Double Contact Regulator
In a double contact regulator, two fixed and one movable contacts are used to vary the current flowing in the alternator field coil.
Before double contact regulators came into general usage single contact types were used. A problem with this design was that during high speed operation, the alternator output voltage was high enough to force sufficient current through the resistor in the regulator to produce a higher than desired alternator output.
In the double contact regulators, a second or "shorting" set of contacts shorts out the field current when a predetermined alternator voltage is reached. While this set of contacts are closed the charging output is reduced to zero. By this method of output control, voltage "creep" is eliminated and output voltage is stabilized regardless of engine speed. The lower contacts control alternator output during low speed operation. The contacts are normally held closed by an armature spring permitting full field current of about two amperes. During this time the alternator is putting out its full output voltage governed only by how fast it is being turned.
When a predetermined voltage is applied to the voltage regulator coil, the magnetic field it produces pulls the armature down, opening the lower contacts, thereby forcing the field current to flow through the resistor. Field current is now reduced to approximately 3/4 of an ampere. The reduced field strength results in a proportionate reduction in alternator output voltage and a reduction of voltage applied to the voltage regulator coil. The reduced voltage decreases the magnetic field of the coil and the lower contacts are allowed to close. With full alternator output restored, the regulator begins to repeat voltage regulation cycle. The vibrating of the relay contacts is very rapid, between 75 to 250 times per second. This cycling action is continuously repeated as long as the voltage limiting action is required.
During periods of high engine speed and low electrical requirements, the increased magnetic field strength of the voltage regulator coil causes the upper contacts to come into operation. When this happens, the alternator field current is reduced to between 3/4 ampere (current flow through the resistor) and zero amperes (upper contacts shorting field current to ground). This cycling action of the contacts sharply limits the alternator output voltage.