26SI Series Alternator – General Information

Illustration 1 g00901038

27-SI Alternator

The alternator is an electromechanical component that converts mechanical energy into electrical energy. The alternator charges the storage battery during engine operation and the alternator supplies power for the machine electrical systems. The alternator is brushless.

The alternator is divided into a front frame and a rear frame. The alternator contains parts that are inside of the frames. The following parts are stationary: the stator, the field winding, the rectifier, the diode assembly, the regulator and the capacitor. The stationary parts are bolted to the frames. The rotor is a moving part. The rotor is mounted to the frames by ball bearings or by roller bearings.

The field winding uses DC voltage to create a magnetic field. The magnetic field magnetizes the rotor. The rotor rotates the magnetic field on the inside of the stator. The stator generates AC voltage. The rectifier changes the AC voltage into DC voltage. Part of the DC voltage returns to the field winding in order to maintain the magnetic field. The remainder of the DC voltage is supplied to the battery and to the electrical components through battery terminals. The regulator limits the DC voltage at the battery terminals by controlling the current in the field winding.

The alternator also contains parts that are outside of the frames. The removable rear cover provides easy access to many of the internal components for troubleshooting and for repair. A belt (not shown) provides the mechanical energy. The belt is attached to the shaft of the rotor. The fan cools the inside of the alternator. Air is drawn through holes that are located on the side of the rear frame. The air exits from the front frame behind the fan.

The battery terminals provide the alternator output. “B+” terminal (1) is a threaded stud. A nut, a washer, and a lockwasher are installed onto the stud. The “B+” terminal is connected to the rectifier. “B-” terminal (3) is a threaded hole. A screw and a lockwasher are installed into the hole. The “B-” terminal is connected to the case.

“R” terminal (2) is a threaded stud that is located between the battery terminals. The “R” terminal is used to restore the residual magnetism of the alternator. This terminal may be used to operate one of the following components:

  • A Charge Indicator

  • A Tachometer

  • An Hour Meter

  • VIMS, CMS, EMS, or a similar device

The “R” terminal supplies the components with a number of voltage pulses for each revolution of the alternator. The components that are connected to the “R” terminal must not draw excessive current.

A paper tag identifies the “B-” terminal on a replacement alternator. Remove the tag and discard the tag. Install the screw and the lockwasher in the housing in order to prevent the entry of dirt and water.

The alternator requires a connection to the positive terminal of the battery for operation. The alternator also requires a path to ground. The alternator’s case is grounded. However, do not operate the unit without an external path to ground. The “B-” terminal is provided in order to connect a ground lead. The ground path should run between the alternator housing and the negative terminal of the battery. Otherwise, the ground path is through the mounting hardware and the brackets to the engine.