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14/11/2005
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Tom Shelley reports on a development that should much to improve condition monitoring of systems both electrical and mechanical
By installing small infra red sensors in front of critical items, it is possible to detect functional deterioration, particularly in bearings and electrical terminations, long before they fail, even when they are deep inside machinery or electrical enclosures.
The sensors are thermopiles, with cold junctions attached to their casings so they respond to detected rises in temperature above ambient. The base technology of the detectors is American, developed by Exergen, based in Boston, but the incorporation into sensing and monitoring systems is in the hands of Q H i, which is based in Harpenden.
One of their big advantages over other systems is that they require no external power source and use no amplification, so are immune to drift and relatively insensitive to inductive pick up of electromagnetic interference. Another advantage is that because they are small, they can be mounted inside electrical equipment and cabinets, whereas thermography using infra-red cameras has to be conducted from outside, and is unlikely to detect warm areas deep inside.
According to Ross Kennedy of Q H i, the sensors are normally set to detect thermal radiation in the range 6.5 to 14 microns, in a 25mm circle 25mm from the device. Many variants, however, are possible, according to application. For example, as used by Eurotunnel, the sensors are optimised to detect thermal radiation in the range 2 to 20 microns and devices can be made with narrower acceptance cones, down to 25mm circles, 250mm from the sensor. The sensors can be installed pointing horizontally or downwards, but not upwards, otherwise they will be reduced to accurately measuring the temperature of dust on their front surfaces rather than what lies beyond. While they are relatively unaffected by electromagnetic induction, it is wise not to run the data cables alongside power cables or busbars and where possible, data cables should be perpendicular to cables carrying heavy currents. They are nonetheless highly suitable for monitoring high current switchgear inside electrical cabinets.
Basic installed price is about £150 to £200 per sensing point. Sensors are linked back via data acquisition cards to a PC or server, to produce trend analyses, graphs and reports. Two separate alarm levels per sensor automatically activate in the event of pre-set temperature levels being exceeded. Standard systems can have up to 348 sensors, while the XL system can accommodate several thousand. Clear screen displays show which sensor has triggered the alarm. Mean time between failures is specified at over 1,000 years. Used with suitable interfacing devices, data can be transmitted by LAN, WAN, Internet or wireless to almost any location in the world.
Other installed applications include cruise ships, and there is considerable interest in installing them in liquid natural gas carriers. They are also suitable for monitoring critical electrical system components in computer data centres, defence and government establishments, city metro systems and process plant as well as motors, pump bearings and gearboxes.
Q H i Group
Ross Kennedy at Q H i Group
Pointers
* Infra red sensors based on thermopiles can be used inside electrical cabinets to monitor the temperatures of critical system components relative to ambient
* They are also suitable from monitoring temperature rise in bearings and gearboxes, which is generally considered to be an unequalled method of detecting problems before they have a chance to result in damage or failure.
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Author
Tom Shelley
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