Generators/Alternators in commercial and GA

[byeplane]

Hi,

I'm trying to get my head around general electrical systems on commercial jet and light aircraft. At the moment I'm getting a little hung up on generators vs alternators.

Could someone please correct me if I've misunderstood:

Generators produce DC, Alternators produce AC. Generators are fairly old technology, shorter lifespan, less efficient. Alternators coupled with transformers and rectifiers to convert current between AC and DC as and when needed are therefore a favourable set up. Yet when I research the electrical systems for say the 737-800, online manuals insist that primary power is obtained from generators! Furthermore, other material I read (which I cannot now relocate) mentions that Alternators are mainly used in GA, and Generators are mainly used in commercial aviation. Am I missing something blindingly obvious?


Any advice would be appreciated

[Lonewolf_50]

It appears that some manufacturers use the terms interchangeably. Here's from an S-70 manual, a few years old ...
One accessory module is mounted on the forward section of each input module. Each accessory module provides mounting and drive for an ac electrical generator and a hydraulic pump module.
Later in the manual, it goes on to explain that this generator supplies three phase AC power, some of which powers AC systems, and some of which goes through an inverter to provide DC power via the DC power busses.

[MathFox]

Generators are the gadgets that produce electricity from mechanical energy. Alternators are a class of generators that produce alternating current. So, one can call every alternator a generator. (There are other classes of generators that produce direct current.)

[Mozella]

Don't confuse the old brush type generator on your 53 Chevrolet with the "generator" on modern Boeing aircraft. I don't know exactly why Boeing calls them generators, but perhaps because they are "self exciting", unlike the ones on your modern auto. A typical "alternator" requires a source of power, like a battery, to excite the field windings. That's what you have on your new car.

On an airliner the "generator" is actually three phase, brushless, alternator with a small DC generator built in. This small DC generator excites the field and that makes it different from the alternator on your new car.

By using high frequency (400hz) AC power, modern aircraft can save weight by using smaller wires and smaller motors.

[byeplane]

Thanks,

That would explain a lot. So Generators (as in units producing direct current) are definitely not in use in modern commercial aircraft, rather alternators are used exclusively?
I'm looking at online manuals now for the DA42 and they would seem to confirm what you're suggesting:
"the electrical master switch enables the ALTERNATOR switches." --> "The L/R ALTERNATOR switches connect the L/R GENERATOR to the L/R main bus."

So I guess any mention of generators should be treated as alternators?

[Smilin_Ed]

Every device that converts mechanical energy to electrical energy is a generator. Initially, all the generators used in aircraft (and automobiles) produced Direct Current (DC). More modern systems employ generators that produce Alternating Current (AC). For systems in the vehicle that require DC, a transformer-rectifier is employed to make the conversion. As was mentioned earlier, the use of AC is beneficial because of weight savings and because there are no brushes required as in a DC generator.

Common usage is that AC generators are called "alternators". DC generators are referred to as "generators" but they are all generators in the end.

Hope this helps.

[Uplinker]

Yeah, 'Generator' is a generic term really - a device to convert one type of power, usually mechanical, into electrical power. The generators in old cars had brushes and commutators to give a direct (ish) current output, smoothed by connection in parallel with the battery. The alternator in your modern car is externally excited, and has a regulator and diode pack built in to give a DC (ish) output. Alternators generate 3 phases of output, 120 degrees apart, the old car type DC generators only had 1 phase, (I think), hence why alternators can give a much larger output.

All electrical power on any aircraft originates from the 'generator(s)' or the battery(s). AC from the generators/alternators is converted to DC for various systems, those DC systems being capable of being supplied by just the batteries in the event of generation failure, as an emergency back-up.

Edit: smilin_Ed, I guess your reply arrived while I was writing mine !

[byeplane]

Thanks guys really but I've opened up another can of worms for myself

I've just found this:


"How is electrical power generated and stored on the DA42?"

-"It is generated via 2 60amp, 28volt DC alternators"

Lol I mean a DC ALTERNATOR?! What the hell?!

This would seem to lean towards what Mozella was suggesting regarding an inbuilt DC generator within the assembly for the purposes of initial excitation. Am I right or wrong Mozella?

A quick look on wikipedia revealed this:

"Direct connected DC generator[edit]
This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft with the alternator. The DC generator generates a small amount of electricity just enough to excite the field coils of the connected alternator to generate electricity. A variation of this system is a type of alternator which uses direct current from the battery for excitation, after which the alternator is self-excited."


So I'm assuming that the reason they are called DC alternators is due to the initial excitation being obtained by a 'smaller DC generator. However a 28V 60A generator doesn't sound so small considering the main battery which can power the main electrical buses (albeit for a limited time) is only 24V 10Ah

Any thoughts?

[3 Point]

Yes, a can of worms indeed! I don't think it is worth raising your blood pressure trying to assign the terms "generator" and "alternator" to specific types of machine. The airframe manufacturers use the terms in a haphazard way which gives them practically the same meaning. I do of course agree with the specific definitions given above and they are strictly correct but, in normal daily usage there is no difference in meaning; they are both machines to convert rotary mechanical energy into electricity.

BTW

"So Generators (as in units producing direct current) are definitely not in use in modern commercial aircraft"

Sorry but the Embraer 145 family of airliners has only 28V DC electrical systems and they have 5 "generators" producing 28V DC at up to 400Amps! A great little aeroplane with very simple and reliable systems!

Sorry!!

[JammedStab]

When it comes to electrical power production, I try to have a completely separate mindset for turbine stuff and piston stuff.

Starting with the piston stuff, if it has a generator, it is the old DC type that needed a somewhat higher rpm to produce current and it is DC current. Installed on the old aircraft. If the piston aircraft has an alternator, it is a modern AC device using a different method to create current. Of course there are likely some exceptions but that is my general rule.

On turbines, the main power sources are all called generators. Starter-Generators on PT-6 turboprops and the generators we see on airliners. I think of them all as AC power producers even though they have the same name as used before on the piston stuff. On the big airliners, you have them producing power for AC systems and then a TR for DC requirements. I suppose on many of the turboprops, they create AC initially but are use an inverter to DC.

Of course you can have alternators on a turboprop. The HS748 had the standard generators changed to DC current and separate alternators for some AC anti-icing capabilities.

So in the end, think of DC generators on the little guys and AC generators on the big guys and alternators on modern little guys.

[Freddie838]

Hi,

Think of what your primary electrical power is on the aircraft thet you are dealing with, if it is AC then you will have AC generators (current terminology) previously known as alternators generating the power. If the primary power on your aircraft is DC then you will have DC generators generating the power. Many modern Helicopters and smaller fixed wing use DC as the primary power and the generators on them are first rate. I work on the AW139 and the generator, like many other aircraft, also is the starter for the engine. It is very reliable and nothing like the DC generators of old.

[Uplinker]

OK. An alternator has three output coils which are static, and spaced 120 degrees apart around the rotating field coil. The field coil is a rotating electromagnet, powered via an external direct current source, via slip rings and brushes. The voltage and therefore the strength of the field coil can be varied to control the output of the alternator, which is 3 phase alternating current.

If an aircraft with AC alternators needs DC voltage, it uses so-called "transformer-rectifier" units. The transformer changes the AC voltage, which is usually 115 volts from aircraft alternators down to 28 volts using a transformer, (which is simply two coils wound around an iron core). The rectifier part uses diodes, (which are electrical "one way valves") to change the alternating 28 volts into direct 28 volts.

A "DC alternator" is presumably like the one in your modern car, which has the diode pack element of a transformer-rectifier built in to channel the AC output of the three coils into one feed of DC. (in your car, the transformer part is not needed since the AC output is designed to be 12 volts).

(an "invertor" is a device that changes DC into AC, {not to be confused with a transformer-rectifier which does the opposite}, essentially by using direct current to run a high power oscillator to produce 115 volts AC at 400 Hz)

[grounded27]

It was a trip for me to understand a Generator was DC and a Alternator was AC on GA aircraft...Only understood the AC generator, TR and battery systems on large jet aircraft before my short stint on corporate jet aircraft.

[latetonite]

Smaller turboprops use often generators, (DC power). The advantage is the process is reversible. If directly connected to the battery, those generators are used as starters for the engines, saving weight and costs.

Another advantage of DC power is, you can connect it straight in parallel with other DC sources, whereas with AC power you cannot due to difference in frequency of the phases. That is why the B737 never interconnects is busses when powered by a different power source. The B707 did, but the alternators had to be first synchronized by the flight engineer.

There are a lot pro and contra's using generators versus alternators.

The misunderstanding comes apparently from the USA, where alternators are called AC generators, or simply generators.

[phiggsbroadband]

Generators...
Alternators....


Nobody has mentioned 'Dynamoes' yet....

[Mozella]

-"It is generated via 2 60amp, 28volt DC alternators"

Lol I mean a DC ALTERNATOR?! What the hell?!

This would seem to lean towards what Mozella was suggesting regarding an inbuilt DC generator within the assembly for the purposes of initial excitation. Am I right or wrong Mozella?

Wrong, but no shame in that given the circumstances. What they're saying is that there are two alternators and they each produce 60 amps.

As we know, they produce AC power. That AC power can be used directly by aircraft with AC electrical systems. But, if the aircraft (usually small) uses a DC electrical system, like your car, then the output of the alternator will be converted to DC. This is exactly what happens to your car except in this case the airplane uses 28 volt DC vs 12 volt DC for your car. This higher voltage saves weight (slightly) because of using smaller wire size.

So, when shopping for a replacement alternator for your car, it is not incorrect to say you want a 12 volt DC alternator (even though we know alternators make AC) since 12 volts is the final output after the power is converted to match the system in your car.

Time for a drink.:}

[byeplane]

Thanks everyone. This has cleared up a lot of my misunderstandings

[cockney steve]

They say there's nowt new in engineering!
Generally, the UK car-parc in the late 1950's / early '60's was Dynamo- equipped.....there was an anomaly, called the "Siba Dynastart" A complex ignition-switch enabled it to act, as stated by other posters, , as a motor and thus start the engine....releasing the key would change connections to the "generator" configuration and it would generate DC and charge the battery via an electro-mechanical regulator box,
The interesting feature was the ability of the switch to be operated to engage alternative contacts and ignition-points.....the engine could thus be run backwards! Anyone who drove a Messerschmitt KR200 3-wheeler with 4 reverse gears, can attest!it was good for 70MPH either direction! :eek:
Early motorcycle alternators were somewhat crude....the Rotor, direct on the crankshaft, was fitted with permanent magnets. the stator fed through a 3-phase rectifierand voltage controlwas via a Zener diode, which, at it's designated voltage, dumped surplus output into a heatsink.

Some modern Alternators still have a residual, low level magnetism in the Rotor, These will self-excite, the output being directed intothe rotor-winding to increase the magnetic field and therefore output. Surplus is switched to battery and acessories, via the regulator,which controls the field -current to regulate output.

On some designs, the initial Field-excitation is via the charge-light this is battery-fed and finds it's return via the Alty field (thus weakly magnetising it, as it's in series with the bulb) When the generator starts producing, the output opposes the battery voltage and current stops flowing .....so the light goes out!
other variants are governed by the regulator and there are also alternators with remote, electro-mechanical (coils and contacts A la doorbell! ) regulators.

You cannot use an alternator as a motor. that is the only disadvantage i can think of.
As the rotor just has a single coil, it'seasily balanced and mechanically robust....contacts are via slip-rings, which, having no segments, make for a lighter, longer-lived assembly.


the stator-windings ar actually in 3 SETS ! Thus they take advantage of both ends of the rotating-magnet's poles, to produce output. As they are static, there is little mechanical stressand , needing no iron casing means lighter weight and better cooling.

I have little knowledge of large-aircraft generators, but, as others have said, high voltage AC has less distribution-lossesand is readily altered via a transformer. Itcan then be rectified to DC if required.
AFAIK, turning DC to AC, via a solid-state Inverter, is less efficient.
In the early 60's there were "rotary convertors" offered by government-surplus dealers....these were essentially a motor and dynamo built into one unit...veryexpensive and very inefficient....but I suppose it's all relative when you are feeding four Merlins on full chat!

[Lightning Mate]

The modern term for alternator is AC generator.

[oldpax]

Todays large gas turbines ( 250Megawatt and upwards)use the alternator as a starter motor as cockney steve has described!!

[latetonite]

And how they manage that then?

[Ozlander1]

Generators...
Alternators....


Nobody has mentioned 'Dynamoes' yet....


And then there's Magnetos, ( Mags to you old people) :hmm:

[Uplinker]

A guess, but if you fed 3 phase into an alternator (one without the diode pack) and energised the field winding, it would rotate wouldn't it?

That could be your starter motor for something like a national grid generator where there is always plenty of 3 phase already present on the network.

[Goldenrivett]

A guess, but if you fed 3 phase into an alternator (one without the diode pack) and energised the field winding, it would rotate wouldn't it?

Correct.
"The 787's engine-start and APU-start functions are performed by extensions of the method that has been successfully used for the APU in the Next-Generation 737 airplane family. In this method, the generators are run as synchronous starting motors with the starting process being controlled by start converters."

AERO - 787 No-Bleed Systems (http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_4.html)

[latetonite]

Complicated process. Not just feeding AC into the stator and have a motor, like the DC starter-generator.

[Uplinker]

Forgive me - your punctuation is slightly confusing, but the principle of using an alternator as a motor is very simple, once you have a source of 3 phase AC*.

The field coil will follow the rotating field provided by the stator coils, once it is suitably energised with DC, so the alternator works in reverse, becoming a 3 phase motor.

You would probably need to limit the current flowing in the stator coils because their impedance would be very low, but this could all be done in the GCU.

*On most airliners the APU supplies this.

[Uplinker]

There are two basic types of generators:

Dynamo: The output comes from rotating coils connected to a commutator, (which is a rotating switch fixed on the end of the shaft), and excited by fixed field coils (or sometimes permanent magnets). It produces DC.

Alternator: The output is taken from fixed stator coils, excited by a rotating field coil. It produces AC, (usually 3 phase).


Two other variants of alternators are:

DC Alternator: Has a built in diode pack. It produces DC.
Magneto: The rotating field is provided by a permanent magnet.

[latetonite]

To uplinker: your alternator will indeed become a relatively low power synchroon motor, but until you have the rotor up to speed, you need precise voltage and frequency regulation.
For that reason I say it is not that simple, to just 'reverse' a standard alternator into a motor if you have AC power at hand.

[Uplinker]

From wiki .....

When attached to......... other alternators, an alternator will dynamically interact with the frequency already present on the grid, and operate at a speed that matches the grid frequency. If no driving power is applied, the alternator will continue to spin at a constant speed anyway, driven as a synchronous motor by the grid frequency. It is usually necessary for an alternator to be accelerated up to the correct speed and phase alignment before connecting to the grid, as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.

[Mozella]

........................as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.

A fact well known by anyone who has experience in the Boeing 707 and similar aircraft of that vintage. As each generator (alternator) is brought on line after engine start, the F/E turns a knob on his panel to carefully match the frequency of the unconnected generator with those already on line. When matched to nearly the exact same frequency, the the F/E then connects the generator with the others. Failure to properly match the frequency (by adjusting the constant speed drive) before bringing the new generator on line could lead to shearing the generator shaft.

[island_airphoto]

Someone who used to sell alternators and engineer DC systems here:
"Generator" is both a generic term for anything that gets spun to generate electricity AND a specific term for a unit that uses the brushes to provide a DC output. This works by having the current generated in the rotating part of the unit (the rotor) and having contacts on the rotor spaced correctly to always have + and - on the same brushes. Advantage is this is a rugged setup that can stand a lot of abuse. Running the current through the brushes is a disadvantage because they spark, generating radio noise, and wear out. Also for mechanical reasons they have a lower maximum RPM that alternators. Generators also make fairly good DC motors if you feed power into them and vice-versa. This can be seen on any aircraft with a starter-generator. 24 volts into it spins the engine and the running engine causes it to produce 28 volts (24 volts batteries charge with 28 volts ;) ) I used to fly a Cessna that had a starter-generator to fire up the PT-6 engine. This is also a danger for a piston engine airplane with the traditional car-type electric starter. If the starter does not disengage you now have a very powerful totally unregulated generator running :eek:

Alternator is in theory a word for generators that produce AC. In practice it is hardly ever used that way. What it usually means is a unit designed to produce DC using built in diodes. Alternators are superior in many ways to generators for engine-driven DC power. The spinning part of the alternator is the field that carries a small amount of DC to provide the magnetic field. Thus there is no large amount of current through the brushes and the contacts are not segmented, thus eliminating the sparking. The stator - fixed windings - is where the power comes from. Alternators can spin faster too, so they can be geared to spin faster than the engine and charge better at idle speed. It is very confusing because alternator=AC, but DC comes out of it. In almost all cases rectifiers internal to the case change the output to DC. An alternator cannot become a motor powered by a battery. The only way to make one become an electic motor would be to bypass the dioes and feed it with 3 phase AC.

It was LONG ago when I worked at Piedmont, but if memory serves the 727s and 737s did not have alternators as the word is commonly used. They had AC producing generators, battery chargers run from the AC bus, and the APU provided air to start the engines. The APU might have had a starter-generator, I remember starting them off the ship's battery when the generator cart had gone missing, but can't recall how I set up the switches after that.

[FE Hoppy]

A typical brushless ac generator as used on commercial aircraft work like this:

Mounted on a single rotating shaft are a permanent magnet (PMG), an exciter coil and a main field electromagnet around which are a PMG stator, and exciter field winding and a main field stator. Rotation of the PMG induces an alternating current in the 3-phase windings of the PMG stator. This alternating current is supplied to the generator control unit (GCU) where it is rectified into direct current. The GCU voltage regulator uses this rectified DC to control the current in the windings of the generator’s exciter field. The magnetic field produced in the exciter field induces a current in the rotating windings of the exciter armature (rotor). This current is converted to DC by the rotating rectifier assembly on the rotor. The resultant current is supplied directly to the windings of the main field to produce a rotating magnetic field. This rotating magnetic field induces an alternating current (AC) in the windings of the main generator stator. This is fed via appropriate contactors the aircraft busbars. In most cases the rotation speed of the armature is controlled by a constant speed drive (CSD) to give a constant frequency (400Hz) but latest design has binned the CSD to save weight so output is "frequency wild" as we used to call it or "variable frequency as they say these days.

[island_airphoto]

Honda and others use "wild frequency AC" to make those nice quiet camping generators. Instead of holding a constant 3600 RPM (for 60 Hz). they vary engine speed with load and use a built in AC>AC inverter to hold the output frequency to 60 Hz. Same deal as you just described.

[barit1]

Not that the subject hasn't been thrashed to within 25.4mm of its life, but to summarize:

1) Simplest is an alternator providing AC output - probably 3 phase, frequency varying with the engine rpm (wild frequency). (May be called a "generator" in defiance of most norms)

2) Said alternator may be packaged with a set of solid-state diodes, which convert AC to a kind of sloppy DC. Typically employed in automobiles for about the last 50 years, and more recently in light aircraft.

3) If a commutator and brushes is packaged in the case to convert the AC to DC, it's a traditional generator from nearly a century ago.

4) If fixed-frequency AC is required, as in transports starting from the 50s, a variable-ratio hydromechanical drive (CSD = constant-speed drive) may be employed driving the 1) alternator. If the CSD and alternator are packaged as a single unit, it's called an IDG (integrated drive generator).

[island_airphoto]

5) Some turbine engines and APUs use the starter as a generator too.

[RCav8or]

Or do they use the generator as a starter motor??:)

barit1,
In regard to your 4th item, I think Boeing has broken that mold. The B787 uses simpler direct geared variable frequency AC alternators in a variable frequency AC to DC, then inverted back to whatever fixed AC frequency is desired for the AC buses.

[barit1]

In regard to your 4th item, I think Boeing has broken that mold. The B787 uses simpler direct geared variable frequency AC alternators in a variable frequency AC to DC, then inverted back to whatever fixed AC frequency is desired for the AC buses.

The last step in this sequence could be either a separate LRU feeding a distribution bus, OR a dedicated module within the end item. (A solid-state inverter driving the gyro in a standby horizon, for example)

I'm not aware which (or both?) the 787 employs.