The white is the common neutral, it is probably a 3 wire unit, and you are missing the ground wire. Check inside the control panel and see if someone wired it using the white wire for the ground, it will be screwed into a terminal lug. Check the schematics to see if the unit requires a common neutral to run the controls.

The black goes to L1, Red to L2. The bare copper wire is the ground which goes to the grounding nut in the rear, it has the green washer on it. The white is the 'common' or 'neutral' wire and isnt used, it gets capped with a wire nut and some electrical tape.