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Need help wiring drum switch.

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Ulma Doctor

Infinitely Curious
Active Member
Feb 2, 2013
Mike: it might help to think of a cap as a sphere with a rubber diaphram across the middle. Pulsations of air or water on one side
will give equal pulses on the other but no actual transfer of air or water will occur.
Caps are like a conductor for AC (depending on the value and frequency). This is true for all caps but motor caps are specially made to withstand AC.
The store and release action for DC takes time which is what gives the phase shift for AC.
ps Tesla was way ahead of his time!
Hi Mark,
thank you for the information


Downloads Moderator
Staff member
H-M Supporter-Premium Member
Dec 25, 2011
I'll add that the magnitude of the DC left in a capacitor that's in an AC circuit like the Start capacitor depends upon where in its sinusoidal cycle the line voltage was at the instant that the switch was opened. And also depends upon what else is still connected in the now OFF motor. If the motor is wired internally to run in only one direction and is being controlled by a single external ON-OFF switch, as soon as the motor slows back down enough to close the centrifugal switch, the capacitor will quickly be discharged through the now series connected start and run windings. So there is no potential shock hazard if you turn the motor off, unplug the line cord, and immediately open up the motor and start fiddling with the wiring. But if the motor is wired to be reversable and connected to most drum switches, neither end of the start circuit is connected to anything when OFF. So if the motor hasn't been OFF for a considerable length of time (dependent upon how good the capacitor is). you may get quite a jolt if you touch both capacitor terminals. The quickest way to guarantee that the capacitor is discharged is to unplug the line cord and then put the drum switch in either FWD or REV for a few seconds. But don't leave it there because Murphy will probably guarantee that you don't remember to put it back to OFF before you plug the line cord back in later. :chagrin:


Active Member
Active Member
Apr 30, 2015
Good points Robert, plenty of ways to get hurt with machinery- cut, maimed, electrocuted, your choice.
Be careful out there folks. Live to play another day. Discharge large capacitors with an insulated screwdriver before handling.
Mark S.
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