388 ELECTRICAL ENGINEERING 



short periods must be high. In some cases when using inter- 

 poles momentary overloads of 200 per cent are permitted. 



242. Compounding. When the field current of a rotary 

 converter is varied wattless lagging or leading currents flow in 

 the armature and magnetize or demagnetize the field and bring it 

 back to its former strength, giving the same generated voltages 

 as before, but there is no change in the direct voltage since it has 

 a fixed relation with the impressed voltage. To vary the direct 

 voltage the impressed alternating voltage must be varied. This 

 can be done (1) by using variable-ratio supply transformers, 

 (2) by connecting potential regulators in the lines or (3) by con- 

 necting reactance coils in the lines and drawing wattless lagging 

 or leading currents through them by varying the field excitation. 

 The first method has the disadvantage of a step-by-step regulation; 

 the second requires expensive apparatus but gives very good regula- 

 tion and can be made automatic; the third is inexpensive and may 

 be made automatic by placing a series winding on the converter 

 fields. The variation of the impressed voltage due to the reactance 

 coils is explained as follows : The e.m.f . of self-inductance lags 90 de- 

 grees behind the current; thus, when the converter is under-excited 

 and a component of current lagging 90 degrees behind the impressed 

 e.m.f. flows through the reactance coil, the e.m.f. of self-inductance 

 due to it lags 180 degrees behind the impressed e.m.f. and there- 

 fore opposes and lowers it. When the converter is over-excited 

 and a component of current 90 degrees ahead of the impressed 

 e.m.f. flows through the reactance coil, its e.m.f. of self-induc- 

 tance is in phase with the impressed e.m.f. and raises it. 



Therefore when reactance coils are connected in the supply 

 lines an increase of the field excitation raises the impressed e.m.f. 

 and so raises the direct voltage, and a decrease of the field excita- 

 tion lowers the impressed e.m.f. and so lowers the direct voltage. 

 The result is the same as in the^direct-current generator but is 

 produced in a different way. 



If the converter is provided with a series winding the reactance 

 coils cause the direct voltage to rise automatically with increase 

 of load. 



243. Starting. Converters may be started in various ways: 

 (1) If reduced voltage is impressed on the slip rings the con- 

 verter will start as a synchronous motor and come up to full speed 

 as explained in Art. 169. Special supply transformers are used 



