3-phase, 2 speed induction motor

[rivecar44]

Hello,
Equipment and application: 50/200 Hp motor, Y and YY dual speed, installed inside HVAC ductwork. fan direct couple to shaft and air flow from NDE to the DE
Assumed Scenario: motor IS NOT ENERGIZED BUT the shaft is rotating in the reverse direction due to air flow coming from the DE to the NDE.
Question: If the motor is energized to start at a high speed while the shaft is rotating in the reverse direction, what direction would the 3-phase motor rotate? Thanks in advance.

 

[Jraef]

It will rotate in the direction it is wired to rotate. There will be a lot of inertia on the shaft when it is energized so the motor will stay in Locked Rotor Torque (and thus Locked Rotor Current) for longer, which will have additional heating effects on the windings and rotational stress on the shafts. The additional time in LRC May cause the OL protection to trip. But barring that, eventually the motor will win out and turn the right way.

It can be different if there is a phase loss before the motor starts though. Normally if there is a phase loss on a 3 phase system and the motor is called to start, the motor will just shake back and forth because there is no relative rotation. But if the motor is already windmilling in the opposite direction when called for, it will continue to run in that direction if given single phase power. So if you are trying to figure out why a motor continued to run in the wrong direction because it was energized while windmilling, it’s likely that it suffered a phase loss first.