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Electric Motor Start Capacitor

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  • Electric Motor Start Capacitor

    I am nearing the completion of my woodworking shop and decided to go home and get my deceased Father's radial arm saw. It was used only once or twice before he died and then sat for many years unused. I found that it would not start. No hum or anything. All the wiring seems to be in new condition and voltage is getting to the start capacitor so I suspect it is bad. But I'm not sure if I can properly test the start capacitor with my digital multimeter.

    If anyone can give me advice on this I would be grateful.
    Can I test it properly with a digital multimeter?
    If I replace it, do I have to get one with the exact same rated MFD range or do I have some room to play with in selecting a replacement capacitor?


  • #2

    Hi are u sure its a start and not a run cap,,,,,,,,, is it blk or silver ?????? tom


    • #3

      voltage dont mater as long as it is higher than the old one, in fack the higher the voltage the better the cap, Tom


      • #4
        Just a thought is the motor wired for 110 or 220? For test purposes you can unhook the wires form the capacitor and hook them together, if the motor starts and runs the capacitor is bad.
        SSG, U.S. Army
        K.I.S.S., R.T.F.M.


        • #5
          It is possible, I went through this exercise last year with my saw motor. If you would like I can get the step by step procedure from one of our E.E. from work.



          • #6
            Start Capacitor Info

            It is black.
            The capacitor is an old Mallory.

            The saw is wired for 110 though it can be wired for 220. However, the capacitor is marked 124-149 micro-farad (MFD) and marked "110VAC".

            Looks like the part number is "235-8029K-02".
            There is also another number stamped on it:

            It is one of those made-by-Ridgid Craftsman radial arm saws that has the safety recall on it. If I can get it running, I'll submit the info to get the safety retrofit and start using it.

            One of the problems that I've encountered in my initial hunt for a replacement is that either the physical size of the capacitor I find is too big or the MFD rating or voltage rating is different.

            The capacitor is 2 3/4" long by about 1 7/16" diameter. Anything physically larger will not fit in its spot and allow the motor housing to be replaced.

            I'll try Tod's suggestion. I wasn't sure if that was safe to do.



            • #7

              Hi as i said voltage on a capasitor is not to important the higher the voiltage the better the cap, as long as the voltage is not less then the old one,,,,Have u checked continuity through the saw ??? the switch might be bad
              connect ur meter to plug, close the switch and meter should read,,,,disconnected from power,, if the cap was bad the saw woud hum and not start,,do u have an anolog meter u can check the cap with that,,,Go to any motor shop and they can check it ..and replace it if bad ..tom


              • #8
                George, No growl or hum at all? If that's the case power is not going through the motor. A bad starting capacitor would have the motor growling but not spinning. Shorting it might let the motor start, but you'll be running too much current through the windings. Only do that once and only for a quick test. As for the ratings, a higher voltage is fine and you can be off some with the MFD ratings, but try to come close. Take the capacitor to a motor repair-rewinding shop. They should be able to test is and being in the business, I'm sure they can find a good replacement even if it has to be ordered and mailed (or sent UPS / FedEx ground) to you.

                If you don't hear any GRRRRRRR when you try to run the saw, it's something more that's wrong. Try (plug pulled) to turn the motor a few turns by hand and then apply power for a second or two. Any GRRRRR now? If not, then look for open connections and maybe a bad switch. You should have a rather low resistance reading (less than 25 Ohms) across the power wires with the switch on and no power applied. Be careful and always be ready to pull the plug fast. Never apply power for more than just a few seconds to a motor that won't budge.


                • #9
                  This may not apply but:

                  I had an old Craftsman table saw that had a button-type overload reset on the chassis (not on the motor). The problem was that once it popped, the metal wafer would not pop back to make contact after it cooled. I had to disassemble the reset switch everytime it overloaded (happened a few times as I was ripping wet treated 2x4s that would bind on the blade).
                  If the saw has a separate reset switch, jumper across it and see if it solves the problem. If it does, the reset is the problem. If you cannot find a replacement, install a circuit breaker or inline fuse holder with a slo-blo fuse.

                  Just a thought. You can check it out with an ohm meter. If there is no continuity through the reset, try pushing the button, and if still no continuity, it is bad.

                  Hope this helps.
                  Practicing at practical wood working


                  • #10
                    If you would like I can consult with one of our electrical engineers or technicians on how to test your capacitor with a multimeter (absolutely free of charge). I went through this exercise last year with my tablesaw motor; I ended up replacing a weak start cap for under $20.00.

                    Last edited by billie_bob; 11-30-2006, 07:44 PM.


                    • #11
                      Here try this, it is relatively easy; first, make sure the motor is unplugged in fact it is usually best if you pull the start cap to perform this test.

                      First discharge capacitor paper clip or screwdriver accross the probes. To check capacitor, set volt-ohm meter to RX 100 scale and touch its probes to capacitor terminals. If capacitor is OK, needle will jump to zero ohms, and drift back to high. Steady zero ohms indicates a short circuit; steady high ohms indicates an open circuit.

                      This method will help you determine whether the cap is dead; however, it is usually not very useful at determining whether it is weak or not. An alternate method is used to determine the latter. Good luck let me know if this helps.

                      Last edited by billie_bob; 11-30-2006, 08:13 PM.


                      • #12
                        Analog meter

                        Hi if u can get an analog meter you can check the cap,,make sure the cap is discharged if it is good,,,,connect the leads from the meter.. watching the meter as u do,,if the meter jumps see how far,, then reverse the leads the meter should go twice as far,,,,, if no reading when meter is connected the cap is open , and no good , if the meter reads and stays up the cap is shorted and no good..,,Good luck,,Tom,,,,,by the way a cap is either good or no good,,,,,,,,


                        • #13
                          It Runs!!!!!

                          I went out there and checked everything again and ran the power down to the reset button on the motor. I don't know how many times I pressed that reset button and it seemed to be closed (that is, not tripped). But this time, I pressed harder and heard a "click". I plugged it up, hit the switch again and she fired right up!

                          Thanks for all the tips. Now I have a "new" radial arm saw to add to my tool collection!


                          • #14
                            You may want to consider checking the amps your motor is drawing and compare it to the theoretical curve. We have found motor capacitors have a shelf life of about 2 years if not used. The dielectric substrate tends to degrade and will cause your motor to run at higher amps; consequently, shortening the life of the motor. Food for thought, good luck.



                            • #15
                              George Congratulations I know it feels great hearing it run finally.