INDUCTION MOTORS 139 



Close the line switches and increase the amperes to 

 twice normal and take readings as before ; also read volts 

 and amperes. Call the reading obtained as the pointer 

 passes the mark as the lever goes up, W-\-F + T, and that 

 obtained as the lever comes down, W F-\-T, T repre- 

 senting torque. 



Readings should be recorded as below, assuming that 

 they were taken on a 440 volt motor: 



Volts, 150; Amps., 40; VT+F, 9 lb.; W-F, 5 lb.: 

 W+F+T, 19 lb.; W-F+T, 15 lb.; T, 10 lb. 



To find the torque: 



2W=14. . .w= 7 lb. 



7 = 10 lb. 



Torque at 1 ft. radius T XL 

 Where L = length of lever arm 

 Torque at 1 ft. radius at normal volts 



= (normal volts) 2 

 (volts read) 



On squirrel cage or wound rotors a value of current 

 should be used which will make W -\-F-\-T at least twice 

 W+F. The maximum and minimum values of W+F 

 + T and of W F-\-T should also be taken. 



All wound rotors and most squirrel cage rotors will 

 show a torque variation depending on the rotor position. 



As a check on the torque readings, the lever should be 

 loosened on the pulley and the pulley rolled forward 

 until the mark on its rim at TV is in line with a second 

 mark on the lever arm, thus changing the relative posi- 

 tions of the rotor and stator. Further readings should 

 be taken and this procedure repeated for four or five 

 different points. The torque should be the same for all 

 points on form K motors. 



A special consideration to be observed in making a 

 test for torque is the maintaining of a constant and 

 correct generator speed. The volts read, when amperes 

 are 200 per cent normal on the first point, should be held 

 constant on all other points, since the torque varies as 

 the square of the volts. The torque also increases as 

 the resistance of the rotor winding increases owing to 



