View single post by Keinokuorma
 Posted: Mon Jan 21st, 2008 12:14 am
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Keinokuorma



Joined: Mon Jun 26th, 2006
Location: Finland
Posts: 1248
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himeros wrote: Also I don't know if the inductance of the motor, in series with  capacitor used in the start winding would come close to the resonance frequence of the 60 Hertz line. I have never seen a start capacitor, of such a low value as they use for the run capacitor however.   And by the way do you know how they deside the value of the run cap?
The run capacitor, if only used for inductance compensation, is wired parallel to the main winding. The value can be decided mathematically or by trial and error. Too big cap would draw extra current and raise the network voltage, so a value is picked that is a wee bit underrated. A close resonance at the network freq is sought, but the motor load status affects this, and must be accounted for.

The start/run cap would be serial to the accessory winding, and the value is looked up by trial and error or the math, too. If one cap is used for both functions, it is usually overshot a little for the run situation. If there is a separate start cap, it can be added parallel to the run cap while starting. Their combined value would then be suitable for starting, and the run cap alone could be matched very close for the desired torque.

Like said before, there are variations of the cap wiring.



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