Chapter 35: DC Generation and Distribution
35.1 Electromagnetic Induction
- The primary source of electrical power on an aircraft is a generator or alternator, which
- converts mechanical energy into electrical energy through electromagnetic induction.
- Principle: When a conductor is moved through a magnetic field (or a magnetic field is
- moved past a conductor), a voltage is induced in the conductor.
- Simple Generator: Consists of a rotating loop of wire (the armature) spinning within a
- magnetic field. This rotation induces an AC voltage in the loop. In a DC generator, a device
- called a commutator is used to mechanically reverse the connections to the loop every half-
- turn, resulting in a DC output.
35.2 DC Generators vs. Alternators
- DC Generator: Has a rotating armature and stationary field magnets. The high-load output
- current must pass through the commutator and brushes, which can cause sparking and wear,
- especially at high altitudes.
- Alternator (Most Common on Piston Aircraft): Has a rotating field magnet and a
- stationary armature (stator). The high-load output current is taken directly from the stationary
- windings, which is more reliable. The small current needed to energize the rotating field is
- supplied through simple slip rings. An alternator's AC output is converted to DC by a solid-
- state rectifier. Alternators are lighter and more efficient at low engine RPM than DC
- generators.
35.3 Voltage Control
- The output voltage of a generator or alternator must be kept constant (e.g., 14V or 28V)
- regardless of engine speed or electrical load. This is done by a voltage regulator. The
- regulator senses the output voltage and automatically adjusts the current flowing through the
- field winding. Increasing the field current strengthens the magnetic field and increases the
- utput voltage; decreasing it does the opposite.
35.4 DC Distribution
- Bus Bar: A central distribution point for electrical power. The generator, battery, and all
- electrical consumers are connected to the bus bar.
- Earth Return System: On metal aircraft, the airframe itself is used as the negative conductor
- for the electrical system. The negative terminal of the generator and battery, and one side of
- every electrical component, are connected to the airframe ("grounded"). This saves a
- significant amount of wiring weight.
- Ammeters:
- Centre-Zero Ammeter: Shows the current flowing into (charging) or out of (discharging)
- the battery.
- Load Meter: Shows the total current output from the alternator. A zero reading in flight
- indicates an alternator failure.