THE INDUCTION MOTOR. 399 



this will be practically equal to the percentage drop of 

 voltage in the windings. 



The method of separating the mechanical losses 

 of the motor from the other losses has already been 

 described on page 389. 



The graphical representation of the various losses at 

 all loads is discussed in connection with the Heyland 

 diagram page 403 and seq. 



SINGLE-PHASE MOTORS. 



If a three-phase motor is running while connected to 

 the supply circuit by three wires in the usual way, and 

 one of these wires is broken, the motor will continue to 

 rotate and to perform its work. The current supplied 

 to the machine under these conditions is a single-phase 

 alternating current, and, although an increase of current 

 in the two remaining wires will have occurred in order 

 to counterbalance the interruption of current in the 

 third wire, the motor will apparently operate quite 

 satisfactorily when supplied with current in a single 

 phase only. 



Further investigation will show that when supplied 

 with single-phase current only, the properties of the 

 motor are somewhat different from those of the ordinary 

 three-phase motor. Thus, if the motor be stopped and 

 supplied with single-phase current while at rest, it will 

 not start itself. If, however, it be started by hand it 

 will probably continue to rotate and gradually attain 

 its normal speed. Further, it will be found that the 

 motor may be started equally well in either direction, 

 and will gain speed and continue to rotate with com- 

 plete indifference as to its original direction of rotation. 



The explanation of this behaviour is to be found in 

 the fact, to be explained immediately, that, when 

 stationary, there is an oscillating field (instead of a rotating 

 field) set up by the single-phase alternating current in the 

 rotor. Let Fig. 193 represent diagrammatically the 

 stator coils of a two-pole motor with squirrel-cage rotor, 

 the phase supplied being shown by a firm line, the idle 

 coils being shown by dotted lines. The flux due to the 

 winding A A 1 will pass through the rotor in such a way 

 as to induce currents in the conductors a, b, c, in one 

 direction, and in the conductors a 1 , ft 1 , c 1 in the opposite 



