2l8 



ALTERNATING CURRENTS 



winding is shown in Fig. 139. The four conductors shown arranged 

 on the rotor periphery form one element of the winding. The 

 zig-zag lines EI and E 2 are high resistances bridged across the end- 

 connections of the winding as shown. It will be easily seen that 

 with a rotating two-pole field (starting position of switch) the total 

 e.m.f. induced around the closed circuit formed by the four conductors 

 always vanishes (Fig. 139 (a)), and that the only paths open to the 

 currents are those through EI and E 2 (which come into play 

 alternately, the currents flowing through them being in quadrature 

 with each other), i.e. the circuit of the winding is a high-resistance 

 one, enabling the rotor to exert a large starting torque. But as soon 

 as the switch is thrown over into the running position, corresponding 

 to a four-pole field Fig. 139 (5) the e.m.f. 's around the circuit formed 



(a) (5) 



FIG. 139. Diagram of Deri Winding for Rotors. 



by the four conductors are added together, the currents flowing 

 around a low-resistance circuit (corresponding to high efficiency) 

 without traversing EI and E 2 . 



The most commonly adopted method for limiting the starting 

 current and securing a large starting torque consists in using wound 

 rotors provided with slip-rings, by means of which starting resistances 

 may be introduced into the rotor circuits. In some cases, these 

 resistances are mounted inside the rotor itself, and an automatic 

 switch, controlled by a centrifugal governor, gradually cuts them out 

 as the motor gains speed. In an arrangement recently patented by 

 H. S. Meyer,* the starting resistance takes the form of iron wires of 

 graded size wound at the bottom of the slots containing the copper 

 winding, so that the wires form part of the magnetic circuit of the 

 rotor. 



* Western Electrician, vol. xxxv. p. 287 (1904). 



