Technical Information

THREE-PHASE MOTOR PROTECTION
Many of Kenergy's large commercial and industrial customers operate three-phase motors at their premises. Due to circumstances beyond Kenergy's control, power may temporarily be lost from time-to-time. This could cause damage to three-phase electric motors, especially when only one of the three phases is lost.

Ideally, upon loss of one phase, the other two phases would be shut down for repair. However, many business owners prefer Kenergy doesn’t shut down power altogether because they can continue to run some equipment on a single-phase.

Since three-phase electric motors are subject to damage upon loss of a phase, and the repair cost is usually substantial, it makes good business sense for business owners to provide phase protection. This can be in the form of a phase monitor that senses loss of a phase and turns off the affected motor.

Most motors are designed to operate with some small phase voltage imbalance without harm, however, the greater the unbalance, the higher the winding temperatures. This leads to insulation breakdown and shortened motor life. Normal motor protective devices such as internal thermal cutouts (heaters) are usually set at a high enough operating point that they offer no unbalance protection.

The complete loss of a phase is sometimes viewed as an extreme case of phase unbalance, but it is more difficult to detect than phase unbalance and damages a motor very quickly. If not detected within as little as five seconds, the motor is a likely candidate for repair.

Loss of a phase will not cause the motor to stop; rather it will continue to operate. However, the internal winding connected to the lost phase is still rotating and cutting the magnetic field of the stator. This causes the winding to act as a generator so that if voltage is measured at the motor terminals, all will appear within a few percent of normal. Although the voltages will appear normal, the full load current is shared between the remaining phases. Due to this overload (about 33%), winding failure quickly occurs.

Phase monitors must sense more than just voltage to detect phase loss and shut off the motor before damage occurs. In most cases, either current or phase angle is also measured. This, in addition to a small voltage change, indicates a lost phase. Phase monitors also protect against over voltage, under voltage, phase reversal and excessive on/off cycling, which can damage motors.

Phase monitors are relatively inexpensive, simple to install, and can provide protection by disconnecting the motor starter circuit when a failure is sensed. Prices usually range between $100 to $200, depending on the features—far below the cost of a new or rewound motor.