Experimental data concerning the safe operating temperature for mica armature-coil insulation

The paper gives the results of three series of tests, made on the first 3750 kv-a. generator installed at Niagara Falls, to determine internal copper temperatures. The object of the tests was to obtain data to assist in the determination of the safe maximum temperature of built-up mica-and-cambric (or mica-and-paper) insulation. The present Institute Standardization Rules recommend the conservative limit of 125 deg, because of the lack of convincing data, justifying the general adoption of a higher limit. In the third series of tests, temperatures of the copper conductors were obtained at the usual operating loads by installing special conductor bars having thermo-couples in actual contact with the copper at its hottest part. The operating history of the generator is given, showing that it has been in operation for twenty years with no interruption to operation due to any effect of temperature on the insulation of the winding. The summary of service and temperature results given in Tables VIII and IX shows that the generator has operated for a time equivalent to nearly seven years without shutdown at temperatures ranging from 120 deg. to 285 deg., for a time equivalent to nearly three years operation without shutdown at temperatures ranging from 145 deg. to 285 deg., and for a time equivalent to fifteen months operation without shutdown at temperatures from 175 deg. to 285 deg. An examination made in 1914 showed the insulation to be in good condition — with the mica unchanged and with the conductors tight in the slots. In conclusion, it is suggested that when suitable mica insulation is used the effect of temperature on insulation becomes of little importance; that the limit to temperature is determined by other factors. The tests show that 150 deg. is a conservative limit giving a liberal factor of safety.