why can't you run two AC motors off the same controller?
I was told it's not possible to run two AC motors off of the same controller. Is this true?
I understand that you can do it with DC motors though. Am I missing something?
The primary problem is that for an AC controller it is very important to know exactly at which position the motor is, so that it can coordinate the output with it. The controller tries to optimise effiency and torque, by creating an output voltage wave form that is timed to the position of the motor. If you have two motors connected to one controller, the two motors will most likely not have exactly the same position, and the controller will be confused. (Unless you fix the two motors mechanically to the same axle, but then it becomes a philosofical question whether it's actually two motors or only one.) Either it won't work at all, or one motor will take more power than the other. Worst of all, you will very likely get a situation where current swings back and forth between the two motors. (Which can be prevented using reactive power balancing but that more or less assumes at least one extra control unit, which we said we wanted to avoid.)
With two DC motors, the controller output is basically a constant voltage (no wave form as in the AC case), and the motors do what they can with it. No need for keeping the motors in sync, or at exact same speed, or match the output to the position of the motors.
You could think of situations where it would be possible to control two AC motors with one controller, but that would be very theoretical.
Finally, very often two controllers cost the same or even less than one, bigger, so normally there's no point.