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

This paper tests the validity of the American Petroleum Institute (API) publication´s premise as applied to flammable vapor ignition on 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 (AIT) (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 AIT. 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. The results of the tests provide data for the IEEE P1349 Working Group´s effort and lead to some significant conclusions for the application of induction motors in classified locations