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Transactions of the Royal Society of South Africa. 
constant excitation. This applies to all fast regulators based on the 
principle of over-travelling. 
Description op Fast Regulator. — We shall deal with the Tirrill 
regulator, which is of the vibrating contact type. Its principle of action 
will be followed from a perusal of fig. 6. Let the main contacts and Kg 
be open, then the differential relay is energised (coil m only) and the relay 
contacts K3 and are open, so that the rheostat r^ is inserted in the field 
circuit. The voltage now drops, as explained before. When it reaches 
c'd" (fig. 5) the attraction of the main magnet is overcome by the spring 
above, which closes the contacts and Ko, thereby passing current 
through winding 71 of the differential relay, which is now demagnitised. 
Fig. 7. — Simplified connections of Tirrill regulator. 
Spring / next closes the relay contacts Ko and and the latter short- 
circuit the rheostat r^ in the field circuit, so that the voltage rises. When 
the upper limit of the voltage is reached the main contacts are open again ; 
and so forth. Both sets of contacts are thus constantly vibrating. The 
frequency depends largely on the load. The times, during which the 
contacts are closed, adjust themselves automatically, so as to maintain a 
uniform voltage at the bus bars or generator terminals. 
For over-compounding, the generator control magnet is supplied with 
a differential series winding, which is joined across a variable shunt in the 
bus bars, or generator leads or feeders. As this series winding opposes the 
shunt winding of the generator control magnet, the relay contacts remain 
comparatively longer closed, and the voltage at the terminals is able to 
