DC Generators, Alternators and Rotating-Machine Maintenance

The motor-types lesson covers motor types and connections; the Article 430/445 lessons carry the Code. General Theory also reaches generators and the maintenance side of every motor task. This lesson is the generator theory (NEETS Module 5) and the maintenance and testing practice of the Army/Navy facilities manual TM 5-683.

1. DC generator principles (NEETS 5-1.6 to 1.9)

• Commutation: the armature coils generate ac; the commutator reverses each coil's connection to the brushes at the instant its voltage reverses, so the load sees dc. The brushes short-circuit a coil only while it passes through the neutral plane (no induced voltage) — sparking at the brushes means improper commutation, most often wrong brush position.
• Armature reaction: the armature's own magnetic field distorts the main field and shifts the neutral plane (in the direction of rotation for a generator, against it for a motor). Small machines correct it by shifting the brushes; larger ones carry compensating windings in the pole faces or interpoles between the main poles — both in series with the armature so their correction tracks the load current, and the brushes never need moving.
• Motor reaction: current in a loaded armature creates a force opposing rotation; the prime mover must supply more torque as the electrical load grows.
• Windings: lap windings give several parallel paths (high-current machines); wave windings put the coils in series (high-voltage machines).