“Most commercial and industrial machines commonly found in heating, ventilation and air conditioning (HVAC) systems, water treatment, material handling, machine tools, marine and aerospace applications are driven by three-phase motors. These reliable pieces of equipment can be damaged by a range of failure conditions that, if not addressed quickly, can lead to failure or at least shortened operational life, resulting in significant downtime and repair costs.
Author: Art Pini
Most commercial and industrial machines commonly found in heating, ventilation and air conditioning (HVAC) systems, water treatment, material handling, machine tools, marine and aerospace applications are driven by three-phase motors. These reliable pieces of equipment can be damaged by a range of failure conditions that, if not addressed quickly, can lead to failure or at least shortened operational life, resulting in significant downtime and repair costs.
Phase monitoring relays can be used to detect these faults and notify the operator to stop the machine before permanent damage is caused. These relays can detect that all three phases are present, have the correct sequence, and keep the phase voltages in the correct range. If an error occurs, the relay can open a set of contacts, initiate an alarm condition or shut down the machine.
A wide variety of phase sensitive relays are available to handle a wide range of voltages, three phase configurations and error conditions. This article briefly discusses potential motor failure modes and how phase sensitive relays work. Then, using Carlo Gavazzi’s actual prototype, the selection and application of three-phase monitoring relays are introduced.
Three-phase motor failure modes
In many industries, the common faults are basically related to the three-phase power supply and its impact on the motor. Loss of one of the three phases or an imbalance of phase voltages can cause higher than normal currents in the remaining phases of the motor, resulting in mechanical power loss, and excessive mechanical vibration. Likewise, undervoltage and overvoltage force the motor to increase current to drive the same load, which shortens the life of the motor.
Another possibility is an incorrect phase sequence that could cause the motor to reverse, which would have catastrophic results on the load side of the motor.
Phase monitoring relay function
The phase monitoring relay has a series of function blocks related to the status of the three-phase power supply. They are powered by the three-phase line they monitor. All these function blocks are monitoring the phase sequence and any phase voltage loss. The relay is activated when all phases are present and the power sequence is correct. In the event of either phase loss or incorrect phase sequence, the relay will de-energize. Some phase monitoring relays also monitor the voltage levels of all three phases. Voltage measurements use true root mean square (TRMS) measurements, deactivating the relay when the amplitude falls below a preset threshold. Some devices offer the same amplitude detection, but with user-adjustable limit settings. Many relay models can monitor phase asymmetry and tolerances. All relays provide a delay before actuating due to voltage levels or asymmetry issues to prevent spurious activation. On some models, the delay is programmable.
Phase monitoring relay selection
Carlo Gavazzi’s DPA01CM44 is an example of a phase-sensitive relay used in a three-wire configuration (Figure 1). The series is powered by three-phase power and offers models that operate at 230, 400, 600 and 690 volts alternating current (VAC). These relays are intended for DIN rail mounting and are also available in plug-in versions. The relay output configuration uses single pole double throw (SPDT) or dual SPDT contacts.
Figure 1: The DPA01CM44 is a DIN rail mounted phase monitoring relay for 208 to 480 VAC three phase line with one SPDT relay contact output. (Image credit: Carlo Gavazzi Inc.)
Under normal voltage conditions, relay activation means that the normally open (NO) contacts of the relay output are closed and the normally closed (NC) output contacts are open. Relay operation occurs within 100 milliseconds (ms). Status LEDs on the front Panel are used to indicate power on and relay activation.
The DPA01 Phase Monitoring Relay can be used to monitor any three-phase line (Example 1 in Figure 2) and can also be associated with a three-phase motor (Example 2).
Figure 2: Application example of DPA01 series three-phase monitoring relay. Example 1 (left) shows its connection to monitor a three-phase line. Example 2 (right) shows its application in monitoring a three-phase motor. (Image credit: Carlo Gavazzi Inc.)
When an error condition occurs in both applications, a contactor properly connected to the relay opens the energized three-phase circuit, while the monitoring relay remains connected to the power supply. When monitoring a three-phase line without a motor when connected, detecting dead phases is no problem, as the missing input level will drop significantly. When the monitor is connected to the motor, the operation of the motor will tend to regenerate the missing phase due to internal inductive coupling. The relay is set so that a phase loss is detected as long as the amplitude is less than 85% of the rated three-phase voltage. Therefore, it is important to verify that the regenerative capacity of the motor is less than 85% of the rated line voltage.
In applications that require custom overvoltage or undervoltage thresholds, Carlo Gavazzi’s DPB01CM48 multifunction phase monitoring relay can be used. The relay can detect overvoltage and undervoltage conditions, as well as loss of phase and improper phase sequence. Overvoltage and undervoltage threshold levels are user adjustable via front panel switches (Figure 3).
Figure 3: The DPB01CM48 multi-function phase monitoring relay monitors phase loss, phase sequence, and selectable overvoltage and undervoltage thresholds using front panel controls. Operation delays for overvoltage and undervoltage start-up are adjustable by the user to minimize transient activation. (Image credit: Carlo Gavazzi Inc.)
This relay is also DIN rail mounted and supports delta and star/wye three-phase topologies. It has a user programmable voltage range with a choice of 380, 400, 415, 480 VAC line-to-line, and 220, 230, 240, 277 VAC line-to-neutral. The circuit topology and voltage range are set via an internal DIP switch. There is a separate model in the DPB01 series to handle the lower voltage range.
The relay has three control knobs on the front panel. The upper button is used to set the lower voltage threshold of -2 to -22% of the rated voltage, the middle button is used to set the upper voltage threshold of the rated voltage of 2 to 22%, and the lower button is used to set the voltage between 10 ms and 30 seconds(s). Operation is delayed. The time delay is used to prevent accidental tripping of the relay due to short voltage transients.
There are three LEDs on the front panel of the relay to show its status. A green LED is used to indicate that the power is on. The yellow LED is used to indicate that the relay is energized. If there is an alarm event, the yellow LED will turn off, the relay will de-energize after a timeout, and the red LED will flash. If the error is an overvoltage or undervoltage condition, the red LED will flash at 2 hertz (Hz) and the relay will be de-energized after a timeout. If a phase loss or incorrect phase sequence occurs, the relay will de-energize within 200 ms and the red LED will flash at 5 Hz for the duration of the error state.
The DPC01DM69 phase monitoring relay has Carlo Gavazzi’s most complete feature set (Figure 4). This DIN rail mounted relay monitors phase sequence and loss of phase, as well as undervoltage and overvoltage, phase symmetry and tolerance.
Figure 4: The DPC01DM69 is a DIN rail mounted phase monitoring relay with LEDs on the left edge indicating relay status (yellow), alarm status (red) and power status (green). Controls are used to set tolerance/asymmetry or over/under voltage thresholds, as well as relay delays. (Image credit: Carlo Gavazzi Inc.)
The operation of the DPC01DM69 is similar to the DPB01CM48 in terms of selection of operating voltage range and three-phase topology. It has additional DIP switch selections for power-on delay (1 sec or 6 sec), output configuration (dual set of SPDT contacts or single set of double pole double throw (DPDT) contacts), and over/under voltage or no Symmetry/Tolerance monitoring function.
The DPC01CM69 operates in three different modes depending on the alarm type. Loss of phase or incorrect phase sequence will cause output relays 1 and 2 to de-energize. An overvoltage or asymmetry event will cause the output 1 relay to de-energize at the end of preset delay 1, while an undervoltage or overrun event will cause the output 2 relay to de-energize at the end of preset delay 2. This dual output arrangement makes the reaction to different alarm events more flexible.
Asymmetry is a measure of power quality and is defined as the absolute value of the maximum deviation of the line voltage divided by the rated voltage of a three-phase system. Tolerance is another measure of power quality and is defined as the absolute value of the maximum deviation of the line voltage from the rated voltage divided by the rated voltage of a three-phase system (Figure 5).
Figure 5: Asymmetry and tolerance equations for a three-phase power supply. (Image credit: Carlo Gavazzi Inc.)
Generally speaking, the unbalance of the three-phase line should be less than 2% to avoid overheating of the three-phase motor. Note that the asymmetry and tolerance on the DPC01CM69 envelope is marked as 2% to 22%.
Across industries, three-phase AC monitoring is essential to avoid costly downtime due to motor damage. To ensure this, Carlo Gavazzi presents a wide range of three-phase monitoring relays, designed with state-of-the-art measurement technology and simple to set up. These relays range from simple devices monitoring phase loss and phase sequence to more complex monitors, and can also sense overvoltage and undervoltage conditions, as well as phase asymmetry and tolerance. These phase monitoring relays ensure that three-phase power problems do not cause damage to expensive machinery.
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