787 electrical system - variable frequency generators?

[waterfalls123]

So how is an aircraft electrical system like the 787 able to be powered by these large, variable frequency AC generators without a constant speed drive (CSD)? My understandings of AC electrics was that motors driven by AC power need stable frequency in order to operate properly and for longevity. Why do "relatively" modern aircraft like the 777 bother with the weight of complexity of constant speed drives (CSDs) and an aircraft like the 787 doesn't need CSDs?

[tdracer]

So how is an aircraft electrical system like the 787 able to be powered by these large, variable frequency AC generators without a constant speed drive (CSD)? My understandings of AC electrics was that motors driven by AC power need stable frequency in order to operate properly and for longevity. Why do "relatively" modern aircraft like the 777 bother with the weight of complexity of constant speed drives (CSDs) and an aircraft like the 787 doesn't need CSDs?
The 787 uses 'VSCF" - Variable Speed, Constant Frequency. In short, the VSCF uses electronics to convert the output to a constant frequency. So they do electrically what the CSD does mechanically. So the aircraft systems don't need to deal with the wild frequency outputs.
The 777 also uses VSCF - but much lower power output (20 KVA on each engine, if memory serves) for 'backup power', as compared to the 150 KVA output of the constant speed IDG.

[TURIN]

The 787 has a very complex power generation and distribution system. It uses large capacity, liquid cooled transformers to convert the frequency wild AC from the generators to square wave +/-270v DC . This drives all the big motors such as hydraulic pumps, cabin air compressors etc. It's difficult to describe without diagrams. Google may help.

[gearlever]

I can't describe it better...

Look here (http://www.afaqscientific.com/icest2013/22-Eid76.pdf)

[waterfalls123]

Through Gearlever-

Never heard of square wave DC. These devices that convert the 235V AC variable frequency to "square wave" DC.......what exactly are they? Are they solid state or do they have moving parts? If they get hot enough where they need liquid cooling, then there certainly must be a lot of waste doing the conversion of 235V AC to 270V DC.

I've googled around and found some stuff on the 787 electrics, similar to what gearlever linked (thanks gearlever). Was just wondering if your instructors or manuals had better "insights" to the magic than ours. Our training seems to be transitioning from "build the system" 30 years ago, to now "teach them the minimum." The pendulum has swung to far.

I can't even get a decent 787 electrical schematic. If you guys have one, pass it along!

[TURIN]

I'll do my best to explain...I've got time.

The transformers are called CMSCs Common Motor/Starter Contollers. They are solid state, the big ones which are used for driving the engine starter motors etc weigh about 400lbs. There are two independent cooling systems called PECS Power Equipment Cooling System. They pump the cooling fluid around the heat exchangers and through the CMSCs.

As for the square wave DC. Imagine a sine wave with vertical and horizontal componants instead of curved. The vertical represents the voltage. the horizonal represents the duration of the peak voltage. So - imagine you are turning a wheel and you place your hands on the top of the wheel and push it along - from a standing start when the engine starter is engaged, the motor needs a big slow push so the horizontal line of your square wave (value +270vDC) is quite long and could last for say half a second, until the motor has rotated 180 deg then the square wave reverses to (-270vDC) and gives a slightly shorter 'push' on the motor in the other direction - your hands are now at the bottom of the wheel and you are now 'pulling' it back towards you as it rotates on it's axel - this flip flopping of the +-270vDC continues with shorter and shorter horizontal componants to the square wave until the motor is up to speed.

This is how it was explained to me (and a bunch of other) mechanical trained LAEs during the Boeing course. Brian from Hawaii, thank you very much by the way.

Here's a few links. This diagram isn't completly accurate but it illustrates the general arrangement.

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/1439x1054/1wbg6_0ae5cd55c17a982ba475156f71df4f7f085e1fa1.jpg


CMSC (http://www.aviationpartsoutlet.com/documents/specials/0PN%207000045H02.pdf)


Square Wave (http://www.aviationpartsoutlet.com/documents/specials/0PN%207000045H02.pdf)

[Chu Chu]

I think part of the confusion is calling it square wave DC. The regular stuff that comes out of the wall outlets in my house is sine wave AC. If you change the wave shapes from smooth peaks to squared-off plateaus, I'd think you'd get square wave AC. (As as long as the plateaus alternate on opposite sides of zero; if they didn't, it seems like it would just be plain old DC.)

[Blacksheep]

The ATRUs produce both plus and minus 270 V DC outputs. The Motor Controllers modulate the duration for which the dc power from each output is alternately applied to the motors - with longer pulses at the start reduced to shorter and shorter pulse width as the motor accelerates.

By the way, the additional weight of the electrical components is counter-acted by there being no need for a separate starter motor, start control valve, pneumatic supply - or even a (notoriously unreliable) pneumatic system at all. Hallelujah!

Now for the hydraulics, Heh! Heh! :suspect:

[Vessbot]

I was gonna pose a challenge to "explain this without using the word 'alternating'" and I see it's already been lost! :8:D
​​​​​​​

[waterfalls123]

All through Vessbot, thanks!

Great schematic Turin. That's the best I've seen. Does anyone know if the 230V AC system is powered when external AC power is plugged in, like at the gate, with the engines and APU not running? If the 230V AC system can be powered by external AC power plugs, do those two AC transformers that convert 230V AC to 115V AC for the L&R AC busses also convert the 115V AC back to 230V AC when 115V AC is the only power available through external power?

As a motorhome owner, I've heard of square wave "sine waves" as cheap inverters on motorhomes (among other uses like in personal pleasure boats) take the motorhome's house batteries (12V DC) and convert their DC power to AC using something other than "smooth" (i.e. pure) sine waves. The use of these cheap converters can hurt appliances (like refrigerators) whose AC motors don't like other than "pure sine" wave AC electricity.

TURIN, you also answered another one of the questions I had in my head after initially reading the 787 electrical system- what in the world were those 3 common motor start controllers (CMSC) needed for? I thought the ATRUs put out "normal" DC electricity (like you'd get from a transformer rectifier in any other jet I've flown). But apparently these ATRUs (as Blacksheep states) aren't putting out "nice" DC voltage like a battery does- it pulses- I assume due to the fact that the ATRUs are receiving wild, plusing, high voltage electricity as an input from the engine and/or APU generators. Therefore, because of the "sort of wild" output from the ATRUs, the CMSCs are needed to properly start (and drive?) these large, DC motors when they're running? Is that correct?

[Dave Therhino]

By the way, the additional weight of the electrical components is counter-acted by there being no need for a separate starter motor, start control valve, pneumatic supply - or even a (notoriously unreliable) pneumatic system at all. Hallelujah!



That was the plan, anyway . . .

[Pugilistic Animus]

Wouldn't the AC generator require excitation from a DC source in order to draw magnetic lines of force? So is that source from BAT? Just curious

[gearlever]

Wouldn't the AC generator require excitation from a DC source in order to draw magnetic lines of force? So is that source from BAT? Just curious

Modern AC generators with field coils are usually self-excited.

[TURIN]

Chu Chu
I think part of the confusion is calling it square wave DC.
Totally agree, we argued that with the Boeing trainers but they weren't having any of it.

Waterfalls123 That schematic isn't completely accurate, lots of info missing from it so it doesn't tell the whole story. It doesn't show the 3rd ext power socket for a start.
But on the whole, yes, you've got the general idea right.

[Pugilistic Animus]

Thanks Gearlever .. I'm not an electrical guy that's my excuse Lol

[mustafagander]

About exciting the field of the alternators, the classic B747 had small magnets in the shaft which were called PMG - permanent magnet generators - to start the excitation process. Not exactly new.

[gearlever]

About exciting the field of the alternators, the classic B747 had small magnets in the shaft which were called PMG - permanent magnet generators - to start the excitation process. Not exactly new.
Indeed. My comment was about older, much older AC generators:)

Anyhow, very interesting development on elec power generation on modern airplanes. I still have something to learn (67).

[FE Hoppy]

the Cseries also has Variable Frequency Generators. (no csd). The AC Hydraulic and Fuel pumps are designed to operate throughout the frequency range.

[HighWind]

The ATRUs produce both plus and minus 270 V DC outputs. The Motor Controllers modulate the duration for which the dc power from each output is alternately applied to the motors - with longer pulses at the start reduced to shorter and shorter pulse width as the motor accelerates.
Are the motors DC motors, and CSMC are some sort of slow pulsing DC/DC converters? Or are the motors AC motors, and the CSMC are PWM modulating a sine?
Traditional brushed DC motors are in decline due to maintenance.
Brushless DC motors are basically a sort of AC synchronous motors with integrated electronics. There is no need for power electronics in both the CSMC's and the DC motors.

[weemonkey]

The 787 has a very complex power generation and distribution system. It uses large capacity, liquid cooled transformers to convert the frequency wild AC from the generators to square wave +/-270v DC . This drives all the big motors such as hydraulic pumps, cabin air compressors etc. It's difficult to describe without diagrams. Google may help.

sounds like a Siemens wind turbine!

[Tinymind]

I am a former ramp and tech support engineer just looking in, I like to keep an eye on whats going on and have been looking for info on the strange hi volts DC I keep hearing about.

It seems that the +/- 270v 'DC' is of course actually AC, the square wave being very much easier, and cheaper, to generate electronically from rectified frequency wild AC.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
It may be this basic or it could be a whole lot more complicated!
Square wave AC is also very messy, being every harmonic possible if I remember correctly, and that can interfere with delicate digital devices, like the rest of the plane.

Frequency wild is alright for galleys that use a lot of power in quiet cruise when other demands may be lower, they are resistive loads that are not greatly frequency sensitive and are designed for normal cruise power settings.

Having been on a couple of Boeing courses, 747-400 and 777, we found the instructor could be very insistent on some matters. We resolved it to 2 reasons.
First.B. S. E. Boeing Standard English was sometimes the cause, they are often teaching engineers for whom English (full of ambiguities) is a second, or even third, language. BSE is very straight forward, but sounds rather awkward to us
The other reason was Boeing proprietary, meaning they had some patent involved, or hoped to. Then the instructors would say quietly that there were 'lawyers involved' and we shut up, knowing that discussion was useless.

[gearlever]

It seems that the +/- 270v 'DC' is of course actually AC, the square wave being very much easier, and cheaper, to generate electronically from rectified frequency wild AC.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
.

The schematics above make me believe it's 270 V DC, coming from the ATRUs. Reason may be for the same amount of elec energy in comparison to 28 V DC the wires are much thinner (less weight and cost).

[bvcu]

A380 the same , no IDG's or CSD's. big weight and power saving using much more DC on the aircraft , going back to 1950's !

[waterfalls123]

I'm the original poster......
It's amazing to me that there is so much confusion over this "magic" electrical system!

[EEngr]

Modern AC generators with field coils are usually self-excited.

Not so much on aircraft. This is true for automotive alternators. But the inability to disconnect the excitation (field) would leave a generator continually feeding into a short circuit. And for a generator capable of producing hundreds of kilowatts, that's a major fire hazard.

The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.

[gearlever]

Not so much on aircraft. This is true for automotive alternators. But the inability to disconnect the excitation (field) would leave a generator continually feeding into a short circuit. And for a generator capable of producing hundreds of kilowatts, that's a major fire hazard.

The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.

YEP.

I should have written

"Modern AC generators with field coils are usually self-excited via PMG"

Sorry

[Uplinker]

Just a general point - and I don't know if it applies here, but "square wave DC" could describe a series of +270 Volts "Direct Current" pulses, with an equal mark space ratio. In other words, the voltage is switched on/off/on/off etc. between +270VDC and zero volts.

True "alternating current", AC, alternates the polarity every half cycle; so on the first half cycle the voltage rises from zero volts to maximum positive voltage and back to zero. In the following half cycle the voltage changes polarity and goes to maximum negative voltage and back to zero again. The AC wave form is symmetrical around zero volts, i.e. it goes from zero - max positive volts - zero - max negative volts, and repeats. What 'Square wave DC' means to me that it is only ever between zero and positive voltage.

Normal AC voltage is a sine wave, owing to being generated by a physically rotating generator. Square wave is produced by electronic switches, and goes from zero to maximum (near) instantaneously, giving the classic square shaped waveform.

Many DC motors now work from square wave DC. This gives higher starting torque, good low speed response and lower heat in the windings. The mark-space ratio can be modulated for varying loads on the motor; the 'on' time of the square wave being very brief for light loads, much longer for heavy loads.

Your battery drill probably uses variable mark-space ratio square wave DC drive, and on some you can hear the high frequency of the square wave generator as you slowly pull the trigger.

[Vessbot]

Just a general point - and I don't know if it applies here, but "square wave DC" could describe a series of +270 Volts "Direct Current" pulses, with an equal mark space ratio. In other words, the voltage is switched on/off/on/off etc. between +270VDC and zero volts.



According to the diagram TURIN posted, the waveform is +/-270V symmetrical around zero, (rather than 0 to +270) so wouldn't that qualify is as AC under your distinction?

[Uplinker]

Probably, if that is the case, but I am not an authority on definitions.

[gearlever]

Hhm I may be wrong but I think +/- 270 V DC is pure DC. No square wave, no alternating polarity.
Look at the colours of the schematic by Turin.

But I don't get the +/-