Relays are used in motor-driven applications to measure and monitor operating parameters such as temperature, current, or voltage, preventing damage to the motor and connected equipment in the case of a fault or abnormal operating condition. Voltage monitoring relays can detect not only under-voltages and over-voltages, but also voltage-related issues such as phase imbalances, phase loss, and phase sequence.

Voltage monitoring relays are designed for either single-phase or three-phase systems. Those that are used in three-phase systems are sometimes referred to as phase monitoring relays.

Single-phase voltage monitoring relays can be used on single-phase AC or on DC voltages. Their primary purpose is to protect motors and connected equipment from either an under- or an over-voltage condition, although some are designed to ensure the voltage remains within a predetermined bandwidth, with both high and low voltage limits.

While different manufacturers use different operating principles (open circuit or closed circuit) for energizing or de-energizing the relay when a set point is exceeded, a simple example of an over-voltage monitoring relay is one that uses a  normally closed (NC) contact. Here’s how it works:

When the operating voltage is below the maximum voltage setting, the relay is de-energized and the contact remains in its default, closed state. If the voltage exceeds the maximum voltage setting (sometimes referred to as the pickup voltage), the relay is energized, the contact opens, and power is removed from the load. When the voltage drops below the maximum voltage setting including a hysteresis value (known as the dropout voltage), the relay is again de-energized and the contact closes, restoring power to the load.

In addition to limits on the allowable voltage, many voltage monitoring relays include a fixed or programmable time delay (also referred to as a tripping delay) for which the fault must be present before the relay will trip. The purpose of the time delay is to prevent nuisance tripping from conditions such as momentary voltage dips (under-voltage). In some relay designs, after the error has been corrected, the time delay will be also be implemented before the relay will automatically reset.

Both under- and over-voltages affect the motor’s output torque, speed, and efficiency, although the primary result of both conditions is motor heating — through higher current draw in the case of an under-voltage condition and through motor saturation in the case of an over-voltage condition. Under-voltages can also make AC induction motors difficult to start and cause unexpected shut-downs.

Three-phase voltage monitoring relays, or phase monitoring relays, monitor additional phase parameters along with over- and under-voltage conditions: namely, phase imbalances, phase loss, and phase sequence (also referred to as phase reversal).

In three-phase systems, over- and under-voltage conditions occur when the voltages in all three of the phases increase or decrease simultaneously. To determine whether there is an over- and under-voltage condition, the relay measures the average voltage of all three lines and compares this to the voltage set point.

To determine if a phase imbalance is present, the relay monitors each of the phases to detect when the voltage in any one phase falls by a predetermined amount below the average of all three phases. Similarly, if complete loss of a phase is detected, the relay will trip and disconnect power from the motor.

Phase imbalances force some of the motor windings to carry more of the load than others, which can cause the motor to experience excessive heat. If a motor experiences phase loss, it can continue to operate by drawing the required current from the remaining phases, but this also causes excessive heat and can damage the motor.

Changing the sequence of any two of the three voltage phases — known as phase reversal — can be extremely dangerous, as it will cause a change in rotational direction for connected equipment such as motors, fans, or pumps. To monitor phase reversal, the relay simply monitors the sequence of the three phases and trips if it deviates from the defined sequence.