268 



INDUCTION MOTORS. 



[Exp. 



Line 



-r R 



WT^-I pvwv- 



sists of a resistance R and a reactance X connected in series 

 across the line, as in Fig. 4. These are thrown out of circuit by 

 opening the switch S, when the motor reaches about half speed. 



30. With R and X in circuit and the motor 

 disconnected, measure the drop around R, the 

 drop around X and the total line voltage. Plot 

 these readings as a triangle to show how nearly 

 a symmetrical 3-phase voltage is obtained. 



31. When starting the motor, take read- 

 ings of the current in the third or starting cir- 

 cuit, c\ also of the currents in the main and 

 motor circuits. 



With the motor running single phase, take 

 readings of the voltage between each two of 

 a the three terminals of the motor and plot as 



a triangle. 



32. Note that the motor may be started with the X circuit 

 open, by means of a single starting resistance between either line 

 and c. Note also that any two terminals of the motor may be 

 taken as the main terminal. 



Note the manner in which the direction of rotation can be 

 reversed. 



FIG. 4. Method 

 of starting single- 

 phase motor as 

 3-phase motor. 



PART III. LOAD TESTS. 



33. The purpose of these tests is to determine the perform- 

 ance of the motor in actual operation under load. Methods for 

 predetermining the performance without load will be taken up 

 in Exp. 8-B. 



34. Any available means may be used for loading the motor 

 and for determining its mechanical output. A Prony brake may 

 be placed on the motor pulley and the power computed from the 

 torque measured by platform scales or spring balance. An objec- 

 tion to the Prony brake is the difficulty in holding the load con- 



