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How to determine mains harmonics?

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  • Dave C
    A synchronous, single-phase 120vac, 3 hp motor is running very hot. The mechanical load is very simple: a chain pulling a hammer up a meter and dropping it. A
    Message 1 of 11 , Jul 5, 2014
      A synchronous, single-phase 120vac, 3 hp motor is running very hot. The mechanical load is very simple: a chain pulling a hammer up a meter and dropping it. A couple of sprockets and a chain, well lubricated and easy to turn by hand. The motor has no physical problems—the bearings are fine and the armature spins freely.

      The supply voltage and mains wiring is sufficient, even over-size.

      So I’m to the point at which I’m suspecting mains harmonics that may be heating the windings.

      How can I confirm or eliminate this as a cause? Can harmonics be easily identified with a scope?

      Thanks,
      Dave
    • John Popelish
      On 07/05/2014 09:17 PM, Dave C davec2468@yahoo.com ... Harmonics are pretty well filtered by the inductance of the motor. I think you would have to have
      Message 2 of 11 , Jul 5, 2014
        On 07/05/2014 09:17 PM, Dave C davec2468@...
        [Electronics_101] wrote:
        > A synchronous, single-phase 120vac, 3 hp motor is running
        > very hot. The mechanical load is very simple: a chain
        > pulling a hammer up a meter and dropping it. A couple of
        > sprockets and a chain, well lubricated and easy to turn
        > by hand. The motor has no physical problems—the bearings
        > are fine and the armature spins freely.
        >
        > The supply voltage and mains wiring is sufficient, even
        > over-size.
        >
        > So I’m to the point at which I’m suspecting mains
        > harmonics that may be heating the windings.
        >
        > How can I confirm or eliminate this as a cause? Can
        > harmonics be easily identified with a scope?

        Harmonics are pretty well filtered by the inductance of the
        motor. I think you would have to have something like a
        square wave to get serious heating.

        Are you certain that the motor is wired correctly? How is
        it started? 3HP is pretty huge for 120 VAC (34 amps?). I
        would think that it is wired for some other voltage than
        120, or the starting mechanism is running all the time, when
        it should be switched off.

        Also, you said synchronous. Are you sure it is not a
        squirrel cage induction motor? What is the name plate speed
        rating?.

        --
        Regards,

        John Popelish
      • harry jenkins
        I agree, 3 hp is huge for a 120 VAC line, even 2 hp is pushing it.  To get 2 hp out of a standard 120 volt line you have to stall the motor and get maximum
        Message 3 of 11 , Jul 5, 2014
          I agree, 3 hp is huge for a 120 VAC line, even 2 hp is pushing it.  To get 2 hp out of
          a standard 120 volt line you have to stall the motor and get maximum torque from it.
          And nobody is going to run a motor like that.  Any device that runs on 120 and has
          a 2 hp motor like an air compressor is using advertising hype,  you can't really get that
          much power from standard wiring.  And the current is  high enough to make line
          resistance a factor.



          On Saturday, July 5, 2014 6:35 PM, "John Popelish jpopelish@... [Electronics_101]" <Electronics_101@yahoogroups.com> wrote:


          On 07/05/2014 09:17 PM, Dave C davec2468@...
          [Electronics_101] wrote:
          > A synchronous, single-phase 120vac, 3 hp motor is running
          > very hot. The mechanical load is very simple: a chain
          > pulling a hammer up a meter and dropping it. A couple of
          > sprockets and a chain, well lubricated
          and easy to turn
          > by hand. The motor has no physical problems—the bearings
          > are fine and the armature spins freely.
          >
          > The supply voltage and mains wiring is sufficient, even
          > over-size.
          >
          > So I’m to the point at which I’m suspecting mains
          > harmonics that may be heating the windings.
          >
          > How can I confirm or eliminate this as a cause? Can
          > harmonics be easily identified with a scope?

          Harmonics are pretty well filtered by the inductance of the
          motor. I think you would have to have something like a
          square wave to get serious heating.

          Are you certain that the motor is wired correctly?  How is
          it started?  3HP is pretty huge for 120 VAC (34 amps?).  I
          would think that it is wired for some other voltage than
          120, or the starting mechanism is running all the time, when
          it should be switched off.

          Also, you said synchronous.  Are you sure it is not a
          squirrel cage induction motor?  What is the name plate speed
          rating?.

          --
          Regards,

          John Popelish



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        • Dave C
          ItÆs async motor (typo). Re. HP, IÆll double check. The label was hard to see so IÆll go look again. Thanks, Dave -=-=-=-
          Message 4 of 11 , Jul 5, 2014
            It’s async motor (typo).

            Re. HP, I’ll double check. The label was hard to see so I’ll go look again.

            Thanks,
            Dave

            -=-=-=-

            On 5 Jul 2014, at 6:35 PM, John Popelish jpopelish@... [Electronics_101] <Electronics_101@yahoogroups.com> wrote:

            > On 07/05/2014 09:17 PM, Dave C davec2468@...
            > [Electronics_101] wrote:
            >> A synchronous, single-phase 120vac, 3 hp motor is running
            >> very hot. The mechanical load is very simple: a chain
            >> pulling a hammer up a meter and dropping it. A couple of
            >> sprockets and a chain, well lubricated and easy to turn
            >> by hand. The motor has no physical problems—the bearings
            >> are fine and the armature spins freely.
            >>
            >> The supply voltage and mains wiring is sufficient, even
            >> over-size.
            >>
            >> So I’m to the point at which I’m suspecting mains
            >> harmonics that may be heating the windings.
            >>
            >> How can I confirm or eliminate this as a cause? Can
            >> harmonics be easily identified with a scope?
            >
            > Harmonics are pretty well filtered by the inductance of the
            > motor. I think you would have to have something like a
            > square wave to get serious heating.
            >
            > Are you certain that the motor is wired correctly? How is
            > it started? 3HP is pretty huge for 120 VAC (34 amps?). I
            > would think that it is wired for some other voltage than
            > 120, or the starting mechanism is running all the time, when
            > it should be switched off.
            >
            > Also, you said synchronous. Are you sure it is not a
            > squirrel cage induction motor? What is the name plate speed
            > rating?.
            >
            > --
            > Regards,
            >
            > John Popelish
            >
            >
            >
            > ------------------------------------
            >
            > ------------------------------------
            >
            > Please trim excess when replying
            > ------------------------------------
            >
            > Yahoo Groups Links
            >
            >
            >
          • Dave C
            WhatÆs the simplest way to determine if the start winding centrifugal switch is stuck closed? Current draw? Orà? Thanks, Dave -=-=-=- ... -=-=-=-
            Message 5 of 11 , Jul 6, 2014
              What’s the simplest way to determine if the start winding centrifugal switch is stuck closed? Current draw? Or…?

              Thanks,
              Dave

              -=-=-=-

              > On 07/05/2014 09:17 PM, Dave C davec2468@...
              > [Electronics_101] wrote:
              >> A synchronous, single-phase 120vac, 3 hp motor is running
              >> very hot. The mechanical load is very simple: a chain
              >> pulling a hammer up a meter and dropping it. A couple of
              >> sprockets and a chain, well lubricated and easy to turn
              >> by hand. The motor has no physical problems—the bearings
              >> are fine and the armature spins freely.
              >>
              >> The supply voltage and mains wiring is sufficient, even
              >> over-size.
              >>
              >> So I’m to the point at which I’m suspecting mains
              >> harmonics that may be heating the windings.
              >>
              >> How can I confirm or eliminate this as a cause? Can
              >> harmonics be easily identified with a scope?

              -=-=-=-

              > Harmonics are pretty well filtered by the inductance of the
              > motor. I think you would have to have something like a
              > square wave to get serious heating.
              >
              > Are you certain that the motor is wired correctly? How is
              > it started? 3HP is pretty huge for 120 VAC (34 amps?). I
              > would think that it is wired for some other voltage than
              > 120, or the starting mechanism is running all the time, when
              > it should be switched off.
              >
              > Also, you said synchronous. Are you sure it is not a
              > squirrel cage induction motor? What is the name plate speed
              > rating?.
              > Regards,
              > John Popelish
            • John Popelish
              On 07/06/2014 10:07 PM, Dave C davec2468@yahoo.com ... You should be able to hear the switch snap, if the motor is running unloaded. You should also see a
              Message 6 of 11 , Jul 6, 2014
                On 07/06/2014 10:07 PM, Dave C davec2468@...
                [Electronics_101] wrote:
                > What’s the simplest way to determine if the start winding
                > centrifugal switch is stuck closed? Current draw? Or…?

                You should be able to hear the switch snap, if the motor is
                running unloaded.

                You should also see a sudden drop in line current, following
                the initial starting surge, as the motor reaches full speed.
                I think I would run the motor, unloaded, at quite reduced
                voltage, to slow the starting process, so I could watch the
                current change more visibly. Do you have a big 12 or 24
                volt transformer you could use for this purpose? Maybe from
                a 10 amp battery charger? You may have to give the motor a
                nudge to start it turning, but it should come up to speed in
                a few 10s of seconds.

                --
                Regards,

                John Popelish
              • Dave C
                Battery charger? This is an async single phase motor. Dave -=-=-=-
                Message 7 of 11 , Jul 6, 2014
                  Battery charger? This is an async single phase motor.

                  Dave

                  -=-=-=-

                  On 6 Jul 2014, at 7:21 PM, John Popelish jpopelish@... [Electronics_101] <Electronics_101@yahoogroups.com> wrote:

                  > On 07/06/2014 10:07 PM, Dave C davec2468@...
                  > [Electronics_101] wrote:
                  >> What’s the simplest way to determine if the start winding
                  >> centrifugal switch is stuck closed? Current draw? Or…?
                  >
                  > You should be able to hear the switch snap, if the motor is
                  > running unloaded.
                  >
                  > You should also see a sudden drop in line current, following
                  > the initial starting surge, as the motor reaches full speed.
                  > I think I would run the motor, unloaded, at quite reduced
                  > voltage, to slow the starting process, so I could watch the
                  > current change more visibly. Do you have a big 12 or 24
                  > volt transformer you could use for this purpose? Maybe from
                  > a 10 amp battery charger? You may have to give the motor a
                  > nudge to start it turning, but it should come up to speed in
                  > a few 10s of seconds.
                  >
                  > --
                  > Regards,
                  >
                  > John Popelish
                  >
                  >
                  >
                  > ------------------------------------
                  >
                  > ------------------------------------
                  >
                  > Please trim excess when replying
                  > ------------------------------------
                  >
                  > Yahoo Groups Links
                  >
                  >
                  >
                • John Popelish
                  On 07/06/2014 11:01 PM, Dave C davec2468@yahoo.com ... Sorry. I was asking if you have the transformer out of a 10 amp battery charger. That would be heavy
                  Message 8 of 11 , Jul 6, 2014
                    On 07/06/2014 11:01 PM, Dave C davec2468@...
                    [Electronics_101] wrote:
                    > Battery charger? This is an async single phase motor.

                    Sorry. I was asking if you have the transformer out of a 10
                    amp battery charger. That would be heavy enough to slowly
                    spin up the motor, so you could monitor the start winding
                    switching by watching the current draw.

                    --
                    Regards,

                    John Popelish
                  • Yahoo
                    i have a 15A variac. will this be up to the task? Sent from MailWise ... From: John Popelish jpopelish@gmail.com [Electronics_101]
                    Message 9 of 11 , Jul 6, 2014

                      i have a 15A variac. will this be up to the task?

                      Sent from MailWise



                      -------- Original Message --------
                      From: "John Popelish jpopelish@... [Electronics_101]" <Electronics_101@yahoogroups.com>
                      Sent: Sunday, July 6, 2014 09:08 PM
                      To: Electronics_101@yahoogroups.com
                      Subject: Re: [Electronics_101] How to determine mains harmonics?

                    • John Popelish
                      On 07/07/2014 01:20 AM, Yahoo davec2468@yahoo.com ... That and an AC amp meter should do it. Set the voltage as low as will make the motor slowly accelerate
                      Message 10 of 11 , Jul 7, 2014
                        On 07/07/2014 01:20 AM, Yahoo davec2468@...
                        [Electronics_101] wrote:
                        >
                        >
                        > i have a 15A variac. will this be up to the task?

                        That and an AC amp meter should do it. Set the voltage as
                        low as will make the motor slowly accelerate to full speed.
                        Look for the current to drop, suddenly, after the motor is
                        almost at full speed. That would indicate a centrifugal
                        starting switch operation.

                        --
                        Regards,

                        John Popelish
                      • epa_iii
                        I strongly doubt that line harmonics are at play here. But in answer to your original question, if you are familiar with the proper sine wave shape, then if a
                        Message 11 of 11 , Jul 8, 2014
                          I strongly doubt that line harmonics are at play here. But in answer to your original question, if you are familiar with the proper sine wave shape, then if a large amount of harmonics are present, then you can see them on a scope. They are usually produced by something like clipping, which would show as flat tops on the sine waveform or some kind of difference between the rising and falling sides of that waveform.

                          If you can not see any distortion on a scope, there probably is not enough harmonic content to be of any concern.

                          But I would look at a lot of other factors first. If this is a new installation (you did not say) then perhaps the motor is wired improperly. I cringe at the number of installations that are done on the basis of just hooking up wires and hoping it will work. Many do. Others produce smoke. You may have a shorted winding. You can use an Ohm meter to check for shorts to the frame of the motor. A winding to winding short would be harder to find, if you have another known good motor of the same kind, comparative resistance readings would reveal it. Otherwise, a call to the manufacturer may help. Or a trip to a friendly motor shop.
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