POLYPHASE CURRENTS.

21

increasing the effort. Upon the load being taken ofE, thecounter-electromotive force increases and less current passesthrough the primary or energizing coils. Without any load thespeed is very nearly equal to that of the shifting poles of thefield magnet.

It will be found that the rotary effort in these motors fully

Fig. 15. Fig. 16. Fig. 17.

equals that of the continuous current motors. The effort seemsto be greatest when both armature and field magnet are withoutany projections; but as in such dispositions the field cannot beconcentrated, probably the best results will be obtained by leav-ing pole projections on one of the elements only. Generally, itmay be stated the projections diminish the torque and produce atendency to synchronism.

A characteristic feature of motors of this kind is their propertyof being very rapidly reversed. This follows from the peculiaraction of the motor. Suppose the armature to be rotating andthe direction of rotation of the poles to be reversed. The appa-ratus then represents a dynamo machine, the power to drive thismachine being the momentum stored up in the armature and itsspeed being the sum of the speeds of the armature and thepoles.

If we now consider that the power to drive such a dynamo

Fig. 18.

Fig. 19.

Fro. 20.

A/VWAAA/V

Fig. 21.

would be very nearly proportional to the third power of thespeed, for that reason alone the armature should be quickly re-versed. But simultaneously with the reversal another element isbrought into action, namely, as the movement of the poles withrespect to the armature is reversed, the motor acts like a trans-former in which the resistance of the secondary circuit would be