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By Larry Massey
Most large industrial plants have adopted infrared (IR) thermography as part of its predictive/preventative maintenance (PPM) operations. The technology has proven its value by revealing conditions hazardous to personnel, and those that are precursors to equipment failures that cause lost production. The use of IR camera scans is based on the fact that many types of equipment anomalies cause a temperature rise associated with an impending failure or fire. IR cameras can identify these conditions in both mechanical and electrical equipment by providing non-contact temperature measurements and thermal images of the equipment. IR scans are especially valuable for improving the safety of electrical equipment by helping personnel identify potential fire and shock hazards.
In electrical gear, one of the main threats to safe, ongoing operation is poor connections. These can be caused by faulty installation, equipment vibration, temperature cycling, and failure to tighten connections properly after repair work. Beyond safety issues and outright failures, there’s another often overlooked problem that loose connections cause – power loss from increased contact resistance.
The temperature rise in most electrical circuits is primarily caused by power dissipation from Ohmic heating. This is particularly true of electrical contact junctions, where inductive and capacitive reactance losses are minimal. Therefore, using Ohm’s Law, the power loss at a junction can be calculated from the equation P = I2R, where P is the power dissipated in watts, I is the current through the junction in amperes, and R is its contact resistance in ohms.
“While its a good idea to identify loose connections, finding them can be a bit like finding a needle in a haystack. ”- Larry Massey
When making electrical connections, the intent is to make contact resistance as low as possible to minimize power loss. If connections are loose, R is higher, as is the power loss and the temperature at that junction. This means the circuit is operating inefficiently and drawing more power from the electrical supply than necessary — the greater the current, the greater the power loss. While one loose connection might not cause much power loss, there can be many of them in a large plant. If current and contact resistance is high enough at these loose connections, they can cause a significant increase in a plant’s electrical utility bill over the course of a year.
Many clients already reap many PPM benefits from IR inspections, but during the economic downturn, some are looking for additional ways to save. Generally, they want a focus on IR scans to locate and fix anomalies that are critical for safe, uninterrupted operation. However, we’ve come to believe there is significant economic benefits in fixing loose connections that drain power unnecessarily. This situation is analogous to using ultrasound surveys to find and repair air leaks, gas leaks and malfunctioning steam traps. These PPM programs can greatly reduce heating costs and the cost of producing compressed air for plant equipment.
Figure 1. Typical IR camera used for PPM thermography (Photo courtesy of FLIR Systems, Inc.)
While it’s a good idea to identify loose connections, finding them can be a bit like finding a needle in a haystack. It’s just impractical to go around taking electrical measurements on every connection in a plant to find the really bad ones. Fortunately, thermography using and IR camera (Figure 1) can make this task a lot easier by scanning large areas of plant equipment for abnormally high temperatures. Still, proper methods must be followed, including the appropriate analysis and calculations.
Some of our larger plants agree that fixing loose connections on equipment that consumes lots of power might have results similar to fixing air and gas leaks, but they want economic justification in the form of hard numbers on the cost savings that could be achieved from IR inspections. This is part of a broader business trend we see, with more companies using efficiency studies to justify maintenance work through real-time cost savings. Therefore, we try to validate the methods for finding loose connections and associated power losses.
The aim is to establish simple techniques and calculations that are easy for thermographers to use. Rather than trying to find exacting methods with a high level of precision, we were looking for good approximations of power loss that, when corrected, would be high enough to show a positive payback on the time invested in IR scans. We recognized that this would vary from plant to plant, because each one could have a different unit cost per kilowatt-hour. However, the general approach would have to be the same for every plant.
The validation procedure was to gather non-contact temperature data from electrical junctions with an IR camera and use that as the input to a mathematical model for calculating thermal heat losses. This model uses conventional equations for heat transfer via convection and radiation. The thermal heat loss was then compared to a calculation of power dissipation using Ohm’s Law. However, instead of using P = I2R, we measured the voltage across the junction (ΔV), current (I) through it and calculated the power loss using P = IΔV. Note that contact resistance can be calculated from R = ΔV/I.