How to Protect Expensive Machinery Using Inexpensive Three-Phase Monitor Relays

Most commercial and industrial machines found in heating, ventilation, and air conditioning (HVAC) systems, water treatment, material handling, machine tool, marine, and aviation applications are powered by three-phase motors. These reliable devices can be damaged by a range of fault conditions which if not addressed quickly, can lead to failure or at least a shortened operating lifetime, resulting in significant downtime and repair costs.

Phase monitoring relays can be used to detect these faults, notify operators, and stop the machine before permanent damage is done. These relays can detect that all three phases are present, have the correct sequence, and have phase voltages within a correct range. If an error occurs, the relay can open a set of contacts that can initiate an alarm condition or power down the machine.

A wide variety of phase-sensing relays are available to handle a broad range of voltages, three-phase configurations, and error conditions. This article briefly discusses potential motor failure modes and how phase-sensing relays operate. It then describes the selection and application of three-phase monitoring relays using real-world examples from Carlo Gavazzi.

Three-phase motor failure modes

Among the common faults across a range of industries are those related to the three-phase power source and its effect on motors. Loss of one of the three phases or an imbalance in the phase voltages can result in higher-than-normal currents in the remaining phases to the motor, leading to loss of mechanical power, and excessive mechanical vibration. Similarly, undervoltage and overvoltage force the motor to increase current in order to drive the same load, which can shorten the life of the motor.

Another possibility is to have an incorrect phase sequence that could cause the motor to reverse direction, which can have disastrous results on the load side of the motor.

Phase monitoring relay functions

Phase monitoring relays have a range of functions related to the state of the three-phase power source. They are powered by the three-phase lines that they monitor. All of them monitor the phase sequence and the loss of any phase voltage. The relays activate when all phases are present, and the power sequence is correct. In the event of a loss of any of the phases or an incorrect phase sequence, the relay is de-energized. Some phase monitoring relays also monitor the voltage levels of all three phases. The voltage determination uses a true root mean square (TRMS) measurement and deactivates the relay when the amplitude drops below a preset threshold. Some devices offer the same amplitude detection but have user adjustable limit settings. A range of relay models monitor phase asymmetry and tolerance. All the relays offer a delay before actuation due to voltage level or asymmetry issues to prevent spurious activation. In some models the delay is programmable.

Phase monitoring relay options

The DPA01CM44 from Carlo Gavazzi Inc. is an example of a phase-sensitive relay for three-wire configurations (Figure 1). The series is powered from the three-phase source, with models available that operate at voltages of 230, 400, 600, and 690 volts AC (VAC). These relays are intended for mounting on a DIN rail, with plugin versions also available. The relay output configurations are either one single-pole double-throw (SPDT) or dual SPDT contacts.

How to Protect Expensive Machinery Using Inexpensive Three-Phase Monitor RelaysFigure 1: The DPA01CM44 is a DIN rail-mounted phase monitoring relay for 208 to 480 VAC three-phase lines with an SPDT relay contact output. (Image source: Carlo Gavazzi Inc.)

Under normal voltage conditions, the relay is activated, meaning that the normally open (NO) contact(s) of the relay output is/are closed, and the normally closed (NC) output contact(s) is/are open. Relay operation occurs within 100 milliseconds (ms). Status LEDs on the front panel indicate power on and relay activation.

The DPA01 phase monitoring relays can be installed to monitor any three-phase line (Example 1, in Figure 2), or it can be associated with a three-phase motor (Example 2).

How to Protect Expensive Machinery Using Inexpensive Three-Phase Monitor RelaysFigure 2: Application examples for the DPA01 series three-phase monitor relay. Example 1 (left) shows it connected to monitor a three-phase line. Example 2 (right) illustrates its use in monitoring a three-phase motor. (Image source: Carlo Gavazzi Inc.)

When an error condition occurs in either application, a contactor connected to the relay in an appropriate way opens the continuing three-phase circuit, while the monitoring relay remains connected to the power source. When connected to monitor a three-phase line without a motor, there is no problem detecting a dead phase because the level of the missing input drops significantly. When a monitor is connected to a motor, the operation of the motor will tend to regenerate the missing phase due to internal inductive coupling. The relay is set to detect a missing phase as long as the amplitude is less than 85% of the nominal three-phase voltage. Consequently, it is important to verify that the motor regeneration is less than 85% of the nominal line voltage.

In applications that require a custom overvoltage or undervoltage threshold, Carlo Gavazzi’s DPB01CM48 multi-function phase monitoring relay can be used. This relay detects overvoltage and undervoltage conditions, as well as lost phase and improper phase sequence. The overvoltage and undervoltage threshold levels are user-adjustable via a front panel switch (Figure 3).

How to Protect Expensive Machinery Using Inexpensive Three-Phase Monitor RelaysFigure 3: The DPB01CM48 multi-function phase monitoring relay monitors phase loss, phase sequence, and selectable overvoltage and undervoltage thresholds using front panel controls. Operational delay for the overvoltage and undervoltage actuation is user adjustable to minimize transient activation. (Image source: Carlo Gavazzi Inc.)

This relay is also DIN rail mounted and supports both delta and star/wye three-phase topologies. It has user-programmable voltage ranges selectable as 380, 400, 415, 480 VAC line-to-line voltages, and 220, 230, 240, and 277 VAC line-to-neutral voltages. Circuit topology and voltage ranges are set using an internal dip switch. A separate model in the DPB01 family handles lower voltage ranges.

There are three control knobs on the front panel of the relay. The upper one sets the lower voltage threshold from -2 to -22% of the nominal voltage, the middle one sets the upper voltage threshold from 2 to 22% of the nominal voltage, and the lowest control is used to set the operational delay between 10 ms to 30 seconds (s). The delay is used to prevent short duration voltage transients from accidentally tripping the relay.

There are three LEDs on the front panel of the relay to indicate its status. A green LED indicates that power is on. A yellow LED indicates that the relay is energized. If there is an alarm event, then the yellow LED is turned off and the relay is de-energized after a timeout, and the red LED will flash. If the error was an overvoltage or undervoltage condition, the red LED flashes at a frequency of 2 Hertz (Hz), and the relay will be de-energized after the timeout. If a lost phase or incorrect phase sequence occurs, the relay is de-energized within 200 ms, and the red LED flashes at a 5 Hz rate for the duration of the error condition.

The DPC01DM69 phase monitoring relay has Carlo Gavazzi’s fullest feature set (Figure 4). This DIN rail mount relay monitors phase sequence and phase loss, as well as undervoltage and overvoltage, phase symmetry, and tolerance.

How to Protect Expensive Machinery Using Inexpensive Three-Phase Monitor RelaysFigure 4: The DPC01DM69 is a DIN rail mount phase monitor relay with LEDs on the left edge indicating relay states (yellow), alarm status (red), and power status (green). Controls set tolerance/asymmetry or overvoltage/undervoltage thresholds, and relay delays. (Image source: Carlo Gavazzi Inc.)

The DPC01DM69 operates similarly to the DPB01CM48 in terms of selecting the operating voltage range and three-phase topology. It has additional dip switch selections for power on delay (1 s or 6 s), output configuration (dual set of SPDT contacts or a single set of double-pole, double-throw (DPDT) contacts), and monitoring functions for overvoltage/undervoltage or asymmetry/tolerance.

The DPC01CM69 operates in three different modes depending upon the alarm type. Phase loss or incorrect phase sequence results in de-energizing output relays 1 and 2. An overvoltage or asymmetry event causes the output 1 relay to de-energize at the end of pre-set delay 1, and an undervoltage or out-of-tolerance event causes the output 2 relay to de-energize at the end of pre-set delay 2. This dual output arrangement allows for greater flexibility in response to different alarm events.

Asymmetry is an indicator of the power source quality and is defined as the absolute value of the maximum deviation of the line voltages, divided by the nominal voltage of the 3-phase system. Tolerance is another indicator of the power source quality and is defined as the absolute value of the maximum deviation of the line voltages from the nominal voltage, divided by the nominal voltage of the 3-phase system (Figure 5).

How to Protect Expensive Machinery Using Inexpensive Three-Phase Monitor RelaysFigure 5: Equations for asymmetry and tolerance of a three-phase source. (Image source: Carlo Gavazzi Inc.)

In general, the unbalance of the three-phase lines should be less than 2% in order to avoid overheating in a three-phase motor. Note that the asymmetry and tolerance scales on the DPC01CM69 cover ranges from 2 to 22%.


Across a wide swath of industries, three-phase AC monitoring is critical to avoid costly downtime due to damaged motors. To ensure this is accomplished easily and effectively, Carlo Gavazzi offers a broad range of three-phase monitor relays designed using the most sophisticated measurement techniques, while also offering easy setup. The relays range from simple devices that monitor phase loss and phase sequence to more complex monitors that additionally sense overvoltage and undervoltage conditions, as well as phase asymmetry and tolerance. These phase monitor relays provide assurance that three-phase power source issues will not cause damage to expensive machinery.