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Single Phasing: Protecting Your Motor Systems

 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.

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

What causes single phasing?

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 (less than 3 percent) 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.

Phase loss protection

Phase loss protection devices are relatively inexpensive and simple to install. They provide protection by issuing a warning or disconnecting the equipment from the circuit when phase loss is detected. Phase or voltage monitors are the most common solution. High and low voltage levels can be selected. Phase Loss Relay

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 monitor can send an audible or visible alarm, or completely shut the motor down. A trip delay (from 1 to 30 seconds) on some units prevents nuisance dropouts. When the fault is corrected, the monitor will automatically reset, but advanced relays incorporate a restart delay setting of up 5 minutes or so. This allows motors to wind down or HVAC compressors to dissipate refrigerant pressure before restarting.

Phase monitors can be installed on each motor or on the secondary side of the circuit. Add a shunt trip capacitor when placing the monitor on the secondary side 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.


Image source: iStock 

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