Aircraft Generators vs Alternators: What A&P Students Need to Know
Aircraft electrical systems need a source of electrical power while the engine is running. That power usually comes from either a generator or an alternator.
Both devices convert mechanical energy from the engine into electrical energy, but they do it in different ways. For A&P students, the important thing is understanding what rotates, what stays still, how the current is produced, and how the aircraft uses that electrical output.
The Big Idea
A generator and an alternator both work because of electromagnetic induction.
When a conductor moves through a magnetic field, voltage is induced into the conductor. That induced voltage can cause current to flow if the circuit is complete.
In simple terms:
Motion + magnetic field + conductor = induced voltage
That is the basic principle behind both generators and alternators.
Generator vs Alternator: Simple Difference
A common way to remember the difference is:
- Generator: usually produces DC output directly
- Alternator: produces AC output first, then usually rectifies it to DC for aircraft use
Many light aircraft use alternators today because they can produce useful output at lower engine speeds and are generally lighter and more efficient than older DC generators.
What a Generator Does
A generator converts mechanical energy into electrical energy by rotating a conductor through a magnetic field.
In a basic DC generator, the rotating part is called the armature. The armature turns inside a magnetic field. As it rotates, voltage is induced into the armature windings.
The generator uses a commutator and brushes to help deliver DC output to the aircraft electrical system.
Main Parts of a DC Generator
A basic DC generator includes:
- Armature
- Field poles
- Field windings
- Commutator
- Brushes
- Frame or housing
The Armature
The armature is the rotating part of a DC generator. It contains windings where voltage is induced.
For A&P test purposes, remember:
In many basic DC generator explanations, the armature rotates.
As the armature rotates through the magnetic field, current is generated in the windings.
Field Poles and Field Windings
The field poles create the magnetic field inside the generator.
The field windings are coils of wire wrapped around the field poles. When current flows through the field windings, they become electromagnets.
The stronger the magnetic field, the more voltage the generator can produce.
A simple way to think about it:
More field strength = more generator output
The Commutator
The commutator is a segmented copper ring connected to the armature windings. It rotates with the armature.
Its job is to help change the internally generated alternating current into a usable direct current output.
This is one of the major parts that separates a DC generator from an alternator.
Brushes
The brushes ride against the commutator. They provide an electrical connection between the rotating armature and the external circuit.
Because brushes physically contact the commutator, they can wear over time. That is one reason generators generally require more maintenance than alternators.
What an Alternator Does
An alternator also converts mechanical energy into electrical energy, but it normally produces alternating current first.
In many aircraft alternators, the magnetic field rotates and the output windings are stationary.
That means the alternator often has:
- A rotating magnetic field
- Stationary output windings
- Rectifiers to convert AC to DC
For many aircraft electrical systems, the alternator output is rectified into DC because the aircraft uses a DC electrical system.
Main Parts of an Alternator
A basic alternator includes:
- Rotor
- Stator
- Field winding
- Slip rings
- Brushes
- Rectifier diodes
- Voltage regulator
Rotor
The rotor is the rotating part of the alternator.
In many alternators, the rotor carries the magnetic field. Current is supplied to the rotor field winding, which creates an electromagnet.
As the rotor spins, its magnetic field cuts across the stator windings.
Stator
The stator is the stationary part of the alternator.
It contains windings where AC voltage is induced. Since the stator does not rotate, it can carry the output current without needing a commutator.
That is one advantage of an alternator.
Rectifier Diodes
Since an alternator produces AC, the output must often be changed to DC before it can be used by the aircraft’s DC electrical system.
This is done with rectifier diodes.
A diode allows current to flow in one direction only. A rectifier uses diodes to convert AC into pulsating DC.
For A&P purposes:
Alternator output is AC first, then rectified to DC.
Voltage Regulator
The voltage regulator controls alternator or generator output.
It does this by controlling field current. If system voltage is too low, the regulator can increase field current. If system voltage is too high, it can reduce field current.
Simple version:
The voltage regulator controls output by controlling the magnetic field.
Why Alternators Are Common
Alternators are common in modern aircraft because they have several advantages over older DC generators.
Alternators can usually:
- Produce useful output at lower engine RPM
- Provide more stable output over a wider RPM range
- Be lighter for the same output capacity
- Require less brush and commutator maintenance
- Supply higher current more efficiently
This is why alternators are often preferred in many aircraft electrical systems.
Generator Output at Low RPM
One weakness of a DC generator is that it may not produce enough current at low engine RPM.
At idle or low RPM, the generator output may be too low to carry the aircraft electrical load. In that case, the battery may have to supply part of the electrical demand.
That can discharge the battery if the engine stays at low RPM for too long.
Alternator Output at Low RPM
Alternators generally perform better at lower RPM than DC generators.
That does not mean an alternator produces full output at idle, but it often produces enough to support the aircraft electrical system better than a generator would.
This is one of the main reasons alternators replaced generators in many applications.
Generator and Alternator Similarities
Generators and alternators are different, but they have several things in common.
Both:
- Convert mechanical energy into electrical energy
- Use electromagnetic induction
- Need a magnetic field
- Need relative motion between a magnetic field and conductors
- Use a voltage regulator
- Supply electrical power to the aircraft
- Help keep the battery charged
Generator and Alternator Differences
| Feature | Generator | Alternator |
|---|---|---|
| Basic output | DC | AC first, then often rectified to DC |
| Common rotating part | Armature | Rotor |
| Common stationary output part | Field structure | Stator |
| Uses commutator | Yes | No commutator for output |
| Uses rectifier diodes | Usually no | Yes |
| Low RPM output | Usually weaker | Usually better |
| Maintenance | More brush/commutator wear | Generally less |
A&P Test Tip: Spinning Coils vs Spinning Magnets
A useful memory aid is:
Generator = spinning coils
Alternator = spinning magnetic field
That is simplified, but it is useful for many A&P-style questions.
A DC generator commonly has a rotating armature, which means the conductors rotate through the magnetic field.
An alternator commonly has a rotating magnetic field, which induces voltage into stationary stator windings.
A&P Test Tip: Armature
If a question asks where voltage is induced in a DC generator, think about the armature.
The armature is the part where the generated voltage is produced in many basic DC generator explanations.
A&P Test Tip: Field Windings
If a question asks what creates the magnetic field, think about the field windings and field poles.
The field winding is an electromagnet. It creates the magnetic field needed for induction.
A&P Test Tip: Rectifier
If a question asks what changes AC into DC, the answer is usually:
Rectifier
In an alternator system, the rectifier diodes convert the alternator’s AC output into DC.
A&P Test Tip: Voltage Regulation
If a question asks how generator or alternator output is controlled, think:
Control the field current
The voltage regulator adjusts field current to control output voltage.
Common Electrical System Indications
Aircraft electrical systems often include an ammeter or loadmeter.
Depending on the system, the indicator may show:
- Whether the battery is charging
- Whether the battery is discharging
- Alternator or generator load
- Whether the charging system is carrying the electrical demand
If the alternator or generator fails, the aircraft may continue operating on battery power for a limited time.
What Happens If the Alternator or Generator Fails?
If the charging source fails, the battery becomes the main source of electrical power.
That means electrical equipment is now using stored battery energy. The pilot may need to reduce electrical load by turning off nonessential equipment.
From a maintenance perspective, a charging system failure could involve:
- Failed alternator or generator
- Broken belt or drive issue
- Faulty voltage regulator
- Open field circuit
- Failed rectifier diode
- Poor wiring connection
- Blown fuse or tripped circuit breaker
- Bad ground
- Weak or failed battery
Troubleshooting Mindset
When troubleshooting a generator or alternator system, do not just replace parts randomly.
Think through the system:
- Is the battery charged?
- Is the alternator or generator being driven?
- Is the belt or drive system intact?
- Is field current present?
- Is output voltage present?
- Is the voltage regulator working?
- Are the wiring connections clean and tight?
- Are the circuit protection devices okay?
- Is the aircraft ground path good?
- Is the cockpit indication accurate?
Electrical troubleshooting should follow the aircraft maintenance manual and wiring diagrams.
Generator Summary
A generator:
- Converts mechanical energy into electrical energy
- Commonly uses a rotating armature
- Uses field windings to create a magnetic field
- Uses a commutator and brushes
- Produces DC output
- May have weaker output at low RPM
- Requires more brush and commutator maintenance
Alternator Summary
An alternator:
- Converts mechanical energy into electrical energy
- Commonly uses a rotating magnetic field
- Has stationary stator windings
- Produces AC first
- Uses rectifier diodes to produce DC
- Usually has better low-RPM output
- Is common in modern aircraft electrical systems
Easy Memory Aids
Here are a few simple ways to remember the topic:
Generator = DC output, armature, commutator
Alternator = AC first, rectifier, stator
Rectifier = changes AC to DC
Voltage regulator = controls field current
More field current = stronger magnetic field = more output
Final Thoughts
Generators and alternators both do the same basic job: they provide electrical power while the engine is running and help keep the battery charged.
The main difference is how they produce and deliver that electrical power.
For A&P students, focus on these key ideas:
- Both use electromagnetic induction
- A generator commonly uses a rotating armature
- An alternator commonly uses a rotating magnetic field
- An alternator produces AC first
- Rectifiers convert AC to DC
- Voltage regulators control output by controlling field current
- Alternators usually perform better at lower RPM than generators
If you remember those points, generator and alternator questions become much easier.