DC Electrics - Aircraft Electrical Power Systems

Aircraft Electrical Power Systems

The power system for a single-engine aircraft consists of a generator or alternator with the control and indication equipment necessary to supply all the electrical power once the system is on line.

The term on line means that the generator or alternator has been switched into the electrical system and is actually supplying power to the system.

With multi-engine aircraft two or more generators or alternators are installed in parallel. The ampere capacity of an aircraft electrical system is determined by the number of powerconsuming devices fitted.

Dipole or Two Wire System

DC Electrics - Aircraft Electrical Power Systems 8

Systems Power Electrical Aircraft - Electrics DC 8

Figure 8.2 Unipole system

This is the most common type of system on an aircraft with metal construction. The metal airframe is used as the negative conductor completing the circuit for the current flow. The negative side of the generator is connected to an ‘airframe earth’ as is the negative side of each load.

Generators and Alternators

Generators or Alternators are used to convert mechanical energy to electrical energy.

A generator produces direct current, DC, by using a rotating armature, stationary field and a commutator as described in the previous chapter whereas an alternator produces alternating current, AC, by using a rotating field and a stationary armature. If it is required to convert the AC output of an alternator to DC, a diode rectifier is used, fitted in the end frame of the alternator.

Most modern light aircraft have a direct current system which is powered by an alternator. The full power output of a generator is closely related to the RPM of the engine and is usually attained with the engine running at half speed whereas the full power output of an alternator can be attained at slow running, one obvious advantage that an alternator has over a generator. The generator is driven at a speed which is approximately three times that of the engine.

Voltage Regulators

The Voltage Regulator maintains the output voltage of the generator or alternator at a constant value, irrespective of the engine RPM or electrical loads. This is achieved by controlling either the current flow in the field coils of a generator, or the current flow in the exciter field of an alternator.

The basic voltage regulator setting controls the generator output to maintain 14 volts for a 14 volt system with a 12 volt battery and 28 volts for a 28 volt system with a 24 volt battery.