How relays protect essential devices and products against problems that can occur with AC power supplies.
Will Darby, of metering, controls and automation specialist Carlo Gavazzi UK, offers his thoughts on how to select the most appropriate machine monitoring relay for various applications
“Machine monitoring relays monitor the status of the main power circuit to ensure efficient detection of any abnormalities on the incoming power supplies, protecting people and equipment. When a fault is detected a monitoring relay will trigger a response, such as signalling a shut-down relay to safely power down the equipment.”
“These compact devices are used as protection in a wide variety of electrical applications including: heating, ventilation and air-conditioning equipment; lifts and escalators; food and beverage production and dispensing equipment; pumps and other motors.”
Multiple monitoring families
“Carlo Gavazzi, for example, manufactures eight different families of monitoring relays to monitor most electric measurements, including 3-phase, voltage, current, frequency, power, power factor, temperature and earth leakage, all designed to protect important devices and products against unlikely problems.
“With such a wide range of capabilities, it is important to select a relay to monitor the aspect of incoming power most critical to protecting an application. For example, inconsistencies in the incoming three-phase power supply quality, such as improper phase sequence, phase loss, and phase imbalance can all cause catastrophic damage to equipment. If a three-phase motor, for example, receives two of the phases out of sequence, it will reverse direction, causing damage to the motor and, more importantly, potentially endangering life where the motor is driving a lift, travelator or material loading plant.”
“Likewise, losing one of the three phases of incoming power can leave a motor unable to operate or, at best, force it to operate at a lower rating. This will lead to a loss in the motor’s performance, generate more heat and will very likely shorten the motor’s life. A similar outcome will occur, if all three phases are present, but one or more of the phases is underpowered, because single phase loads are drawing power unequally on one of the phases. A monitoring relay, such as Carlo Gavazzi’s DPA52 and DPB52 models, has been developed to protect loads from wrong phase sequence and phase loss.
“Voltage is another issue where variations can cause damage to equipment. If a voltage is lower than is required by a motor, then the motor will draw more current, be less efficient and produce more heat than it should, which may shorten its life. Conversely, if the voltage is higher than that at which a component has been designed to operate, the component will become stressed and could be permanently damaged. Voltage monitoring relays are available that measure over- or under-voltages from a few millivolts up to several hundred volts.
“Carlo Gavazzi’s DPD series is a 3-phase multifunction monitoring relay with NFC communication. This compact, 3-phase voltage and frequency monitoring relay comes with two factory default settings, which can be modified, if they are inappropriate for the application, through an App via smartphone or PC and uploaded into the DPD by means of its NFC communication. For hazardous environments, Carlo Gavazzi manufactures the DUB72, a double under-voltage monitoring relay which has two set levels and two outputs to enable one to be used as a pre-alarm and the second as an alarm.
“For DC supplies, voltage relays can be used to provide the status of battery-backup power supplies on generator systems, for example.”
Current monitoring relays
“Monitoring the current is another option for indicating the performance of a component. Equipment that suddenly starts drawing more current may be experiencing a voltage imbalance or it may be the sign of a mechanical problem, such as an impending bearing failure, or some other issue. Current monitoring relays, such as Carlo Gavazzi’s DFB01 model, help in detecting these variations and in taking action before damage to the machine occurs.
“Low current AC or DC inputs can be connected to most monitoring relays directly. Higher currents generally need to be monitored through a shunt or current transformer, although in some instances AC currents up to 100A can be directly monitored by some modules.
“Power monitoring relays can be used to detect variations in the power consumption for various loads; depending on the relay they can be used to react to either excessive loads or no-load, motor load variation
“Temperature monitoring relays can be used to help prevent equipment overheating and to prevent fires. These relays use PTC thermistors; these have a positive temperature coefficient, which means that their resistance increases as the temperature increases. They can monitor the temperature of up to six motors, or motor windings, at once and can be designed to shut down a motor, if it does start to overheat, to prevent any damage occurring. Carlo Gavazzi’s DTA series of motor thermistor relays protect electric motors from overheating, ensuring continuous production and preventing machine downtime.”
Proactive insurance policy
“They can be supplied with any voltage from 24 V to 240 V either AC or DC with two Normally Open NEMA B300 rated, or two SPDT relay outputs. The DTA04 and DTA72 models can be configured for auto or manual reset, whereas the DTA71 is only auto. The DTA71 and DTA72 come with DIN 43880 housing, making them suitable for the distribution boxes and panels for typical domestic and industrial building installations.
“Relays can be designed to shut down power to a motor, for example, similar to the operation of a fuse. The relay’s response will depend on the application and the consequences of the abnormality occurring. More commonly, the monitoring relay will signal a shutdown relay, which will then safely power down the equipment; or the relay will signal a programmable logic controller, which can then trigger a variety of responses. For this reason, relays can be thought of as a proactive insurance policy. They typically cost less than 10% of the equipment that they are protecting and, in addition to protecting individual machines, they can also help minimise lost production time.”