7/16/2019 Letter01!01!02 - Insulation - Defining Class Temperature Ratings

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IN5ULATION:DEFINING CLA55 TEMFERATUKE RATING5

Insulation Class Temperature Ratings

Close inspection of a typical motor namep latereveals that there is a designation for theInsulation Cass of the winding. Of tenabbreviated "Ins. CI. ", the letter codes usedwi ll be A. B. E or H. These are the valuesused on motors that conform to NEMA(National Electrical ManufacturersAssociation) standards.

Aword of caut ion about tempe ratu redesignations on namep lates The maximumambien t. i.e., surrounding air temperature, is

also indicated on the namepla te. Most oftenthe ma ximum ambient, typically des ignated"Max. Amb" on the nameplate, will be 40"C.

As will be discussed later, the Max. Amb. hasa relationsh ip to the insulation class rating;but Max . Amb. is not an indicato r oftempera ture rise of a motor. It is an indicatorof the maximum temperature of thesurrounding ai r, or "room tempe ratu re", thatthe motor should be operated in. The pointhere is that this temperature is not a measureof motor surface temperature, or motortempera ture rise.

The le tter codes each relate to a numericaltempera ture rat ing, A fo r 105°C, Bfor 130°C, Ffo r 155°C and H for 180°C. NEMA still usesthe letter codes, but scientif ic/engineering

communities prefer the numericaldesignations. Figu re 1 belO'N illustrates therelationship between time and tempe rature forthe four insulation classes.

Figure1: Winding tempera ture vs. hoursof li fe for el ec trica l insulation classe s.

What do these temperature des ignationsmean? They specify the the rmal classi fication

of the insulation used in the stator windings ofa motor or generator. If the rotating elementof a motor or generato r has insulatedwindings, e.g., awound roto r motor, thedesignation also applies to the rotatingelement. The temperature value associatedwith the rating is the maximum continuoustempe rature that the windings can operate at.Temperatures in excess of the insu lation classdesignation will resu lt in premature failuredue to reduced winding life . In extreme cases,immediate winding failure may be the result.

Up until the 1960's most motors had Class Awinding systems. The advent of the Tframe

motor series in the mid· 1960's resulted inwidespread use of Cass B nsulation. Somemanufacturers are now using Class Fwindingsin the ir more special ized motors; and inexceptional cases, motors have been builtwith Class Hsystems.

Many have been using Class H o r decades.The primary reason for Class H s rel iability.Most of the rewind cost is for labor, whi lemost of the mate rial in the motor is reused,e.g., the stator core and frame, rotor, andend be lls. While it makes economic sense fora manufacturer to have mu ltiple insulationsystems. the repair se rvice center doesn't

perform rewinding en-mass as themanufacturers do. By using Class H only, theservice center buys the highest temperaturerated winding mate rials at the best buyingleve l they are capable of. The net resu lt is awinding with the best ach ievable tempera turerating. That gives the service center added

Temperature vs. Life Curves for Insulation Systems

(per IEEE 117 & 101)

- + - C ~ S A • C ~ - u - C ~ S F - o - C ~ S HCI') 100000

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10000

100 0

100

40 90 140 190 240

VVlNOING TEMPERATURE

confidence in the longevity of the rewind, aassures the end user that the rewound mo

will outlive the original winding under thesame operating conditio ns. Figure 2 belowillustrates awinding that might have survivan over loadcondition if it had been upgradto Class H rom Class B.

Figure 2: An electric motor winding thhas been thermally degrad ed by anoverload condition.

Insulation system thermal classifications aimportant because they establish the differtempe ratu re capabil ities associated withwind ing materials. Insulation engineersdete rmined long ago that increasedtempe ratu re degrades awinding, and that

ra te of degradation was measurable. For o50 years designers have gone by thescientifically based rule of thu mb tha t a lOdecrease in temperature reduces wind ing

thermal life by 50% . If we consider thedifference between a motor as·built with aClass B (130°C) winding, and a Class H(180°C) rewind, the 50 degrees increase incapability equates to 2 to the fifth power(that's 2 x 2x 2 x 2 x 2J. or 32 t imes moretheoretical life . We say theoretical becausthe real wo rld other factors, typicallymechanica l in nature, wil l intervene such ta 32-fold increase in moto r (not just windinlife is not to be expected.

Another cons ideration is the relat ionshipbetween insulation class, temperatu re riseand ambient temperature. The insulationclass rating designates the maximumoperating temperature for a winding. Thetempe ratu re rise is the difference betweenambient and the operating temperatures.

To be continued inthe nextLongo Letter