Rotation of a loop in a magnetic field and the waveform produced; operation and construction of revolving-armature and revolving-field alternators; single-phase, two-phase and three-phase alternators; star and delta connections; frequency, phase and power calculations.
Notes
AC generators — summary notes
Free
Main ideas
An alternator generates AC. In aircraft practice the field rotates and the armature is stationary, which avoids passing the main output through slip rings.
Modern aircraft alternators are brushless three-stage machines: a permanent magnet generator, an exciter, and the main generator with rotating rectifiers.
Output frequency depends on poles and speed: f = (P × N) / 120, where P is the number of poles and N is in revolutions per minute.
Three-phase output has three windings 120° apart. It gives smoother power and lighter machines than single phase.
Constant 400 Hz output requires constant rotor speed, maintained by a constant speed drive (CSD) or integrated drive generator (IDG).
⚠ Exam trap: more poles or higher rpm gives a higher frequency. The /120 form uses rev/min; if speed is given in rev/sec the divisor is 2.
Key formulas
Frequency from poles and speed
f = (P × N) / 120 (N in rev/min)
Three-phase star
V_line = √3 × V_phase, I_line = I_phase
Three-phase delta
V_line = V_phase, I_line = √3 × I_phase
Solved examples
A 6-pole alternator runs at 8000 rpm. What is its output frequency?
f = (P × N)/120 = (6 × 8000)/120 = 48000/120 = 400 Hz.
A star-connected alternator has a phase voltage of 115 V. What is the line voltage?
V_line = √3 × V_phase = 1.732 × 115 = 200 V — the familiar 200/115 V aircraft system.