DC Generators and Regular Systems By Scott Fureman With the onset of spring and summer many rebuilders were once again pre- sented with generators, mechanical regulators, cutouts, and their inherent problems. While the “old pros” can rebuild and diagnose generator systems with ease, the “new guy” on the bench will often find himself working on a unit that was built before he was born. Generators provide output through the induced voltage produced when an armature rotates within a magnetic field. As the wire coils in the armature rotate within a magnetic field produced by the field coils, the output voltage is produced. The voltage induced in the armature is picked up on the commuta- tor by carbon brushes and conducted to terminal studs or case ground. Because the carbon brushes are very soft the commutator must be undercut. Undercutting removes the mica insula- tion from between the commutator bars in order to prevent premature brush wear. The mica is relatively hard when compared to the carbon brush, and, if not removed, will cut the face of the brush every time it passes. The mica should be cut clear the full width and length between the copper bars, and to a depth equal to the distance between the bars. The commutator should then be sanded smooth. Because the entire output of the gen- erator passes through the commutator- brush connection, it is vitally important that the brushes be seated properly to the radius of the commutator. This can be done by running the generator on the test bench and gliding an abrasive “comm stone” along the length of the commutator. As the generator charges with unseated brushes, you will see sparks at the base of the brushes. As the brushes seat, the sparks will subside and become evenly distributed along the length of the brush face. An additional indication that the brushes are seating properly can be seen when the charging voltage and amperage increase as the brushes begin to seat fully. The current for the field coils is sup- plied by either the hot brush (A circuit) or “F” field terminal (B circuit). On three brush generators, the field current is provided by the armature via the third, usually adjustable, brush. The main brushes are positioned 180 degrees apart to collect positive and negative output from the commutator. The third brush is positioned up to 45 degrees away from the output brush and collects current in order to supply the field coils. The clos- er the third brush is placed to the output brush, the higher the voltage potential of the field brush. The higher the brush voltage, the stronger the field magnetism and higher the output. Although generators are essentially motors, the fine field coil windings inhibit the strength needed to create a useful amount of torque. The starter- generator combination creates a useable amount of torque by combining a heavy gauge starter coil with a fine gauge generating coil in one unit. These com- binations are, by design, low output units, but the cranking torque is high enough to start a small gasoline engine. The starter-generator combination is typically used in applications such as small lawn and garden tractors where the electrical requirements and cranking torque are minimal. The starter-generator combination has been discontinued and will only be found on older equipment. Unlike alternators, gen- erators are not polarity sensitive. Also, unlike alternators, generators must be polarized before they will operate prop- erly. Polarizing the generator sets up the magnetic fields around the field coils. The generator uses this residual magnet- ism to begin the generating process. The polarization process differs between “A” and “B” circuit, but the basic principle remains the same. In order to polarize a generator, we must know the ground polarity and whether the unit is an “A” or “B” circuit. In the event the unit in question is an “A” circuit (like most Delco-Remy units), negative ground, we know that the positive side of the field coils is connected to the “ARM” terminal and the negative side is connected to the insu- lated “FLD” terminal. In order to polar- ize the “A” circuit generator with the reg- ulator hooked up we must momentarily jump “BAT” to “ARM” with the genera- tor and regulator grounded. It is impor- tant to limit the time the jumper wire is connected to the “ARM” terminal, because polarizing applies system volt- age to the armature and field coils, caus- ing a relatively high amperage draw. Once the generator is installed and polarized, and the battery checked and charged, we can start the engine. As the armature rotates within the residual mag- netic field around the field coils, voltage is induced in the armature. This voltage causes current flow through the field coils and back to ground via a “field resistor” mounted on the regulator (“A” circuit). This resistor bypasses the regula- tor circuits in order to allow faster turn- on. The output of the generator continues to increase until the voltage i