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Motor Systems: Phase Protection Solutions

 Key Points
  • Loss of a single phase on three-phase motors can damage equipment and lead to costly repairs.
  • Phase loss can result from a downed power line, a blown fuse or equipment failure.
  • Voltage monitors can detect phase loss and disconnect equipment to prevent damage.

Motor test
Three-phase motors are commonly used for pumps and fans, as was well as industrial process equipment. Loss of a single phase can damage equipment, resulting in reduced productivity and costly repairs. In applications where three-phase equipment is critical to operations, phase protection makes good business sense.

What causes phase loss?

The loss of a single phase on a three-phase line may be due to a downed power line or a blown pole-top fuse on the utility system. Phase loss may also result from a single-phase overload condition causing one fuse to blow, or an equipment failure within a facility.

A small phase voltage unbalance will not typically harm a motor. However, greater unbalances often lead to insulation breakdowns and shorter equipment life. The complete loss of a phase is more difficult to detect than simple phase unbalance, and it can damage a motor quickly. If not detected within as little as five seconds, it may lead to costly repair or replacement of the motor.

Phase loss protection

Phase loss protection devices are relatively inexpensive and simple to install. They provide protection by disconnecting the equipment from the circuit when phase loss is detected. Phase or voltage monitors are the most common solution.

In addition to voltage, most phase monitors also measure current and phase angle, which can indicate phase loss, as well. Phase monitors detect over-voltage, under-voltage, phase reversal and excessive on/off cycling—all of which can damage motors. They react to total phase loss or phase unbalance. The action taken by the monitor can range from audible or visible alarms to complete motor shutdown.

Phase monitors can be installed on each motor, or on the secondary side of the circuit. A shunt trip capacitor should be added when the monitor is placed on the secondary side of the circuit so that it will still operate if there is a loss of power. Choose phase monitors based on the incoming line voltage and the detected frequency.

While phase protection may cost a small amount of money upfront, it can help to save thousands of dollars in lost production and equipment repair or replacement.  

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