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.