CONVERTERS 393 



If the motor is synchronized and it is found that the generator 

 is out of synchronism the circuit must be opened and the motor 

 allowed to slip back a pair of poles at a time until the correct 

 position is reached. 



249. Induction Frequency Converter. The induction fre- 

 quency converter may be used instead of the synchronous-motor 

 generator set. It consists of an induction motor with a wound 

 rotor driven by a synchronous motor connected to the 25-cycle 

 supply lines. The stator of the induction motor is also connected 

 to the supply and produces a revolving field. At standstill the 

 frequency of the e.m.fs. generated in the rotor windings is 25 

 cycles. When the rotor is driven backwards at synchronous 

 speed the frequency is 50 cycles and when driven at 140 per cent 

 of synchronous speed it is 60 cycles. If a receiver circuit is con- 

 nected to the rotor slip rings 60-cycle power can be supplied to 

 it. f or T 5 2- of the power output is supplied to the rotor by 

 transformer action from the stator and the remaining T 7 is sup- 

 plied by the synchronous motor as mechanical power. 



The principal disadvantage of the induction frequency con- 

 verter is its poor voltage regulation. Due to the presence of the 

 air gap in the magnetic circuit the reactances are large and the 

 e.m.fs. consumed by the reactances are large. 



The exciting current required by the induction motor may be 

 provided by over-exciting the fields of the synchronous motor 

 and making it draw a leading current. In this way the power 

 factor of the set may be made unity, but the increased current 

 in the synchronous-motor windings increases the copper losses 

 and the heating. 



Since the rotor of an induction motor may be wound for any 

 number of phases irrespective of the number of stator phases the 

 induction frequency converter may be used to change the number 

 of phases as well as the frequency. The same result may of 

 course be obtained with the synchronous-motor generator set. 



250. Mercury Arc Rectifier. The mercury arc rectifier or 

 mercury vapor converter is used to convert alternating current 

 to direct current for charging storage batteries supplying arc 

 lights and many other purposes. 



Fig. 372 shows the diagram of connections of a rectifier set. 

 The essential parts are the exhausted bulb B and the two react- 

 ance coils E and F called sustaining coils. In addition there are 

 various controlling reactances and resistances. 



