Page:Electricity (1912) Kapp.djvu/227

Rh of them are therefore not in the most advantageous position for exerting mechanical force. The improvement necessary to overcome this imperfection is obvious. If we replace the copper cylinder forming a coating to the iron core by a regular copper winding embedded in the surface of this core, we constrain the currents to flow along definite paths, which relatively to the currents in the fixed part or "stator" are always in the most efficient position for the production of mechanical force. The modern non-synchronous motor is therefore provided with a winding both on the stator and on the rotor; and both windings are embedded in slots, so that the reluctance of the magnetic circuit is mainly that of the air-gap between the outer surface of the rotor and the inner surface of the stator. This need not be larger than a mere mechanical clearance allowing the inner part to revolve without touching.

A lesser imperfection of the motor shown in Fig. 24 is due to the employment of only two currents. The result of this arrangement is that the strength of the revolving field is subject to certain fluctuations. At the moment that the current in A has crest value the maximum value of the induction in the air-gap