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    Motor run capacitor physical size vs voltage rating & measurements?

    (TL;DR skip the first paragraph)
    I encountered my first dead motor run capacitor at a customer this week.
    For some fields I guess it is a very common occurrence but the usual way for industrial machines here is to use 3-phase motors.
    (Unless a servo motor makes more sense that is).
    But for some reason this 3-phase machine uses a 1-phase motor for its liquid coolant pump.
    The pump would run but sometimes trip the 10A breaker.
    I monitored it starting and stopping for over an hour until it finally stopped in a specific spot between its windings:
    Then it could not start rotating again, so instead of drawing below 5A it suddenly took 15A
    And that led me to the motor run capacitor, its print reads 25uF 400v but it measured 0.4nF, no wonder the motor was unhappy

    The capacitor was mounted in a little sheet metal box opposite the input terminal connections of the motor.
    Now the original capacitor measures 40mm wide by 74mm long and is manufactured by 미래콘덴서 in South Korea.
    This does not seem to be a common size, the major distributors like Mouser and Elfa lists caps that are 45x74mm or 40x92mm.
    At my job I found a 45x70mm capacitor that I did manage to squeeze in with some persuasion into the metal box, however it is only rated at 370v.
    And here comes my question: when I measured the AC voltage across the capacitor with the motor running it was 363v.
    The machine is american and has a 400v step-down transformer, the motor seems to be fed by something we never see in Europe: split phase.
    That is from one hot or the other to ground I measure 133v.
    But from both hots I measure 230v, this checks out with: √3 x 133v = 230v.
    What I do not understand then is how can the voltage be 363v on the capacitor, should it not also see 230v?
    I gratuitously stole this diagram of how I guess the motor is wired, but the 363v reading at Vcap then makes no sense to me.
    Is the meter just being fooled by the phase-shift imposed by the capacitor?

    Attached Files
    Last edited by Per Hansson; 09-23-2020, 01:20 PM. Reason: Replaced Elfa URL with the one originally intended
    "The one who says it cannot be done should never interrupt the one who is doing it."

    #2
    Re: Motor run capacitor physical size vs voltage rating & measurements?

    See if this one will fit your enclosure

    https://sales.marsdelivers.com/item/...-MFD-370V-RND/

    This type of motor are very similar to air conditioning compressor motor
    the voltage on capacitor is very important to follow you could even use a higher voltage one if you can find one to fit

    The voltage on the capacitor has do with the voltage generated by the start and run winding of the motor
    This is why when the capacitor loose the UF value the motor might start with very little load and if it does run it will not be at the right speed and will cause the motor motor to over heat

    One note if you can not a capacitor to fit the enclosure just find a capacitor to fit an enclosure you can find the wires that go to the motor can be extend away from the motor this is done all the time

    One other note make sure that the capacitor that you use is a run capacitor not a start capacitor they are completely different

    Using a start capacitor as a run capacitor will cause it to explode

    A start capacitor only has power to when starting the motor
    while a run capacitor has power on it all the time ( just in case you did not know the difference between the two of them )
    Last edited by sam_sam_sam; 09-20-2020, 07:46 AM.
    9 PC LCD Monitor
    6 LCD Flat Screen TV
    30 Desk Top Switching Power Supply
    10 Battery Charger Switching Power Supply for Power Tool
    6 18v Lithium Battery Power Boards for Tool Battery Packs
    1 XBox 360 Switching Power Supply and M Board
    25 Servo Drives 220/460 3 Phase
    6 De-soldering Station Switching Power Supply 1 Power Supply
    1 Dell Mother Board
    15 Computer Power Supply
    1 HP Printer Supply & Control Board * lighting finished it *


    These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

    1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
    2 Hem Saw Computer Stack Board

    All of these had CAPs POOF
    All of the mosfet that are taken out by bad caps

    Comment


      #3
      Re: Motor run capacitor physical size vs voltage rating & measurements?

      Thanks sam_sam_sam but that capacitor at 1.75" (45mm) width is the same as the used CSC capacitor I already squeezed in there.
      In this case since it is a liquid coolant pump the capacitor can not be mounted externally, that would be a safety hazard.
      And the voltage rating is 370v just like my replacement (the original was 400v allegedly).
      And this was the point of my thread: it is more about electrical theory than finding an actual replacement.
      (The link to Elfa in my original post shows a replacement that should work).
      But I am more interested in understanding the electrical theory about how the cap can see 363v from 230v line to line voltage.
      Attached Files
      "The one who says it cannot be done should never interrupt the one who is doing it."

      Comment


        #4
        Re: Motor run capacitor physical size vs voltage rating & measurements?

        Originally posted by Per Hansson View Post
        But I am more interested in understanding the electrical theory about how the cap can see 363v from 230v line to line voltage.
        This video is very simple explanation of this type of motor

        https://youtu.be/jNWlWzFzHi4

        Not the best one I seen

        The capacitor that was in your first post should work if you have to take off the mounting bolt be careful not to crack the plastic of the capacitor or just drill a hole for the mounting bolt and use a rubber washer on the enclosure end to keep the rain out of it

        What I was talking about earlier was you use a plastic enclosure for the capacitor and the wires for the capacitor go through a liquid thigh connector from where the capacitor was originally to the plastic enclosure with the same liquid thigh connector but I would only do this if you have no other choice

        One note I would not recommend using this capacitor to run this motor other than testing weather or not to see if you have a motor problem or not ( the one you show in your last post )
        Last edited by sam_sam_sam; 09-20-2020, 10:34 AM.
        9 PC LCD Monitor
        6 LCD Flat Screen TV
        30 Desk Top Switching Power Supply
        10 Battery Charger Switching Power Supply for Power Tool
        6 18v Lithium Battery Power Boards for Tool Battery Packs
        1 XBox 360 Switching Power Supply and M Board
        25 Servo Drives 220/460 3 Phase
        6 De-soldering Station Switching Power Supply 1 Power Supply
        1 Dell Mother Board
        15 Computer Power Supply
        1 HP Printer Supply & Control Board * lighting finished it *


        These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

        1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
        2 Hem Saw Computer Stack Board

        All of these had CAPs POOF
        All of the mosfet that are taken out by bad caps

        Comment


          #5
          Re: Motor run capacitor physical size vs voltage rating & measurements?

          The capacitor in my last post is the dead one that was in the unit.
          I showed another picture of it being measured by my Fluke in the OP reading 0.4nF
          With the Keysight U1733C it reads 17pF, yea, that is not going to be reused.
          As said in the OP I have put in a replacement capacitor, that is probably an order of magnitude better.
          For one it has a production date, and it has a P2 safety rating.
          What concerns me is the 370v rating vs 400v of the original, and how in the world there can exist 363v on it?
          Last edited by Per Hansson; 09-20-2020, 02:28 PM.
          "The one who says it cannot be done should never interrupt the one who is doing it."

          Comment


            #6
            Re: Motor run capacitor physical size vs voltage rating & measurements?

            You would be better off with a 470 volt one if find one it would last longer

            Here is where you can find some

            https://www.aliexpress.com/item/3300...c36170215acd-0
            Last edited by sam_sam_sam; 09-20-2020, 03:23 PM.
            9 PC LCD Monitor
            6 LCD Flat Screen TV
            30 Desk Top Switching Power Supply
            10 Battery Charger Switching Power Supply for Power Tool
            6 18v Lithium Battery Power Boards for Tool Battery Packs
            1 XBox 360 Switching Power Supply and M Board
            25 Servo Drives 220/460 3 Phase
            6 De-soldering Station Switching Power Supply 1 Power Supply
            1 Dell Mother Board
            15 Computer Power Supply
            1 HP Printer Supply & Control Board * lighting finished it *


            These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

            1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
            2 Hem Saw Computer Stack Board

            All of these had CAPs POOF
            All of the mosfet that are taken out by bad caps

            Comment


              #7
              Re: Motor run capacitor physical size vs voltage rating & measurements?

              The Voltage on the cap is higher than the AC line Voltage due to back EMF generated by the motor winding.
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment


                #8
                Re: Motor run capacitor physical size vs voltage rating & measurements?

                Thank you budm that was a very short and concise answer that really explained it well for me!
                I should have understood it intuitively since I know a motor acts as a generator when speeding down.
                It just did not occur to me that it does it all the time even when at speed.
                Maybe if I am curious enough I will take some very careful scope measurements.
                Here was a good Wikipedia article on the subject: https://en.wikipedia.org/wiki/Counte...romotive_force
                "The one who says it cannot be done should never interrupt the one who is doing it."

                Comment


                  #9
                  Re: Motor run capacitor physical size vs voltage rating & measurements?

                  Can you access the center point to be able to measure Vb and Vc?
                  Never stop learning
                  Basic LCD TV and Monitor troubleshooting guides.
                  http://www.badcaps.net/forum/showthr...956#post305956

                  Voltage Regulator (LDO) testing:
                  http://www.badcaps.net/forum/showthr...999#post300999

                  Inverter testing using old CFL:
                  http://www.badcaps.net/forum/showthr...er+testing+cfl

                  Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                  http://s807.photobucket.com/user/budm/library/

                  TV Factory reset codes listing:
                  http://www.badcaps.net/forum/showthread.php?t=24809

                  Comment


                    #10
                    Re: Motor run capacitor physical size vs voltage rating & measurements?

                    No, the motor manufacturer designed this a bit strange:
                    In the input terminal box are just two wires (plus earth) with spade connectors that you put the incoming single phase on.
                    Then in the opposite side of the motor in the box for the capacitor are also only two wires.
                    So I guess one of those will be in common with one of the input wires, like in the schematic posted inline with my OP.
                    I attached a photo I took for my own use while there, it was the best I had of the terminal box.
                    Incidentally this shows the actual power draw of the motor with a working capacitor: 2.7A instead of ~5A
                    Attached Files
                    "The one who says it cannot be done should never interrupt the one who is doing it."

                    Comment


                      #11
                      Re: Motor run capacitor physical size vs voltage rating & measurements?

                      Open-circuit run cap- must have tripped the internal pressure switch.
                      The cap running voltage depends on the motor's load, so at no load I believe you are seeing worst case 363VAC.
                      I don't see a lot of airflow from ventilation holes there on the entire bottom so this might be a bit hot. The fan likely just recircs hot air.
                      EIA-456-A is a standard for metallized film capacitors (motor run) polypropylene oil-filled. The lifetime test is 125% of rated voltage for 2,000hrs to simulate 60,000hrs operation goal.

                      Comment


                        #12
                        Re: Motor run capacitor physical size vs voltage rating & measurements?

                        There some interesting about motor back EMF.
                        http://physics.bu.edu/~duffy/sc545_n.../back_emf.html

                        This is very interesting to me now, will study more.
                        BTW, the reason I ask if you can get access to the center tab of the motor to be able to measure the Vb and Vc is because I like to know if the Voltage on the running cap is the vector sum of the Vb and Vc winding which is 120 degree apart.
                        I am also wondered about speaker back EMF as how it affects the speaker function.
                        Last edited by budm; 09-23-2020, 02:54 PM.
                        Never stop learning
                        Basic LCD TV and Monitor troubleshooting guides.
                        http://www.badcaps.net/forum/showthr...956#post305956

                        Voltage Regulator (LDO) testing:
                        http://www.badcaps.net/forum/showthr...999#post300999

                        Inverter testing using old CFL:
                        http://www.badcaps.net/forum/showthr...er+testing+cfl

                        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                        http://s807.photobucket.com/user/budm/library/

                        TV Factory reset codes listing:
                        http://www.badcaps.net/forum/showthread.php?t=24809

                        Comment


                          #13
                          Re: Motor run capacitor physical size vs voltage rating & measurements?

                          It's more of a resonant (tank) circuit between the run capacitor and the motor's stator windings inductance - which changes with load (slip). I think the goal is 120 phase-shift at nominal load.

                          I look at back-EMF as the voltage spike from a quick collapse in current flow through something inductive. A loudspeaker also acts as a generator though due to the moving cone and the magnet. Good power amps have the clamp diodes to prevent damage from that.

                          Comment


                            #14
                            Re: Motor run capacitor physical size vs voltage rating & measurements?

                            budm: Thanks for that link, yes I understood why you where after the center tap point.

                            redwire: It did not trip a pressure switch, the motor was operating ok-ish pumping coolant liquid albeit at visibly reduced flow and pressure with the dead cap when it drew ~5A.
                            With the working cap and drawing 2.7A the coolant flow and pressure is better but above all stable / consistent.
                            At no point was the motor run/tested without a load, the pump was always submerged in coolant liquid.
                            "The one who says it cannot be done should never interrupt the one who is doing it."

                            Comment

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