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  • 3 h.p. Single Phase Compressor motor question

    Dayton 3 h.p. compressor motor, relay clicks when it attempts to start, but no go. On the motor, leads T2, T3 and T8 are tied together, but not to anything else. T4 and T5 go to one side of the source, with T! to the other. I see 220 v. in and out of the relay and at all three of the motor connections.

    I unloaded all pressure, still no go. Could this be the start capacitor only?

    Thanks,

    Dan L


    Salem, Oregon

  • #2
    The capacitor is usually a good bet.

    Does the motor hum?

    More to the point, is it drawing current?

    You can take off the belts, and give it a spin in he correct direction, if it runs, yes it is likely to be the capacitor.

    You can measure the capacitor value if your digital multimeter has a capacitance function. Many don't measure as high a value as the one you have, though.

    For a quick test, set multimeter to ohms. First discharge the capacitor. It is probably discharged already, but discharging with a low value power resistor is a good plan. You can first check for voltage with the multimeter on volts, and if only a low voltage, short with a piece of wire, screwdriver, etc.

    When discharged, set to ohms, measure across the capacitor as if you were measureing resistance. It SHOULD measure low, and then keep climbing up. With that value of part, it may take a while if the part is good. Try both ways. This is not a "measurement", but an "indication". If the part is good it will take at least a few seconds to go from a low value to a high value.

    If you are worried about the motor, you can use the multimeter to check resistance.

    If I were doing it, I would check to make sure the motor was drawing current, with a clamp-on meter, or just by hearing the hum. If so I would use a capacitor checker. But if I didn't have one handy I would use a multimeter as a rough check.

    If I had one handy the right size, I'd probably just replace the part. Faster than over-analyzing the situation

    If the motor was not drawing current I'd check connections, make sure the contactor was making a good connection through, and verify the motor connections were good with the ommeter function.
    2730

    Keep eye on ball.
    Hashim Khan

    Everything not impossible is compulsory

    Comment


    • #3
      Thanks for responding, Jerry.

      My clamp meter went south on me, so can't check current.

      I'm reading less than 2 ohms across the different connections to the motor itself. No hum in the motor when activated.

      The start capacitor climbs from low to high within a few seconds, which may be an indicator that it is healthy.


      Dan
      Salem, Oregon

      Comment


      • #4
        OK, did you check continuity?

        With power off, contactor open, is there low resistance from line one to line 2 ON THE MOTOR SIDE of the contactor? Meaning "through the motor"... The motor has to be a complete circuit in order for the thing to draw current and hum.

        Contactors are not immortal. Neither are motors, start switches in motors, nor the devices that make wire connections.

        While checking continuity. also check the wires that go to the capacitor.
        2730

        Keep eye on ball.
        Hashim Khan

        Everything not impossible is compulsory

        Comment


        • #5
          Another thing to look at is the start relay. It replaces the mechanical centrifugal switch. It has the contacts but instead of weights it has a relay coil in series with the motor that only sees enough current to close when the motor starts. If that coil is open the motor never sees any power

          Sent from my SM-G900V using Tapatalk

          Comment


          • #6
            A simple capacitor check can be performed with a 9V or 12V battery, and a 1k to 10k resistor. Charge the capacitor from the battery through the resistor. In a period of time equal to the RC time constant, the capacitor should charge to about 2/3 the source voltage. So for the 1000 uF starting capacitor and a 10k resistor, with a 9 volt battery, it should charge to about 6 volts in 10 seconds. In 50 seconds it should be charged to the full 9V. Allow the capacitor to remain charged for a few minutes and measure again. This will give an idea of the self-discharge leakage. You can also discharge it through the same resistance and it should drop to about 1/3 voltage in the TC (10 seconds for 1000 uF and 10k). This should give a pretty good idea of the capacitor's state of health, although it may have problems at working voltage.
            http://pauleschoen.com/pix/PM08_P76_P54.png
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030

            Comment


            • #7
              Originally posted by 6PTsocket View Post
              Another thing to look at is the start relay. It replaces the mechanical centrifugal switch. It has the contacts but instead of weights it has a relay coil in series with the motor that only sees enough current to close when the motor starts. If that coil is open the motor never sees any power

              Sent from my SM-G900V using Tapatalk
              Right you are! Here is my relay, the left side of which is apparently shot:
              I think this is very old, possibly called a "Maxstar" years ago.

              I've bypassed it temporarily, but am wondering what a replacement would look like....

              Dan
              Salem, Oregon

              Comment


              • #8
                Actually a motor that size uses a potential or voltage relay that picks up the voltage across the start winding and drops out the relay/start cap. I forget all the details and Jerry can fill in what I missed.
                That is not a potential or starting relay but a contactor or heavy duty relay, in any event it looks shot.
                Last edited by wmgeorge; 01-12-2017, 06:35 PM.
                Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

                Comment


                • #9
                  Originally posted by wmgeorge View Post
                  Actually a motor that size uses a potential or voltage relay that picks up the voltage across the start winding and drops out the relay/start cap. I forget all the details and Jerry can fill in what I missed.
                  That is not a potential or starting relay but a contactor or heavy duty relay, in any event it looks shot.
                  So the start capacitor is not even used in this setup?

                  I'm hoping there is something I can use instead of this old thing, since it does not appear to be repairable.

                  Dan L
                  Salem, Oregon

                  Comment


                  • #10
                    If L1 is shot just move it to L2. You are only using 2 out of 3 contacts now.

                    Comment


                    • #11
                      Originally posted by ww_big_al View Post
                      If L1 is shot just move it to L2. You are only using 2 out of 3 contacts now.
                      I'll try it, thanks.
                      Salem, Oregon

                      Comment


                      • #12
                        That appears to be a thermal motor overload relay, with two "heaters" (the spring-shaped elements), that trip the relay if the motor takes too long to start or otherwise draws too much current for some period of time. Such units can be repaired, and it may be that the contacts need to be cleaned up or there may be dust and dirt or even bugs that cause failure. The heaters are also replaceable, although it may be hard to find ones for something so old. They are rated for current and should match the motor FLA (Full Load Amps).

                        I found a similar device on eBay for $75:
                        http://www.ebay.com/itm/ARROW-HART-C...-/291805798913

                        Here is a heater element but I don't know if the rating matches yours:
                        http://www.ebay.com/itm/New-Arrow-Ha...-/191716081292

                        I tried to find a suitable motor starter for your 3 HP compressor, but it's a bit complicated. You may need a 120V or 240V or even 24V coil, and you may need an overload with a contactor. For 3 HP and 240V single phase you need something like 10-15 amps. Here are some items that should work:

                        https://www.automationdirect.com/adc...s/GH15DN-3-10A (16 amp contactor $57)

                        https://www.automationdirect.com/adc...ads/RTD32-1400 (10-14 amp overload $67)
                        http://pauleschoen.com/pix/PM08_P76_P54.png
                        Paul , P S Technology, Inc. and MrTibbs
                        USA Maryland 21030

                        Comment


                        • #13
                          You can check and see but L2 might be a dummy terminal as this is only single phase. The starting relay and cap would be after this contactor. The two spring looking things are overload heating elements.

                          Do a google for potential starting relay. Here is one > http://www.achrnews.com/articles/924...tarting-relays
                          Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

                          Comment


                          • #14
                            If you're not a Chinese hater, you can pick up the whole shebang for under 60 bucks with no shipping.

                            http://www.ebay.com/itm/MAGNETIC-MOT...-/321134649990

                            Comment


                            • #15
                              Originally posted by wmgeorge View Post
                              You can check and see but L2 might be a dummy terminal as this is only single phase. The starting relay and cap would be after this contactor. The two spring looking things are overload heating elements.
                              You may be right. Never thought about it being a dummy terminal.
                              Might want to check that Red wire. It looks like it's been smoked.

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