How to Connect a 3 Phase Motor

Updated July 11, 2018

Three-phase motors are used in commercial and industrial applications for loads above the fractional and low horsepower ratings serviced by single phase motors. Three-phase motors are preferred where three-phase power is available because they are more efficient and of simpler construction than single-phase motors. They must be connected to the power supply via motor starters which provide short circuit protection, a means of disconnecting the motor and protect it against overloading. Connecting cable sizes must match the over-current protection of the breaker or fuse in the motor starter.

Disconnect the power at the motor starter. The starter will have a fused disconnect switch or circuit breaker which must be opened. Place a padlock into the mechanical interlock which prevents the re-closing of the switch or breaker, and lock into place.

Check the nameplate data for voltage and current on the starter and make sure it matches the nameplate data on the motor. Check the cable size against AWG cable tables to make sure the cable is the right size. Check cable markers to make sure the correct cable is being connected. Check for voltage at the motor end of the cable before making any connections.

Note the insulation colours of each connection -- the starter will have the cable connected to terminals labelled T1, T2 and T3. The motor will have terminals labelled T1, T2 and T3. Connect the same colours to the same terminals as noted for the starter.

Remove the padlock and close the disconnect switch or breaker. Start the motor. When the motor has started and is operating normally, verify the rated operating voltage and current to make sure they match the motor nameplate data.

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About the Author

Bert Markgraf is a freelance writer with a strong science and engineering background. He started writing technical papers while working as an engineer in the 1980s. More recently, after starting his own business in IT, he helped organize an online community for which he wrote and edited articles as managing editor, business and economics. He holds a Bachelor of Science degree from McGill University.