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RPC run capacitor balancing question?

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  • RPC run capacitor balancing question?

    Hello Group,
    Got my RPC up and running tonight and now need to ask a question? I have a 7.5 hp idler and read 240 between L1, L2 and 208 between L1, L3 and 192 between L2, L3 with no load turned on. L3 is my produced leg. I know that I need to size the run cap's for the larger load in the shop for best balance. So far I have 10mf and 15mf cap's at 370v That I'm going to start with.
    Now to my real question. How do you know what size cap to put with which leg? Is it as simple as just try one and read the voltage? I also found out tonight I need a bleed resistor across the cap's, what size and wattage do those need to be?

    A newbie here on this one. Done some reading and searches on this forum with limited success. But as you all know the internet has all the right answers so I'm well prepared.

    Look forward to your responses.

    TX
    Mr fixit for the family
    Chris

  • #2
    There is an old but popular original by Fitch Williams here, look for the FRW files.
    http://www.mwdropbox.com/dropbox/_1998_retired_files/
    Max.

    Comment


    • #3
      Read this and follow it. Easy.

      http://www.practicalmachinist.com/FitchWConverter.pdf

      Comment


      • #4
        Chris

        First about 100,000 ohms at 1 or 2 watts for a drain resistor

        With all due respects for the fine article mentioned by Max and Lakeside , I have the following comment
        I have built over 60 RPC's in my life and I would say to forget your voltage readings.
        You want "AMPS"
        That's what does the work.
        Voltage is only "Potential" , not HP
        Balance the legs with amp readings, as Voltage numbers will drive you nuts
        I use 15 MF per HP as a start and go from there.
        I found motors to vary tremendously and you will be tuning to them specifically
        Older motors with more mass are easier. Some of the "efficient' motors of late are NOT easy !
        If you want to fine tune , Select the most prevalent load and use it.
        Do not try to do both low and high speeds on your lathe for example, it will be different .
        Then , you can use or add a higher voltage Cap ( 600 V) as that is really less capacity at 370 volts.
        Lets say you have a 10 Mf at 370 and need 2 more MF, well a 4 Mf @600v will work great to add the small amount needed
        So you can wire in parallel,to get the added capacity
        If this is a RPC , you can look at Volts, but as soon as you drop your slave motor in the circuit, it will change
        Then you can add Caps to the slave motor to be rebalanced. This is especially needed if the RPC runs multiple motors either single or at the same time

        Not trying to be the fly in the ointment but it is not as complicated as some believe it to be.

        Rich

        Edit
        PS For starting , use 100 MF per HP
        I have used , amp relays,voltage relays, pneumatic relays, push buttons and timer relays (0-5 sec)
        By far , the most consistent , reliable and trouble free are timing relays with a contactor...hands down
        Last edited by Rich Carlstedt; 11-18-2014, 11:50 PM.

        Comment


        • #5
          Why do you think a capacitor's rating is based on it's voltage rating or am I reading what you wrote incorrectly? a 4mf 600v capacitor is 4mf at 10 or 600v.

          Comment


          • #6
            Originally posted by Rich Carlstedt View Post
            Chris

            First about 100,000 ohms at 1 or 2 watts for a drain resistor

            With all due respects for the fine article mentioned by Max and Lakeside , I have the following comment
            I have built over 60 RPC's in my life and I would say to forget your voltage readings.
            You want "AMPS"
            That's what does the work.
            Voltage is only "Potential" , not HP
            Balance the legs with amp readings, as Voltage numbers will drive you nuts
            I use 15 MF per HP as a start and go from there.
            I found motors to vary tremendously and you will be tuning to them specifically
            Older motors with more mass are easier. Some of the "efficient' motors of late are NOT easy !
            If you want to fine tune , Select the most prevalent load and use it.
            Do not try to do both low and high speeds on your lathe for example, it will be different .
            Then , you can use or add a higher voltage Cap ( 600 V) as that is really less capacity at 370 volts.
            Lets say you have a 10 Mf at 370 and need 2 more MF, well a 4 Mf @600v will work great to add the small amount needed
            So you can wire in parallel,to get the added capacity
            If this is a RPC , you can look at Volts, but as soon as you drop your slave motor in the circuit, it will change
            Then you can add Caps to the slave motor to be rebalanced. This is especially needed if the RPC runs multiple motors either single or at the same time

            Not trying to be the fly in the ointment but it is not as complicated as some believe it to be.

            Rich

            Edit
            PS For starting , use 100 MF per HP
            I have used , amp relays,voltage relays, pneumatic relays, push buttons and timer relays (0-5 sec)
            By far , the most consistent , reliable and trouble free are timing relays with a contactor...hands down
            Rich has it right!

            And there is no sense in balancing anything unless the machine tool is under typical load.

            Current! Voltage means squat!

            Comment


            • #7
              AFAIK the capacitance does not depend on the voltage rating. You may be thinking about the amount of energy that can be stored, which is linear with capacitance but goes by the square of the voltage. Polypropylene capacitors seem to be very good and have very low temperature coefficients and are often self-healing. I don't have much experience with using capacitors to generate a third leg for a three phase motor, but you might estimate the value by using the phase voltage (240 VAC) and the expected current draw of the load (about 3 amps per HP). This is an impedance (reactance) of 240/3=80 ohms, and X(c) = 1/(2*PI*f*C). For 60 Hz, C = 1/(6.28*60*80) = 33 microfarads. Rich's starting point of 15 uF/HP is in the same ballpark. The actual voltage and phase angle will change depending on the load, and once the "idler" motor is running, it will help stabilize the values, especially if it has a bit of a flywheel connected to the shaft.
              http://pauleschoen.com/pix/PM08_P76_P54.png

              Paul: www.peschoen.com
              P S Technology, Inc. www.pstech-inc.com
              and Muttley www.muttleydog.com

              Comment


              • #8
                Originally posted by lakeside53 View Post
                Why do you think a capacitor's rating is based on it's voltage rating or am I reading what you wrote incorrectly? a 4mf 600v capacitor is 4mf at 10 or 600v.
                It does not act like a 4 Mf at 370 volts. It acts smaller

                Explanation:
                Think of it as a rubber bladder and water. A rubber bladder made to handle 4 gallons at 600 PSI , will be smaller at 370 PSI and not have the volume
                A 4 gallon 370 PSI rubber bladder will have 4 gallons at full pressure , but will explode above that as the rubber will not stretch any more.
                Caps are not rigid containers

                Rich

                Comment


                • #9
                  Sorry.... I don't buy that explanation at all. Capacitance is a constant not affected by the applied voltage.
                  Last edited by lakeside53; 11-19-2014, 09:01 PM.

                  Comment


                  • #10
                    Originally posted by lakeside53 View Post
                    Sorry.... I don't buy that explanation at all. Capacitance is a constant not affected by the applied voltage.
                    +1 100% completely.
                    Max.

                    Comment


                    • #11
                      Originally posted by Rich Carlstedt View Post
                      It does not act like a 4 Mf at 370 volts. It acts smaller

                      Explanation:
                      Think of it as a rubber bladder and water. A rubber bladder made to handle 4 gallons at 600 PSI , will be smaller at 370 PSI and not have the volume
                      A 4 gallon 370 PSI rubber bladder will have 4 gallons at full pressure , but will explode above that as the rubber will not stretch any more.
                      Caps are not rigid containers

                      Rich
                      Ummmm............ NO!

                      You are definitely thinking of capacitors rated in kVA. THOSE are a particular value at a particular voltage. They WILL NOT pull the same kVA at a different voltage.

                      But a 4uF is 4uF no matter what.

                      And, "farads" which is the measure of capacitance, are defined as 1F is 1 coulomb of charge stored per volt applied. 2 volts? 2 coulombs. It is a relation between charge and voltage, so it is a constant (more or less) at any voltage within the rating.
                      Last edited by J Tiers; 11-19-2014, 09:11 PM.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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