Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email:
In business for 39 years


• Article 1 - May, 1989


Checking out a capacitor and how it's used to start a motor©


I like the idea of including "Tech Tips" - Troubleshooting, in the COSEIA newsletter. These are always nice bits of information to store away for future reference. However, Greg, three points are worth noting in your discussion. First, if the capacitor check is made with an analog meter (the ones with the needle), the ohmmeter selector switch should be set on a relatively high setting, e.g., the R x lK, Rxl0K (best), or R x lM setting to get the best needle deflection and return to infinity. Second, a digital meter can also be used (set on about the 200K or 300K scale - meter brand dependent). However, here you will see the readings start at low numbers and advance up to the maximum reading available for the selector switch setting (a 1 or 2, depending upon the meter brand). Third, the purpose for using a capacitor is indeed to get the pump motor started, but is not to provide "a burst of energy" as you stated. The pump motor contains two windings; a start and a run winding. The capacitor is in series with the start winding. These winding circuits are then wired in parallel with each other. The purpose of the capacitor is to provide an out-of-phase magnetic field in the start winding (i.e. the magnetic field in the start winding will be out-of-phase with the run winding). Without this out-of-phase magnetic field, the motor will not move. Most of the motors used in the Taco and Grundfos pumps (residential applications) can be used with proportional controllers. The motors that use a centrifugal switch (like on a belt driven furnace fan motor), would soon burn out if used with a proportional controller.


One last thing worth noting, since the start and run windings are wired in parallel, whenever power is removed from a PROPERLY operating permanent capacitor pump motor, the capacitor will discharge through the windings. In this case you would not be shocked if you were to touch the capacitor wires/terminals. However, if the run windings were open circuited, the capacitor would not discharge. Hence, it would be best to short the two capacitor wires/terminals together before touching them. Also trying to measure a charged capacitor with your ohmmeter could very likely damage your meter.


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