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tlfamm
01-28-2013, 09:30 AM
Stumbled upon this interesting discussion (a copy of a thread from rec.antiques.radio, circa 1999):

http://www.behlman.com/pdf%20WF/B-Origin%20of%20400hz%20in%20aircraft.pdf



Two extracted posts:


Neil S.:

-- quote --
There is nothing particularly magical about 400Hz, but by going to such a frequency, the size and weight of the generators and transformers can be cut to a fraction of that for, say, 60Hz. As the frequency goes up, the thickness of iron must go down, and for a given power and voltage, the copper must have a given cross section, so at some point (with the technology of the time) the advantages of increasing frequency are overtaken by iron losses and inadequate cooling potential. This fell in the area of 400Hz back about when you suggest, and once adopted, naturally was hard to change. The larger generators were commonly 3 phase for even better space/weight utilization. This was also used for mil ground mobile equipment.

Some post war Radars used 1000Hz, and they used this to time the 1ms pulse repitition rate, as well as make the motors and transformers even smaller. Man did they whistle.
-- end quote --

Randy Guttery:

-- quote --

1) think about (most) all of the airborne radios used during WWII and Korea. They used 400Hz? nope - DC (converted by dynamotors and in a few cases - vibrators).

How about into the mid fifties - about the time R-390/1/As were hitting the street - were they 400Hz capable? - nope - the R-390 "airborne / mobile" used a dynamotor...

Just when DO we see 400hz radio start popping up??? Early 400hz set - LORAN receiver - used in a submarine - around 1952... (there are some other equipment examples used in subs at the time - but since I don't know if they've been declassified - we'll just note that there were "some" and let it go at that).

Neither subs nor aircraft were the "driving" force in miniaturization - missiles were. In the early 50's - Russia started beating us badly in the arms race. The Air Force had become it's own branch - and had taken over missile development - like the Red stone, etc. In these early missiles - guidance was the key - and this is where a lot of weight (and space) savings could be had. Some of the earliest solid state digital computers were developed using 400 hz. It was used in the power supplies - transformers to convert 115V 400hz to a couple of DC supplies. It also turned the magnetic memory "platter" which served not only as the memory for the system - by as a real time integrator. Since the 400 hz being generated was extremely accurate - it was used to convert acceleration against time = speed - and speed against time = distance. 400 hz is a handy number in time conversions for several reasons - but basically it's also a good compromise between high efficiency to use (vs. a lower frequency) while still not too difficult (then) to generate accurately (vs. even higher frequencies). So inertial navigation of missiles was one of the prime motivators towards 400hz.

...
-- end quote --

Optics Curmudgeon
01-28-2013, 09:41 AM
The reduction in iron mass and capacitor size are nice, but they are indirect offshoots of the original reason for 400hz AC power in aircraft. In the late 1930s Sperry Gyroscope was trying to develop an autopilot gyro that didn't use carbon brushes because the carbon dust was getting into the gyro bearings and causing precession problems. Their solution was an AC motor, and to get the speed they needed with a realistic number of motor poles the needed about 400hz. A small motor generator was installed in the aircraft to convert 28vdc to the AC. For most of WWII aircraft radios used the 28vdc, but tapping into the 400hz supply gradually caught on and AC generators were added to take the load.

Duffy
01-28-2013, 09:47 AM
An aside to this 400 hz thread, in the 60's the last generation electric homing torpedoes, (Mk 44?) used 2300 Hz. I believe it was based on the argument that the ultrasonic transducer was the BIG power drinker, and it saved a lot of iron, (strictly speaking, nickle wire.)
At that time, Canada still did a bit of weapons reasearch and developement. In the case of this torpedo, the US design had a problem with transmission noise requiring a very expensive work-around. The Canadian answer to the same problem was a contra-rotating DC motor that elliminated the transmission entirely.

Stuart Br
01-28-2013, 10:31 AM
Interesting, when I worked in the aircraft industry in the 1980's. I was told that 400Hz was used to reduce vibration, but the "less iron" theory makes much more sense when every ounce counts in aircraft design.

MrSleepy
01-28-2013, 11:03 AM
Bosch made a range of 400hz powertools ...I only ever saw one, a breaker , but it was a big step in quality from the the normal blue and green production.

Wirecutter
01-28-2013, 12:36 PM
If you ever listen to air traffic control, the tone you often hear in the background when a plane communicates with controllers is 400 Hz. I've done a bit of listening myself. (Sept 11, 2001 was an interesting time to listen here in the DC area.) The bleed over of 400 Hz into radio transmissions doesn't seem to be limited to any particular type or age of aircraft, either. Sometimes it's a big craft (like a 747) and sometimes it's not as big (737). I've never heard it from a smaller prop plane, or even small private jets. The majority of traffic I hear is commercial - big airlines and such - not private. Perhaps somewhere in such birds, a ground is not connected properly, or perhaps ground loops exist.

Most interesting to me is that airlines make such a big deal about electronics on board potentially interfering with radios and navigational aids, but apparent wiring problems allow the 400 Hz noise into their radio transmissions.

BTW, I read once that, back in the late 80's anyway, the electronic device that caused the worst interference on commercial aircraft was a professional (news) video camera.

darryl
01-28-2013, 08:54 PM
I've watched a few plane crash investigations lately, just out of interest as to why, and what has been done since to avoid the problems. One of the images of a suspect part showed a nameplate- 115vac- 400 hz. This was in a 737 I believe, so it's probably late '80s or newer. I wonder if the latest planes still use that frequency? I don't see why not, if ac is required, as even older iron laminations can handle it- up to about 1000 hz as I recall reading somewhere. That would be for thin laminations of course, not the thick ones used in low duty cycle motors like window crank, asb systems, many starters, etc.

Alan Douglas
01-28-2013, 09:32 PM
I've been following rec.antiques.radio+phono for a long time, though there's not nearly the volume of discussion there now. I understand that Randy Guttery died recently but the other participants are still active. In any event, Optics Curmudgeon has probably nailed the original use.

Lew Hartswick
01-28-2013, 10:13 PM
I don't see why not, if ac is required, as even older iron laminations can handle it- up to about 1000 hz .
Are you forgetting Audio amplifiers that used iron laminated cores for the
entire audio range ? Roughly 20 cps to 20 kc :-) Hz and KHz :-)
And petty good constant characteristics.
...lew...

J Tiers
01-28-2013, 11:00 PM
Are you forgetting Audio amplifiers that used iron laminated cores for the
entire audio range ? Roughly 20 cps to 20 kc :-) Hz and KHz :-)
And petty good constant characteristics.
...lew...

Indeed.

But they do use thinner laminations than most power transformers. And the iron is typically a better grade.

if you want to do bulk production of equipment, and you do NOT want it limited to a small group of specialist companies, you go with covering 80% of the goal, and that would probably be the use of fairly standard laminations, regular wire, ordinary winding techniques, etc. Under those limits, 400 Hz is a good choice.

The next octave or so probably starts to cut into the performance, and is only around a halving of the weight from the 400hz case. but 400hz is a bit better than a six-fold reduction relative to 60.

The folks making the decision no doubt didn't let the perfect get in the way of the pretty darn good.

At work, we make a little custom 400Hz VFD for a government entity..... the frequency is not for weight, but rather to turn a couple of motors at something over 11,000 rpm. it happens that the motors have to be small to do their job, and small in this instance requires high rpm, and higher input frequency.

Oh, yeah, we actually saved the entity money, and have better performance than the prior rather expensive solution.

dp
01-29-2013, 12:22 AM
At work, we make a little custom 400Hz VFD for a government entity..... the frequency is not for weight, but rather to turn a couple of motors at something over 11,000 rpm. it happens that the motors have to be small to do their job, and small in this instance requires high rpm, and higher input frequency.


I used to work for an aerospace company that specialized in high density power supplies and which used copper tape windings and carefully machined ferrite cores. The density was such there was barely room for cooling air. It was also my first exposure to CNC. The result was tiny boxes with huge power transformations.

J Tiers
01-29-2013, 08:22 AM
Yep. Things get much smaller at high frequencies.

Did a commercial audio amplifier of 2500 watts (almost 3.5 HP) in which the main power transformer would fit in the palm of your hand. Things get much smaller at 100 kHz even than 400 Hz. That one didn't use tape, but others did.

Eventually the size reductions stop, because you still have to make connections that carry actual current, there are spacings required due to required safety isolation (mains side vs isolated "customer side"), etc.

if the power is small, and there is no particular isolation required, sizes can get pretty silly-small.

ironmonger
01-29-2013, 09:14 AM
Taking a look at the power transformer in my Miller Maxstar 200 which operates at a frequency of 20 to 100 kilocycles... it's about the size of a large orange and handles an output over 5kw. This one fits in a trunk and is one hand-able.

paul

Mark Rand
01-29-2013, 04:53 PM
As a datum, switch-mode power supplies often work around 2MHz. Transformer windings are even made with PCB tracks on adjacent layers of a multi-layer board. As time has gone on frequencies have risen to get higher power densities from transformers and motors. The same effect is in use (but not so obvious) when you get drill motors with universal motors spinning at 12,000 rpm for a 3,000 rpm chuck speed.

Black_Moons
01-29-2013, 05:03 PM
As a datum, switch-mode power supplies often work around 2MHz. Transformer windings are even made with PCB tracks on adjacent layers of a multi-layer board. As time has gone on frequencies have risen to get higher power densities from transformers and motors. The same effect is in use (but not so obvious) when you get drill motors with universal motors spinning at 12,000 rpm for a 3,000 rpm chuck speed.

Only *micro* power SMPS run at 2Mhz. Switching losses and skin effect would kill any high power SMPS running at those speeds.
Computer SMPS usally run more around 30khz, with some up to around 90khz (I have taken several *dozen* of them apart and checked the timing capacitor/resistor values verus the controller IC's datasheet)

kf2qd
01-29-2013, 08:33 PM
The Human Body seems to have a resonance close to 60Hz. Meaning that the muscles will tend to lock and you are unable to break free. 400 Hz is used for safety reasons. Thus its first use in Naval vessels.

darryl
01-29-2013, 08:53 PM
We've been buzzing to 60 (and 50) hz for so long now that it's probably a natural thing to resonate with. :) But it's interesting- I wonder at what frequency we begin to be able to tolerate current through the body- offhand it seems that 400 hz would still be low. I'm sure it's not all to do with skin effect, and I'm also sure the effects on the body are going to be related to how much power the body is actually absorbing. Obviously, too much and you will burn up-

With current flowing through an organic system, chemical changes occur. If the current reverses quickly enough, maybe those changes don't in themselves cause the inability to 'let go'. I can just imagine that at some certain low frequency, the organism would convulse.

Been a long time since I've read anything about this.

The Artful Bodger
01-29-2013, 10:21 PM
We've been buzzing to 60 (and 50) hz for so long now that it's probably a natural thing to resonate with. :) But it's interesting- I wonder at what frequency we begin to be able to tolerate current through the body- offhand it seems that 400 hz would still be low. I'm sure it's not all to do with skin effect, and I'm also sure the effects on the body are going to be related to how much power the body is actually absorbing. Obviously, too much and you will burn up-

With current flowing through an organic system, chemical changes occur. If the current reverses quickly enough, maybe those changes don't in themselves cause the inability to 'let go'. I can just imagine that at some certain low frequency, the organism would convulse.

Been a long time since I've read anything about this.

Thomas Alva Edison, who is on record as having invented everything, answered this question long ago with his public executions of stray dogs.

J Tiers
01-29-2013, 11:49 PM
Thomas Alva Edison, who is on record as having invented everything, answered this question long ago with his public executions of stray dogs.

Wrongly.........

DC, which he advocated, is of course more dangerous, as it penetrates farther into the body. The so-called "skin effect" is less of an issue with a resistive substance like your body, but is still a factor.

The higher the frequency, the less the penetration. More skin burns, less deep current through your heart. Maybe.

If you get into high voltage at a low impedance (high current), you are likely dead anyhow.... and whatever extremity touches the source will likely explode due to steam pressure, so you won't like it even if you are not dead.....

My suggestion is that you not get into the situation where you find out..... most of this stuff is academic / forensic, and no longer a matter of interest to the central protagonist........

EVguru
01-30-2013, 04:37 AM
Only *micro* power SMPS run at 2Mhz. Switching losses and skin effect would kill any high power SMPS running at those speeds.

So you wouldn't believe a 30Kw battery charger with a switching frequency of around 2MHz?

They also had 50Kw and 100Kw models. Resonant switching technology (zero current or zero voltage switching) can result in essentially zero switching losses. The transformers used stacked PCB windings with minimal skin losses.

Vicor produce modules to at least 600 Watt using resonant switching in the MHz range, which is hardly 'micro' power.

http://www.vicorpower.com/cms/home/products/brick/mini-maxi-micro-converters

Black_Moons
01-30-2013, 05:42 AM
So you wouldn't believe a 30Kw battery charger with a switching frequency of around 2MHz?

They also had 50Kw and 100Kw models. Resonant switching technology (zero current or zero voltage switching) can result in essentially zero switching losses. The transformers used stacked PCB windings with minimal skin losses.

Vicor produce modules to at least 600 Watt using resonant switching in the MHz range, which is hardly 'micro' power.

http://www.vicorpower.com/cms/home/products/brick/mini-maxi-micro-converters

Can't find anything on there webpage that says what switching freqency they actualy use. Can you tell me where they spec that?
Forgot about resonant switching. I guess that does boost the high power range some. Didn't realise that had become more common.

Weston Bye
01-30-2013, 11:20 AM
I do not know the origin of 400Hz, but I know where some of it ended up. When I was stationed on the Enterprise, there were three 400Hz transformers (3-phase), for supplying aircraft deck power, located in our shop compartment. Those things whined 24-7. A person could almost get used to it.

chipmaker4130
01-30-2013, 01:04 PM
. . . Perhaps somewhere in such birds, a ground is not connected properly, . . .

You got it. This problem is nearly always caused by a fault in the pilot's headset cord shield. Our headsets get moved around a lot, and some pilots aren't very careful about how they handle the cord. The 'radiator' for the interference is the cockpit windshield heat. The 400Hz tone will vary in strength as the pilot changes headset orientation relative to the windshield.

dave727
02-01-2013, 10:33 PM
Yes , until you get to the Boeing 787 which uses 235 volt Variable Frequency Starter Generators which operate at 350-800 hz depending on engine rpm. No more constant speed generator drive transmission.

The 787 has 1.4 megawatts generating capacity between the 2 generators on each engine and the 2 on the auxilliary power unit.

The gens on the engines are rated at 250kw each, the apu gens a little less.

The engines no longer have pneumatic starters.




- 115vac- 400 hz. This was in a 737 I believe, so it's probably late '80s or newer. I wonder if the latest planes still use that frequency?.

darryl
02-01-2013, 10:50 PM
So now it's variable frequency. Good- it would start to get a little annoying after 60 years of hearing the same 400 hz.

I am curious- how do those generators operate from jet engines? I can only imagine that they would be external to them, and use some kind of pto?

Optics Curmudgeon
02-01-2013, 11:22 PM
The Human Body seems to have a resonance close to 60Hz. Meaning that the muscles will tend to lock and you are unable to break free. 400 Hz is used for safety reasons. Thus its first use in Naval vessels.

Since this has bubbled back up, power system decisions are, and have always been, made on the basis of technical requirements. Electrical power systems are all considered dangerous, and the safety requirements are taken care of in other ways. Many ships in WWII had live front DC switchboards, on submarines there was a lot of DC available with a very low source impedance, the solution was, and still is, guards, interlocks and procedures. That and being careful. There is plenty of 60hz power on board ship, it's the primary mode. Lots of systems used 400hz, some of the Nike system radars used it, there were ground based radios that were actually aircraft radios put in vans, the APN-4 LORAN set used on subs was designed for aircraft and it wasn't worth redesigning, pretty soon 400hz seemed to be everywhere. There were also some oddballs, the Navy TBM transmitter used an 800hz motor generator for the input to its power supply. But the first proposed use is still Orland Esval's 1937 patent application for a gyroscope.

dave727
02-02-2013, 02:08 AM
Yes there is a bevel gear set off one of the core shafts that turns a shaft that goes to the accessory gearbox that is mounted under the engine- and the generators, oil pump, fuel pump , etc are attached to this gearbox.




So now it's variable frequency. Good- it would start to get a little annoying after 60 years of hearing the same 400 hz.

I am curious- how do those generators operate from jet engines? I can only imagine that they would be external to them, and use some kind of pto?

darryl
02-02-2013, 02:58 AM
Thanks Dave. I had guessed it would be something like that- I just thought it should be as simple and foolproof as it could get, considering the high input shaft speed and the heat. Bevel gears, one right angle turn- probably kept lubed by the same oil flow that feeds the main bearing in that area. Can't see a better way to get shaft power out.

J Tiers
02-02-2013, 10:22 AM
Is it not so that there is in many such (at least older ones), a variable speed transmission to keep the frequency fairly constant?

dave727
02-02-2013, 12:55 PM
Yes a Sundstrand constant speed drive transmission kept the generator at 400 hz ( 6000 rpm ) whether the engine was at idle or takeoff power. When the New generation 737 came out they have an integrated drive generator that had the drive unit and generator as a one piece unit that weighed about half of the old setup.

Hopefuldave
02-03-2013, 12:49 PM
SNIP-----8<----
A bit about Edison electrocuting dogs...




Wrongly.........

DC, which he advocated, is of course more dangerous, as it penetrates farther into the body. The so-called "skin effect" is less of an issue with a resistive substance like your body, but is still a factor.

The higher the frequency, the less the penetration. More skin burns, less deep current through your heart. Maybe.

If you get into high voltage at a low impedance (high current), you are likely dead anyhow.... and whatever extremity touches the source will likely explode due to steam pressure, so you won't like it even if you are not dead.....

My suggestion is that you not get into the situation where you find out..... most of this stuff is academic / forensic, and no longer a matter of interest to the central protagonist........



Edison proposed AC electric chairs for executions, and did a lot of political lobbying to get it as the Edison Company wanted the US to standardise on DC power distribution (on which they held a lot of patents) - their main competitor was Westinghouse, who used the incredibly elegant designs (and patents) of Nikola Tesla to distribute AC polyphase power, so there was a lot of money riding on it...

The idea was that by associating AC with executions Edison would seed the idea in the public's mind that their DC power was "safer" - which it wasn't! DC distribution was far less efficient, at least at the time, using motor-generators for voltage conversion. Luckily, Westinghouse and AC won out, or we'd still be changing motor brushes in the 'fridge every few months...

AC is a lot easier to mechanically switch too, as there's an inherent spark-quench 100 or 120 times a second - take a look at a high-power (ten or more KVA) DC relay and you'll see the effort that has gone into making it break the circuit - magnets to steer the arc, "flame chutes", high-pressure air blasts to cool the arc and reduce its conductivity...

Some high-voltage high-power is now being distributed as DC (e.g. between power stations and major switching points), as the skin effect comes more into play at hundreds of KV an KA, and DC allows the use of *all* the cable, but solid-state voltage conversion still puts it back to AC polyphase for delivery to "medium current" users and substations (below a few tens of megawatts).

Just my ha'pennorth,
Dave H. (the other one)

rdhem2
02-03-2013, 08:59 PM
USN circa 1972. On destroyers 400 Hz Motor Generator sets were in the passageways outside radio central. Electricians (EM's) got power to and maintained the motor portion and the Twigets (Radiomen) cared for the generator end and associated user equipment. Always being curious I wanted to know what that 400Hz stuff was for and what it did. Never did have the chance, not in my rate I was told. Another golden NAVY rule.