S-A] 



OPERATION AND LOAD TEST. 



267 



a phase-splitter, an induction motor must be wound as a 2-phase 

 or 3-phase motor. 



27. Starting as a Two Phase Motor. The stator is wound 

 with two circuits, a main circuit A and an auxiliary starting cir- 

 cuit B, in series with which is a resistance or phase-splitter, R. 

 The circuit A has high reactance and low resistance, while B has 

 low reactance and high resistance. The starting circuit B is in 

 parallel with A, as in Fig. 2, and is opened 

 when the motor reaches about half speed. 



28. With the motor at rest, measure (at 

 half voltage) the currents in A, in B, and in 

 the line circuit. Plot the readings as a triangle 

 to show the phase difference between branch 

 currents. Note the effect of reversing the line 

 terminals. In what way may the direction of 

 rotation be reversed? 



Connect the motor in circuit with the start- 

 ing circuit open. Note that the motor remains 

 at rest, but will start slowly in either direction 

 when started by hand. 



29. Starting as a Three Phase Motor. The motor is wound 

 as a 3-phase motor. The starting device,* or phase splitter, con- 



* (2pa). Use of Condenser as Starting Reactance. The reactance X, 

 Fig. 3, may be an inductance or a condenser. To reduce the size of the 

 condenser, it may be connected through a transformer, the low voltage 

 side of which is connected in the circuit; the high 

 voltage side is connected to the condenser. The con- 

 nections for an auto-transformer are shown in Fig. 

 4. The voltage is increased and the current is de- 

 creased as the ratio of transformation ; the necessary 

 reactance (X = -*-/) is thus decreased as the 

 square of the ratio of transformation. A ratio of 

 transformation of 1 13, as in Fig. 3, reduces the size 

 of condenser to 1/9. See i6a, Exp. 5-8. Aside from 

 the present application, this is a useful laboratory 

 device for obtaining large capacity from small condensers. The limit is 

 the voltage that the condenser will stand. 



FIG. 2. Method 

 of starting single- 

 phase motor as a 2- 

 phase motor. 



P. 



a c 



FIG. 3. Use of 

 condenser with auto- 

 transformer. 



