Temperature change is often a red flag of impending harm to people or equipment. Today, accurate temperature measurement is easy; if you know what to do and where to look.
Equipment that conducts, generates, or consumes power emits heat. Heat becomes a problem when current flow is excessive and temperatures rise beyond maximum specified values. At that point, you begin to see the threat of arcing, fires, power outages, insulation damage, and other situations that create hazards and downtime.
Harmonic current overload is a major source of excess heat. Harmonics exist where you have computers, adjustable speed drives, lighting ballasts, and other such nonlinear load equipment. You'll also find excess heat where you have faulty windings or connections; as well as many other conditions. Measuring temperature is an efficient and reliable way to anticipate or pinpoint problems in electrical circuits and equipment.
By monitoring these critical temperatures, you can identify developing problems in time to take preventive action. The easier it is to do the monitoring, the more likely your team will do it. The harder it is to do, the less time busy plant technicians will find to do it. This is the argument for noncontact infrared (IR) temperature measurement. Consider, for example, how fast IR instruments are. You can take a measurement in one quarter of a second. This means you don't waste time waiting for thermocouples to stabilize.
The "Cadillac" of such measurement is thermographic imaging (TI). TI, like a Cadillac, carries a premium price. And, like a Cadillac, it's not always the appropriate vehicle to take you where you need to go. Nor can everybody afford one. For much less money and much less training, the IR thermometer stands out as the tool of choice for most noncontact IR measurement tasks. These convenient point-and-shoot devices are fast, accurate, and efficient. Their technology has proven itself in over 40 years of use.
Safety and convenience. IR thermometers are small, lightweight, and require little training to use effectively. You simply point the device at the target, pull the trigger, and check the readout. The thermometer monitors surface temperature from a distance by measuring the infrared radiation naturally emitted from an object (Fig. 1, in original article). The detector reads the energy signal, and the instrument's electronics converts that reading to a temperature value; which you see displayed on an LCD. On most instruments, accuracy is 51% of the reading or 51DegrC (52DegrF).
Because you can take a precise reading from several feet away, you reduce or eliminate the risk of burned fingers, electrocution, and falling. The distance depends on the spot size desired and the focal length of the instrument.
Flexibility. There are three common ways to take readings.
• Spot measuring: This method determines the absolute surface temperature of objects. While it's an effective way of using IR devices, it's not nearly as productive as the following two methods.
• Temperature differentials: This method compares two separate spot measurements, either in different locations or at the same location over time. It's an extremely productive method, because it allows you to eliminate variables that spot measuring cannot account for. It also allows you to use trending to predict what will happen.
• Scanning: This method focuses on targets such as circuit breakers, cable connections, or transformer housings to identify hot spots (high temperature points). With this method, you simply pull the trigger and slowly move the instrument along the target. When you see sudden fluctuations in temperature, you've found hot spots.
Using the instrument. IR thermometers measure only the surface temperature of objects, and do not read through panels or covers. However, sometimes a panel will heat up, if the connections behind it are very hot. The surface temperature of motors, oil-filled transformers, and circuit breakers correlate directly to operating conditions. Let's take a look at some ways to use this instrument.
Pinpointing nuisance tripping of circuit breakers. Checking circuits individually by conventional means can take forever; and cause major disruptions in operations. But with an IR thermometer, you can isolate suspect circuitry just by scanning the panel.
Identifying sources of energy loss. Connectors or other components may be draining energy because of heat created by loosening and wear or by the buildup of dirt, carbon deposits, and corrosion; all of which create unwanted resistance. A portable IR thermometer allows you to check connectors and components from a variety of distances and angles. Temperature differentials of 10DegrC (18DegrF) from ambient indicate the early stages of a problem you need to investigate. Differences of 30DegrC (45DegrF) warrant immediate investigation.
Protecting electric motors. Polyphase induction motors represent about 85% of all electric motors used in industry. An industrial plant may run hundreds of these motors daily. A motor circuit has a breaker (or fuses), starter, thermal overload protection device for each phase, and the motor itself. Any one of these can overheat unexpectedly. A small voltage unbalance is lethal to a motor, but you can use your IR thermometer to verify balanced phase-to-phase power distribution; as well as check connections. IR is also ideal for checking motor bearing and gear box temperatures.
The speed and convenience of an IR thermometer gives you the ability to respond quickly to a motor shut down. Using it as a forensic tool, you can see if the problem is in power supply connections, circuit breakers, fuses, thermal overload protection devices, or elsewhere. An IR scan can even determine if you have a clogged air filter.
Protecting transformers. Regular scans of transformer housing, winding, and power terminal temperatures provide a better understanding of what's going on in this equipment. You can even detect winding flaws.
Other devices. Checking low-voltage batteries ensures proper connections and identifies poorly attached cell strap connections in a battery string that may heat up enough to burn posts and create hazards. You can spot heat buildup in lighting ballasts before they start to smoke. A large UPS has hundreds of battery connections, all prone to loosening and corrosion. You can also look for cold spots in the UPS, which may indicate an open DC filter circuit.
Interpreting results. So, you measure with an IR thermometer and see temperatures you don't like. But how do you know if you really have a problem? The answer lies in the combination of published rating information and personal experience. Electrical equipment manufacturers normally define maximum permissible operating temperatures, frequently listing this on the equipment's rating plate.
The real benefit comes from having temperature histories for your equipment. Recording and trending temperatures will allow you to see when a problem is beginning. Catching problems before symptoms are obvious is the best way to keep expenses and downtime to a minimum.
Selecting the right tool. When choosing an IR thermometer for electrical maintenance, consider the size of the targets you monitor, their distance from where you'll monitor them, the temperature ranges to cover, whether you want laser sighting and variable emissivity settings, and your budget. Let's look at each of these, starting with cost.
IR thermometers suitable for electrical maintenance range from under $99 (basic models without laser sighting) to over $1200 for more sophisticated models with built-in data logger and a host of advanced features.
We express the optical resolutions of IR as a distance-to-spot size (D:S) ratio. To get the D:S, simply divide the distance (from the object to the thermometer) by the size of the spot you're measuring. The D:S is critical, because accuracy requires your target to be at least as large as the diameter of the spot you're reading. The object should fill the sensor's field of view. You need to position the object so background infrared radiation doesn't enter the optics' field of view (Fig. 2, in original article). So, base your choice of IR thermometer on how far the instrument will be from the target and the size of the target.
Select an instrument with a temperature range most closely matching your application range. Some instruments measure temperatures as low as 125DegrF (132DegrC) for subzero applications. Others measure temperatures as high as 5400DegrF (3000DegrC) for specialized applications involving metals and glass.
Most users opt for laser sighting. Some of the newest models even offer a multi-point laser sighting circle with center aiming point to delineate the precise area you're measuring. Other options include such features as data storage. Some IR thermometers can store readings for later downloading to a computer. Make sure to use your IR thermometer on a regular basis.