Electrical insulation is continually being improved. The motor manufacturers make use of this and other technological developments to put more power into smaller, lighter, more efficient packages. Modern insulating materials can withstand heat, moisture, and corrosive atmospheres, and new metals can withstand more mechanical punishment. Computer design techniques are also helpful.

Insulation systems were first classified according to the material used, and permissible temperatures were established based on the thermal aging characteristics of these materials. For example, Class B insulation was defined as inorganic materials such as mica and glass with organic binders; 130°C was the allowable maximum operating temperature. The present definition of insulation system Class B stipulates that the system be proven "... by experience or accepted tests ... to have adequate life expectancy at its rated temperature, such life expectancy to equal or exceed that of a previously proven and accepted system." The definition is now functional rather than descriptive.

The newest catalogs show standard induction motors designed with Class B insulation for operation in a 40°C ambient with 80°C rise by resistance at 100% load for motors with 100% service factor. Class F insulation, with the capability of operating up to a 105°C rise by resistance, is today frequently offered as standard for machines with a Class B rise, particularly the larger sizes. Many users specify this as a standard. Previously, induction motor ratings were based on temperature rise by thermometer.

These changes require an explanation. National Electrical Manufacturers Association (NEMA) standards previously allowed three methods of temperature determination: (1) thermometer, (2) resistance, and (3) embedded detector. Motor engineers have long recognized that measuring temperature rises by placing a thermometer against the end windings does not give the best indication of insulation temperatures near the conductors in the slot. The average temperature rise of any motor can be measured by resistance. This will give a better indication of the temperature in the hottest part of the winding than will thermometer measurement. On machines equipped with temperature detectors, there will usually be a difference in the readings taken by an embedded detector and by winding resistance, with the detector reading usually slightly higher, because it is embedded near the hottest part of the winding. Adequate placement and selection of the embedded detectors can be a factor in determining the quality of this type of monitoring. For example, longer detectors may provide more of an average temperature, and detectors near the fan end of a long TEFC motor may not see the hottest winding temperatures.

The resistance method gives an average temperature of the whole winding. Some parts will be hotter than others; usually the end turns will be somewhat cooler than parts of the winding in the middle of the iron core. NEMA committee members have been collecting test data on many machines to determine the correlation between temperature measure ments by detector and by resistance, and the standards are periodically updated to reflect any of the technology improvements.

NEMA standards do not give any fixed maximum operating temperature by any class of insulation. Briefly, NEMA states that insulation of a given class is a system that can be shown to have suitable thermal endurance when operated at the temperature rise shown in the standards for that type of machine. Standards for synchronous motors and induction motors with a 100% service factor specify 80°C rise by resistance for Class B insulation. Also, the total temperature for any insulation system is dependent upon the equipment to which it is applied. For example, railway motors with Class B insulation have rated standard rises by resistance of 120°C on the armature. Induction motors with service factor have 90°C rise at the service factor load.

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