If you are using an AC, BLDC or some other brushless drivetrain, (like siemens, azure, hpgc, etc) then the motor / inverter already support running in either direction. There will be a reversing signal you can hook up someplace on the inverter.
If you are using series or shunt DC motor (like a warp 11" or something) then you need a reversing contactor. This is basically a giant DPDT (double-pole-double-throw) switch that is wired up so that it can reverse the direction of electrical current through the field winding relative to the armature. This reverses motor direction. Buying a reversing contactor like an albright SW202 will set you back a couple of hundred but will do the job nicely. That's what I've got.
Normal operation will be one contact energized and one turned off. Reversing the motor will mean de-energizing one side and energizing the other side. Note that when you wire it, ensure that "fault" states (both on or both off) do not short out the motor controller. You can wire it so these "fault" states result in an open circuit or in a short circuit. Go for open circuit.
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