Test and Measurement company Fluke Australia shares some methods using infrared (IR) thermometers to test electrical systems and equipment.
Monitoring electrical systems
Infrared thermometers (IR) are useful for both diagnostic and predictive inspection of electrical systems and equipment. As electrical currents generate heat, temperature monitoring is an efficient way to predict potential equipment failure. Infrared non-contact thermometers let users quickly gather temperature information.
Infrared thermometers can provide precise readings with 0.75 to 2 per cent accuracy, from as far away as 35 feet, depending on the model used. They require no set-up and have a response time of less than a second. These instruments are lightweight, rugged, and easy-to-use.
Measuring electrical components
Non-contact IRs measure the surface temperatures of an object from a safe distance. Infrared thermometers measure surface temperature so accurate results are obtained only when the target is directly visible, which means that covers and enclosures need to be removed to expose the object to be measured.
Motors and oil-filled transformers and circuit breakers can be measured directly because the surface temperatures of their enclosures generally correlate to the internal temperature. The following applications can be part of a comprehensive preventative maintenance program to prevent equipment failures and unscheduled down time.
Connectors
Normal on/off current loading and environmental temperature changes result in repeated heating (expansion) and cooling (contraction) of connections, which can gradually loosen the connector. As the loosened connector has higher resistance to current flow, it dissipates power and, as a result, generates heat. Similarly, dirt, carbon deposits, and corrosion in connections also cause higher resistance.
When evaluating connections, it is important to know the temperature difference between the connector and the ambient temperature. If ambient temperature is unknown, it can be quickly determined with the non-contact thermometer. Increases of 10 °C from ambient temperature indicate a poor connection, ground in circuit, or unbalanced load. It is likely that a temperature reading of 30 °C or more above ambient indicates a serious problem.
Electric motors
Industrial plants often have hundreds of polyphase motors in operation. To ensure a motors life span, temperatures must be monitored to verify balanced phase-to-phase power distribution and correct operating temperatures. The National Electrical Manufacturers Association (NEMA), recommends a ± 1 per cent power balance to prevent damage or motor burn-out, and the IR thermometers can be used to inspect supply power connections and circuit breakers (or fuses) for equal temperatures.
Motor bearings
Heat is generated when bearings break down, causing the motor to vibrate and become off-centered. Scanning bearing temperatures with an IR lets the maintenance engineer detect hot spots and schedule repairs or replacements before the problem leads to an equipment failure.
Motor winding insulation
The life of motor winding insulation is drastically shortened if operating temperatures exceed rated maximums. The life of normal winding insulation is about 10 years.
Certain standard test procedures, such as the IEEE ‘Meg-Ohm’ test for motors 50 horsepower and above, require knowledge of the motors temperature to obtain accurate results. In such cases, the instantaneous digital temperature display of an IR is invaluable. IRs are also effective in determining the source of the problem when a thermal overload protection device does not work and the motor shuts down.
Phase-to-phase measurement
High-voltage, three-phase power circuits are common in industrial electrical systems. This is of importance to induction motors, large computers, and other equipment, which require balanced phase-to-phase power. If the power balance is not maintained due to an overload or ground in the circuit, damage and downtime can result. Checking cables and connectors with non-contact thermometers for equal phase-to-phase temperatures will quickly show if there’s a difference of 5 °C (9 °F) or more, indicating a problem.
Transformers
Maximum permissible operating temperatures are usually listed on the transformer. The windings of air-cooled units can be measured directly with an IR to verify overall temperature, any hot spots indicate winding flaws.
Wires and cables
Wires and cables can be monitored with a non-contact thermometer to identify heat caused by cracks, corrosion, or deterioration. When comparing two cables, the one with the higher temperature is carrying the larger current.
Uninterruptible power supplies (UPS)
DC battery connections are susceptible to loosening and corrosion, which can create excess heat. Hot localised connections in the UPS output filters can be identified with an IR. A cold spot may indicate an open dc filter circuit.
Back-up batteries
Low voltage batteries should be checked with a non-contact thermometer to ensure proper connections. Poorly attached cell strap connections in a battery string may heat up enough to burn the posts.
Ballasts
Aging electrical components cause lighting fixtures to overheat. Using an IR can detect an overheated ballast before it begins to smoke.
Utilities
Infrared readings can quickly and cost effectively identify hot spots in connections, cable splices, transformers, and other equipment. Routine temperature audits will help prevent the enormous costs that result from equipment failures and system shutdowns.
Interpreting results
Determining if there is a true problem once a temperature reading has been taken is a combination of the technician’s own experience with the equipment and the ratings provided by the manufacturer of the electrical components being monitored. Electrical equipment manufacturers usually list on the rating plate the maximum allowable temperatures.
Fluke Australia can be contacted by calling 02 8850 3333, or visiting www.Fluke.com.au