Therefore Class F insulation with a Class B temperature rise gives us a thermal margin of 25☌, potentially increasing the life of the motor by up to 5 times. With modern designs, a 'Class B' temperature rise is readily achievable. Modern insulation materials means Class F insulation is commonly used for motor windings. This practice means that a motor with Class F insulation and an 80☌ rise is referred to as an 'F/B' motor. For example, an 80☌ rise is often referred to as a 'Class B', as 80☌ is the maximum allowable temperature rise for a 1.0 S.F. motor with Class B insulation and a 40☌ ambient temperature. Motor Insulation Temperature Ratings (NEMA)Īlthough not specified by NEMA, it is now common practice within industry to refer to the allowable temperature rise for a given class of insulation, as a temperature rise letter.
The overall winding resistance is of course the sum of the resistance of the cooler end turns, and the warmer windings in the stator slots. This gives us a 10☌ temperature differential between a Class F insulation maximum temperature rating (155☌) and an allowable maximum temperature (145☌) which gives an allowance for the "hotspot" temperature in the interior of the winding.
motor, if we add the NEMA allowable rise of 105☌ to the reference ambient temperature (40☌), results in the motor having an operating temperature of (105+40)=145☌. motor, the NEMA allowable temperature rises (at service factor) are A=įor a Class F insulated, 1.0 S.F. The allowable temperature rises (at full load) for a 1.0 S.F. The resistance of the winding is a function of temperature of the winding. The allowable temperature rises are based upon a reference ambient temperature (40☌) and are determined by a "resistance method", once the motor has achieved thermal equilibrium under load, the resistance of the windings is measured.
NEMA specifies allowable temperature rises for motors at operating under full load (and at service factor, if applicable).
Motor winding insulation max temperature ratings carry NEMA designations. To put it simply, today's motors can simply be too hot to handle, even when all is working as it should be. This doesn't necessarily mean it is running too hot or operating improperly (by the way, we strongly advise against touching anything that is 125☌). So if we look at a Class F insulated motor which will be happy running at 155☌ and then subtract 30☌, we get a surface temperature of 125☌. The Surface temperature of the motor is typically 30☌ lower than it is at the windings. NEMA Insulation Classes do away with guessing, and give the motor manufacturer a defined framework to operate in. )/Type/Annot/Rect/CreationDate(D:20041028101341-06'00')/NM(EpnTh_FuPfFN6rcQim_PFD)/Name/Comment/Subtype/Text/C/M(D:20041028101341-06'00')/T(jsloan)/Popup 1296 0 R/AP>endobj1296 0 objendobj1297 0 objstreamĬorel WordPerfect - Volume 3-4 FIST Manualġ 0 objendobj3 0 obj/PageMode/UseOutlines/Names 1272 0 R/Outlines 127 0 R/Metadata 1298 0 R/AcroForm 66 1 R/Pages 56 0 R/OpenAction 128 0 R/StructTreeRoot 1024 0 R/Type/Catalog/Lang(en)>endobj4 0 obj/ProcSet>/Type/Page>endobj63 1 obj/Ff 65536/MK/AP>endobj66 1 objendobj84 1 objendobj253 0 objendobj1037 0 objendobj1298 0 objstreamĪcrobat PDFWriter 4.Heat kills, and the old standard of using your hand to judge the temperature of a motor and if it was over heating no longer applies. )/Contents(None of the form fields on this page have descriptions. None of the form fields on this page have descriptions.