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Powering a VFD with an RPC's output?

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  • Powering a VFD with an RPC's output?

    Powering a VFD with an RPC's output?

    Any negative consequences? An reason it just should not be done?

    Why you ask? I've a 600V surface grinder that's wired and works perfectly. Run's off an RPC with 240 1P, that is then stepped up to 600V with a 3P transformer. I want to add a VFD to the grinder for soft stops and starts. I have the VFD and want to just run it off the 600V rpc generated power to the machine. The alternative is a lot of work, rewire it to be 240 1P, buy a transformer, etc.

    afaik power goes into the VFD, then gets rectified and it does its thing thereafter from a DC buss....so why would it care? Or what am I missing?

    thanks!
    in Toronto Ontario - where are you?

  • #2
    Sounds reasonable. Only thing I can think of is to watch the balancing of the phases so that the voltage of any given phase does not exceed the VFD input rating.

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    • #3
      thanks for the answer....but I screwed up....grinder is 240. I have the 600 VFD and thought it would be perfect.... could have sworn the grinder was 600....memory must be going...now I have buy one. Sorry for the time waste

      edit amazon has 2.2 kw vfd;s for 109 cdn today.....don't leave them plugged in
      Last edited by Mcgyver; 05-09-2021, 01:07 PM.
      in Toronto Ontario - where are you?

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      • #4
        Originally posted by Mcgyver View Post
        thanks for the answer....but I screwed up....grinder is 240. I have the 600 VFD and thought it would be perfect.... could have sworn the grinder was 600....memory must be going...now I have buy one. Sorry for the time waste

        edit amazon has 2.2 kw vfd;s for 109 cdn today.....don't leave them plugged in
        So what about the transformer that steps up the RPC output for the grinder? What are the voltages in/out there? OP said 600 out of the transformer but apparently not.

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        • #5
          The RPC is 240 which some machines run on (like the SG) and I also take that 3P output and bump it to 600 for some other machines. Shop is wired for both 600 and 240 3P, I just forgot/mixed up which voltage the grinder was. I also have 240 1P through out so will hook up the new VFD to that then there's no need to turn on the RPC to run the grinder
          in Toronto Ontario - where are you?

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          • #6
            I got the VFD running on the surface grinder today....most of my wiring for machines is on the ceiling in conduit and in a shop where there is hardly room to use a step ladder, it is a misery working overhead running a new 240 1P line to the SG locale...one foot on a ladder and the other on machine and much as I want to delude meself, I'm not 25 anymore. But....that's over, and is it every nice! I must have sat there turning it on and off for 1/2 an hour. I used a cheapo VFD and think they should be disconnected when not in the shop so used the OEM on/off contactor turn power on/off to the vfd.

            Why do this? For the guys without grinders, there's always a bit of clearance between the shaft and wheel. Initially that creates an imbalance on the outside of the wheel as the outside is spinning slightly eccentrically . When you dress the wheel only the smaller imbalance close to the shaft remains which is much less a factor (imbalance is a function of mass and distance from the axis). Without a soft start/stop, the wheel gets jarred slightly and that balance the dressing created is lost. For best results, every time you turn the spindle on and off, redress. A soft start/stop should get rid of that. My current impoteus is around being able to dress an angle, then remove the dresser and indicate in a setup without having to keep the wheel running.

            I think i'll go back out and turn it on and off a few more times before dinner arrives
            Last edited by Mcgyver; 05-18-2021, 06:58 PM.
            in Toronto Ontario - where are you?

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            • #7
              Originally posted by Mcgyver View Post
              I got the VFD running on the surface grinder today....most of my wiring for machines is on the ceiling in conduit and in a shop where there is hardly room to use a step ladder, it is a misery working overhead running a new 240 1P line to the SG locale...one foot on a ladder and the other on machine and much as I want to delude meself, I'm not 25 anymore. But....that's over, and is it every nice! I must have sat there turning it on and off for 1/2 an hour. I used a cheapo VFD and think they should be disconnected when not in the shop so used the OEM on/off contactor turn power on/off to the vfd.

              Why do this? For the guys without grinders, there's always a bit of clearance between the shaft and wheel. Initially that creates an imbalance on the outside of the wheel as the outside is spinning slightly eccentrically . When you dress the wheel only the smaller imbalance close to the shaft remains which is much less a factor (imbalance is a function of mass and distance from the axis). Without a soft start/stop, the wheel gets jarred slightly and that balance the dressing created is lost. For best results, every time you turn the spindle on and off, redress. A soft start/stop should get rid of that. My current impoteus is around being able to dress an angle, then remove the dresser and indicate in a setup without having to keep the wheel running.

              I think i'll go back out and turn it on and off a few more times before dinner arrives
              I have a VFD on my surface grinder. Soft starting as you described is a very beneficial feature. I also really like the ability to set the deceleration. Grinders take a long time to spin to a stop, I have mine set to stop in about 10 seconds. Stopping in 10 seconds isn't fast enough to jar anything into imbalance yet you don't have the long wait. Its a nice safety feature also, less time working close to a spinning wheel.

              I don't use the VFD's ability to vary the rpm but as I understand it there are some cases where this can be very desirable as well.

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              • #8
                The firm where I used to work used to grind ultra high tensile steel for aircraft, (250 ton / sq") and it was so easy to burn. The only way around was to make sure the wheels were fully balanced as well as dressed and all grinding was done wet. I'm not sure how you could address the balance problem with the clearance you are having. Maybe some different thickness shims of brass to minimise clearance, as every wheel would vary slightly.

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