This article is written as much for me as for you, since it will try to collect my understanding of how alternators and generators work.
Generator
The advantage of a generator are:
- Doesn’t require power input to operate
- Output is Direct Current (DC)
- A DC Generator is a DC Motor – so it can be used as a Starter/Generator combination.
A DC Generator consists of magnets surrounding a coil of conducting wire. The coil is mounted on bearings and is free to rotate. When the coil is rotated within the magnetic field a DC current is generated in the coil. Through the use of a commutator and brushes the power generated is retrieved from the central coil.
The coil is rotated either through pulleys and belts or gears from the rotation of the engine.
The faster the coil rotates the more power is generated – so generators need to be rotated quickly to produce their fully rated output.
The power pickup comes through the brushes, so large amounts of power can be fed through that linkage which can cause wear in the brushes and commutators.
In the starter/generator combination the application of power to the rotating coil will cause the coil to rotate allowing the starter to operate. Upon completion of the start by removing power from the coil the unit reverts to generator power.
Alternator
The advantage of an alternator are:
- Produces full rated power at lower rotation speeds
- Unit is lighter than a comparable alternator.
An AC Generator consists of 3 fixed coils (stators) surrounding a coil free to rotate. When an electric current is applied to the rotating coil AND the coil is rotated then power is induced in the 3 fixed coils. The 3 fixed coils produce Alternating Current (AC) which can be changed to DC current through the use of a rectifier bridge (normally 6 diodes). This combination of AC Generator and Rectifier is an Alternator.
The coil is rotated through pulleys and sometimes through gears.
While higher rotation speed produces more power, the rotation required for equivalent power from a DC generator is lower.
The power pickup comes through the fixed stator coils. The field current is fed to the rotating coil through slip rings and brushes. Since the field current is typically low (1-7 amps) the wear on the slip rings and brushes is lower.
Operational Concerns
The alternator system will have a cockpit switch to provide power to the field coils. This switch may be a split master switch or a separate switch. Failure to turn this switch on will prevent the alternator from providing output leading to eventual discharge of the battery.
With an alternator you must have some power in the electrical system (from the battery) in order to provide current to the field coil and hence allow the alternator to create electricity. If the entire electrical system is inoperative – even if the engine is started the alternator will not produce electricity. The same is NOT true in a DC generator based system – only rotation is needed to produce electricity.
Long periods of low speed engine rotation can lead to insufficient rotation to produce power. In an alternator system low idle may not be sufficient, but in the typical small reciprocating engine 1,000RPM idle will provide sufficient rotation. In a generator system idle may not be sufficient to provide enough power and engine must periodically be run-up. In addition long periods of descending flight with the engine(s) at idle may mean insufficient rotation speed to maintain enough output. Watching the voltage and ammeter indications will help manage the electrical system charging system.