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Start Capacitor for A.0. Smith Motor

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  • Start Capacitor for A.0. Smith Motor

    My old Craftsman 33 gallon hot dog compressor went on the fritz ... whats happening is it's tripping the breaker on start up . I narrowed the problem down to the start capacitor a 136 - 163 Mf measuring 210 Mf.

    I ordered one a Bo jack 135-162 Mf from Amazon .. My question is the old one has a 15 K ohms resistor across both terminals , it measures 14.85 K within the 5 % tolerance band . Should I solder the resistor to the new capacitor which comes without one ? .



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  • #2
    That's a bleed resistor isn't it intended to drain the cap when the unit it powered down.
    Yes, I would say move the resistor over to the new cap or get a new resistor of the same value and wattage.
    "It's a table saw, do you know where your fingers are?" ? Bob D. 2004
    "?ǝɹɐ sɹǝƃuıɟ ɹnoʎ ǝɹǝɥʍ ʍouʞ noʎ op `ʍɐs ǝlqɐʇ ɐ s,ʇı"

    https://www.youtube.com/user/PowerToolInstitute
    http://www.cordlessworkshop.net
    https://youtube.com/@bobd.
    ----

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    • #3
      Is BoJack a good quality brand for capacitors? I'm planning to buy a spare run capacitor for my A/C and don't know the good brands to choose from.

      Comment


      • drainman scott
        drainman scott commented
        Editing a comment
        I only got the BoJack because that's all I could find with the values and dimensions I needed ... Titan HD seems to be popular among some of the HVAC You Tube channels I subscribe to and is made in USA .

      • AverageHomeowner
        AverageHomeowner commented
        Editing a comment
        Thanks for input! I searched around and Titan HD does indeed appear to be the best value with regard to quality and price. AmRad is another brand that is regarded highly but more pricey.

      • blue_can
        blue_can commented
        Editing a comment
        Never heard of BoJack. Titan HD has a good reputation as does AmRad. I believe the Turbo caps from AmRad are good but pricey. I sometimes keep one in my toolkit since they can be configured to different values it saves carrying a stack of caps of different values and can get a system running for customer right away. I usually pass the cost of the turbo caps on to the customer. But if you already know the value you are probably better off with something cheaper. Another thing to consider is the age of your system - not much point spending money on an expensive cap for a old system that does not have too many years left.

    • #4
      Originally posted by Bob D. View Post
      That's a bleed resistor isn't it intended to drain the cap when the unit it powered down.
      Yes, I would say move the resistor over to the new cap or get a new resistor of the same value and wattage.
      Definitely a good idea to reinstall on new cap ... After some research a bleed resistor's main objective on a start cap is to prevent arcing of the switch contacts prolonging life a motor .

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      • #5
        When testing capacitor with a bleed resistor attached it will give you a high ( false ) reading you must detach one lead to get a true reading . I found out the hard way and ordered a new one when old one was good .

        Right now I'm waiting on the arrival of a new start switch . The old one I tried to sand down the contacts and bent one of the contact tangs . I'll continue this thread with my findings for someone who might gain some insight if there air compressor goes on the fritz ( slow startup, trips breaker ) .

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        • #6
          Just out of curiosity, do you know what caused the start capacitor to fail? A bad relay?

          Comment


          • drainman scott
            drainman scott commented
            Editing a comment
            The start switch being stuck close would cause a failure of start cap ... sometimes it's the failure of the centrifugal clutch or thermistor start switch is at fault.

        • #7
          I installed new start switch still no go ... I tried to find a wiring diagram for the A O Smith motor no luck ... I still have my original manuals for the 21 years old compressor but nothing there either .


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          What did next was to attempt to measure the resistance of the start winding and run winding ... The resistance of motor ground to earth ground ... and resistance of L1 to L2 power going in to motor ... my findings .

          Blue common to Red 1.2 ohms start winding

          Blue to purple off run cap 1.2 ohms run winding

          L1 to L2 .4 ohms

          motor ground to earth ( used ground plug on cord ) .5 ohms ( good )

          So appears I have short in the motor ... I believe start winding should have more resistance that run ... and L1 to L2 should have high resistance ... in which .5 ohms would be a short between them .


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          • #8
            I think what cause the short was my ground on the plug ... when I disconnected plug from outlet the ground stayed in outlet ... I replaced the end .. but damage was already done .



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            • #9
              Originally posted by drainman scott View Post
              So appears I have short in the motor ... I believe start winding should have more resistance that run ... and L1 to L2 should have high resistance ... in which .5 ohms would be a short between them .
              Depends on the type of motor - PSC type motors that keep the start and run windings operating at all times during operation can have resistances very similar to each other.

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              • #10
                Originally posted by blue_can View Post

                Depends on the type of motor - PSC type motors that keep the start and run windings operating at all times during operation can have resistances very similar to each other.
                I have a capacitor start capacitor run induction motor I guess you could call it 2 phase because the start winding is 90 degree's from run ... I don't know that the resistance being about the same for start /run windings are normal for that motor ... I would need the specs and that will never happen . The horse power is not on the motor but rather boosted by Craftsman so no idea for a replacement motor.

                I really don't use it that often so I will put it on the back burner with some other tools with motor problems .. I am sure there are no shorts to ground .


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                • #11
                  Matthias YouTube School Of Electric Motors





                  See his channel list of videos for more on motors.

                  https://www.youtube.com/playlist?lis...-fnsHV-x_j6X-g
                  Last edited by Bob D.; 10-12-2022, 03:07 PM.
                  "It's a table saw, do you know where your fingers are?" ? Bob D. 2004
                  "?ǝɹɐ sɹǝƃuıɟ ɹnoʎ ǝɹǝɥʍ ʍouʞ noʎ op `ʍɐs ǝlqɐʇ ɐ s,ʇı"

                  https://www.youtube.com/user/PowerToolInstitute
                  http://www.cordlessworkshop.net
                  https://youtube.com/@bobd.
                  ----

                  Comment


                  • #12
                    Originally posted by drainman scott View Post

                    I have a capacitor start capacitor run induction motor I guess you could call it 2 phase because the start winding is 90 degree's from run ... I don't know that the resistance being about the same for start /run windings are normal for that motor ... I would need the specs and that will never happen . The horse power is not on the motor but rather boosted by Craftsman so no idea for a replacement motor.

                    I really don't use it that often so I will put it on the back burner with some other tools with motor problems .. I am sure there are no shorts to ground .


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                    The main difference between this and a PSC is that it also has a start cap which drops out once the motor is up to speed - but the run cap supplies continuous current to the start winding but shifted 90 degrees out of phase. Since both windings remain in operation I would expect the resistance to be similar so I don't see both being the same as an issue. But specs would help for sure.

                    Motors that have a start winding that is only used during starting and not during running are typically much weaker than the run winding and there you will typically see higher resistances.

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