Well.... sort of...mostly...bpaige wrote: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.Zreyn wrote: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.
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...
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.
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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.