through. The loss on the run cap
> must be much lower or it will overheat. Run caps are usually much
> larger than start caps(for a given voltage and capacitance) and more
> expensive for this reason.
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
> "Wireless Weather Stations", and lot's more. Call Toll Free,
> 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
--
Barry Kutner, W2UP Internet: w2up@mindspring.com
Newtown, PA Frankford Radio Club
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|