VFD wired to mill circuitry
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  1. #1
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    Default VFD wired to mill circuitry

    I have an old 3 phase mill which has what appears to be a starter of some sort and other than that is simply switches and fuses. Is there any reason I can't feed the mill with a VFD into the starter as opposed to wiring the motor directly?

    The reason I ask is the mill is a combo vertical/horizontal mill (Rambaudi VSR-2) with separate motors on the horizontal and the vertical. Instead of wiring a separate VFD to each motor I'd like to just power the mill at 100% and use the original circuitry/gears/belts for speed control.

    Is there a good reason this wouldn't work?

    thanks
    philip

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    It is not good practice to interupt the connections between a VFD and a Motor. This can let the smoke out of the VFD. Unless the two motors are identical, the parameters in the VFD should be set for each motor.

    Since you are not looking for speed control from the VFD I would recommend that you use a rotary phase converter. It will work seamlessly with the controls and multiple motors.

    Bill

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    Quote Originally Posted by Bill Simmons View Post
    It is not good practice to interupt the connections between a VFD and a Motor. This can let the smoke out of the VFD. Unless the two motors are identical, the parameters in the VFD should be set for each motor.

    Since you are not looking for speed control from the VFD I would recommend that you use a rotary phase converter. It will work seamlessly with the controls and multiple motors.

    Bill
    Thanks Bill. I have a 10hp homebrew RPC. I bought a lathe which came with a VFD and I envy the quietness but I really like my vintage machines and twiddling the knobs and belts. So I'll probably stick with the VFD.

    Thanks.

    philip

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    You need to directly wire the motor to the VFD, depending on the VFD type and if you are running sensorless vector you can damage/destroy the VFD if suddenly disconnected from the motor while running. Running it in a V/Hz mode may be less likely to cause damage is my understanding.

    Some VFDs are setup so you can load parameters for two separate and different motors, you can switch the parameters via one of the VFD's low level inputs. I have used this setup on a few occasions where I use a suitable rotary switch to switch the VFD to motor connections. I use a 3 position switch with an extra poles which switches the low level signaling for the other motor parameters. There is a neutral middle position on the switch that disconnects the low level signaling (run commands) to the VFD, so more of a safety interlock so the other motor can't start up. You could also send a free-run command to the VFD. In simple installs I typically use 3 wire control on mills, otherwise some form of latching relay system that disconnects run signaling with power loss and certain safety modes. Using this approach your run commands remain the same, so not recommended if you are planning to use different run controls between the motors.

    Depending on the size of the motors and if you have other drives, then an RPC may be a better turn key choice. If just two smaller motors, say 3 Hp and under then possibly two inexpensive VFDs might be a simpler option.


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