Abstract :
The possibility of determining the frame size of a motor on economic instead of physical grounds is first discussed, and a basis is laid down-for economic comparisons. The capital and working charges are considered in turn, and in connection with the latter the efficiency of a shunt motor when suitably under-run is examined. Applying this method to the case where ¿ h.p. at 1000 r.p.m. is required, the economical position is indicated for various conditions of service and prices of energy, and it is found that a ¿-h.p. or ¿-h.p. frame is preferable in all but exceptional cases. Applying the method to sizes ranging from ¿ h.p. to 15 h.p. similar results are obtained, and it is found that the usual physical criteria are only satisfactory as a sole basis of choice when energy is exceptionally cheap or hours of service are short. The possible extension of the method and the effect of various changes are briefly reviewed.
