XII. Starting of Synchronous Motors 24. In starting, an essential difference exists between the single- phase and the polyphase synchronous motor, in so far as the for- mer is not self-starting but has to be brought to complete syn- chronism, or in step with the generator, by external means before it can develop torque, while the polyphase synchronous motor starts from rest and runs up to synchronism with more or less torque. In starting, the field excitation of the polyphase synchronous motor should be zero or very low. The starting torque is due to the magnetic attraction of the armature currents upon the remanent magnetism left in the field poles by the currents of the preceding phase, and to the eddy currents produced therein. Let Fig. 72 represent the magnetic circuit of a polyphase synchronous motor. The m.m.f. of the polyphase armature currents acting upon the successive projections or teeth of the armature, 1, 2, 3, etc., reaches a maximum in them successively; that is, the armature is the seat of a m.m.f. rotating synchro- nously in the direction of the arrow A. The magnetism in the 1 Since with lower impressed voltage the current is leading, with higher impressed voltage lagging, in a synchronous motor. 152 ELEMENTS OF ELECTRICAL ENGINEERING face of the field pole opposite to the armature projections lags behind the m.m.f., due to hysteresis and eddy currents, and thus is still remanent, while the m.m.f. of the projection 1 decreases, and is attracted by the rising m.m.f. of projection 2, etc., or, in other words, while the maximum m.m.f. in the armature has a position a, the maximum magnetism in the field-pole face still has the position b, and is thus attracted toward a, causing the field to revolve in the direction of the arrow A (or with a station- ary field, the armature to revolve in the opposite direction B). Lamination of the field poles reduces the starting torque caused by eddy currents in the field poles, but increases that caused by remanent magnetism or hysteresis, due to the higher permeability of the field poles. Thus the torque per volt-ampere input is approximately the same in either case, but with laminated i FIG. 72. — Magnetic circuit of a polyphase synchronous motor. poles the impressed voltage required in starting is higher and the current lower than with solid field poles. In either case, at full impressed e.m.f. the starting current of a synchronous motor is large, since in the absence of a counter e.m.f. the total impressed e.m.f. has to be consumed by the impedance of the armature cir- cuit. Since the starting torque of the synchronous motor is due to the magnetic flux produced by the alternating armature cur- rents, or the armature reaction, synchronous motors of high armature reaction are superior in starting torque. Very frequently in synchronous motors a squirrel-cage wind- ing is used in the field pole faces, to give powerful starting torque by the induced currents therein, on the induction motor principle. Such squirrel-cage winding should have fairly high resistance to start well from rest, but low resistance to give powerful syn- chronizing, that is, to pull its load promptly into synchronism. SYNCHRONOUS MACHINES 153