what would be the results of one phase open-circuiting while the motor was running?

Answer 1

If one of the three phases is lost, the current drawn by other two phases would rise and following consequences would follow:
1. If motor was not fully loaded and current drawn by remaining two phases (after loss of one phase) of motor are less than its rated current, motor would continue to run.
2. Motor would get heated and thermal over load should protect it.
3. If phase failure or negative sequence or unbalance protection was provided motor would be stopped by activitation of these devices.
4. If neither of above protection is provided and motor is loaded in such a way that it results in over heating, motor may burn if temperature rise is more than temperature which its insulation can withstand. There are different limits for different class of insulation.

Answer 2

If one phase of three is open while the motor is running it will slowly come to a stop with a loud humming sound -depending on the size of the motor. The temprature of the motor will also rise very fast. If not stopped immediatly, the motor will over heat and the coil will get burnt. I have seen this happen somany times

Answer 3

2-phase motor? Immediate drop of H.P. and R.P.M. by 50%. Then, depending on the load that the motor must bear, it's rapidly downhill from there. If there is a failsafe relay in the circut, then the whole thing shuts down until both supply lines become available again. Then it's just a matter of restarting the motor at either the control panel or the starter reset in the utility room.