Alternator vs. Generator

[BradG]

Can anyone tell me what the technical differences between an alternator and a generator are? Details would be GREAT

 

[Rvrrat]

Alternator.. Alternating Current (AC) which can be passed through a transformer by which amperage and voltage can be changed to a different value from what came out of the alternator originally.

Generator... generates a Direct Current (DC) which can not be transformed. The voltage that comes out of a generator is what you get. DC can be passed through a converter and made into AC

The reasons for having one over the other is a whole different ball game and a bit more complex than I can make a simple explanation of however, the above is the basic differences between the two. Hope that helps.

 

[Acestick]

Alternators are lighter and put out their rated power even at lower rpm's or power settings, but they need a little "juice" form the battery to work...The ration is negligent though, for sake of argument say an alternator can put out 60 amps, t would only require maybe 1/2 an amp to work...(This is just for sake of argument)

Generators used to be cheaper, but they are getting older so probably pricier as well...They CAN be inverted into AC power, but inverters are a little too fancy for GA aircraft...One advantage though, is that if your battery was dead and you had to hand prop the aircraft, the generator would recharge the battery...The alternator would not, because it needs some power to operate...

 

[Kaman]

Hello,
An alternator has a much higher power output than an generator. The gernerator may have only 2-4 poles whereas the alternator may have as many as 12-14.
Alternators are pretty much a must for operating AC driven systems like avionics and computers, because that type of gear needs a stable high frequeny power source (400hz). A DC convertor will be installed to convert the AC to DC for DC powered systems. The alternator has an advantage also because it will put out it's full power capabilty no matter what the RPM of the driving force (accessory drive, belt, etc...) Thats where the "pole" thing becomes important as I mentioned above.
Generators are seldom seen in newer aircraft that demand more power and at higher freqs (400Hz). For instance a Luscombe has a generator, and many older airplanes like it and because electrical power demands are low in these airplanes, however, a minimum RPM has to be maintained to have adequate electrical power...
some examples of AC and DC driven sources:

28VDC: Electric Flap motors or gear motors.
26VAC: Rotating Radar antennas (Remember AC is a more stable source, think Freq. radar works a helluva lot better if the antenna RPM is constant. DC motors wouldn't cut it for that job)
115VAC (3 phase 400 Hz): Just about ALL your modern EFIS and avionics runs on that stuff.

Hopefully, some FE-type peoples will have better info than an old "scope dope" like myself

Regards,

ex-Navy rotorhead

 

[BradG]

Ok, so in a light airplane w/ a generator, does it need a convertor to tunr the DC into AC for the avionics?

 

[A Squared]

The real difference between an alternator and a generator.

The difference between an alternator and a generator is this:

In a generator, the coils that create the magnetic field are fixed and the coils in which the current is induced go round and round.

In an Alternator, the coils that create the magnetic field go round and round, and the coils in which the current is induced are fixed.

It's that simple. But it's not that simple, sometimes people speak of a AC generator, when they are really talking about an alternator (by the definition above)

Anytime you create current with a rotating device, you get AC. Doesn't matter if it's a Generator or an Alternator. About the only way to create true DC is a galvanic Cell (battery)

A DC generator converts the AC to DC through the use of segmented commutators. Actually, they only output Quasi-DC, as there is a always a residual voltage ripple.

An alternator creates AC electricity also.
In a general aviation aircraft (and in your automobile) the DC alternator converts the AC into DC through the use of a set of diodes called a rectifier. This DC will also have a residual voltage ripple. The rectifier is built into the alternator assembly, so although, strictly speaking, the alternator generates, AC, only DC comes from the terminals of the alternator.

A typical GA aircraft has no AC systems. All avionics fopr GA aircraft are designed to opaerate from a DC power supply. If they requre AC internally, the AC is created by an internal inverter.


confused now?

 

[avbug]

A Squared covered most of it. Someone had mentioned that they thought an aircraft with a dead battery could be started and charged with a generator. This isn't true. While an alternator needs a regular field current to be applied, a generator needs polarized current to flash or initiate the production of electricity; this may not be available if the battery is dead and a start is attempted. Further, using the aircraft generator to charge the battery is poor practice, and may result in an overload on the generator, a battery thermal runaway, etc.

From a pilots perspective, the alternator will give better regulated voltage at reduced RPM's, where a DC generator will not.

Light aircraft avionics are generally all 28 VDC, or 12 VDC.

 

[tarp]

BradG:

Everything in your GA airplane is probably DC (Direct Current).

As to your original question - what are the fine points of difference between a GA aircraft with a generator and one with an alternator?

Generator:

1.) usually in older aircraft
2.) usually heavier
3.) creates DC electricity by turning an armature (coil of wire) inside permanent magnets - a generator requires almost no battery input and may continue to generate electrity even if the battery is removed from the circuit (while running).
4.) suffers from power fluctuations i.e. a generator on an idling engine may not deliver its rated voltage and/or amperage and may need some minimum RPM to deliver full power.

Alternator:

1.) usually found in newer aircraft
2.) usually lighter
3.) creates AC electrity by turning an armature (coil of wire) inside a "field coil" which is another wire that has a "trickle charge" of electricity provided by the battery. The field wire creates an electromagnetic field in lieu of the permanent magnets found in a generator. If the battery circuit is removed from the field coil an alternator will not operate (i.e. battery that will hold a charge is essential). The alternator usually contains an internal "diode pack" that rectifies AC current to DC required for the airplane.
4.) does not suffer the power fluctuations of a DC generator due to efficiencies and the action of the diode pack to create a constant source.

So when you are teaching your students, remember to keep them thinking about what a weak battery would mean to the airplane. The fact that you need a jump start could be a bad omen on most airplanes because you need a capable battery to keep the alternator running.

While you are in this mode, you might as well think about the difference between a voltmeter and ammeter on the panel. A voltmeter gives you the voltage put out by the "system" but in an alternator system allows you to view the charging health of that system. A healthy system will develop 14 volts or 28 volts. A sick system will show 12 volts or 24 volts (i.e. the power of the battery) or less. An ammeter shows the current load of the system and basically shows the balance between the power in the system vs. the consumers of electricity. The electrical system should always be developing more current than what is required by the consumers or you will drain the batteries.

Enjoy the CFI.

 

[reaperman]

Light aircraft avionics are generally all 28 VDC

So are Heavies.

Actually 28VDC is just the standard power bus voltage. The avionics boxes themselves each have thier own DC-DC converters that supply all sorts of voltages to the avionics +5, +/- 12, +3.2, +6000, whatever the particular applications require.

 

[Clearsky]

On a Cessna 172 what makes up the difference between a 28 volt producing alternator and a 24 volt battery? Is this difference what the alternator uses to run?

 

[Speedtree]

The system voltage should always be higher than the battery so the alternator can charge the battery. If the voltages were equal the two would fight and the potential would be for the battery to share some of the load and subsequently be run down. A 24v battery will be with a 28v system and a 12v battery will be with a 14v system.

Another point. Some larger airplanes. I am basically familiar with mine, a conquest, have starter generators and we convert to AC for a few select avionics. We run a turbine engine so the rpm is pretty constant at 41,000 rpm, reduced probably to some lower rpm for the generators. The varying rpm problem is not a factor.

I would guess that even the newer King Airs and other turbo props have something similar.

 

[A Squared]

Sigh..so much mis-information, so little bandwidth.


>>>>generator...creates DC electricity by turning an armature (coil of wire) inside permanent magnets.

No, the magnetic fields are created in field coils, not by permanent magnets. It is true that there are permanent magnet generators, but they don't produce much power, and they are impossible to regulate (except by varying RPM) You may find small permanent magnet generators on an airplane in instrumentation or control circuits (like a tachometer generator), but you won't find them supplying the power for the aircraft electrical systems.


>>>>>Alternators...creates AC electricity by turning an armature (coil of wire) inside a "field coil" which is another wire that has a "trickle charge" of electricity provided by the battery.

No, an alternator creates electricity by tuning the field coils inside the stator, the current is induced in the stationary stator windings...... just like I stated in my previous post.


>>>>>If the battery circuit is removed from the field coil an alternator will not operate (i.e. battery that will hold a charge is essential)

No, once the field windings have been excited, the alternator is producing electricity and becomes self sustaining. You may in fact remove the battery from the airplane once the alternator is producing power and it will continue to do so. The alternator only needs an outside source of voltage initially. There are alternators which need no outside voltage to start producing electricity. They are called "self exciting" alternators. I don’t know whether they are used in any aircraft applications.


regards

 

[V-1]

For Clearsky,

In simplest terms, voltage is a measure of pressure. High pressure always seeks a low pressure. If the battery and alternator/generator were both of the same voltage (pressure), then electricity wouldn't flow properly between them.

Think of two water reservoirs, seperated by a dam. If the water level on both sides of the dam were the same, no water would be exchanged between the two reservoirs (barring temperature differences). If one reservoir was filled to the top of the dam, and the second nearly empty what would happen when the sluice gates were opened?

To move electricity, there needs to be sufficient pressure (voltage) to overcome resistance at the other end (and inefficiencies in the conducting material, of course).

Hope this helps.