To study the different starting methods of three-phase induction motors & also study how to reverse the direction of rotation in a three-phase induction motor.

Experiment No.: – 12

Reversing the direction of rotation in a three-phase induction motor:-

The rotation of a wye- or delta-connected motor can be changed by exchanging any two of the three phases of the incoming voltage. Fig. 12.5 shows diagrams for a star-connected motor and Fig. 12.6 shows the diagrams for a delta-connected motor for clockwise (forward) and counter-clockwise (reverse) rotation. From these diagrams notice that T₁ and T₂ supply voltage terminals have been exchanged in the diagram for motor reversal.

In industrial applications, terminals T₁ and T₂ are generally switched by the contacts of the reversing motor starter. These diagrams will be useful for installation connections and troubleshooting.

This wiring configuration is also used when the motor is connected for plugging use. When the motor is used for plugging, it’s reversed while running at full rpm. When the motor’s stator is quickly reconnected for reverse rotation by switching two of the three input voltage lines, it will quickly build up a reverse magnetic field that will begin to rotate in the opposite direction. The rotor will follow this quick change in rotation and begin to rotate in the opposite direction. This will cause the load to decrease its rpm from full in the clockwise direction to zero, and begin to rotate counter-clockwise. The moment the rotor begins to rotate in the opposite direction, the power is de-energized and the rotor shaft is stopped from rapid reverse torque.