g-A] FREQUENCY CHANGER. 293 



P" wherever located. The short-circuit point P" may be located 

 anywhere on the semi-circle, from A (when s = o) to P'" (when 

 s= oo ), according to the slip. P'" would be reached only if the 

 speed were infinite, or, if the secondary resistance, internal as 

 well as external, were zero. See also Fig. I, Exp. 9~B. 



5. Apparatus. When a commercial frequency changer is 

 available, it should be run under its rated conditions and appro- 

 priate measurements made of input and output. 



In the laboratory, any phase-wound induction motor can be 

 conveniently used as a frequency changer. It may be belt-con- 

 nected to a direct-current shunt machine which may be driven at 

 different speeds and will serve as a driving motor or as a 

 generator. 



6. Preliminary Test. With primary voltage constant, make 

 a run at different speeds from synchronous speed in the same 

 direction as the rotating field to the same speed in the opposite 

 direction. (In changing the relative direction of rotation, it 

 may be simpler to reverse the direction of the rotating field by 

 changing primary connections than to reverse the direction of 

 the driving motor.) For different speeds, note the frequency 

 (which may be computed) and the voltage of the delivered cur- 

 rents. Plot voltage for different speeds, frequencies or slip. 



7. Load Run at Constant Speed. (These tests may be cur- 

 tailed or expanded as desired.) With the rotor driven against 

 the field, so as to convert from a low to a high frequency, vary 

 the load of the receiver circuit consisting of non-inductive resist- 

 ances. Measure the primary input and secondary output of the 

 induction machine, and the input of the driving motor. The 

 mechanical power supplied by the driving motor is the motor 

 input less losses determined as in Exp. 2-B. This mechanical 

 power added to the measured primary power gives the total 

 power supplied to the induction machine. 



Compute the efficiency of the induction machine alone and of 



