Using VFD to get 3 phase power
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  1. #1
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    Default Using VFD to get 3 phase power

    Ok so I have an old 1987 CNC milling machine that I'm changing over to Linuxcnc.
    So I need 3 phase to run the spindle motor, what I'm wondering is when the main 240volt power comes into the machine should that go straight to the VFD and then the 3 phase coming out of the VFD wire into the main power when the 3 phase originally wired into?

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    No. Generally you want the output of the VFD connected directly to the motor, and convert the other control circuits in the machine to trigger the inputs on the VFD.

    VFDs can't handle a motor being connected to the output while running; they will fault out unless the motor is much smaller than the drive rating as they can't supply the starting current.

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    Oh ok, and can I delete the relays also, and use the Mesa 7i77 board for the input signals, or will I still need the relays?

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    You should be able to connect it directly to that board, but check the manuals of both. I would probably just use relays for peace of mind; dry contacts are easier to deal with and troubleshoot.

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    I commend you on Linuxcnc. Been using Linux for more than 20 years.

    You really have to have a diagram of the entire system first.

    I think what you are asking is if a VFD can run a motor and at the same time power the other components, at the same time.
    What happens when the stop command is given to the VFD? Chicken and egg.

    The VFD is always directly connected to the motor.

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    I've often wondered if properly synchronized inverters (from inverter generators) would perform this task OK. The generators seem tolerant of switched loads. Speed control is not a necessary feature in many cases. I know there's a crossover in circuitry between VFDs and inverter generators, but haven't done the deep dive.

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    If three phase was delivered to everybody think of all the people who would be out of a job. This section would be gone.

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    Hi
    I have only retrofitted a couple of lathes with vfd but in both cases, I discarded the entire electrical protection and control system.
    I kept the control switches (door micro-switches, operating switches etc) but all the fuses, relays, circuit breakers went in the bin.
    I also replace the 3 phase 415VAC 7kW motor with a 3phase 240VAC 4kW motor. I used a 3.8kW Yaskawa V1000 vfd. This is the most powerful industrial quality drive I could find that can run off 1ph 220VAC.

    I converted all controls from 110AC to 24VDC.
    I fitted a RCBO to the switchboard.
    I replaced the fuses with a combination of MCB and ultra-fast fuses in accordance with the manufactures recommendations.
    I fitted a EMI filter to the input of the vfd. Schaffner makes a version specifically for the V1000.
    I converted all indicator lamps to 24V LED.

    I found that it was easier to rip out and replace the entire electrical system. The only thing I kept was the switch that controls fwd/stop/rev on the saddle.

    This is the original electrical control/protection.
    img_3952.jpg

    This is the vfd version.
    img_0056.jpg

    My lathe is LinuxCNC ready in that it is now compatible with a CNC retrofit.

    To answer your question.
    The vfd needs to be connected directly to the spindle motor, and nothing else.
    Each servo motor needs its own driver. If the servo motors run 400VAC, you may need to replace them.
    You should avoid using relays if you can.


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