passenger jet electrical power generation

Factual Questions
1

AIUI, commercial aircraft electrical power is AC, operating at 400 Hz; contrast this with the ground-based electrical grid, which operates at 60 Hz. The higher frequency of aircraft electrical power is achieved with a faster-spinning alternator (24,000 RPM, versus 3600 RPM for the alternator at ground-based power plants); this is more efficient, enabling the use of a smaller, lighter alternator.

You can hear this 400 Hz as the whine in the background when the pilot gets on the PA system. And that’s the interesting thing: it’s the same 400 Hz whine, whether the engines are at a low-RPM idle prior to takeoff, or operating at a high RPM associated with full-speed cruising. How do they manage this? Is the alternator completely separate from the engines, fed by bleed air delivered in a closed-loop/controlled fashion to maintain the alternator at exactly 400 Hz? Is there a CVT belt drive or something?

I know there’s an emergency RAT for use if/when all the engines have stopped working. Are there multiple alternators (i.e. one per engine), so that if one engine shuts down the other working engine(s) can still generate electrical power? Or is there one alternator that draws mechanical/pneumatic power from all of the engines?

2

Yes, a device similar to CVT called a constant speed drive is used:

Prior to that inverters (electro-mechanical motor generator or solid state) were used, and alternators also provided variable frequency AC for applications that didn’t care.

3

Tangentially related - older Amtrak locomotives ran the diesel engine at a constant 893 Hz because it also ran the alternator which supplied head-end power (i.e. power to the passenger cars) at 480V 60Hz. I imagine this was not very efficient.

4

Engine RPM and the number of poles. 3600 RPM alternators use 2 poles. My 1800 RPM alternator uses 4 poles. 1200 RPM uses 6 poles, etc. Onan makes them in those variations but there doesn’t seem to be any difference in alternator size, although that may be for efficiency of manufacturing. On my alternator, there are many combinations of 50, 60 cycle, 1 or 3 phase and different voltages available. Each is just a different rotor. They also make 180 and 400 Hz but I’m not sure how large they are.

Dennis

5

All 100% correct as far as it goes. CSD is the now-old way. It’s still semi-current tech, but the direction of the future is different.

The future is eliminating any mechanical speed conversion and just letting the alternator spin at a fixed proportion of engine RPM. Which of course means electrical frequency varies by a factor of roughly 2 between idle and max engine RPM.

A few bizjets are already using variable frequency AC. As is the 787.

In the 787 each engine directly turns two 235VAC variable frequency generators. So 4 total. Most of the high-draw items on that aircraft directly consume the 235VAC at whatever freq.

There is a 325 VF -> 115VAC 400Hz converter system for powering some of the smaller loads and some of the avionics. They seem to be simply a standard switching power supply = modern wall wart, just seriously super-sized. There’s also a 28 VDC system fed off the 115VAC system via fairly standard-tech transformer/rectifier units.

6

As **andrewm **outlined above, the typical (non-787) installation is one generator and one CSD per engine. So the CSD is responsible for maintaining approximately 400Hz and the generator creates the juice. RPM regulation is not very precise, with 380Hz-420Hz being a typical regulation range.

One older airplanes (727 era & before) the generators were paralleled together in operation. Each generator directly powered a bus with its own dedicated set of loads, but there was an intertie that was normally switched in to harness all generators and all loads into a single common system.

Modern practice (737 & subsequent, Airbus, McD-D) is to not parallel. Instead each half of the aircraft’s AC system is normally electrically separate from the other half. If one generator quits, the two load busses are connected and the surviving generator carries both sides. That also triggers automatic load reduction where some large consumption items cut out to avoid overloading the one surviving generator.
You didn’t ask, but typically the APU that provides electricity on the ground has the same sort of generator as the engines but no CSD. Instead the APU jet engine’s RPM is regulated to maintain 400Hz-ish. Said another way, the generator gearing is set up so the engine runs at 100% RPM all day and that happens to correlate to 400 Hz. As with the engine-driven generators, the APU can supply all the busses or just some. But any given bus has exactly one power source at a time. In most (all?) 2-engine aircraft the APU can be run in flight as a backup or replacement generator if needed. I did that some time in the last month. One engine generator was broken, so we planned and flew the flights with the APU carrying that side while the other engine-driven generator was doing normal duty on its side. That’s not uncommon.
The RAT is yet another system on some, but not all, big jets. The electrical system on a big airplane consists of 6 or 8 layers of importance. As the supply of power dwindles the outer peripheral layers shut off to preserve juice for the more important inner layers. In serious *extremis * about 3/4ths of the cockpit and 100% of the rest of the airplane is dark & dead. Instead of running on 100s of kilowatts we’re running on 10s of watts.

The RAT is one step up from there. It provides a long-term source of low wattage AC power to (typically, each make & model is slightly different) drive some low voltage AC stuff and a DC power supply to feed the most important DC stuff. As such, RATs are common on long range ETOPS-certified aircraft but not shorter-ranged overland aircraft.

The point being that the general response to major electrical failures is “land ASAP before something catches fire or the battery runs out.” Over land you don’t need a battery-stretcher; you’re never far from an airport. Halfway across the ocean you do. The RAT is just that, a battery-stretcher.

7

just as an aside, modern cars do this too. we call it “load shedding;” if the charging system (alternator) can’t keep up or has failed outright, ECUs on the bus are programmed to shut off based on importance. So stuff like the radio and infotainment will power down first, and the PCM and BCM will stay up until they simply can’t anymore.

8

To add another wrinkle, most smaller business jets and turboprops, and nearly all turbine-powered civilian helicopters, have 28VDC electrical systems, with starter-generators to both start the engines and generate electrical power once the engine has reached it’s self-sustaining speed. They have inverters installed to provide AC power for any avionics/cabin loads that require it.

9

Here’s a link to a basic schematic for an older jet. It is still modern enough to have the generators powering their own side independently rather than in parallel as mentioned by LSLGuy.

You’ll see there are two channels, which just refers to a collection of services normally supplied by its own dedicated power source, and four possible sources of AC power. Ext AC is just for use on the ground so that leaves three sources airborne. The APU and engine generators are identical with the only difference being that, as mentioned in the above posts, the APU acts as it’s own constant speed drive while the engine generators have a separate CSD.

In normal conditions engine 1 generator (GEN 1) powers channel 1 and engine 4 generator (GEN 4)* powers channel 2. If either engine driven generator fails then the AC transfer system connects the opposite side generator and that generator can run the whole system. Alternatively the APU generator can run the failed side but the APU can not run both sides at the same time. If both GEN 1 and 4 have failed then the APU generator will supply channel 1 which is basically the captain’s side of the aircraft.

TRUs power the DC system and charge the batteries.

There is further redundancy from the green hydraulic system which is powered by engine 3. If both AC busses are not powered, a hydraulically driven standby generator comes on line and powers the essential AC services. This results in the loss of several important hydraulic systems so it is only used as a last resort.

If all generators are lost including the standby, then the batteries can power some vital services off the emergency AC bus via the standby inverter. Things are looking pretty bad at this stage as you only have about 30 minutes of electrical power left and then everything goes dead.

I have had two situations recently that involved the electrical system. In one, an engine generator was unserviceable and had been MEL’d**. This means we could legally fly with the generator out of service. To do this the APU had to run for the entire flight with its generator powering the failed side to provide adequate redundancy in case of a further failure.

In the other, the generator control unit for generator 1 failed which took the generator offline and prevented the bus transfer system from working. Instead of the failed side being taken over by the good generator, it just went dead. That meant a loss of various non-essential systems (autopilot for one) and no lights or instruments on the captain’s side of the cockpit. A bit of a pain at 2am but not life threatening.

We could have recovered more of the failed side by turning the standby generator on but what you gain in electrics does not make up for what you lose in hydraulics, so it is not recommended.

Ref JHBoom’s comment, on a turbo-prop you’d expect to see the main generators on the DC side of the schematic with the AC portion being smaller and mainly supplying avionics.

An example for a Dash 8 shown below.

Note the paralleling control box. In the Dash 8 the three inverters supply the AC busses in parallel rather than the system being split.

**Generators are named for the engine powering them, there are only three including the APU generator. Engines 2 and 3 power the hydraulic pumps.

** Maintenance required to fix the generator is deferred in accordance with the aircraft minimum equipment list.*

10

There are techniques for creating a stable AC frequency from an alternator that is driven from a variable speed power source. These designs go by the name of Double Fed Electric Machines. The common idea is that you can power the field coils of the alternator from a variable frequency source. So you watch the speed of the input drive, and vary the frequency of the field coils so that the output frequency is stable. This is how wind turbines are designed now. (I like to visualise it as the field coils creating their own rotating magnetic field inside the alternator, and the power generating coils rotate relative to that field, you don’t need to know what the external world is doing, just what your motion is relative to the field.)
However I can see that such a system would not be popular with air-plane designers. Far too much to go wrong, and the systems are not mass efficient. In the modern world, with high efficiency power converters being ubiquitous, there is scant need to require a highly regulated frequency anymore.

11

Not 893 Hz. 893 RPM.

It was efficient. Basically its an EMD which means the diesel motor generated electricity, and electric motors propelled the train.

So all that running the diesel engine at 893 RPM meant was that it used a slight bit more fuel than otherwise if it wasn’t using the minimum power it would supply at 893 RPM.

it saves some fuel to be able to run the diesel at a keep warm of approx 400 rpm, when its got absolutely nothing to do. eg when sitting still, or rolling down a steep hill.

12

The 757 and 767, if used for EROPS, have a hydraulic motor generator (HMG) which uses a hydraulic motor to generate about 10KVA of electricity in the case of both engine-driven generators dying. The generators, at the time of 757/767 EROPS certification, were not considered reliable enough that the probability of losing both on a long overwater flight was less than 10E-9. So the HMG was added to provide a backup electrical power source.

By the time the 737 was certified for EROPS operation, generator reliability had improved and the FAA’s understanding had evolved, and an HMG was not required.

13

Very clever. I’d never heard of that approach. Thank you. It’s obvious once you think of it. But it’s very much non-obvious until the *a ha *moment. Or until somebody else tells you about it. :slight_smile:

14

It actually more efficient to write “AIUI” rather than “as I understand it”? I had to Google that. Or is it that everyone writes this way, and I’m the outlier?

15

I use it all the time, and I know I’ve seen others use it as well. Abbreviations like that get used all the time on this site and others. Examples, watch out ISTM (it seems to me) and IANA* (where IANA = “I am not a” and * = D for doctor, L for lawyer, or some other letter(s) for a profession you can usually discern via context).

And yes, four keystrokes is generally more efficient than eighteen. :smiley:

16

As a side note, modified jet engines are a surprisingly efficient way of generating relatively large amounts of electrical power on the ground. When modified in this way they’re called “aeroderivatives” and are essentially powerful gas turbines. Among the ones that GE makes is one based on the CF6 engine used on 747s, which can generate 48 megawatts. I remember being on a tour of a nuclear power plant once and they showed us the backup power facility that could be turned on in the event of a nuclear shutdown – it was a lineup of jet engines.

17

You can use a multipole alternator if you’re trying to produce a multiphase AC system (e.g. ground-based 3-phase power), or if you’re immediately going to rectify it to produce DC power (e.g. an automotive alternator).

Which leads to another question. What’s the situation on commercial aircraft? I assume it’s AC power all the way to the devices being powered (this would explain the whine heard over the PA system), but are there multiple phases/legs being produced that power isolated circuits, or is there just one phase coming out of the alternator?

18

Three-phase. The electric-powered hydraulic pumps use 3-phase, 400 Hz.

19

Continuing for the typical fairly modern big airplane …

Yeah, as **Rocketeer **says, engine generators put out 3-phase 115 VAC. Big motor loads are also 3-phase. Hydraulic pumps, fuel pumps, ventilation fans, etc.

Mid-load things like galley ovens are multi-phase; if there are 6 ovens in a galley, the first 2 are wired into phase A, the second pair into phase B, and the third pair into phase C. Likewise things like cabin lighting that’s large in aggregate; different zones will be on different phases so it comes out more or less even.

The rest of the various AC loads for this or that gizmo are spread between the 3 phases with one eye on balancing the total load between phases and one eye on redundancy for the case a single phase drops dead.

Very definitely redundancy is the main consideration as between stuff connected to the left or A power system versus the right or B power system.

The DC systems run off AC-powered transformer+rectifier units (“TR” or “TRU” depending on the manufacturer) which are typically driven off a single phase but you may have more than one, each on a different phase. There’s also AC-powered battery charger(s).

There is a relatively small amount of low voltage AC, typically 28VAC. This is powered by a transformer unit off one phase.

The bottom line result is the load balance between the three phases coming off any given generator is never perfect, but it’s close enough.

20

When you have a 110 socket on a passenger airplane, is it 60Hz or 400?

Incidentally, someone told me a long time ago that the reason for the 400Hz was a weight saving move because they could use air core transformers instead of iron core. Does anyone know if that is true?