Helping motors keep their cool

Chris Medinger

An electric motor’s insulation system separates electrical components from each other, preventing short circuits and, thus, winding burnout and failure. Insulation’s major enemy is heat, so it’s important to be sure to keep the motor within temperature limits. There is a rule of thumb that says a 10-degree Celsius (18-degree Fahrenheit) rise reduces the insulation’s useful life by half, while a 10° C (18° F) decrease doubles the insulation’s life. This implies that if you can keep a motor cool enough, the winding will last forever. However, that thinking ignores factors such as moisture, vibration, chemicals and abrasives in the air that also attack insulation systems.

The real issue is at what temperature the motor windings are designed to operate for a long and predictable insulation life — 20,000 hours or more. The National Electrical Manufacturers Association (NEMA) sets temperature standards based on thermal classes, the most common being A, B, F and H. The table above provides a summary.

Class B or Class F insulation systems are usually used in today’s industrial-duty NEMA “T frame” motors. Many manufacturers also 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 altitudes.

Class A insulation, while not used on today’s industrial-duty motors, was standard on industrial “U frame” motors built in the 1960s and earlier. Because Class A insulation has such a low temperature rating, older motors were required to have far lower maximum temperatures. This accounts for the perception among many long-time motor users that modern motors “run hot.” In fact, they do when compared with older motors, but modern insulation systems are so much better that the reliability and durability of new motors are equal to or better than older-design motors. Plus, better insulation systems allow motor manufacturers to put more horsepower in a smaller package.

Insulation Classes and
Their Thermal Ratings
Insulation Class
Maximum Winding Temp.
A
105°C (221°F)
B*
130°C (266°F)
F*
155°C (311°F)
H
180°C (356°F)

*Most common classes for industrial-duty motors

Table shows highest allowable stator winding temperatures for long insulation life. Temperatures are total, starting with a maximum ambient of 40° C (104° F).

Table 1

Determining Correct Operation
Though many people believe they can judge a motor’s operating characteristics by feeling its surface, this isn’t a very effective method. Design ratings for temperature apply to the hottest spot within the motor’s windings, not how much of that heat is transferred to the motor’s surface. Unless you have intimate knowledge of a specific motor model’s design — including 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. This is true even if temperature measurement methods far more sophisticated than the human touch are used. In addition, for safety reasons, it’s unwise to touch operating motors anyway.

Specifying motors with inherent overload protectors, thermostats or resistive temperature devices, or installing similar protection in motor controls, can help ensure that a motor is taken off-line before winding damage occurs. Motor protection of one sort or another is advisable in almost any application. A common and reliable field test for motor heating involves checking the motor’s amperage draw with a clamp-style ammeter. Use this to confirm that actual amps are less than or equal to the nameplate rating. A precise test for winding temperature is the resistance method. This involves careful measurements with sensitive equipment, calculations and several hours of time. Procedures to conduct such tests can be found in technical manuals. Or, contact your motor manufacturer.

Common-sense Precautions
Sometimes a motor overheats because of a manufacturing or design defect. But far more often, overheating can be traced to misapplication. Overloading is the leading cause. This could take the form of 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 percent 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 or misalignment of power transmission linkages. Plus, unanticipated changes in environment, aging of equipment, misuse and other factors can subject the motor to stresses for which it was not intended.

Environmental conditions that can result in motor overheating include high ambients (especially look at the near vicinity of the motor for any heat-generating device) and high altitudes (above 3,300 feet or 1,005.84 meters, where the thin air has less cooling potential). You might 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.

Power supply problems are another overheating cause. Low voltage will result in the motor drawing higher current to deliver the same horsepower, and the higher current means higher winding temperatures. A 10 percent drop in voltage could cause nearly that much rise in temperature. Excessive or sustained high voltage will saturate a motor’s core and lead to overheating, as well. In three-phase motors, phase imbalances can result in 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 result in motor burnout.

Often overlooked as a cause of overheating is the number of start/stop cycles. It’s not uncommon for a motor at starting to draw five times or more the current it does while running. This accelerates heating dramatically. 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 rating. This is highly dependent on application, so it’s best to check with your motor manufacturer if you’re facing a high-cycle application.

Finally, pay special attention when applying adjustable-speed inverter drives, especially if you are introducing an inverter in a system of older motors. Some additional heating to the motor windings will inevitably occur because of the inverter’s “synthesized” AC waveform. A greater cooling concern involves operating for an extended time at low motor speed, which reduces the flow of cooling air. Modern inverter-duty motors have higher insulation ratings to help alleviate this concern, and the robust insulation systems used in most of today’s general-purpose industrial motors are also adequate for many applications. In extreme cases, though, a secondary cooling source may be required.

Chris Medinger is the national service manager for LEESON Electric Corporation, a manufacturer of motors, gearmotors and drives. To learn more, visit www.leeson.com or call 262-377-8810.

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