Module 3.17-3.18 AC generators and AC motors

Knowledge levels

Sub-module	A	B1 / B2 / B2L	B3
3.17 AC generators	—	2	1
3.18 AC motors	—	2	1

AC generation - the rotating loop

• A loop rotating at constant speed in a uniform magnetic field produces a sinusoidal EMF: zero when the loop sides move parallel to the field, maximum when they cut it at right angles.
• One revolution of a 2-pole machine produces one cycle.

Frequency f = (P × N) / 120, where P = number of poles, N = speed in rpm.

Worked example

How fast must a 6-pole alternator turn to produce 400 Hz?

• N = 120 f / P = 120 × 400 / 6 = 8 000 rpm

Equally, an 8-pole machine at 6 000 rpm gives 400 Hz.

Types of alternator

Type	Description	Notes
Revolving armature	armature rotates; output taken through slip rings and brushes	small, low-power machines only
Revolving field	field rotates (fed via slip rings or brushless exciter); output taken from the stationary stator	standard type: high output current does not pass through brushes
Brushless	an exciter generator and rotating rectifiers on the shaft feed the main field; often with a permanent magnet generator (PMG) pilot exciter	modern aircraft IDGs, VFGs - no brushes to wear, low maintenance
Permanent magnet generator	field from permanent magnets; output not regulated by field control	supplies the generator control unit (GCU) and dedicated loads such as FADEC

• Single-phase: one winding, one output. Two-phase: two windings 90° apart (rare; used in some servo and instrument systems). Three-phase: three windings 120° apart - standard aircraft supply (115/200 V, 400 Hz).

Frequency control

Aircraft engines run at varying speed, but many loads need a fixed 400 Hz. Solutions:

• Constant speed drive (CSD) between engine and generator; combined in one unit = integrated drive generator (IDG).
• Variable frequency generator (VFG) - frequency follows engine speed (about 360-800 Hz) and loads are designed to accept it.
• Voltage is regulated by the GCU controlling the exciter field current.