wtm@neoucom.UUCP (Bill Mayhew) (10/10/87)
I have done some work on power distruibution systems-- though not too much. Most 400 Hz stuff is designed for use in mobile applications where saving weight and size is very important. I'll side with Larry (as usual) on this issue. Working with 400 Hz power is much less fun becuase there is much more loss due to direct radiation from the transmission line. Yes, indeed, at high power levels skin effect at 400 Hz is a problem. 400|800|1200 whine is much more annoying than 60|120|180 Hz hum from poorly regulated power supplies. Of course, one of the big advantages of using 400 Hz is that filter capacitors can be smaller to achieve the same level of regualtion. Unfortunately, rectifier diodes are less efficient at 400 Hz. The best win is that 400 Hz power transformers are very small. Torroidal construction is much more common for 400 Hz applications thatn it is for 60 Hz. Many 400 Hz transformers are 95-98% efficient. By the way, most 60 Hz transformers are over 90%. We have several 50 KVA transformers here that are air cooled and are only about 18 inches (~50 cm) on a side cubes. If memory serves me right, Cray computers are equipped with 6 power supplies disguised as benches mounted around the lower outside part of the CPU. Each power unit is about 5 KVA, I think. Bill (wtm@neoucom.UUCP)
larry@kitty.UUCP (Larry Lippman) (10/11/87)
In article <726@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes: > I have done some work on power distruibution systems-- though not > too much. Most 400 Hz stuff is designed for use in mobile > applications where saving weight and size is very important. Interestingly enough, frequencies higher than 400 Hz have been used to power aircraft equipment. While 400 Hz is now the standard, much experimentation was conducted during World War II. Based upon the design and manufacturing capability that existed during WW II, a weight savings of 50% in transformer "iron" could be achieved by going from 60 Hz to 400 Hz. While toroids represent an even higher weight savings at 400 Hz, toroidal manufacturing was extremely limited during WW II. Frequencies higher than 400 Hz were tried, but with diminishing returns. Going from 400 Hz to 800 Hz produced only 10% reduction in weight. Nevertheless, 800 Hz was pretty much adopted as the standard frequency for U.S. Navy aircraft during WW II. Stable power frequencies could only be produced by means of a DC powered inverter; it is obviously impossible to produce a constant frequency by an alternator that is directly driven by an aircraft engine. However, to "keep things simple", engine-driven alternators were indeed used on some WW II aircraft; they produced a constant voltage (carbon pile regulators no problem here), but a variable frequency. Common frequency ranges used were 400 to 800 Hz and 800 to 1,600 Hz for U.S. aircraft; the British went one step further, they used alternators which went from 1,200 to 2,400 Hz. That's right - 2,400 Hz 3-phase power! The philosphy with variable frequency AC power was to use small inverters for frequency-critical power like servomechanisms. With some hindsight, there was probably no practical advantage in using frequencies above 800 Hz, but a lot of "strange" things were done during WW II. Frequencies higher than 400 Hz did carry over into the 1950's, but by the 1960's 400 Hz remained as the survivor. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
john@hpcvlo.HP.COM (John Eaton) (10/13/87)
<<< < The best win is that 400 Hz power transformers are very small. ---------- I once worked on a military HF transmitter that could be operated off either 60 or 400 hz depending on what power supply was installed. The 60 hz supply came in its own 19" enclosure and took two men to carry. The 400 hz model could be held easily in one hand. BTW the 60 hz power used by the navy on board ships is different than what you get from your local utilities. It comes as two hot leads balanced at 55 volts to ground instead of a hot and a neutral. You have to be careful about what you plug into it. John Eaton !hplabs!hp-pcd!john
john@tower.UUCP (John Moore) (10/15/87)
In article <2103@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: >In article <726@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes: >U.S. Navy aircraft during WW II. > Stable power frequencies could only be produced by means of a DC >powered inverter; it is obviously impossible to produce a constant frequency >by an alternator that is directly driven by an aircraft engine. However, >to "keep things simple", engine-driven alternators were indeed used on ><> Larry Lippman @ Recognition Research Corp., Clarence, New York It is not at all impossible to produce a constant frequency by and alternator directly driven by an aircraft engine. I used to fly in the US Navy P-3 Orion, a Turboprop aircraft. The engines ran at constant speed and were in fact PHASE LOCKED at the A/C power distribution phase. They ran alternators that directly powered the equipment. -- John Moore (NJ7E) (602) 861-7607 (day or evening) The opinions expressed here are obviously not mine, so they must be someone else's.
rees@apollo.uucp (Jim Rees) (10/28/87)
It is not at all impossible to produce a constant frequency by
and alternator directly driven by an aircraft engine. I used to fly
in the US Navy P-3 Orion, a Turboprop aircraft. The engines ran at
constant speed and were in fact PHASE LOCKED at the A/C power
distribution phase. They ran alternators that directly powered the
equipment.
The original message from Larry referred to WWII aircraft, which used
piston engines. It isn't easy to get a constant-speed drive from one
of these. The Orion (military Electra) has turboprops. I'm not sure
how their aux power takeoff works, but if it's like a turbojet, the
alternator runs off a separate constant-speed turbine fed by the
compressed air from the engine. This is easy on a turbine engine
(turboprop, turbojet, fanjet, etc). Unfortunately, these engines
were not available during WWII, mostly because of materials technology.
So the alternator ran directly off the piston drive shaft, and the
power frequency varied with engine speed.