Machinery's Handbook, 31st Edition
2654 Electric Motor Applications Temperatures.— The applicability of a given motor is limited not only by its load starting and carrying ability, but also by the temperature which it reaches under load. Motors are given temperature ratings which are based upon the type of insulation (Class A or Class B are the most common) used in their construction and their type of frame (open, semien closed, or enclosed). Insulating Materials: Class A materials are: cotton, silk, paper, and similar organic ma- terials when either impregnated or immersed in a liquid dielectric; molded and laminated materials with cellulose filler, phenolic resins, and other resins of similar properties; films and sheets of cellulose acetate and other cellulose derivatives of similar properties; and varnishes (enamel) as applied to conductors. Class B insulating materials are: materials or combinations of materials such as mica, glass fiber, asbestos, etc., with suitable bonding substances. Other materials shown capa ble of operation at Class B temperatures may be included. Ambient Temperature and Allowable Temperature Rise: Normal ambient temperature is taken to be 40 ° C (104 ° F). For open general-purpose motors with Class A insulation, the normal temperature rise on which the performance guarantees are based is 40 ° C (104 ° F). Motors with Class A insulation having protected, semiprotected, drip-proof, or splash- proof, or drip-proof protected enclosures have a 50 ° C (122 ° F) rise rating. Motors with Class A insulation and having totally enclosed, fan-cooled, explosion- proof, waterproof, dust-tight, submersible, or dust-explosion-proof enclosures have a 55 ° C (131 ° F) rise rating. Motors with Class B insulation are permissible for total temperatures up to 110°C (230 ° F) for open motors and 115 ° C (239 ° F) for enclosed motors. Motors Exposed to Injurious Conditions.— Where motors are to be used in locations imposing unusual operating conditions, the manufacturer should be consulted, especially where any of the following conditions apply: exposure to chemical fumes; operation in damp places; operation at speeds in excess of specified overspeed; exposure to combus tible or explosive dust; exposure to gritty or conducting dust; exposure to lint; exposure to steam; operation in poorly ventilated rooms; operation in pits, or where entirely enclosed in boxes; exposure to inflammable or explosive gases; exposure to temperatures below 10 ° C (50 ° F); exposure to oil vapor; exposure to salt air; exposure to abnormal shock or vibration from external sources; where the departure from rated voltage is excessive; and or where the alternating-current supply voltage is unbalanced. Improved insulating materials and processes and greater mechanical protection against falling materials and liquids make it possible to use general-purpose motors in many locations where special-purpose motors were previously considered necessary. Splash- proof motors having well-protected ventilated openings and specially treated windings are used where they are to be subjected to falling and splashing water or are to be washed down as with a hose. Where climatic conditions are not severe, this type of motor is also successfully used in unprotected outdoor installations. If the surrounding atmosphere carries abnormal quantities of metallic, abrasive, or non explosive dust or acid or alkali fumes, a totally enclosed fan-cooled motor may be called for. In this type, the motor proper is completely enclosed but air is blown through an outer shell that completely or partially surrounds the inner case. If the dust in the atmosphere tends to pack or solidify and close the air passages of open splash-proof or totally enclosed fan-cooled motors, totally enclosed (nonventilated) motors are used. This type, which is limited to low horsepower ratings, is also used for outdoor service in mild or severe climates.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online