The VFD already has high and low voltage connections in the same box, and high and low voltage wiring directly to it. Installing a switch or a potentiometer in a couple of those low voltage circuits does not seem too bad to me, and I am quite sure it is commonly done. Naturally, it is a very good idea to wire things so the two sides do not present a threat to each other or a shock or fire risk. Or am I missing something? I am for sure not an electrical genius...+1 to what Jim said.
I'll add (just for additional clarification) that with a VFD, you'll have wiring in two different voltage regimes. The main on/off switch, between the line and the VFD, runs 240 (or whatever) volts of AC at several amps. The start/stop, speed control, and direction controls, and maybe a remote speed readout, are low voltage DC. It's bad electrical practice (and in some instances a code violation) to mix high and low voltage circuits in the same enclosure (OK, can be done if suitable barriers are installed). Anyway, the easiest solution is frequently to add a separate box/panel/whatever just for the low voltage controls (switches, speed pot, etc.)
Use a contactor for the main on/off. You'll have at least one made redundant by the VFD.+1 to what Jim said.
I'll add (just for additional clarification) that with a VFD, you'll have wiring in two different voltage regimes. The main on/off switch, between the line and the VFD, runs 240 (or whatever) volts of AC at several amps. The start/stop, speed control, and direction controls, and maybe a remote speed readout, are low voltage DC. It's bad electrical practice (and in some instances a code violation) to mix high and low voltage circuits in the same enclosure (OK, can be done if suitable barriers are installed). Anyway, the easiest solution is frequently to add a separate box/panel/whatever just for the low voltage controls (switches, speed pot, etc.)
if I cut the plug off of the motor electrical cord, hook it up to the motor connections of the VFD, then run some 10/3 from the junction box to the connections on the VFD what changes?
You can not have a switch between the motor and the VFD, but the motor is going to work fine. Your existing switch reverses the motor by swapping two wires, the VFD does this electronically by inverting the phases. (OK, not exactly inverting, but close enough for this discussion)Why does the motor and its switches not work any longer?
It boils down to this, if you want to use the machines original controls you must connect them directly to the VFD.