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How to Reverse a 3-Phase Motor

Big electric motor in a marble quarry. Alentejo, Portugal image by mrfotos_fotolia from Fotolia.com

Three-phase motors rely on a polyphase system for delivering continuous alternating current. Instead of the single power lead used in single-phase electrical systems, three-phase systems use three power leads. Each power lead is a phased 120 degrees apart from the other two leads, making them perfectly balanced. When each phase of the three-phase system is connected to its proper terminal in the three-phase motor, the motor runs forward, as it was designed. However, if so desired, the motor can be reversed by switching the order of the phases.

Shut the breaker off that supplies power to the three-phase motor, then lock the breaker with a padlock to prevent the breaker from being turned on while you are working on the motor.

Unscrew and remove the cover of the motor using a flathead screwdriver, and determinate two of the three phases using a flathead screwdriver. It doesn’t matter which two phases are removed: T1, T2 or T3.

Swap the two phases and refasten them using a flathead screwdriver. Reinstall the cover.

Restore the power. The motor now runs in reverse because two of the phases have been swapped.

Troubleshoot A 3-phase Electric Motor

Three-phase motors are normally used in commercial-type operations. These motors have fewer parts and are more sturdily built than their single-phase counterparts. Take input voltage to the motor using the volt ohmmeter. The motor-specified voltage must be present on all three phases. Examine the motor's electrical connections and terminals. Remove power to the motor and repair any damaged or loose connections. Remove motor voltage and disengage the motor from the machine that it is running. If it does, check the machine for serviceability. Replace motor if it will not restart.

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