DC machines
The commutator-based machine — alternating internally, direct at the brushes. Most flexible single-phase rotational machine.
Lessons
The commutator in action
A purely mechanical inverter/rectifier — AC inside the coil, DC at the brushes. The single most elegant trick in machine design.
DC: generated EMF and torque
E_a = K_a·Φ·ω, T = K_a·Φ·I_a — one machine constant, two equations, four numbers in lockstep.
DC excitation: shunt vs series vs compound
Three field arrangements, three personalities — constant-speed shunt, high-torque series, balanced compound.
DC armature reaction and commutation
Cross-magnetising distortion, MNA shift, and how interpoles + compensating windings cancel them automatically.
DC motor speed control — the three knobs
V_t below base speed, field weakening above base speed, armature resistance for starting. Modern 4Q drives integrate all three.
DC generator external characteristics
V_t vs I_L curves: linear droop (sepex), regenerative droop (shunt), rising (series), over/flat/under-compound.
DC motor starter design
Stepped armature resistance, 3-point and 4-point starters, NVR + OLR coils, and the modern current-limited converter.
Cross-field DC machines (Amplidyne, Metadyne)
Rotary power amplifiers using shorted q-axis brushes for 2-stage cross-field amplification. Gain 10⁴-10⁵. WWII radar / naval gun / elevator / mill servos. Replaced by power electronics — Bimbhra / GATE syllabus.