There are many reasons for motor failures, such as overloading, bearing failures, rotor failures, worn shafts, and short circuits or ground faults within the motor windings, and contaminants that may enter the motor, causing short circuits or overloading. These failures occur for both single- and three-phase motors. However, single-phasing is one of the most common causes for motor failure for a three-phase motor. It can occur from a failure at the utility company supply or anywhere downstream from the incoming supply system.
What is the reason for the failure and how can this costly problem be solved? Can properly sized overload devices or fuses correct single-phasing or should phase or voltage monitors, current monitors, or multifunctional monitors be installed in addition to the overload or fuse protection devices? These failures can be very costly and may result in equipment or structural fires, further compounding hazards to property and people that violate 90.1(A) of the National Electrical Code (NEC).
Motor overload devices, as stated in 430.31, are designed and intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. This section also states that overload protection is not required where power loss would cause a hazard, such as in the case of a fire pump.
A fire pump should not be protected for overload because failure to operate the pump could create a greater hazard from the fire. Overload protection of three-phase motors may not operate fast enough to respond to a single-phase problem with a motor since mechanical overload devices may take as long as 20 seconds to operate during a single-phasing situation. The single phasing of the three-phase motor results in two of the three windings carrying the entire load of the motor, resulting in potential heat damage to the windings and the insulation surrounding the windings.
Electronic phase monitors, unlike mechanical overload devices, can detect voltage unbalances, as well as high and low voltages, within a much shorter time interval (within one or two seconds) and can respond to shut the motor circuit off before damage to the motor, the motor control apparatus and the motor branch-circuit conductors can occur. Most motor phase monitors can detect single-phasing, voltage unbalances and reverse motor rotation before the motor even is started and will prevent the motors from starting, thus protecting the system from potential damage.
A multifunctional monitoring device is designed to protect against high- and low-voltage, single-phasing, reverse-phasing (where the three-phase motor is running in reverse operation), and voltage difference between the three phases. It also is designed to monitor the current in all three-phase conductors. This added protection provides total monitoring and is desirable for large motor applications. The best current monitors use three current transformers, one for each phase, and compare the current in each phase to determine if there is an overcurrent or undercurrent imbalance or any fault current in any of the phases. Multifunctional monitors provide monitoring of phase parameters, phase sequencing, phase loss, phase unbalance and both overvoltage and undervoltage monitoring. Where any anomalies appear, the motor is shut down, thereby protecting the motor and the circuit components from damage.
There is a major NEC issue with any of these monitoring devices. Ensuring that these devices are appropriately and correctly connected is critical in its safe operation. Many of these monitoring devices are not directly connected and must be appropriately connected through either lugs on the line side of the motor-control contactor or through lugs on the load side of the contactor.
If the lugs are not designed for double lugging or will not accept larger conductors for the motor and smaller conductors for the monitoring devices, accessory lugs must be ordered and installed or the contactor lugs must be changed. Care must be taken when changing the existing lugs within the motor contactor to ensure the proper clearances are maintained between the energized lugs at the line or load connections and between the new lugs and the grounded metal contactor enclosure. As the voltage of the motor increases, the clearances required will also increase, based on the voltage and the listing requirements for clearances in UL-508, the UL Standard Industrial Control Equipment.
The question of whether phase monitoring and similar equipment cost is justified can be answered based on the replacement cost of the motor and related equipment, the downtime involved in the replacement, and the cost of the labor to change out the damaged equipment.