322 ELECTRICAL ENGINEERING 



In Fig. 303 the m.m.f. of the load current in the secondary 

 cannot be opposed by any primary m.m.f. since the coils are at 

 right angles. To supply the m.m.f. required to balance the 

 secondary load m.m.f., and so prevent a large reactance drop in 

 the winding, the coil T called the "tertiary" coil is placed on the 

 movable core at right angles to the primary coil. It is short 

 circuited and exerts an m.m.f. equal and opposite to the second- 

 ary m.m.f., and so reduces the secondary reactance to the value 

 corresponding to the leakage flux. 



The only current carried by the primary coil in this position 

 is the exciting current. 



In intermediate positions of the rotor the secondary m.m.f. is 

 partly balanced by the induced current in the tertiary or com- 

 pensating coil and partly by a load current in the primary coil. 



(2) The polyphase induction regulator has a polyphase wind- 

 ing on the moving core, which is connected to the polyphase 

 supply. The secondary or stator is wound with the same number 

 of phases, but the phase windings are kept entirely separate so 

 that they can be connected in the different lines to raise or lower 

 the voltage. 



When polyphase currents flow in the primary windings a 

 revolving magnetic field is produced of constant value as in the 

 alternator or induction motor. This field cuts the secondary 

 windings and generates e.m.fs. in them of the same frequency as 

 the primary impressed e.m.fs., but less in the ratio of turns. 



Ss 



Si 



FIG. 306. 



As the primary is turned the magnitude of the revolving field 

 is not changed, and therefore the magnitude of the secondary 

 e.m.fs. is not changed but their phase relations with the im- 

 pressed e.m.fs. are changed and the load voltage is varied as 

 shown in Fig. 306. By turning the primary through 180 degrees 

 the phase of the secondary e.m.f . is changed from direct opposition 

 to the line voltage to direct addition to it and thus the load volt- 

 age is varied by an amount equal to twice the secondary voltage. 



