Temperature rises in electrical machines on variable load and with variable speed

In selecting an electrical machine it is required to estimate how hot the windings will become when the most onerous duty anticipated is performed. This has been difficult when the duty involved varying loads and speeds. Relationships are given by which such estimates may be based on the results of temperature measurements on a machine of the type in question resulting from sustained operation of the on-off type. The data required to specify the capabilities of a machine can be given by two curves, and so may be included on the data sheet for a machine of given type. No measurement or specification of the energy losses as such is required, the method being essentially a comparison between the set or combination of various operating conditions that constitute the duty, weighted according to the time each condition holds, with measurements for representative operating conditions on the test-bed. Methods of carrying out suitable type-tests are discussed, and the proposals are illustrated by the results of measurements and calculations on a typical 25 h.p. direct-current mill-type motor, operating both with forced ventilation and totally enclosed.