250 



ALTERNATING CURRENTS 



capacity fuses at the motor are sometimes omitted, the line 

 fuses giving the required protection at starting. 



As the squirrel-cage motor at starting is equivalent to a short- 

 circuited transformer, it is necessary to reduce the starting 

 current in the larger sizes. One simple method (Fig. 237) is to 

 use a delta-connected motor. By means of a triple-pole, double- 

 throw (T.-P. D.-T.) switch the windings are first thrown in Y 

 across, the line, thus applying only l\/3 or 58 per cent, of the 

 normal voltage to each coil. This makes the line current one- 

 third the value it would have if the motor were directly across 

 the line. When the motor has attained sufficient speed, the switch 

 is thrown over, connecting the motor in delta across the line. 



(a) Starting (6) Running 



FIG. 237. The Y-delta method of starting an induction motor. 



The most common method of starting the squirrel-cage motor, 

 however, is to use an auto-starter or starting-compensator, 

 similar to those shown in Figs. 238 and 239. In the General 

 Electric compensator shown in Fig. 238, the three coils of a 

 three-phase auto-transformer are connected in Y. When the 

 switch is in the starting position, the compensator is connected 

 across the line with only the line fuses for protection. Under 

 these conditions the three motor lines are connected to three 

 taps, one in each phase of the auto-transformer. Hence the 

 motor voltage is reduced, usually to one-fourth or to one-half 

 its rated value. When the switch is in the running position, the 

 compensator is entirely disconnected from the line and the motor 

 is connected directly across the line through the running fuses. 

 In Fig. 238 the heavy lines show the path of the current when 

 the compensator is in the running position. It should bo re- 

 membered that a compensator supplying a motor with half 

 voltage reduces the line current to one-fourth its normal value. 



v OF ELECTR: 



JLTY OF APPLIED SCIENCE. 



