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.
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 cannot keep your hand on a surface that hot long enough to discern differences, and if you try, you could get a nasty burn.
Even if feeling a motor’s surface is not 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 (National Electrical Manufacturers Association) frame motors, the key insulation 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 (20°F) 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 is 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 at which the motor windings are designed to operate so they give a long and predictable insulation life of 20,000 hours or more. 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 for motors with a 1.0 service factor. Greater than 40° C ambient may require special application considerations or special motor designs.
Class B or Class F insulation systems are most common in today’s industrial-duty motors. Smaller sizes 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. Beyond 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 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.
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, unanticipitated 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 damages occurs.
Resistance method. A 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 .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 stablises. This usually takes 3 or 4 hours, possibly longer depending on motor size. Disconnect the motor from 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:
Tt = Tc + (Rh – Rc)/Rc x (Tc + 234.5)
Where:
T(t)=total winding temperature
T(c)=Cold motor (ambient) temperature, C (The motor should be in the ambient environment long enough to reach that temperature.)
R(h)=Hot motor resistance
R(c) =cold motor resistance
234.5 = constant for copper windings
In the 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 a leading cause. 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 the 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 motor to stablise to its maximum continuous operating temperature. This is highly application-dependent.
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.
Be especially careful when operating an inverter-powered 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 general purpose industrial motors are adequate for many applications. In extreme cases, however, a secondary cooling source may be required.
