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| Illustration 1 | g00864661 |
(2) Rotor (3) Slip rings (6) Rectifier (7) Stator (9) Capacitor (10) Regulator (11) Diode assembly (12) Brushes | |
The 21-SI series alternator is an integral charging system. The alternator has a built-in diode rectifier and a voltage regulator. The system produces DC current for electrical systems.
The solid-state voltage regulator that is built into the 21-SI alternator limits the system voltage by switching the ground circuit for the field coil at a rate between 10 hertz and 7000 hertz. When the ground circuit is on, field current passes from the diode assembly through the field coil. Nominal regulated voltages of 13.5 to 14.5 volts are available for 12 volt systems. The nominal regulated voltage for the 24 volt system is between 27.0 and 29.0 volts.
If the alternator has an "I" terminal, the terminal may be used in order to supply excitation current. The current flows from the keyswitch to the "I" terminal. The "I" terminal must have an indicator light in series between the keyswitch and the "I" terminal. The indicator light provides a verification of alternator excitation. This maintains the field current around 0.17 amperes. Once the alternator begins charging, the field current is supplied from the diode trio. Current stops flowing through the "I" terminal and the indicator lamp turns OFF.
The initial voltages at start-up are generated by residual magnetism in the rotor. As the speed increases and the output increases, the voltage that is available at the diode assembly begins to supply field current for normal operation. When output voltage exceeds battery voltage, the alternator begins to drive the system voltage.
While the alternator output voltage is below the setting of the voltage regulator, the regulator turns on the field current. This allows the alternator to produce the maximum output. When the voltage setting is reached, the regulator turns off the field current. When the field current is off, the magnetic field in the rotor decreases and the alternator output voltage begins to fall. The falling voltage causes the regulator to turn on the field current and the current rebuilds the magnetic field. This cycle continues at a rate between 10 hertz and 7000 hertz. The rate of the cycle is dependent on the system load. The cycle keeps the output and the system voltage very close to the voltage setting. The cycle will continue unless the electrical demands of the system cause the system voltage to fall below the voltage setting. If the system voltage falls below the voltage setting, the regulator will allow continuous field current to enable the alternator's maximum output.
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| Illustration 2 | g00671151 |
Terminal Locations of the Alternator (1) "B+" terminal (output) (2) Ground terminal (3) "R" terminal (4) Location for "I" terminal | |
External connections to the 21-SI alternator are made to the terminals shown in illustration 2. The "B+" terminal (1) on the 24 V alternator is a 1/4 inch stud. The 12 V alternator has a 5/16 inch stud.
A threaded 1/4-20 hole in the rear frame is provided as the ground terminal (2) .
The "R" terminal (3) is a #10 stud.
The "I" terminal (4) is also a #10 stud. This terminal is only available on the following alternators.