How to change the speed of a three phase motor?

A typical 2 speed three phase motor will have 6 leads on the motor windings which will require 3 contactors to incorperate both the "high" and the "low" speeps.

Inorder for the motor to run on "low" speed you need to have a contactor (c1) that pulls in for the L1, L2,L3 leads... that should achieve the "low" speed of the motor..

To achieve the "high" speed you will need a contactor (c2) to shun or short L1,L2,L3 together... and another contactor (c3) to pull in the L4,L5,L6 leads...

to recap ... Low speed will requrie c1= L1,L2,L3 powered by phase 1, phase 2, phase 3 of the primary source voltage.

High speed requires c2 to short or shun L1,L2,L3 together while c3= L4,L5,L6 to be powered by Phase 1, Phase 2, Phase 3 fo the primary source voltages.

Be sure to check for proper rotation of the motor and by reversing any 2 leads of the primary source voltage you will change the rotation for the motor.

This can also be done using two contactors by having L1,L2,L3 connected to the first contactor and also the additional contacts mounted on top of the second contactor.

L4,L5,L6 are connected to the second contactor. The important thing to remember is to use an interlock which fits into both contactors when they are clipped together. This stops both contactors operating at the same time!!! ( the additional contacts mount on top of a contactor and are mechanically connected to it so that they act together)