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VFD 2-speed motor question

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  • VFD 2-speed motor question

    I have a Emco super 11 lathe that is 220v 3ph. I'm currently using a rotary phase convertor that is starting to act up (running warm and making odd noises on start-up).

    What I would like to do is replace the convertor with a VFD but I'm not sure if having the 2-speed motor will cause any problems. Is it as simple as basically wiring it up as the phase convertor was, leaving the lathe power switch in either the high or low speed position and using the VFD to power the lathe on and off? Or will I have to wire the VFD directly to the motor and bypass the original switch all together?



  • #2
    You can't have a switch between the VFD and the motor. All the switching has to be done by the VFD. Your external switches will connect to the VFD now.
    "Patriotism is the last refuge of a scoundrel"


    • #3
      I've got a similar situation on my milling machine: a 2-speed motor and a VFD.

      My solution has been to take the handle off the motor switch (so in a moment of stupidity I can't switch it), leaving the switch set to the motor speed I use most (low). I then control everything with the VFD.

      If I want the motor's high speed, I turn off (unplug) the VFD, turn the motor switch to high, then plug in the VFD again. I find that I hardly ever use the high speed.

      I have speculated that as long as the VFD has the motor switched off when the speed change is made, the motor switch can be changed even with the VFD turned on (i.e. no need to unplug) but I have not confirmed this. (Does anybody know???)

      A lathe may require more frequent use of both the high and low motor speeds, in which case this technique may get a bit cumbersome, but based on my usage I don't find it particularly onerous or inconvenient. If one can in fact change the motor switch safely with the VFD powered on, as long as the VFD has the motor turned off, it wouldn't be a big deal at all.
      Last edited by SGW; 10-27-2011, 08:15 PM.
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      • #4
        Yes it can be done, I had a situation where 2 identical motors were set up to to be alternated on the VFD output while the VFD was powered.
        It was a semi automatic (conveyor) system, the VFD stop/start was interlocked together with a VFD output set to 'At Zero Speed' with the change over contactors, so it can be done.
        Manually of course, you have to ensure the VFD is at 0hz before the change.


        • #5
          Great! Thanks for the info guys.


          • #6
            John... Your Emco motor is a Delta wound 2 speed. The speed is changed by shorting delta segments (so take care if you remove the switch and do ths manually) - it doesn't have separate winding for "high and low". IIRC, it is 2hp on high speed, and 1hp on low (or something close to that).

            The best way to run your lathe on a vfd is simply to select the "high speed" mode. When you run the VFD at 60HZ, you have your full 2 hp. Run it at 30hz and you'll have your low speed and the same 1hp as you had with the switch in low speed... You can't materially beat the speed/hp curve with any other combinations. For Foward/Reverse, just use the VFD inputs, not the selector switch.
            Last edited by lakeside53; 10-28-2011, 02:40 AM.


            • #7
              Cerus has vfds that will do everything for you. They have an output to drive a contractor to switch between motors. The unit handles stop and switch with a input. It also has dual parameters, one for each speed.


              • #8

                The VFD will be set up for only one operating condition, not both speeds as the current draw will be different. Set up the system for the high speed connection and get working.


                • #9
                  When I scrapped the various burned out contactors, relays & field selectors on my 2 speed 5 hp, 15" YMZ lathe and replaced it all w/ a VFD, I just used the high speed windings. Works like a charm; I even managed to save the jog button (the VFD has a logic input for that).

                  - Bart
                  Bart Smaalders


                  • #10
                    You won't "kill" any decently designed VFD if you DO switch the output..... even if it is driving a motor.

                    But generally, you can't drop the motor on the VFD output and expect it to start the motor... the surge is too large for the overcurrent settings. While it is "possible" to design a VFD to take that (I have) it is not usual, and as a rule it costs more.

                    Since you can't turn the motor "on" with a "matched-size" VFD using a switch to connect it, there is little point in putting in a switch.

                    But, there is nothing to stop you putting in a switch to shift speeds, or connect an entirely different motor, etc.

                    Now, if you have a "vector" drive, you may confuse it by switching motors..... that is why a few can switch "programs". That type measures the motor and uses the results to predict performance, allowing it to have a number of advantages. With multiple programs, it can be switched to the appropriate one when changing speeds.

                    For the motor-switching type application, a plain old "V/Hz " VFD is fine.
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                    • #11
                      I've converted my Super 11 with a nice Allen Bradley VFD, electronic brake, all new switches (sold the original for $70) and it works like a charm.

                      Send me a PM and I'll email you the diagram.



                      • #12
                        I run the 2-speed motor on the vertical head of my Elliott mill from a VFD, can't remember the make. It runs fine on either speed, changing the speed switch with the inverter switched on is not a problem but it does trip out if I forget & try to change speed while the motor is running. With the 2-speed motor I haven't felt any need to change the belt position in over 2 years since fitting the VFD.



                        • #13
                          Thanks again guys for all of the info! Jason I'll take you up on your offer, pm sent.

                          Looks like I will be ordering the vfd next week, I"m leaning towards like the Teco models from Dealers Electric.



                          • #14
                            Originally posted by johnc View Post
                            Thanks again guys for all of the info! Jason I'll take you up on your offer, pm sent.

                            Looks like I will be ordering the vfd next week, I"m leaning towards like the Teco models from Dealers Electric.

                            did you get the information for this from Rustyclockwork?