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Motor protection

Single phasing and voltage unbalance are power system problems that can cause motors to fail. In addition to lost productivity, this can lead to substantial repair or replacement costs for damaged equipment. Phase protection devices can safeguard your facility from the potentially damaging effects of these electrical disturbances.

What is single phasing?

A three-phase motor needs three equal power phases in order to operate correctly. The loss of one of these phases is known as single phasing. Inside your facility, phase loss can result from poor switchgear maintenance. Outside, a downed power line or blown pole top fuse on the utility system could be the issue.

Single phasing can cause serious problems in three-phase motors:

  • Above normal current in the remaining phases
  • Reduced motor speed
  • Uneven torque, which may cause the motor to vibrate
  • Overheated or burned-out motor windings

Voltage unbalance

Voltage unbalance is less extreme than complete phase loss, but it may have similar consequences. With the installation of new, three-phase power systems, careful attention is given to balancing the loads on each phase. As single-phase loads are added to these balanced systems, an unbalance may occur. Thermal overloads, magnetic breakers and other such devices will not detect this gradual unbalance and, therefore, may not provide adequate protection.

Voltage unbalance degrades the performance of three-phase motors and it can shorten equipment life. At motor stator terminals, it causes phase current unbalance far out of proportion (up to 10 times) to the voltage unbalance. Unbalanced currents lead to torque pulsations, mechanical stress, increased losses and overheating.

Protection solutions

Phase protectors or phase monitors help shield your motor systems from the harmful effects of single phasing and voltage unbalance. These devices sense more than just voltage; they can detect phase loss and shut motors off before damage occurs. Current or phase angle is also measured in many cases. This, in addition to a small voltage change, indicates a lost phase.

Motors can be damaged by over-voltage, under-voltage, phase reversal or excessive on/off cycling. Phase protectors detect these scenarios and react to phase unbalance or total phase loss. During phase loss, the protector sets off an alarm or shuts down equipment entirely. Choose phase protection equipment based on the incoming line voltage and frequency that will be detected.

Why take the risk?

Protection equipment is relatively inexpensive, typically ranging from $100 to $300, and simple to install. Compared to the cost of purchasing or repairing critical equipment, as well as downtime, phase protection is worth the investment. Consult with a qualified electrician to determine if your motor systems are adequately protected.