Every electrician and electrical maintenance technician should know how to reverse a three phase motor. Luckily this is quite simple once you know how to do it.
I need to reverse at least one motor a week. Often because the conveyor it powers is jammed up with debris.
How to Reverse a Three Phase motor
To reverse a three-phase motor you must swap any two phases. This can be done at the motor JB, local and remote disconnects, or the MCC.
- Make sure it’s safe to reverse the motor. Some equipment like pumps and compressors are only designed to run in one direction. I’ve broken more than one drive chain by trying to reverse a conveyor that had no business being reversed. Unfortunately, there’s only one way to learn what can and can’t go in reverse!
- Stop the motor and lockout the power source. Follow your company lockout tag-out procedures.
- Test for zero energy with your multimeter or voltage tester. Make sure there is no power going to the equipment you are going to work on before disconnecting any wires.
- Swap the connection of any two phases. For example, if the current configuration of the T-leads is Black-Red-Blue, change to Red-Black-Blue.
- Remove your lock and verify the desired rotation has been achieved. A good habit to get into is testing any work you’ve done. Mistakes can be made even when doing something as simple as reversing a motor.
Why Does This Work?
While running, an induction motor generates a rotating magnetic field. This field rotates the rotor which is connected to the output shaft of the motor. Switching two of the phases causes the magnetic field to rotate in the opposite direction, thus turning the rotor in the opposite direction.
If you’d like to learn more about the components in a three phase motor and how they work, check out this article by Electrical4U.