Loss in the Armature Circuit

Author: E.E. Kimberly

The armature-winding power loss is proportional to the square of the armature current. The armature-circuit resistance is the sum of the component resistances of the armature winding, commutator, brushes, brush holders, leads, and contact between the brushes and the commutator. At any fixed temperature, the resistances of all the component parts are constant except those of the brushes and brush contacts. The phenomenon of brush-commutator contact resistance is not fully understood. Contact resistance between copper and carbon decreases as the current density increases. With current densities commonly used, the contact-resistance voltage drop between carbon brushes and the commutator is approximately 2 volts (1 volt at the positive brush set and 1 volt at the negative brush set) and is substantially constant at common operating current densities.

Copper leaf brushes are commonly used on generators designed for low voltage (below 40 volts). In some machines brushes molded or sawed from blocks of a mixture of carbon and metallic powder are used. However, in most machines, brushes of carbon or carbon and graphite are used.

When brushes of carbon are used, their commutator-contact resistances are found to decrease with an increase in current density. Because of the inverse resistance characteristic of carbon-brush contacts, the resistance of an armature circuit may be much less at full load than at no load. Therefore, in measuring the armature-circuit resistance of a direct-current generator to be used in calculating its losses at full load, the rated full-load current should be used.