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  • VFD and a CAP

    Will a VFD work with a motor with a capacitor?

    Building a 72x2 belt sander, I found a very high speed motor but it has a capacitor.

  • #2
    No. VFDs are for 3-phase motors, which generally don't have or need a starting capacitor. If your motor has such a thing, it's almost certainly a single phase, and in general likely won't work with any sort of electronic speed control.

    Doc.
    Doc's Machine. (Probably not what you expect.)

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    • #3
      Originally posted by Doc Nickel View Post
      No. VFDs are for 3-phase motors, which generally don't have or need a starting capacitor. If your motor has such a thing, it's almost certainly a single phase, and in general likely won't work with any sort of electronic speed control.

      Doc.
      Negative, a variac can be used to control the speed of a single phase motor, although that centrifugal starter switch will make things interesting. Perhaps your analysis is correct.

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      • #4
        Yes there are VFD's for certain types of single phase motors.

        https://www.wolfautomation.com/blog/vfds-for-single-phase-motors/

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        • #5
          So I am back to looking for a 3 phase motor. Thank you

          Comment


          • #6
            Originally posted by RB211 View Post
            Negative, a variac can be used to control the speed of a single phase motor, although that centrifugal starter switch will make things interesting. Perhaps your analysis is correct.
            What is the differance between triac like on variable corded hand drills and a variac.I was going to use a Luton heavy duty dimmer switch on my Gang Drill setup but read the dimmer switch won't perform like factory trigger switch.

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            • #7
              Variac are just variable transformers. The output is a variable voltage sine wave.

              "Triac" or SCR control for motors changes the average voltage based on trigger phase angle of the sine wave, and adjusts the actual trigger point (and therefore the average voltage) based on load, often by just back emf biasing the gate. Light dimmers function the same way BUT lack any feedback from the motor to adjust the voltage based on load applied. Low speed torque and speed regulation suffers when trying to use a light dimmer type on a motor.

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              • #8
                Originally posted by outlawspeeder View Post
                Will a VFD work with a motor with a capacitor?

                Building a 72x2 belt sander, I found a very high speed motor but it has a capacitor.
                Yes, for certain types. BETTER with a capacitor motor than a standard motor, in some ways.

                It WOULD work for a regular single phase motor, except for the start switch. If you tried to slow down the motor, the switch would close and put it back in start mode. Other than that, no particular problem.

                For a PSC motor, having a capacitor but no centrifugal switch, it works fine, but the VFD must be able to handle the start current for a short time. I've designed VFDs of single phase that did that over the MIL temp range. Had to start up at -40C when pumping JP8.

                Most single-phase VFDs need to start the motor at full speed (rated frequency) to develop good start torque.
                Last edited by J Tiers; 11-28-2017, 02:33 AM.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  We have two votes for "yes, for certain types".

                  What are those certain types; what's the nomenclature for those that work and those that don't?

                  Dan
                  At the end of the project, there is a profound difference between spare parts and extra parts.

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                  • #10
                    If you go to the link in post #4 there is a good list. Here it is:

                    Compatible:

                    Permanent Split Capacitor (PSC)
                    Shade Pole Induction
                    AC Synchronous Motor

                    Incompatible:

                    Capacitor Start
                    Split Case
                    Repulsion Induction
                    Series Universal (AC/DC)
                    Any motor with starting switch (centrifugal or relay)
                    Any motor with separate starting winding
                    Last edited by lakeside53; 11-28-2017, 01:11 PM.

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                    • #11
                      Originally posted by danlb View Post
                      We have two votes for "yes, for certain types".

                      What are those certain types; what's the nomenclature for those that work and those that don't?

                      Dan
                      The "PSC" motor (has a start/run capacitor but no start switch), and the shaded pole motor will work. In fact, any induction motor that does NOT have a speed dependent switch would work. A standard single phase motor would work, except that the switch cuts back in at 60 to 80% of full speed, and messes things up.

                      Essentially, any motor that uses AC, has a speed dependent on frequency, and does not have a speed dependent switch, will work if you can get it started, and it will vary speed. Even a universal motor will work, but the speed will not vary in the same way. (Speed will vary as voltage varies, normally in proportion to frequency, but not BECAUSE OF frequency)

                      It is conventional to say that PSC and shaded pole are the only ones that work, because they are the most common motors that have no start switch. But if the motor runs on AC, which the VFD produces, it will run on a VFD, but it may not vary speed.

                      Now, the VFD must be made, or selected, to be able to supply start current to the motor. A standard VFD has only the current capability to run the motor at full load, it normally cannot supply the extra surge to start a motor at full speed. And, the single phase motors must be started at or near full speed in order that the phase shifting mechanism (capacitor, high resistance winding, pole shading coil) will provide enough phase shift and phase shifted current, to give suitable starting torque.

                      With a 3 phase motor, a 3 phase VFD provides its own phase shifting, and so it can start from a low speed, and have good torque.
                      Last edited by J Tiers; 11-28-2017, 01:18 PM.
                      1601

                      Keep eye on ball.
                      Hashim Khan

                      Comment


                      • #12
                        Originally posted by lakeside53 View Post
                        If you go to the link in post #4 there is a good list. Here it is:
                        Thanks. Somehow I skipped over that post. The forum + firefox combination is often skipping to the wrong spot when I "go to first unread".

                        Dan
                        Last edited by danlb; 11-28-2017, 02:47 PM.
                        At the end of the project, there is a profound difference between spare parts and extra parts.

                        Comment


                        • #13
                          Originally posted by RB211 View Post
                          Negative, a variac can be used to control the speed of a single phase motor, although that centrifugal starter switch will make things interesting. Perhaps your analysis is correct.
                          The one common case where this will work is work is driving a fan. It turns out that the voltage-torque ( constant freq.) curve of an induction motor matches the speed-torque curve of a fan fairly well. So, a fan speed control can be implemented with a variac , rheostat , or a tapped transformer winding. This works best on motors with no start mechanism so max speed range can be obtained.
                          RWO

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                          • #14
                            The key question to ask about speed control, is whether the speed is controlled at "no load". If so, it is a true speed control. If not, it is a "load balancing" speed control, as just mentioned concerning a fan.

                            A "true" speed control will give good consistent control of a lathe , drill, etc. A "load balancing" control will not give good speed control without a feedback mechanism that reports speed do the control circuit.

                            Variac, rheostat, tapped inductor or transformer, etc, are basically "load balancing" as it related to induction motors. Induction motors "want to" run at the speed inherent to the number of poles. The only "true" speed control for an induction motor is one that varies frequency.

                            Some other motors have to use a "load balancing" method, because they have no inherent speed for a given voltage. A shunt DC motor, with constant field (or a PMDC) can give decent regulation, because it has a stable inherent feedback mechanism. But series motors, including "universal" motors, have somewhat of the opposite of speed control as an inherent mechanism They tend to have load-dependent varying speed and torque..
                            Last edited by J Tiers; 11-28-2017, 02:43 PM.
                            1601

                            Keep eye on ball.
                            Hashim Khan

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