Why 3ph motor will work in reverse direction?

Answer 1

no

Under most conditions the answer would be no, however I experienced this very occurrence myself. This is the condition it happened under: Wiring a three phase power band saw with a hydraulic pump. The pump was used to clamp the work in place and raise the cutting head. One of the wires for the pump motor had come out of the factory installed terminal strip. When the saw was started the pump ran in reverse, we changed the rotation at the disconnect. Started the pump again, the pump again ran in reverse. Thinking we had reconnected it incorrectly, we again changed the phasing. Started the pump again, again it ran in reverse. We started looking the wiring over and found the wire out of the terminal strip, corrected this problem and the motor ran in the right direction. Why did this happen? A three phase motor will run with only two phases connected as long as there is no load on the motor. The pump only loaded the motor when it was turning in the right direction and pumping oil. The pump provided no load when turning in reverse and the motor could easily turn it.

Answer 2

No it can not. To make a three phase motor run in the opposite of normal rotation it has to be wired to go in that direction. Vertical turbine pumps have ratchets on the top of the motor housings to prevent this condition from happening. When a motor starts it has a high inrush current for a 5 second time interval. If a pump was turning backwards when the pump was started the motor rotor would have to come to a stop and then start turning in the correct rotation direction. This would take it beyond the 5 second natural time interval and would prolong the high current on the motor stator. This would cause the motor to trip the overloads and shut down. Nowadays most pump motors are controlled with VFD's (variable frequency drives) and use a ramp up feature to overcome the inrush current by starting the motor to rotate slowly. Even this type of starting is engineered to start from a stopped rotor making the ratchet is still viable on the pumps of today.

Answer 3

If the AC drive that you mention is a VFD then it has to be programmed to rotate the way you want it to turn. Some have the ability to be changed right from the face hand controller others have to be programmed. Just by connecting the wire colouring code to the output of the drive does not guarantee which way it will turn. Go into the program set up and select from the menu, the rotation that the motor requires to operate correctly.