The Third-Brush Generator

Author: E.E. Kimberly

The generator used on automobiles for battery charging must operate over a wide range of speed with a reasonably uniform current output. This characteristic may be achieved in several ways, the most popular of which is by use of the "third brush" method of exciting.

Fig. 29-3. Third-Brush Generator

In the third-brush generator, a schematic diagram of which is shown in Fig. 29-3, the shunt field is not excited directly from the main brushes, but is connected between one main brush and the third brush, which may be adjusted at will within limits around the periphery of the commutator between A and B. The voltage available for excitation is only that generated in the armature conductors producing voltage on the commutator between B and C. When the generator is started and begins to build up, the growing flux density is practically uniform at the pole faces. When the voltage rises to a value somewhat above that of the battery, the reverse-current cutout closes the charging circuit. There being no interpoles or compensating windings, the armature current produces a severe distortion of the field flux, as shown in Fig. 29-3. Much of the flux, which at first cuts conductors between B and C to produce excitation voltage, is thus forced into the leading pole tip nearer A and is lost for the purpose of excitation. Furthermore, there is a net loss of flux because of saturation of the pole tips.

As the speed is increased above a minimum speed determined by design, the losses of voltage by armature reaction equal the gain in voltage by gain in speed, and the charging current can rise no higher.

In addition to the field distortion by the armature reaction, the strength of the field is further reduced by the demagnetizing action of the short-circuit current in the conductors being commutated by the third brush. The ultimate result is a reduction of charging current as the speed is increased beyond a limit determined by the design proportions of the generator.