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Generator vs an Alternator

Posted by on February 23, 2009 14 Comments Category : Flight Instructor Blog Tags : ,

alternatornewArthur writes in asking:

What’s the difference in a generator and an alternator?

Good question and I’m sure that it is one that many student pilots have asked themselves before.  The key difference between an alternator and generator is how the current is created.  In other words what is “fixed” and what spins.

A little electricity 101 first.  Anytime you have electricity in a wire, there is a small magnetic field generated around the wire.  The opposite is true too.  If you move a wire through a magnetic field, a small amount of current or electricity is generated in the wire.  Basically, anytime an electrical conductor moves through a magnetic field, voltage is generated and voltage then causes current to flow.

So getting back to our question, what is the key difference between a generator and an alternator?  Well, in a generator you are creating electricity by moving a wire armature (imagine a square wire loop) within a fixed magnetic field. You can alter how much electricity is generated by altering either the speed at which the armature rotates or by altering the strength of the magnetic field which is the purpose of those wire windings on the stator magnet in the picture below.


Now in an alternator, a magnetic field spins in a series of windings called a stator.  In affect, the opposite of what happens internally in a generator.  The net result is the same, just an alternator uses a much more efficient method.  Even though an aircraft alternator is a bit more complex than a generator, there are many advantages of using an alternator.  First, the construction of an alternator allows for the windings to be directly wired to their output points vs having inefficient slip rings and such.  Also, due to the way that alternators are constructed (with 3 separate windings) it allows for much smoother electrical output (imagine driving a V8 vs a straight four cylinder car).  A alternator can also be brought up to speed much quicker and can develop sufficient power even at lower, idle RPM.


Now, time for my disclosure.  Most of my knowledge on this subject comes from 3 sources.  One, my coworker who is an A&P mechanic and 2 other sources, “Aircraft Systems for Pilots” by Dale De Remer and this article at Rowand.net titled “Alternator and Generator Theory“.  If you want to learn more about either aircraft alternators or generators I would suggest either of these sources (my mechanic isn’t available as I am already picking his brain for the next article!).

Until the next time….

Fly Safe!


  1. Arthur on Mar 08, 2009

    Thanks a lot Paul for your reply..

  2. Kenry on Mar 18, 2009

    Commenting usually isnt my thing, but ive spent an hour on the site, so thanks for the info

  3. vardhan on Mar 24, 2009

    short and precisely explained

  4. Dave on Feb 04, 2011

    Whoever wrote this, hasn’t got a clue as to how either functions. A generator uses a physical magnet, and an alternator uses an electromagnet which outputs AC power that must be rectified to DC power through a rectifier bridge.

    The stator in the alternator spins within the windings, which are fixed!!!!!

  5. frank on Jul 15, 2011

    you failed to mention the presence and purpose of diodes

  6. Vibin vinay on Sep 12, 2011

    I think this explanation is much longer,you may explain the same oprn in simple words……

  7. Ryan McCall on Oct 17, 2011

    Thanks! Great Explanation!

  8. alex on Mar 01, 2012

    an alternator consumes 10 (TEN) times the energy that it puts out…….
    making the old fashion generator used on cars in the early 60’s More efficuent.
    we use them today especially because of all the extra circuits put into our cars.
    from DVD players to Heated seats! and it costs us!

  9. O on Sep 03, 2012

    actually there are alternators of both types. a rotating armature type (that includes the windings) and the revolving magnetic field type were the magnets (rotor) revolve around the windings (previously called the armature but in this type of alternator the armature windings are stationary and called a stator). the principles are the same but basicly each one rotates different portions of the device. the only real difference between a generator and a alternator is that alternators have no comutator to change the alternating current produced in the armature (windings) to direct current. Both generators and alternators produce ac current because any votage produced by rotation of the magnetic field, or windings in the field induces alternating voltage in the windings. as stated previously, generators merely have a commutor that changes the ac voltage to dc voltage.

    Source – Delmars standard textbook of electricity

  10. sohel on Jan 29, 2013

    actually it was ok but i want to know more about the basic difference between alternator and generator. i also want to know why we use an alternator in a diesel generator?

    Thank you

  11. flowfish on Mar 30, 2013

    Good simple explanation. Thx

  12. mahadevaswamymahadevaswamy1994@gmail.com on Apr 29, 2013

    thank u

  13. rasul on Dec 08, 2013

    state the difference between generator and alternator in DC aircraft?

  14. Doug Beck on Sep 15, 2016

    Currently converting my Continental O-300 equipped plane from a generator to an alternator.

    Alternator advantages:
    1. Lighter weight 7lbs vs 14lbs in my case
    2. Greater power output 50amps vs 25amps in my case
    3. Develops power at low (idle) rpm … This is the big one … With a generator, during taxi and final approach below 1500rpm, you are typically operating on the battery.
    4. New equipment vs 70 year old generator that had been rebuilt many times and refused to not leak on the shaft, regardless of what seals and orings were tried
    5. Alternator kit comes with new solid state regulator vs mechanical unit that generator used (although this is an option for an old gen as well – Zeftronics unit comes recommended)

    Generator Advantages:

    1. Will self-excite …if battery is ever completely dead and plane is started by hand propping, a generator will start to charge battery, whereas an alternator needs some current from a battery to excite field to begin charging … This is likely a non issue to most pilots unless you are flying taildraggers away from civilization.

    2. Infrastructure – not really a nod to gens, but be aware that the additional amps produced by the alternator may require rewiring with heavier guage wire, as well as replacing fuses and your ammeter.

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