292 INDUCTION MACHINES. [Exp. 



3. The secondary frequency n 2 varies with the slip s ; that is, 

 n 2 = sn 1 . At synchronous speed, s = o and n 2 = o; at stand- 

 still, s=i and 2 = w 1 . When the rotor is driven against the 

 rotating field, with a speed equal to synchronous speed, 5 = 2 

 and 7*2 = 2^; etc. In this case the frequency is increased and 

 the driving motor is supplying power proportional to the increase 

 in frequency and voltage. Thus, when the frequency is increased 

 50 per cent., two thirds of the power is supplied by the primary 

 of the induction machine and one third is supplied by the driving 

 motor; losses are here neglected. 



When the frequency is decreased, the rotor revolves in the 

 same direction as the field at less* than synchronous speed; the 

 secondary voltage and power are decreased so that the electrical 

 power supplied to the primary is more than the power given out 

 by the secondary. If the surplus power is more than enough to 

 supply the losses, the induction machine runs as a motor and 

 furnishes mechanical power. Frequency changers are not com- 

 monly used to decrease the frequency. 



4. Referring to the circle diagram, Fig. I, Exp. 8-B, the 

 short-circuit point has the position P" when the rotor is at stand- 

 still (s=i); the machine then acts as a stationary transformer, 

 the range of working as the secondary external resistance changes 

 being from A on open circuit to P' at full load and P" on short 

 circuit. The excessive current, however, prohibits going much 

 beyond full load, as in any transformer. 



When the rotor is turning with the field (s < i), the short- 

 circuit point P" shifts to some point as 8 and when turning 

 against the field (^ > i) to some point as 9, the full range of 

 working being from A on open circuit to the short-circuit point 



* (3 a )- The rotor could be driven above instead of below synchronous 

 speed, with a negative instead of a positive slip. Electrical power would 

 then be given out by the primary as well as the secondary, the machine 

 being simultaneously a primary and secondary generator (see Exp. Q-B). 

 but it is doubtful whether there is any useful application for such 

 operation. 



