296 ALTERNATING CURRENTS 



reduced. Therefore, the horizontal field becomes less than the 

 vertical field and a so-called elliptical field results. 



At standstill, <f> A is zero and the rotating field becomes a pulsat- 

 ing field, which has already been described. 



It might be supposed that the above rotating field would react 

 on the rotor in the same manner as the rotating field in the poly- 

 phase induction motor. Since 4>A originates in the armature and 

 also because of its quadrature position, it cannot of itself react 

 on the stator to cause a power current to flow in the stator, and 

 therefore it cannot of itself contribute power to the rotor. How- 

 ever, due to the resultant rotor currents produced by the com- 

 bined action of the two fields <f> A and <t> M , it can be shown that the 

 resulting torque acting on the rotor under the above conditions 

 acts in a clockwise direction, and so produces rotation. 



118. The Operation of the Polyphase Motor as a Single-phase 

 Motor. The single-phase induction motor is distinctly inferior 

 to the polyphase motor. For the same weight, its rating is about 

 50 per cent, of that of the polyphase motor, it has a lower power- 

 factor and is less efficient. 



If one phase of a polyphase motor be opened, the motor will 

 operate as a single-phase motor, although it will not start under 

 these conditions. The rating and the break-down torque of a 

 polyphase motor, operating single-phase, are considerably re- 

 duced and if rated polyphase load is applied continuously, the 

 motor may overheat. 



Ordinarily in starting a polyphase motor, all three lines are 

 closed when the compensator is in the starting position and the 

 motor starts as usual. When the compensator is thrown to the 

 running position, however, a phase may become open through 

 the compensator. This would occur if one of the fuses were 

 blown, Fig. 238, page 251. The motor then operates single- 

 phase and the only indication that it may give of this condition is 

 overheating if the load is near the rated value. The best test 

 for an open phase is to insert an ammeter in each line. 



119. Starting Single-phase Induction Motors. As the single- 

 phase induction motor is not self-starting, auxiliary means must 

 be used to supply initial torque. One method is to split the 

 phase by the use of inductance, resistance or capacitance. 



Figure 275 shows one method of splitting the phase, a two-pole 



