AC Induction Motor Forward and Reverse Operation
In certain applications like lifts, cranes, etc., it is required that the motor be capable of reversing. This requires a reversible motor, i.e., one in which the shaft has a bi-directional rotation, both clockwise and anti-clockwise. In the induction motor, this is quite simply achieved by changing over any two of the leads.
In Figure 4.14, the direction of an induction motor with DOL starter is reversed. Phases L1 and L2 are interchanged; you can always change direction of the induction motor by interchanging any two phases. In larger ratings of motors the cooling fans may be suitable for one direction of rotation only. If bi-directional operation is anticipated, the same should be specifically stated to the manufacturer while ordering.
This can be achieved by changing the leads, either at the motor terminals or at the starter end connection. It is always advisable, to make changes to the lines going to the starter, as there may be some confusion in case of motors arranged for star–delta starting while making changes at the motor terminals.
In Figure 4.15, reversing connections of a three-phase induction motor with star–delta starter are shown. Phases L1 and L2 are interchanged; you can always change direction of the induction motor by interchanging any two phases.
As discussed earlier, it is advisable to make changes in the lines going to the starter. There may be some confusion, in the case of motors arranged for star–delta starting, while making changes at the motor terminals.