The Thermal Time Constants of Dynamo-Electric Machines

Considering the thermal behavior of a dynamo-electric machine intended for continuous service, its acceptance tests require, at present, only a limiting temperature elevation under a continuous rated load. For the intelligent operation of a machine after it has been put in service, additional information is desirable concerning its thermal behavior under changes of load. This subsidiary thermal information concerning a machine with a continuous rating may consist of (1) its final temperature rise under some steady load other than its rated load, such as either 75 per cent or 125 per cent of the rated load, and (2) its thermal time constant. The thermal time constant of a machine, assumed as conforming strictly to an exponential law of temperature rise above a constant ambient temperature, after being transferred suddenly from one steady load to another, is taken as the time required to attain 1-ϵ
^-1
or 63.2 per cent of the final temperature change. This may be called the exponential thermal time constant. This is a fundamentally scientific quantity; but is very awkward to remember or to explain to a person not well versed in the mathematical theory of the subject. It is recommended in the paper that for all practical engineering work, a new time constant called the binary time constant be used. It would correspond to the ''Period,'' or ''Half-value period,'' already used in the Science of Radio-activity and in measurements of Radio active decay.