VFD Wiring a 2 Speed Mill
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  1. #1
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    Default VFD Wiring a 2 Speed Mill

    Hello,

    I am working on powering up my 3-phase milling machine.
    It has a 2 speed motor, [delta/YY].
    It has a step-pulley head, so the majority of speed changes will be done by switching the belt.


    How should the motor be connected to the VFD?
    Should the motor be connected to high speed or low speed?

    I don't want to lose the torque in back gear for drilling/tapping.

    This will be my first 3-phase machine, I am new to this, and any advice is appreciated.

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    If it's rated at the same HP for both speeds (close to the same current in both is a good clue) then it'll make more torque in the low-speed connection (double-ish what it does in high), if the VFD can run to double the usual 50 or 60 Hz then you should have the same speed range, using 50/60 to run the low speed range, 100/120 for high speed.

    It IS possible to run it with both speeds and the original speed switchgear, BUT you won't be able to switch under power and you may need a VFD with multiple motor configurations (particularly for auto-tune VFDs) if the motor current and other parameters are substantially different - look for a VFD with two or more "parameter sets" and work out a way to signal the VFD that you've changed between them (usually you can use one of the low-voltage control inputs for this). It would be a good idea to add an interlock to the speed switch to signal the VFD to cut its output while switching if there's any chance that the speed switch could get flipped under power - look for a "gate block" function and how to signal it, which will do this?

    Whether you retain the 2-speed switchgear or not, if you're tapping and want a fast stop-and-reverse it's a Very Good Idea to add braking resistors for very rapid stops and reverses, and you'll need to do some trial-and-error with braking settings...

    Dave H. (the other one)

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    100% on what Dave (the other one) says.

    My Bridgeport BRJ has a 2hp 'pancake' motor, step-pulley and backgear.

    I have the step pulley set to center (1:1 ratio). This makes the amount of belt-wrap maximum between drive and driven... that means maximum power transfer for the belt. I use the VFD to go from 5hz or so, up to 120hz, and that covers basically everything I need. I have, on occasion, needed slower spindle speed to swing a large diameter tool, I've shifted backgear to low, but that doesn't happen often.

    For tapping, you WILL need dynamic braking resistors, and get the braking parameters and DC injection set right. In my experience, it's best to limit the motor torque for tapping, to prevent twisting off taps (don't ask me how I know, and if you need any broken taps... ) Having a dual-switch footpedal is good too-... pedal up = stop, pedal down full = forward, pedal halfway up = reverse. Address the tap to the hole, step down full. get it started a turn and a half or so, then ease up on the pedal to back up a smidge, then back down... repeat 'till you're at depth... then back it out, and stop.

    Whatever you do, DO NOT change D/Y with the machine running. Bring VFD to STOP.

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    If it is constant Hp, or the Hp is highest on the lowest speed winding, then I use the low speed winding and then over speed the motor to the high speed winding RPM. This retains maximum Hp and torque. I have done a few VFD installs with dual speed motors and in some cases have had incompatibility problems with the motor running poorly despite auto-tuning and adjusting the parameters. Also did the dual motor settings with a switchover switch for the winding connections, with an interlock switch. I find running a dual speed motor off of one winding works best in most cases.

    On tapping I setup the mill installs with an auto start/auto reverse triggered off of proximity sensors set for the depth. On smaller taps might consider something like a Tapmatic.

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    Thanks for the replies, I've made a few chips now.
    The VFD I have is a Lense SMVector 2hp, it seems to work well.

    Right now I have the VFD wired into the drum switch instead of the motor.
    I have the switch set to low speed FWD and I removed the knob to eliminate the temptation of using it.

    I will probably order up some low voltage switches for FWD/REV and JOG instead of using the VFD buttons.
    I will also look into adding the resistors for tapping and I like the footswitch idea as well..

    I will probably get banned for posting this (its an MSC china mill), but here is the motor plate if anyone is curious,
    mill-motor-plate.jpg

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    Quote Originally Posted by LIFTED View Post
    ... Right now I have the VFD wired into the drum switch instead of the motor.
    I have the switch set to low speed FWD and I removed the knob to eliminate the temptation of using it.
    ...
    That's how I wired up my 2hp 2-speed *nco mill to an Allen Bradley 1305 5hp VFD. Kept the knob in the machinist box on the opposite side of the shop to minimize the chance of someone switching it under power. I had the braking resistor unit for it which helped it stop a lot faster, but it would still trip out if I tried stopping it in a second or less even with light tooling. At risk of joining you in the banned for talking about a home shop import machine, this was how I had mine setup:
    Attachment 255389
    I remote mounted the VFD control panel, used AB 800T switches and speed pot to control it, and stuck on a Tachulator for RPM reference which I could also set to display the correct RPM when in back gear (I was lazy and picked the speed off the top of the quill pulley). I optimized the drive settings for the motor in low speed where it spent most of its time. I never had any issues with the drive when running it in high speed, but I'm never pushing it much in high speed as I'm usually in high due to small diameter tooling. It was a real joy to operate this mill.


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