INDUCTION MOTORS 145 



Vary the load on L by changing the water resistance or 

 the load on the motor to which it is connected to suit 

 the testing conditions required on M. The efficiency 

 of M may be required for a series of speeds or of loads. 

 Read the input and speed of M and the volts and amperes 

 ofL. 



The ' ' counter torque ' ' must now be obtained to com- 

 plete the calculations. To obtain this, disconnect M, 

 connect L to a source of current which can be varied 

 so as to give L different speeds, keeping L separately 

 excited. If the "pumping back" method for loading L 

 has been used, the connection will probably not require 

 any change. Run L as a motor driving M, keeping the 

 field current of L constant at the same value that it had 

 when L was used as a generator. 



Vary the speed of L so that the speed of M can be 

 varied slightly from below its previous minimum speed 

 to slightly above its maximum speed. Take a number 

 of readings at varying speeds, reading volts and amperes 

 input of L and speeds of L and M. If the electrical 

 efficiency alone is desired (Case A), sufficient readings 

 have been taken. If the commercial efficiency is desired 

 (Case B), take off the belt from L and run it light as a 

 motor. Vary its speed from slightly below to slightly 

 above the speeds used before when running as a motor 

 and take a number of readings at different speeds, reading 

 volts and amperes input and speed; L being separately 

 excited with the same value of current that was used in 

 the two previous cases. The necessary readings are now 

 complete for calculating the efficiency. 



Case A 



Let Wm be the total input of M. 



Let Wl be the product of volts and amperes read for L. 



Let Fm be M's friction, windage, etc. 



Let Fl be L's friction, windage, etc. 



Divide the belt friction equally between L and M, 

 including this in Fm and Fl. 



Let R be the hot resistance of Us armature, which 

 must be measured. 



Let C be the current in Us armature. 



