Flammable vapor ignition initiated by hot rotor surfaces within an induction motor-reality or not?

Abstract

Work by the IEEE P1349 Working Group (Motors in Hazardous Locations), which is working on a recommended practice for application of motors in Class I, Division 2 areas, has questioned if the rotor of an induction motor becomes too hot for safe application in a classified area. API Publication 2216, Ignition Risk of Hydrocarbon Vapors by Hot Surfaces in the Open Air, concludes that ignition of vapors on hot surfaces in open air requires temperatures well above (at least 200/spl deg/C) the laboratory-determined (ASTM E 659) minimum ignition temperature of the material involved. This paper tests the validity of the API Publication's premise as applied to hot surfaces within an induction motor. Several motors of various sizes are instrumented with thermocouples, a flammable concentration of a material of low autoignition temperature (e.g., diethyl ether, n-hexane, n-heptane, or tetrafluoroethylene) is introduced to the motor interior, and the motors are subjected to a series of short-duration locked-rotor tests where the rotor surface temperature is brought, in defined increasing steps, above the material's listed autoignition temperature. The temperatures at which ignition occurred are reported. These tests are intended to simulate the condition of a fully-loaded motor being suddenly stopped in an atmosphere of flammable gas or vapor, such as during an emergency shutdown of a processing unit during a release. Other tests were conducted on running motors at overload to heat the rotor well above normal operating temperature to simulate somewhat abnormal operating conditions.