What will concerned parents do when their child says, “Mommy and Daddy, I think I’m sick”? Feel the little one’s forehead, of course. It’s a logical first step — followed, one hopes, by measurement methods more precise than the human touch before making a final diagnosis.

Perhaps it’s this common human scenario that causes people, even those who probably know better, to attempt to judge an ac induction motor’s condition by feeling its “forehead.” Here’s a story from our archives at Leeson that illustrates the perils of making motor diagnoses based on feel:

A motor user facing a humid environment in a part of his plant called for advice on what kind of motor he might use for maximum durability. We recommended he try a washdown-duty motor, which is designed to withstand not only humidity, but even frequent direct blasts from a hose, as in a food processing area. He agreed that though his wasn’t a food plant and he wouldn’t be hosing down the motors, the added moisture-resistance of the washdown motor made sense. So he installed one of our washdown motors, which have among other features a white epoxy exterior finish.

Some time later, this same customer called to say that while the washdown motor was holding up well and had no apparent performance problems, it appeared to be “running hot.” The reason for his concern was that the motor’s white surface was discoloring. Upon investigation, we found that maintenance people had been feeling the motor’s surface, leaving the dirt, oil, and grease of their day’s work behind on the white surface. This “hot motor” problem was solved with spray cleaner and a shop rag. And a check of the motor’s current draw, a much better way to gage performance, showed it to be operating properly.

You can’t tell by touching

Point is, you can’t accurately judge a motor by feeling its surface. Design temperature ratings apply to the hottest spot within the motor’s windings, not how much of that heat is transferred to the motor’s surface. The heat transfer will vary greatly from motor to motor based on frame size and mass, whether the frame is smooth or ribbed, whether open or totally enclosed, and other cooling factors. Even the efficiency of the motor may have little effect on the surface temperature. For example, a premium-efficiency motor, though its internal temperature will be cooler as a result of lower losses, may not have lower surface temperatures, because the ventilation fan will likely be smaller to reduce windage losses. Plus, any motor’s frame surface is a quilt of hot spots and cool spots related to internal air circulation patterns.

Unless you have benchmark lab readings of heat runs that show “normal” surface temperatures for that specific model in exact locations on the frame, a motor’s skin temperature provides little if any evidence of what’s going on inside.

Another point: for safety’s sake, no one should be touching most electric motors in the first place, unless they are specially designed to have safe surface temperatures. Such motors include those used on bench grinders, power saws and the like. For those applications, Underwriters Laboratories sets maximum acceptable surface temperatures for a metal “surface subject to casual contact” at 70 C (158 F) after 30 minutes of operation in a 25 C (77 F) room. Even at that temperature, however, you don’t want to touch the surface for long.

The surface temperature of a continuously (and correctly) operating general purpose industrial electric motor will easily be 80 C (176 F) and perhaps as high as 100 C (212 F). You can’t keep your hand on a surface that hot long enough to discern differences, and if you try, you could get a nasty burn.

There are no published standards regarding surface temperatures of general purpose motors, though UL does set such standards for explosion-proof motors. Also, machinery manufacturers sometimes specify unusually low maximum surface temperatures for certain applications. Your motor manufacturer can help you work through the specifics.

Heating, a valid concern

Even if feeling a motor’s surface isn’t the way to judge operating temperature, a motor’s winding temperature is important. The concern, of course, is for the integrity of the motor stator’s insulation system. Its function is to separate electrical components from each other, preventing short circuits and, thus, winding burnout and failure. In most NEMA frame motors, the key insulation system components include magnet wire coating, which insulates wires within a coil from each other; slot cell and phase insulation, typically high-strength polyester sheets that are installed in stator slots to provide phase-to-ground protection; and insulating varnish into which the wound stator is dipped to provide moisture resistance and overall better insulating performance.

Most people who work with motors have heard the common rule of thumb that a 10 C rise cuts the insulation’s useful life in half and a 10 C decrease doubles the insulation’s life. That rule of thumb does not mean that if you can keep a motor cool enough, it will last forever, because there’s more to a motor than just its windings, Also, insulation can have other enemies such as moisture, vibration, chemicals and abrasives in the air that might shorten its life.

The more pertinent issue is the temperature that the motor windings are designed to operate so they give a long and predictable insulation life of 20,000 hours or more. The National Electrical Manufacturers Association (NEMA) sets specific temperature standards for motors of various enclosures and having various service factors. These standards are based on thermal insulation classes — the most common being A, B, F and H. The table summarizes these standards into maximum winding temperatures a motor can attain and still have long insulation life. These are total temperatures, based on a maximum ambient of 40 C, (104 F), plus the additional heat (temperature rise) generated by motor operation. Greater than 40 C ambient may require special application considerations or special motor designs.

The temperatures shown are for motors having a 1.0 service factor. Many industrial motors have 1.15 or higher service factors, indicating a higher overload tolerance, and meaning they could operate safely at higher temperatures. But why push it? Use these maximums and you won’t go wrong.

Class B or Class F insulation systems are most common in today’s industrialduty motors. Smaller sizes, say up to 5 hp, are typically Class B. From 5 to 10 hp, many ratings move toward Class F. That’s also true of premium-efficiency and inverter- duty motors. Larger than 10 hp, Class F becomes most common. Beyond that, many manufacturers design their motors to operate cooler than their thermal class might allow. For example, a motor might have Class F insulation but a Class B temperature rise. This gives an extra thermal margin. Class H insulation systems are seldom found in general-purpose motors, but rather in special designs for very heavy-duty use, high-ambient temperature, or high-altitude conditions.

Class A insulation is not used on today’s industrial-duty motors, though it can be found on some small appliance motors. Class A insulation was, however, standard on industrial motors built in the 1960s and earlier — the so-called NEMAU frame motors, as opposed to today’s NEMA-T frame designs. Because Class A insulation has such a low temperature rating, these older motors were required to have far lower maximum temperatures than today’s Class B and Class F insulated motors. This accounts for the perception among many long-time motor users that modern motors “run hot.” In fact, they do compared with older motors, but their insulation systems are so much better that the reliability and durability of new motors are equal to or better than older-design motors. Plus, older motors achieved cooler operation through the expense of larger frames and more material. Better insulation systems have allowed motor manufacturers to put more horsepower in a smaller package for maximum cost effectiveness.

Determining correct operation

Provided you have purchased a motor from a reputable manufacturer, correctly sized, applied and installed it and are operating it under the conditions for which it was built, you have very little reason to be concerned about it overheating. However, unanticipated changes in environment, aging of equipment, misuse and other factors can subject the motor to stresses for which it was not intended.

Specifying motors with inherent overload protectors — such as thermostats, thermocouples, or resistive temperature devices (RTDs) — or installing motor protective devices in motor controls, can help ensure that a motor is taken off-line before winding damage occurs. Since protectors and thermostats are typically very reliable devices, if a motor is constantly “tripping out,” it usually means it is overheating. Motor protection of one sort or another is advisable in almost any application. Your motor supplier can help sort out the details.

A good field test is to check the motor’s current draw using a clamp-style ammeter. If current draw is less than or equal to the nameplate rating, you can be confident all’s well with the windings, including their temperature, if the motor is operating in an application it is design for.

Resistance method. A more precise test for determining winding temperature is the resistance method. This test requires an ohm meter capable of measuring very low resistance. For motors up to about 2 hp, the meter should be accurate to 0.1 ohm; from 2 through 20 hp, 0.01 ohm; and for larger motors, 0.001 or better yet to 0.000001 ohm.

With the motor disconnected from power lines, first use the ohm meter to determine resistance across the motor leads on a cold motor. Then connect the motor and operate it under normal load conditions until the running temperature stabilizes. This usually takes 3 or 4 hours, possibly longer depending on motor size. Disconnect the motor from the power source and, as quickly as possible, make another resistance check.

Then enter these cold and hot resistance readings into the following formula to determine the winding temperature

Where:

Tt = Total winding temperature, C Tc = Cold motor (ambient) temperature, C (The motor should be in the ambient environment long enough to reach that temperature.) Rh = Hot motor resistance, Ω Rc = Cold motor resistance, Ω 234.5 = Constant for copper windings

In a laboratory environment, such as a motor manufacturer uses, resistance testing is often done in conjunction with correlating tests involving thermocouples placed in the windings and at specific locations on the motor’s surface. This testing produces a heat-run profile for a particular motor model. Only by referring to such design-specific data can any correlation be made between surface and winding temperatures.

Guarding against overheating

Motor manufacturers are not perfect. Sometimes a motor overheats because of a manufacturing or design defect. But far more often, motor overheating problems can be traced to misapplication. Overloading is the leading cause. This involves using an undersized motor, a situation that may become more common as concern for energy efficiency puts the emphasis on eliminating oversized motors. Use an 80% loading as your guide. Most electric motors reach their peak efficiency at that load, and a comfortable overload margin remains. Other common causes of overloading include a load seizing up, causing a locked rotor condition on the motor, misalignment of power transmission linkages, and increased torque requirements of the driven load.

Environmental conditions that can result in motor overheating include high ambient temperatures (look especially at motor surroundings; is the motor near a heat-generating device?) and high altitudes. Above 3,300 feet, the thin air has less cooling capability. You may have to derate a motor under these conditions, probably choosing the next size up. Another environmental concern is dirt and fibers, which can clog ventilation openings, coat heat dissipating surfaces, and cause a variety of mechanical problems. If it’s dirty, use a totally enclosed motor versus an open one.

Power supply problems are another overheating cause. Low voltage will cause the motor to draw higher current to deliver the same horsepower, and the higher current means higher winding temperatures. Figure that a 10% drop in voltage could cause nearly that much temperature rise.

Excessive or sustained high voltage will saturate a motor’s core and lead to overheating as well. In three-phase motors, phase imbalances can cause high currents and excessive heat, the extreme being the complete loss of voltage in one phase (called single phasing), which if correct protection is not in place, will burn out the motor .

Often overlooked as a cause of overheating is the number of start-stop cycles per hour. While starting, a typical motor draws five to six times the rated running current. This starting current accelerates heating dramatically. Most continuous-duty motor designs are intended to do just that — operate continuously. Though various provisions are made relative to loading and off-time, NEMA essentially limits a three-phase continuous-duty motor to two starts in succession before allowing sufficient time for the motor to stabilize to its maximum continuous operating temperature. This is highly applicationdependent, so it’s best to check with your motor manufacturer if you face a highcyclic application. A custom design may be required.

Finally, pay special attention when applying adjustable-speed inverter drives, especially if you are connecting an inverter to an older motor. The inverter’s “synthesized” ac wave form increases motor heating. However, technological advances continue to improve the wave form to more closely approximate an ac sine wave. More importantly, be especially careful when operating an inverterpowered motor at low motor speed (less than 50% of base speed) for extended periods, unless the motor has a separately powered cooling fan, which delivers a constant volume of cooling air over the motor regardless of motor speed.

Modern inverter-duty motors have higher insulation ratings to help alleviate this concern, and the robust insulation systems used in most of today’s generalpurpose industrial motors are adequate for many applications. In extreme cases, however, a secondary cooling source may be required.

Chris Medinger is national service manager for Leeson Electric Corp., Grafton, Wisc. During his 20 years in the motor industry, he has also served as quality engineer and prototype motor administrator.