DC Generators, Alternators and Rotating-Machine Maintenance

Lesson thry-01 covers motor types and connections; nec-07-01 to -04 carry Article 430/445. The Class II task list also names "install/replace D-C generator" 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).