What is a phase failure?

A phase failure is when one of the phases feeding power to a three-phase motor becomes disconnected for any reason while the motor is operating. The motor continues to run on the remaining two phases. This condition causes the motor to run at a reduced speed and experience vibrations.

The current also increases considerably in the remaining phases, leading to internal motor heating. If not promptly disconnected, this can result in damage due to the rise in temperature. This failure can be detrimental, even if it occurs when the motor is not in operation, as sometimes the motor can start with only two connected phases depending on the load.

In summary, a phase failure is never a desirable condition for the operation of an induction motor, and appropriate measures should always be taken to protect it.

Causes of a phase failure:

• Loss of one of the phases from the power supply.
• Damage to one of the cables feeding the motor.
• Broken connection terminals due to vibrations or aging.
• Loose connection terminals.
• One of the three fuses in the three-phase circuit opens.
• The starter contactor is damaged/oxidized and leaves a phase open.
• Damaged relay contacts.
• Improperly configured protections.

Effects of a phase failure:

• The motor operates at a reduced speed.
• Considerable loss of the motor’s relative power.
• As only two phases are providing power, the current in the remaining phases increases considerably. This causes the insulation in the windings to be unable to withstand the increased current and heat. When the insulation is damaged, a short circuit occurs in the windings, causing the motor to burn.
• It can cause generator overload.
• If the motor is off when the failure occurs, it may not start when needed, and if it does, the motor will burn.

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