larry@kitty.UUCP (Larry Lippman) (10/07/87)
In article <6424@apple.UUCP>, bldrnr@apple.UUCP (Brian Hurley) writes: > >(or, as Jay says, 416) Hz. Ditto for the noise and energy loss. There is > >nothing inherently wrong with big 60 Hz transformers; just walk out to your > >nearest power substation and take a look. > > This is not quite true. The reason for a Cray using 400/416 Hz power > has to do with the source more than weight considerations. The amout > of power used is very large (I don't know the figures off hand, but it may > fall into the hundreds of KW) the olny source for this kind of power > is a Sub-station feed. Which is NOT 60Hz. I'm not certain what you mean by a "Sub-station feed", but utility companies can deliver some megawatts of power to a customer site without having to resort to voltages above 480 volts AC within the customer site. As an example, a 1200 ampere 480/277 volt wye service will deliver about one megawatt. 480 volt "busbar" service is available to 3,000 amperes or more (a couple of megawatts, minimum). If you are implying that incoming utility company service at high power is NOT 60 Hz, this is incorrect. > If you transmitted even 100KW of > power @ 440VAC that would result in a curennt flow of approx 318 AMPS! You really need to figure 480 volts, since 440 delta or grounded delta service is almost non-existant; on the other hand, I'll give you some reactive KVA allowance - we'll say a power factor of .95. So, the I = 100,000 / (1.732 * 480 * .95) = 127 amperes > Copper ain't that cheap! If I remeber my basic transmission line math, the > higher the AC frequency, the smaller the transmission line needs to be, > but I don't remeber the math that would show a direct corralation. The > end results of high frequency AC power include lighter equipment as well as > lower cost and lower loss transmission. What you have is skin effect, which means you can use hollow conductors, but not conductors of smaller diameter. From a practical standpoint, there is no advantage in power transmission based upon using 400 vs 60 Hz in the relatively short-distance environment of a computer system. > There are disadvantages to this type > of power as well, the equipment for regulating 440VAC @ 400Hz is not as > inexpensive as 60Hz, but imagine the transformer that could handle 100KW > @110VAC, 60Hz... I'm not certain how large you think this transformer would be, but I have seen 100 kva transformers used in electroplating power supplies that measure about 22 inches on a side, are about 42 inches high, and weigh about 600 pounds. In a computer application, however, I don't believe any designer in their right mind would NOT use distributed power supplies, with each supply being no greater than a couple of kva in capacity. > Again the weight is not the issue. The solution to the interface is simple. > Design a motor that works on 400Hz and have it drive a 60Hz Generator. If > the design is mechanicly sound, the losses are comparable to a transformer > of comparable size. Transformers, especially those of toroidal design, can achieve efficiencies of 98% or more. Ain't no way that any rotating component power converter can match that efficiency! As a point of information, even HUGE transformers, like those used in utility substations are more efficient than you may think. As an example, I have seen a 5 megawatt three-phase transformer at an industrial plant down the road converting 34.5 kV delta to 4.16 kV delta that has a maximum loss of 5% (i.e., 95% efficiency). I read the nameplate; WHY I got close enough the read the nameplate is another story... <> 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?"
larry@kitty.UUCP (Larry Lippman) (10/07/87)
[I am trying to move this discussion to sci.electronics, where at this point it really belongs.] In article <3011@ames.arpa>, msf@amelia (Michael S. Fischbein) writes: > Not only aircraft, but much shipboard equipment uses 400Hz. The goal is > certainly not saving weight. In most case, the answer is fairly simple: > > Most electronic equipment, especially digital stuff does not run on AC > at 60Hz, 400Hz or any other sine wave; it runs on DC. Now, how do you > convert AC to DC? (Basically) A full wave rectifier and a capacitor to > fill in the ripple. How big does the capacitor have to be? Depends on the > frequency of the incoming AC. Higher freq, smaller cap. Look at your > computer's power supply sometime; bet more space is taken up by capacitors > than any other component (not counting heat sinks; that's not electronics > but mechanics). Take a look at some of the big iron power supplies; > they have some truly monster caps in some machines. Space is generally not a problem on shipboard electronic equipment, nor is weight, so 400 Hz power is used on some shipboard apparatus for neither of these two reasons. The reason 400 Hz is used on some shipboard equipment is to accommodate the needs of navigation, fire control and antenna positioning systems which use gyros and servomechanisms. The majority of military gyro motors, servo motors, resolvers, synchros, control transformers (the synchro variety), other types of magnetic angle position encoders, etc. operates from 400 Hz power. The reason for this is that 400 Hz results in smaller packages, better resolution, and faster response than does a 60 Hz counterpart for a servomechanism or gyro application. While there are shipboard fire control and antenna positioning (as in radar) systems which use 60 Hz for servomechanisms, today it is more common to see 400 Hz being used. <> 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?"