The Theory of Automatic Regulators. 
277 
maximum value. When a load rush occurs, the voltage drops. This is 
equivalent to a reduction in the excitation. In our case it thus means that 
the excitation has been reduced from a value corresponding to a« and to the 
exciting current a„l, to one corresponding to am and the exciting current 
a„J. Let the contact voltmeter act when the voltage exceeds the limits 
Ymax and Ymin (see fig. 3), the difference between the two values being aY 
corresponding to Aa = 28. 
As soon as Y„,/„ is reached the relay puts the regulator into motion, some 
little time elapsing, however, before the next contact is reached. The ex- 
citing current then commences to rise, and with a large number of contacts 
the enveloping curve of the various exponential curves, corresponding to 
1 
Vntax \ 
5m^/ /Ci 
time. 
Fig. 4.— Hunting of regulator. 
different positions of the regulator, will be a straight line, with the inclina- 
tion % against the abscisse axis. The first and last curves will alone be out, 
the current for the final step following the last exponential curve. 
If the speed of the regulator is too great, the regulator will overtravel 
the correct position by one or more steps, since, although the relay has 
disconnected the driving motor, the magnetisation lags behind. The excitation 
may even still be rising when the relay has already reversed the direction 
of travel of the regulator. This is clearly indicated in fig. 4. 
When the voltage reaches V^ax, point P, the relay reverses the driving 
motor, moving the switch lever backwards, but a little time U elapses before 
a step is actually cut in. On account of magnetic lag the voltage keeps on 
rising until several steps have been inserted. If no excess regulation is to 
