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question for the induction motor gurus

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  • #31
    You can calculate the capacitive reactance to determine the current. Use an on-line calculator or use the formula X(c) = 1 / (2 * PI * f * C). For 70 uF and 60 Hz it is about 38 ohms, so at 120 VAC that would be about 3.1 amps. That is maximum - once the motor is running, back EMF will reduce the effective voltage on the capacitor, and thus lower current. The 8 uF capacitor will have reactance of 331 ohms, so less than 1/2 amp. I think the start or PSC winding might be the one with higher resistance. It would likely have more turns so the ampere-turns will be greater, thus supplying more starting and running torque.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #32
      OK , so it's the same formula for a motor, cool! That makes it a bit easier.
      I know about the clamp meters but it's definitely not in the budget for a while yet. I have a Fluke 117 and a Simpson 260 available in service right now, but my stock of shop supplies such as test leads, clips, and fasteners has gone to zero over the last few years, so I'm stuck with whatever I can scrounge for now. The tax return is making me drool but it isn't here yet.
      25 miles north of Buffalo NY, USA

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      • #33
        That higher resistance winding could be right for a PSC, but washing machines usually need more torque than PSC motors supply on starting.

        No harm in trying the other winding, but it would be good to have the current reading and compare with full-load current (marked on it) to see if it makes sense. Be good to have the existing current as-is, to see if THAT makes better sense.

        I'd expect to see about 40% of max load current (FLA, Full Load Amperes) with the motor just running unloaded. One of the windings will show that, the other will show either a goodly amount more, or a goodly amount less, depending on which winding is start, and which is run.

        I might have an extra Amprobe meter laying around, I'll have to look.
        Last edited by J Tiers; 03-19-2020, 09:28 PM.
        CNC machines only go through the motions.

        Ideas expressed may be mine, or from anyone else in the universe.
        Not responsible for clerical errors. Or those made by lay people either.
        Number formats and units may be chosen at random depending on what day it is.
        I reserve the right to use a number system with any integer base without prior notice.
        Generalizations are understood to be "often" true, but not true in every case.

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        • #34
          Wouldn't it pull the max amps anyway if the rotor wasn't spinning? I thought it would... could well be wrong tho. FWIW the winding wire could give a clue if you want I can get a measurement of the diameter. It looks like the same stuff they used to wind auto alternators with, so I would say 14 ga as a guess.

          And why would it sound like a switch inside?? is there one buried all the way inside the rotor??
          I am having bionic ears implanted, they pick up everything.. I have one already installed and waiting for the 2nd one
          Last edited by nickel-city-fab; 03-19-2020, 09:37 PM.
          25 miles north of Buffalo NY, USA

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          • #35
            Originally posted by nickel-city-fab View Post
            Wouldn't it pull the max amps anyway if the rotor wasn't spinning? I thought it would... could well be wrong tho. FWIW the winding wire could give a clue if you want I can get a measurement of the diameter. It looks like the same stuff they used to wind auto alternators with, so I would say 14 ga as a guess.

            And why would it sound like a switch inside?? is there one buried all the way inside the rotor??
            I am having bionic ears implanted, they pick up everything.. I have one already installed and waiting for the 2nd one
            if the rotor is NOT spinning, it is in the "locked rotor" condition, and will usually pull somewhere around 4x to 7x rated amps.

            As for the switch sound, it would make LOTS OF SENSE if it had a switch. The switch would be in series with the start winding, and if it has a working start switch, you can connect a regular start capacitor and have a perfectly fine motor, maybe.

            If you can see the switch, that would surely assist in determining what to do with it, and would positively ID the start winding. That would be the one that measures very high resistance when the switch is open, which it would be as soon as the motor gets up to maybe 65% of rated speed.

            The start switch is closed when the motor is not turning, and a set of weights on the rotor move outward by centrifugal force when the motor speeds up, which shifts a "collar" and lets the switch open. That is set up to go "over center" so that it "snaps" open and also probably "snaps" closed when it slows down.
            CNC machines only go through the motions.

            Ideas expressed may be mine, or from anyone else in the universe.
            Not responsible for clerical errors. Or those made by lay people either.
            Number formats and units may be chosen at random depending on what day it is.
            I reserve the right to use a number system with any integer base without prior notice.
            Generalizations are understood to be "often" true, but not true in every case.

            Comment


            • #36
              Could I learn something by hooking up the ohm meter and spin the shaft with a drill? See which winding goes "open"? Just looking inside the frame I can't imagine how they got a switch in there...
              25 miles north of Buffalo NY, USA

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              • #37
                You would need around 1200 rpm to open that switch if there is one in there.
                CNC machines only go through the motions.

                Ideas expressed may be mine, or from anyone else in the universe.
                Not responsible for clerical errors. Or those made by lay people either.
                Number formats and units may be chosen at random depending on what day it is.
                I reserve the right to use a number system with any integer base without prior notice.
                Generalizations are understood to be "often" true, but not true in every case.

                Comment


                • #38
                  Originally posted by J Tiers View Post
                  You would need around 1200 rpm to open that switch if there is one in there.
                  The mini lathe claims 2500 RPM. I shifted it into high range and did some simple measurements... the beast of a motor just fits if I remove the tailstock. Will spin it up tomorrow and report back (getting late here)


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                  25 miles north of Buffalo NY, USA

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                  • #39
                    I bow to your mastery of the "kludge", sir.

                    I thought I was pretty good at the "lashed-up test rig", but it would not have occurred to me that it would fit on the minilathe...!

                    May I suggest a piece of cereal box between the motor and the bed?
                    CNC machines only go through the motions.

                    Ideas expressed may be mine, or from anyone else in the universe.
                    Not responsible for clerical errors. Or those made by lay people either.
                    Number formats and units may be chosen at random depending on what day it is.
                    I reserve the right to use a number system with any integer base without prior notice.
                    Generalizations are understood to be "often" true, but not true in every case.

                    Comment


                    • #40
                      Originally posted by J Tiers View Post
                      I bow to your mastery of the "kludge", sir.

                      I thought I was pretty good at the "lashed-up test rig", but it would not have occurred to me that it would fit on the minilathe...!

                      May I suggest a piece of cereal box between the motor and the bed?
                      hehehehehe not so sure its mastery so much as desperation Good idea on the cardboard. There is just enough gap (about the thickness of a finger) that I don't want anything bashing the bed. I consider the lathe itself to be junk, but the ways are like new.
                      25 miles north of Buffalo NY, USA

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                      • #41
                        Hrmmm... can't do that, spinning it up in the lathe. I didn't like the way the whole setup was jumping around, so I abandoned that idea. Will look into increasing the capacitance.
                        25 miles north of Buffalo NY, USA

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                        • #42
                          Originally posted by nickel-city-fab View Post
                          ???

                          I have a good motor from my old washing machine, 1/2 HP 1725 RPM. There is no start or run cap inside, nor is there any provision for one.
                          Someone educate me here?

                          ???
                          Weirdo! Why do we tear machines apart to get the guts?

                          Those motors are multi wound I think. Good motor just put a meter on it, HF is giving them away, and they work.

                          Wanna see some HP digital measuring tools. Yup, right here in my house.

                          And some very nice HF mech and dig volt ohm meters!! I like free. And they measure well.. JR

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