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  • Question for Electricians

    I'm looking at buying a relay for my rotary phase converter. I need a cheap relay to kick a start capacitor in to spin up the idler motor.

    I found one at Surplus Center at it's rated at 30 amps up to 300 volts or 2 hp at 240 volts. Why such a difference? According to the FLA motor chart I'm looking at, 30 amps at 230 volts would be the FLA of a 5 horse motor.

    Anyway, I need to kick in my start capacitor only momentarily but it is for a 10 horse idler motor. I reckon it will have to handle 30-40 amps but for no more than about a second. Any reason I couldn't use this relay? I don't think I would want to use it for a relay that was going to be on for any length of time, but just for a second or two, surely it would be ok?


    I should add that it had 30 amp FLA at 277 and a 75 amp LR at 277 ratings.
    Last edited by Fasttrack; 03-09-2011, 07:07 PM.

  • #2
    Need more information of course, but discrepancy may be a three phase VS. single phase issue...IE. number of legs.

    I think makes and breaks are more important to service life than run time, but you MIGHT get away by running several lines from the capacitor in paralell across the contactor legs.

    Comment


    • #3
      Of course it makes sense that it would be the number of "makes/breaks" that determines the life. I was busy thinking about it fusing, but I'm sure the fusing point is well beyond the rated current

      Here is the relay in question:
      https://www.surpluscenter.com/item.a...tname=electric

      Comment


      • #4
        Arghuahha...I don't know-seems barely acceptable, but I'd rather have a beefier motor starter for ease of connection if nothing else. I don't get along with spade terminals real well, but quick scan through their catalog didn't show anything perfect either.

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        • #5
          Well I'll look on ebay. I found a much beefier contactor that I was going to use as my main contactor for a pretty reasonable cost. Maybe I'll just buy two and then I won't have to worry about it...

          Comment


          • #6
            Full load amperage (FLA) of any motor is still a small fraction of instantaneous inrush current whenever the motor starts. Contact welding can be a problem even when the contacts are rated for the long term current draw.

            Dave

            Comment


            • #7
              Relay

              Check old air conditioning condensing units as they have a good relay for the compressor motor.

              Comment


              • #8
                If all you want is a relay for the start cap,then you don't need 30amps.Your capacitior won't handle more than 5amps and only then it's momentary.

                You need one of these-
                http://compare.ebay.com/like/3703719...=263602_304662

                Around here AC shops and Electrical supplies carry them new for about $15.

                You say kick the capacitor in,how are you going to kick the capacitor out?

                Just a thought,when I build a RPC I drill and tap the shaft and mount a centrifugal switch and use it to control the start circuit.Makes for perfect starts with no timers,push buttons,bi-metal relays etc in the system.

                http://www.torq.com/centrifugal.htm
                I just need one more tool,just one!

                Comment


                • #9
                  A time delay relay would be just fine, set if for a couple of seconds. Dayton makes them, they will have a knob on top for adjusting the time.
                  Last edited by squirrel; 03-09-2011, 11:09 PM.

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                  • #10
                    Originally posted by Fasttrack
                    Well I'll look on ebay. I found a much beefier contactor that I was going to use as my main contactor for a pretty reasonable cost. Maybe I'll just buy two and then I won't have to worry about it...
                    that's what my 10 hp RPC uses, two big beefy contactors...one turns on the motor, the other the start caps....start cap contactor is controlled by a an industrial timer set for a second or two. All bought used for not much money. Surprised you're going to get away with such a puny RPC
                    .

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                    • #11
                      Fastrack, take a look at this site and see if it has anything that interests you.
                      http://www.torq.com/phaseconverters.htm
                      James

                      Edit, should have read the thread first, looks like Weird beat me to it.
                      Last edited by J. Randall; 03-09-2011, 11:48 PM.

                      Comment


                      • #12
                        Originally posted by J. Randall
                        Edit, should have read the thread first, looks like Weird beat me to it.
                        Not to worry,I beat Torq to the idea
                        I just need one more tool,just one!

                        Comment

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