Can I run both 240 and 208 volt 3 phase motors of of a 208 Y system? My understanding is that the 240v motors will just run a little slow. Is this correct?

 

Hi,

A motor wants to use a certain amount of power. With low voltage, it will pull more current. It might get hotter than the coil insulation likes.

 

That's probably right..
Most 3 phase motors designed for less than 2 hp run pretty much the same on either 208v or 240v. Newer three phase motors are almost always wound to run pretty well from about 200 to 250v. Keep in mind that you never see a single capacitor on the outside of a three phase motor. If you see a capacitor, it's probably a single phase motor.
The motor doesn't know if its a "Y" or Delta supply. The supply voltages are 120 degrees out of phase in each case.
Most of the buildings that have 240v delta transformer systems are older, because their "floating ground" (no neutral) systems are difficult to make safe. 208 volt motors usually run pretty well on 240v delta secondary systems. Larger motors sometimes had taps that would allow for low or high line conditions, but that was rare for small three phase motors.
I hope this helps,
Rich