(Click here to view article in digital edition)Machine monitoring relays detect abnormalities in incoming power supplies to protect 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.Carlo Gavazzi manufactures seven different families of monitoring relays to monitor most electric measurements including 3-phase, Voltage, Current, Frequency, Power, Power factor, and Temperature which are designed to protect important devices and products.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 cause catastrophic damage to equipment. If a three-phase motor 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 most 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. Monitoring relays such as Carlo Gavazzi’s DPA52 and DPB52 models have 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 Near-Field Communication (NFC). This compact, 3-phase voltage and frequency monitoring relay comes with 2 different 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. 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.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. 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 variationTemperature 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 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 starts 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. 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 PLC, 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. For help in selecting machine monitoring relays contact Carlo Gavazzi UK by visiting www.carlogavazzi.co.uk About the author:Will lives in Leeds with his family and has been with Carlo Gavazzi for over 9 years. As a keen sportsman and when not playing golf, Will can be found watching various sports especially football, cricket and golf; and if he’s not at home, you might just spot him at Guiseley AFC/CC!
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