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  • RPC Wows...how to test a cap?

    Yesterday the RPC stopped working. I was on a machine and it lost power, rpc kept spinning. Later tried restarting the RPC, contactors kicked in, then it the motor hummed, no rotation. Its a 10 hp 240V RPC. A contactor and timer turns on the starts caps for a few seconds. The photo below shows obvious issues with the start caps, however its looked like this for a while and still worked. The start caps all check out with the multimeter - capacitance and no open circuit.

    I'm guessing its possible for a cap to look ok at meter voltages but fail at operating voltages? As there a more thorough way to test them? I'm a little confused on why it failed yesterday while already running? i.e. if it was a start cap failure would it be noticeable while running or only when you went to start it again?

    thanks

    Last edited by Mcgyver; 10-11-2020, 12:56 PM.
    in Toronto Ontario - where are you?

  • #2
    If those haven't actually failed by now (and it sounds like the starting one has for sure) they will soon. Just replace them.

    Comment


    • #3
      The starting caps likely didn't cause the RPC to fail while it was at speed. I've seen this before and almost without exception the problem was connection related. Burned motor connections or burned connections on running capacitors or burned wires caused buy poor connection etc. Every now and then a capacitor or two will be bad but that has been rare in my experience.

      Start by changing the start capacitors then check every connection on the run capacitors and motor connections for signs of heat and security. I'd also check the capacitors one at a time (disconnected of course) with a capacitor checker. Pictures of your RPC and control box would be helpful. Burned contactor points is also a possibility but this is also rare in my experience.

      You can test capacitors with a VOM meter but you kind of have to have a feel for it, it's hard to explain. So it's best if you get a capacitor checker.

      Ron
      Last edited by nc5a; 10-11-2020, 01:56 PM. Reason: Added content to answer question

      Comment


      • #4
        Starting cap are typically electrolytic and are unmarked as to polarity, due to being bi-polar and also solid electrolyt for this application.
        They do not take kindly to being to subjected to AC power for more than a couple of seconds.
        Probabally hence the appearance of electrolyte corrosion.
        Max..

        Comment


        • #5
          Another point, the caps are only in the circuit to get the RPC motor started, after that they are not in the circuit. SO.... if you were running and your power from the RPC quit, the start caps wouldn't be the blame, they were not in the circuit once running.

          Comment


          • #6
            In a pinch you can charge them up individually and then discharge with a jumper(screwdriver). You would have to remove that discharge resistor first. Bulged or leaking usually means bad. Anytime those are in the circuit more than a few seconds they will fail, sometimes violently. Look for stuck shut contacts on your contactor or timer malfunction.

            Comment


            • #7
              They have already lost electrolytic... note the white powder. Replace them, then carefully start up. If it doesn't start "normally", shut off and investigate (assuming you have one) the potential relay or timer (stuck contact will burn up the start caps) and/or the motor. If you have a variac you can test the potential relay on the bench prior. If you only have a timer, check that with a light bulb or...

              Lost power when under load - fuses, breaker, wire, whatever.
              Last edited by lakeside53; 10-11-2020, 03:54 PM.

              Comment


              • #8
                I should have noted that they are being replaced, that's a given. Was more bothered that it quit while running suggesting like Sparky said its not them. Might caps that check out at multimeter voltages not work at high voltages...or I am I making crap up? If they do behave like that, whats a better way to test?

                Max, those two caps are just for starting, they're not the running caps...they kick out after a few seconds via a timer.

                Anyway, as they need replacing regardless, I'll do the trial and error route...replace them then determine if I have to keep going. thanks all
                in Toronto Ontario - where are you?

                Comment


                • #9
                  Yes, capacitors can leak or exhibit intermittent shorts at higher voltages. You can charge them to full rated voltage and then see if they hold that charge for a minute or so. You can also measure capacitance by using a 3 volt white LED and 100k resistor in series across the capacitor. Apply a 9V battery to the capacitor to charge it to 9V, and let it discharge until the LED goes out, which will be about 37% of 9 volts, or 3.33 volts, while keeping track of the time. T = RC, so C = T/0.1 Meg. So 10 seconds would be 100 uF. This is only an approximate method, but should be enough to verify if the capacitor is within about 20% of stated value. I found that it is also necessary to add a resistor across the LED, or directly across the capacitor, or else the current just drains slower and slower. Here is a video of a 330 uF capacitor with an LED and a 10k resistor in series and 20k across the LED. The LED goes out in about 4 seconds. You could tweak the components to get better accuracy.

                  http://enginuitysystems.com/pix/elec...t_LED_6650.AVI
                  http://pauleschoen.com/pix/PM08_P76_P54.png
                  Paul , P S Technology, Inc. and MrTibbs
                  USA Maryland 21030

                  Comment


                  • #10
                    I also built a 10hp unit. I start mine with an on button that brings the start capacitors into the circuit. If I hold the button for "a couple of seconds" as you say your timer is doing my start caps will fry. I push the on button for less than a second. I basically just give it a quick push and it's running. This doesn't help your problem but may save you a cap or two in the future.
                    I used the Fitch design fwiw and used mainly oil filled caps with a couple of electrolytic caps because I ran out of the oil filled.

                    Comment


                    • #11
                      Originally posted by challenger View Post
                      If I hold the button for "a couple of seconds" as you say your timer is doing my start caps will fry. I push the on button for less than a second. I basically just give it a quick push and it's running. This doesn't help your problem but may save you a cap or two in the future.
                      .
                      I mentioned a few seconds, but it might be less. I will check what the timer is set at when next out there. When I get it working again I guess it would make sense to reduce that time to the minimum required to get it going

                      in Toronto Ontario - where are you?

                      Comment


                      • #12
                        My phase converter has a current sensing relay.
                        It is electronic and not the mechanical Supco that
                        you used to see in times past. The spec sheet
                        says it also has a 5 second fail safe timer, in case
                        the motor does not start, to save the caps from
                        blowing up. Kinda nice, but my RPC is within
                        earshot of my start and stop buttons, so I can hear
                        if it just hummms and will not start.

                        -D
                        DZER

                        Comment


                        • #13
                          Originally posted by Sparky_NY View Post
                          Another point, the caps are only in the circuit to get the RPC motor started, after that they are not in the circuit. SO.... if you were running and your power from the RPC quit, the start caps wouldn't be the blame, they were not in the circuit once running.
                          Unless, the controlling circuit for start caps failed itself,
                          Can you start your RPC with a pull rope?
                          If RPC is based on a plain 3ph motor, you can start it with a pull rope like a old go-kart engine.
                          In stone hammer simplest of worlds, you don't need any caps at all in your RPC, it will run & give 3ph without caps at all.
                          The start caps give you instant start without a pull rope
                          The run caps give you better balanced power across the generated leg and the other legs
                          For that matter, you don't need contactors either, you put 1ph across leg 1&2-pull the rope, and it will run an give you the 3rd leg
                          Contactors give you a safe and controlled way to start the thing.

                          Comment


                          • #14
                            Originally posted by Mcgyver View Post
                            Yesterday the RPC stopped working. I was on a machine and it lost power, rpc kept spinning. Later tried restarting the RPC, contactors kicked in, then it the motor hummed, no rotation. Its a 10 hp 240V RPC. A contactor and timer turns on the starts caps for a few seconds. The photo below shows obvious issues with the start caps, however its looked like this for a while and still worked. The start caps all check out with the multimeter - capacitance and no open circuit.

                            I'm guessing its possible for a cap to look ok at meter voltages but fail at operating voltages? As there a more thorough way to test them? I'm a little confused on why it failed yesterday while already running? i.e. if it was a start cap failure would it be noticeable while running or only when you went to start it again?

                            thanks
                            ........

                            You almost certainly have a bad connection. The bad connection is probably involved with the start circuit and 3rd leg, but if your powered motors stopped, TWO wires must not be connected. Just the generated leg opening would NOT stop the load motors.

                            Go through the entire thing, looking for bad connections or bad contactors. You may find the issue in the coil circuit of the contactors, as that could cause the various issues.

                            I assume the start must work before the load can be applied in yours, via an interlock, but that may not be the case. I assume you DID turn OFF the load before trying to restart. A load on the generated leg will not let the thing come up to voltage, and may cause other issues.

                            Capacitor problems are not necessarily indicated here. The start caps are failing and need replaced (which you state you will do), but as mentioned will not affect the load when running. The "run" capacitors could fail open (possibly after shorting inside), since they DO have an internal protector (or most do). But open run caps will not stop the load motor.
                            1601

                            Keep eye on ball.
                            Hashim Khan

                            Comment


                            • #15
                              It could be a single wire bad connection. If the machine was running under heavy load, the loss of one phase might cause it to stall, and then it would not restart on single phase - as indicated by the hum. Two open connections would not cause any noise.
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

                              Comment

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