26SI Series Alternator – Normal Operation

Illustration 1 g00620877

26-SI Alternator Schematic

The 26-SI series alternator is a brushless, heavy-duty 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 integrated circuit voltage regulator that is built into the 26-SI alternator limits the system voltage by switching the ground circuit for the rotor field on and off. When the ground circuit is on, field current passes from a diode trio through the stationary 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.

As the alternator begins operating, the regulator detects battery voltage that is applied to the ignition terminal. The initial exciting current then flows through the rotor coil. When the ignition switch is turned ON the regulator turns on and off. This occurs intermittently in order to reduce battery discharge. Current is kept to a minimum of approximately 0.17 amperes.

The voltage regulator turns ON the field current and the voltage regulator turns OFF the current many times in one second in order to keep the voltage at a preset level.

For 12 volt systems, an output rating of 85 amperes is standard. For 24 volt systems, output ratings of 50 or 75 amperes are available. The output ratings of a specific 26-SI model are located in Specification, “Alternator”.

The 26-SI alternator is designed for a one-wire charging system configuration. One wire refers to the minimum number of connections that are necessary at the alternator for operation. The output of the alternator must be connected to the positive terminal of the battery. A ground path is also required. The ground path should run between the alternator housing and the ground terminal for the battery.

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 trio becomes sufficient to supply field current for normal operation. When output voltage exceeds battery voltage, the alternator begins to drive the system voltage. The residual magnetism can be lost. This results in no output. Loss of the residual magnetism can be caused by extended downtime or a severe shock to the alternator. Refer to Troubleshooting, “T6 Residual Magnetism Restoration”.

While the system 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 collapses 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 rapidly. 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 full field current to flow so that the alternator’s maximum output is realized.

Illustration 2 g00649290

(1) Positive Terminal

(2) Relay Terminal

External connections to the 26-SI alternator are made to the terminals that are shown in illustration 2. The standard output terminal is a female type. The connecting bolt is insulated. The cable for charging has a special connector. When the connection is installed, the electrical connection is sealed from moisture. No voltage exists at the exposed parts. The head of the connector bolt is normally stamped “NO VOLTS” in order to indicate the insulated type.

All of the electronic parts of the alternator are sealed in a compartment in order to keep out the moisture and the dirt.