AC Generators
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.
AC generators, summary notes
- 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.
- 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
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.
Three-phase AC generation
AC generators concept map
AC Generators
AC generators quiz
AC Generators, quiz
1. A 6-pole alternator running at 8000 rpm produces a frequency of:
400 Hz800 Hz200 Hz2. In an aircraft alternator, the arrangement normally used is:
Rotating field, stationary armatureRotating armature, stationary fieldBoth rotating3. In a star-connected three-phase system, the line voltage is:
√3 × phase voltageEqual to the phase voltagePhase voltage ÷ √34. The three windings of a three-phase alternator are spaced:
120° apart90° apart180° apart5. Constant 400 Hz output frequency is maintained by:
A constant speed drive or IDGVarying the field currentChanging the number of poles in flight6. If the number of poles is doubled at constant speed, frequency:
DoublesHalvesIs unchanged7. A brushless alternator's three stages are:
PMG, exciter and main generatorStator, rotor and commutatorPrimary, secondary and tertiary windings8. In a delta-connected system, the line current is:
√3 × phase currentEqual to the phase currentPhase current ÷ √39. A 4-pole alternator must run at what speed to give 400 Hz?
12 000 rpm6000 rpm24 000 rpm10. Three-phase generation is preferred on aircraft mainly because it gives:
Smoother power from lighter machinesA higher output voltageSimpler single-wire distribution