POL YPHASE CURRENTS.

65

this speed is not reached, owing to the fact that the armatureand field currents do not properly correspond, so that when thecurrent is passed through the armature (the motor not beingquite up to synchronism) there is a liability that it will not “holdon,” as it is termed. It is preferable, therefore, to so wind orconstruct the motor that on the start, when the armature coilsare short-circuited, the motor will tend to reach a speed higherthan the synchronous—as for instance, double the latter. Insuch case the difficulty above alluded to is not felt, for the mo-tor will always hold up to synchronism if the synchronous speed—in tlie case supposed of two thousand revolutions—is reached orpassed. This may be accomplished in various ways; but for allpractical purposes the following will suffice: On the armatureare wound two sets of coils. At the start only one of these is

short-circuited, thereby producing a number of poles on the ar-mature, which will tend to run the speed up above the synchron-ous limit. When such limit is reached or passed, the current isdirected through the other coil, which, by increasing the numberof armature poles, tends to maintain synchronism.

In Fig. 52, such a disposition is shown. The motor having,say, eight poles contains two field-circuits a and b, of differentself-induction. The armature has two coils f and u. The formeris closed upon itself, the latter connected with the field and linethrough contact-rings a b, brushes c cl, and a switch f. On thestart the coil v alone is active and the motor tends to run at aspeed above the synchronous; but when the coil a is connectedto the circuit the number of armature poles is increased, whilethe motor is made a true synchronous motor. This disposition