How are Generators and Alternators different?

[GAHorn]

Somebody remind me,why did we rush to alternating current generators in the first place? I don't remember having any big problems with the old DC generators & as Gahorn mentioned the DC generator has certain advantages.

Alternators have the great advantage over direct-current generators of not using a commutator, which makes them simpler, lighter, less costly, and more rugged than a DC generator. The stronger construction of alternators allows them to use a smaller pulley so as to turn faster than a DC generator, improving output when the engine is idling. Alternators use a set of rectifiers (diode bridge) to convert AC to DC. To provide direct current with low ripple, alternators have a three-phase winding. Alternators are also lighter and easier to package on the engine.

I'm in Florida vacationing so I missed the fly-in. How was the event? I have 10 acres on that strip right across from Fishing Joe's place. I hope to build a small get-away cabin on it someday...

Well.... sort of...mostly...

Alternators differ from generators primarily in that they rotate their FIELD coils rather than their secondary (current-producing) coils.

Electrical current is "generated" whenever a magnetic field (usually created by an electromagnet, called a "field") is passed near or in the vicinity of a conductor or set of conductors (coils of wire). The electricity generated is what we wish to put to work.

In a generator, the secondary coils are mounted upon a shaft, commonly referred to as the armature, which is rotated by the engine. The armature-coils spin within/near the field coils, which are mounted and stationary around the inside of the generator CASE. Battery power is used to excite the field, which creates the magnetism that in-turn induces current in the armature-coils. The problem is that as each armature coil passes each field coil, due to the North/South reversal of that magnetic influence, the current tries to alternate within the armature coils (which if uncorrected would cancel itself out). So the armature coils are connected to a multiple set of split, copper, bus-bars called a "commutator" .... (because it "commutes" the current elsewhere)... which due to it's location upon the armature is also rotating at the same speed as the armature coils. Fixed-position carbon brushes are therefore alternately fed that generated current at exactly the same speed/frequency as the natural current-reversal occurs. In such a manner, the current exits the generator in only one-direction as DC current.

All that mechanism weighs a bit more than a simpler alternator system which reverses the components relationship with each other. In the alternator it is the field which is placed upon the armature (rotating shaft) and the secondary coils are mounted inside the case, surrounding that field-armature. Because the armature/field rotates...it is called a "rotor", and because the secondary, current-producing coils are stationary... they are called a "stator". Battery power is passed to the rotor via brushes that run against to solid copper rings (commutator), which unlike the generator.... each commutator is one-piece. The electromagnetism of the rotating field passes by the stator.... inducing a current in the stator's coils. The problem of the North/South alternating current that is created is corrected by using solid-state, electrical check-valves called "diodes", ... which straighten out ("rectify") the AC current into DC current.

The advantage of the alternator over a generator is that the commutator of the alternator, supplying DC current to a rotating field, does not have to be switched by a series of copper bus-bars...in other words the current can be passed to the field and not have to deal with reversing itself to correct the North/South effect. (Remember, that alternating correction will occur at the output of the stator using rectifier/diodes.) So the rotating mass can be lighter.
Further, the field is more completely surrounded by the secondary (current-producing) coils of the stator, so more efficient induction of current occurs.
Since the rotating mass (armature/rotor) of an alternator is lighter, it can be spun faster with less danger of centrifugal force damage IN SOME DESIGNS, but that does not occur with the geared alternators that are used on our O-300/C-145 engines.
The reason alternators can be excited to produce more current at lower RPMs than generators is because of the efficiency gained by surrounding the field with the secondaries (stator)...not by increased RPM due to smaller pulleys. (Our installations do not use pulleys.)

So, bottom line: Alternators are lighter and produce more current at low rpm, ... but require an electrical current (charged battery) to work. Have a dead battery? You have a dead alternator.
Generators are heavier (for similar output capacity) but since their fields are mounted directly to the outer-case, they slightly magnetize the body/case of the generator and therefore will generate a current regardless of a dead battery. (Although battery power is used to excite their fields, they retain a slight "polarity" from residual magnetism. This "polarity" is what is re-established when we perform the proceedure known as "polarizing" the generator. It is this residual polarity that also allows the generator to make current with a dead battery. Alternator "rotors" are not made of heavy iron/steel in order to save rotating-mass/weight, therefore they do not retain residual magnetism.) Generator commutators, being more complex, usually require brush-replacement about twice as often as alternators. (Recommend about half-TBO, or 800 hours, whichever comes first.) Generator armatures, being heavier than alternator armatures, should have their bearing replaced at the same time as brushes. (Although anyone who pulls an alternator's brushes for repacement and does not change the bearings simultaneously is an optimist.)

I only gave this detailed explanation because I knew bluEldr was on vacation and wouldn't gripe about my explaning the Big Bang theory just because someone asked what time it was.


PS: Why is George not a proponent of Alternator Conversions?

Answer: It depends.

It depends on the reason for the conversion.

If more electrical power is needed, and the largest generator possible (in our cases, 35 Amps) is still insufficient, then an alternator is the only solution. The cost is about $1,000 by the time the conversion is complete.

But if the present generator is sufficient for "continuous" loads, then changing to an alternator is like taking the gasoline engine out of your car and installing a diesel, because diesel engines are slightly more efficient.

If you have an original 12 or 20 Amp generator, and you have a CONTINUOUS electrical load of more than 15 Amps, then an upgrade to a 35* Amp gen/regulator is a less expensive solution (about $200) and a lot simpler paperwork episode (simple logbook entry) than an alternator conversion. ($1K plus a Form 337/STC.)

If you are worried about the fact that your landing/taxi lights exceed your generator capacity when everything else is on also..... then remember that your BATTERY is intended to supply that additional, temporary, electrical demand. After you turn those land/taxi lights off your gen will recharge your battery.

Typical, full-IFR-equipped 170's rarely exceed continuous 25Amps with modern avionics. I recommend spending extra money on other, more important maintenance items. JMHO.

============

A gen can be exchanged/overhauled for about $200. An alternator (after you've accidentally reversed the jumper-cables from your car) will cost $500-$750. (The generator will have been unharmed.)

*-There are also 25Amp gens available for those that do not have a "dampered" crankshaft. (35A gen requires a dampered crank.) But a common error is to install a 25A gen... which is controlled by a 12 or 20 Amp regulator. Remember, a generator will only put out what a regulator asks of it. If the regulator is a 20A unit,.... a 25A or 35A gen will only deliver 20Amps. Match the regulator to the generator.

**-It never hurts to regulate a large capacity gen with a low-capacity regulator.....but NEVER attempt the reverse situation of installing a larger-capacity regulator on a smaller-capacity generator. Example: A 35A regulator will demand a 20A gen to produce 35Amps, and the generator will BURN UP making the attempt. Obversely, a 20A reg will only ask 20A from a 35A gen. Although the 35A gen will be "loafing"... the aircraft battery will never receive a current higher than 20Amps.

***-It is imperative that when upgrading to a higher-capacity generator OR alternator that appropriately-sized wires be installed at the same time. In particular, the ARMature wire on a generator to BATtery terminal of the regulator must be upgraded to the proper size. (Usually 8ga.) The BATtery terminal of the reg to the main aircraft BUS must also be upgraded. For alternators, it's the STAtor to REgulator to BUS or BAT that must be upgraded.
(Field and Rotor wires may remain the same, in most cases.)

Additionally, the fuses or circuit breakers that protect those wires must also be upgraded.

[bpaige]

Both the generator and the alternator 'generate' electricity and produce AC current. They are both generators and they are both alternators. Development of solid-state diodes in the early '60s enabled engineers to design a new, lighter, more compact 'generator' by replacing the split ring commutator/brush assy in the earlier generators with solid rings/brushs/diodes. Those of us that have belt driven alternators benefit from the higher rpm of the alternator, most noticably at idle.

[GAHorn]

Both the generator and the alternator 'generate' electricity and produce AC current. They are both generators and they are both alternators. Development of solid-state diodes in the early '60s enabled engineers to design a new, lighter, more compact 'generator' by replacing the split ring commutator/brush assy in the earlier generators with solid rings/brushs/diodes. Those of us that have belt driven alternators benefit from the higher rpm of the alternator, most noticably at idle.

It's true that the commutators are simpler in alts than they are in gens, but the weight difference of the commutators is insignificant. (About 6 ounces.) The real weight savings is in the lighter armature, usually aluminum, in the alt. The total weight differences are not that great, only about 5 lbs.

I take it you have a Lycoming or rear-mounted TCM engine. While a pulley and belt can be sized to provide higher rpms, the actual rotational speed differences are not great. The "coming in" speed of the generator is similar to the alternator. The true reason the alt produces more current than gens at low rpms is due to the fact that the secondary coils in the alternator are larger, and recieve more flux-field from the field due to placement. (Think about a device limited by outside-size restrictions. If the generator (either AC or DC) has large secondaries mounted as a stator, such as in an alternator, versus secondaries that must remain small because they have to fit within the field such as in a generator, then the larger stator of the alt can receive more induced current than the small armature of the gen.)

The drawback to the front-mount Lycoming device is the problem of a broken belt and the requirement to pull the prop just to change the belt. (But I know of no generator-equipped front-mount Lyc's anyway so it becomes a moot point.)

It's a fine argument which device is "AC" and which is "DC". They both ultimately produce DC, but in common parlance the alternator is said to produce alternating "AC" and the gen "DC". This is determined by what type current is produced by the secondaries.

[Brad Brady]

The drawback to the front-mount Lycoming device is the problem of a broken belt and the requirement to pull the prop just to change the belt. (But I know of no generator-equipped front-mount Lyc's anyway so it becomes a moot point.)

O-235's on PA 12's......Brad

[LBPilot82]

Just so that I can be clear. I am upgrading my avionics a making a few small changes to my electrical system. I have a 50 amp Jasco alternator installed which means I can't exceed 40 amps of continuous draw on the system (80% capacity). George you said that the battery is intended to take the temporary load of the landing/taxi light circuit so when I do a load test on the system, I do not include the landing light circuit?

[jrenwick]

More and more, ASF and the FAA are recommending leaving landing lights on full-time to help others see you. Many of our ancient aircraft weren't designed with this in mind, but if you have an upgraded electrical system and want to do that, the generator or alternator would need to have the capacity to run the lights with everything else full-time.

[GAHorn]

Landing/taxi lights are considered an "intermittent" load and are not included in the "continuous" electrical load calculations.

And, even if you turn them on when in the traffic pattern, that is still a small portion of most ordinary flights. (The only time you'd leave them on continuously would be a flight whose sole-purpose was to perform takeoff/landing circuits. Even if you did that, you'd turn them off for taxi-back, unless you did only touch-and-goes, and in that case you'd likely burn only the taxi lamp...not both the land AND taxi lamp. That would be worth about 100 watts, (less than 5 amps) and would still be considered intermittent, on top of which most of that operation would be at RPMs sufficient to keep the battery fully charged, even with a generator. He has a 50A Jasco alternator and would be in great shape.)

[bpaige]

It's a fine argument which device is "AC" and which is "DC". They both ultimately produce DC, but in common parlance the alternator is said to produce alternating "AC" and the gen "DC". This is determined by what type current is produced by the secondaries.

Neither unit inherently produces DC. In the case of the generator the secondary windings are spun through the magnetic lines of flux (field) one monemt north to south and the next moment south to north. The induced current in the secondaries change direction (AC). The commutator alternately switches the ends of the secondary windings so that the current direction is always the same direction at the output (DC). The alternator, on the other hand, spins the magnetic lines of flux (field) through the secondary windings, also inducing alternating current. Instead of a split ring commutator as in the generator, the secondary windings are connected to a bridge rectifier (diodes) and the AC is rectified to DC at the output. The output of both devices is pulsating DC which the battery smooths out much as a capacitor would filter the pulses following a bridge rectifier in a DC power supply. None the less, the appearance of semiconductor diodes in the early 60s was when the alternator began to appear in the auto world because it has the advantage of compactness, lightness, higher rpms, longer brush life, and lower cost. All of that applies to the aviation world except the lower cost. And of course the disadvantage as you have pointed out is the need for a dc source to excite the field at startup.

[GAHorn]

...Neither unit inherently produces DC. In the case of the generator the secondary windings are spun through the magnetic lines of flux (field) one monemt north to south and the next moment south to north. The induced current in the secondaries change direction (AC). The commutator alternately switches the ends of the secondary windings so that the current direction is always the same direction at the output (DC). The alternator, on the other hand, spins the magnetic lines of flux (field) through the secondary windings, also inducing alternating current. Instead of a split ring commutator as in the generator, the secondary windings are connected to a bridge rectifier (diodes) and the AC is rectified to DC at the output.

Yes, I thought I said that.

...The output of both devices is pulsating DC which the battery smooths out much as a capacitor would filter the pulses following a bridge rectifier in a DC power supply. ..

While this is mostly semantics, the battery in any circuit does behave much as a capacitor...that is not it's function in this case, and it does not perform any function to rectify the alternating current to DC. The secondaries in the two systems are different because at the brushes (where it is harvested for use) the generator is producing DC current, while at the stator output(where it is harvested for rectification) the alternator is producing AC current.

... None the less, the appearance of semiconductor diodes in the early 60s was when the alternator began to appear in the auto world ....

My references tell a little different story. Semiconductor diodes predate the '60's by quite a while. John Fleming invented the diode in 1904 and by 1926, L.O. Grondahl and P.H. Geiger had produced rectifying (semiconducting) copper oxide-copper diodes. By 1938 it was widely described in many journals and widely used in electronics even before WW-2 to replace vacuum-tube diodes. What was to become the modern P-N/Zener diodes was in full production before 1950. My resource gives credit to the widespread automotive conversion to 12-14 volt electrical systems in the early 1960's that made alternators popular because the previously-used 6-volt systems were not amenable due to amperage requirements (although Delco produced some for a short while. I know of at least one early N-Ford tractor equipped with a 6-volt Delco alternator.)

I agree that alternators completely had replaced generators by the mid-60's in most automotive applications. This was likely due to the popularity of air conditioning and dual headlights which place HUGE electrical demands on the charging systems.

Modern cars are even worse! My 2007 Grand Cherokee has a single alternator rated at 190 (one hundred and ninety) Amps! And it's a tiny little thing, not much larger than an old Scintilla magneto. If I could just figure out a way to put a gear on it and stuff it inside a 35A generator case so Bruce wouldn't notice......

[Zreyn]

I knew that,uh George what time did you say it is?