Alternating-current motors for the economic operation of mine fans

In this paper the writer emphasizes the steadily increasing tendency toward more highly specialized equipment for definite classes of service with resulting improvement in efficiency and operating characteristics. Mine fan applications are considered under two general heads, (a) those requiring constant speed and (b) those requiring adjustable speed drive. The necessity of selecting the proper type of motor for each case is pointed out and the statement made that in view of recently perfected methods of speed regulation, the polyphase induction motor is equally applicable for both constant and adjustable speed service. Several methods of obtaining speed control, which have been used in the past, are briefly discussed and their limitations indicated. The latest development involving speed regulation by “dynamic control” as opposed to “rheostatic control” and the striking advantages of the former in the way of strictly adjustable speeds, each constant under varying load, high operating efficiency throughout the entire range of speed reduction and the important possibilities of high power factor correction, are described. An example of net saving effected by dynamic as compared with rheostatic control is cited. For smaller fan installations, a new type of polyphase motor is described with which starting, stopping, and forward and reverse speed control are obtained by shifting the brushes. In conclusion the writer finds the polyphase motor in some one of its several forms eminently suitable for any and all fan installations, save possibly in cases where an existing direct-current system is already installed.