rgb@PHY.DUKE.EDU ("Robert G. Brown") (02/14/90)
Thanks to all the nice people who responded, and the folks at SG who
(eventually) arrived at the One True Word. For we mortals who are
confused by a wiring specification that calls for 220V single phase
(ground, neutral, hot) power for a machine sold in a country where
there ain't no such beast (or at least, where it is extremely rare)
there follows a short Discourse on Wiring:
Feeding the Beast
The receptacle for the 220S Power Series Rack is a Nema 6L 30R, twist
lock. Its specification is for "30A, 195-240V, 50-60Hz single phase"
or words to that effect. The plug has three prongs:
|_ Ground
Neutral \ / 220V
that are >>supposed<< to carry the potentials indicated in an ideal
world.
Ground (for those ignorant of the Code), is supposed to be real, live
(or actually dead) >>ground<<. It should be connected to the moral
equivalent of the plumbing, or the steel girders in a building that go
deep into the ground. It should >>never<< (intentionally) carry
current. When you touch something connected to ground, you should be
as safe (electrically) doing so as when you touch the plumbing.
Ground is >>very important<< to computers as a signal shield, too, so
your ground should be electrically "quiet" in higher frequencies.
"Neutral" in your 110 V household wiring is the white one in the
white, black, and copper triplet in standard three-wire cable. It,
too, is connected to ground, but >>it carries current<<. In fact, all
the current that flows "out" the black (hot) wire flows "back" to
ground on the white neutral line. Since it carries current, it is
easy to electrocute yourself on a neutral line. In the days before
the code, standing on a wet bathroom floor and flicking on a light
switch shorted to the neutral wire was more than adequate to get the
10 mA or so through the torso needed to defibrillate the heart.
(Today, a "ground fault" protected switch compares the current on the
black and white wires and if it is not equal -- because some of it is
being diverted through your torso, for example -- it cuts off the
line).
"Hot" in a 110V circuit or the 220V circuit shown above has a
potential difference of 110 or 220V >>relative to a grounded wire at
the transformer<<!
The thing that does "work" in an electrical circuit is the potential
difference between the two poles of the current carrying circuit.
That is the key to the solution to the problem.
The SG-220S in the rack mount has no 110V circuitry in it at all.
Nowhere does it use "neutral" as anything but a potential reference
for the "hot" wire. The only thing that matters is the net potential
difference between the two poles.
For that reason, the following are perfectly acceptable power sources:
|_ Ground
110V sin(wt) \ / 110V sin(wt + Pi)
(what most humans would call 220V two phase power). This is typical
of the power supplied to a normal house for running the range or the
drier. In the household there may or may not be a >>fourth<< wire to
serve as a current carrying neutral which can be used to split the
line into two 110V circuits. Note that the potential difference
between the poles is:
110V sin(wt) - 110V sin(wt + Pi) = 220V sin(wt) at 60 cycles.
Also acceptable to the power series rack is:
|_ Ground
120V sin(wt) \ / 120V sin(wt + 2Pi/3)
============================================
(not used)
X 120V sin(wt + 4Pi/3)
This wiring uses two out of three legs of a "three phase 220"
circuit. This is typical output of a "Wye" transformer and is common
in Universities and offices. Again, there may or may not be a current
carrying neutral allowing it to be split into three 120V lines. The
potential difference is:
120V sin(wt) - 120V sin(wt + 2Pi/3) = 207.8V sin(wt + Pi/3)
where the phase shift is completely unimportant (when >>did<< time
start, anyway ;-). This (208V) is well withing spec for the 220S.
The reasons for running 220V lines in this way are to minimize risk --
unless you touch two lines simultaneously you can only get a 110V
shock -- and to allow appliances to draw 30A or 45A in circumstances
where each 120V or 110V line is fused to draw no more than 15A. 15A
is code for 14 Gauge wire up to 50 feet from the distribution panel
and 12 Gauge up to 100 feet. I recall that 20A can run on 12 for 50
feet, and 30A requires 8 or 10 Gauge, but I don't have a reference
handy and don't quote me on that. The power cable for the 220S is 10
Gauge, three wire.
One reason for the confusion is that >>electricians<< call all three
wirings of the above plug/receptacle combo "single phase" 220V AC!
God only knows what they would call two phase or three phase.
I'm going to wrap it up here, without telling you the story of the
Ground Loop, boys and girls. That's what you get (sort of) if you
treat a current carrying "neutral" as ground and connect it to
something that is a bit closer to "true" ground for the circuit. This
is surprisingly easy to do, especially in a machine that is running
several lines of power with different phases and a reference neutral.
In the week or so since I've posted, I've heard lots of funny stories
about blowing up coffee pots plugged into Vax power strips, blowing up
Vaxes by connecting them when they were plugged into different phases
on different sides of the room, and lots of other stuff. The moral of
the story is: beware ground loops. I suspect that SG made the power
220 only in order to avoid this very problem.
The bad news, of course, is that our machine doesn't work (still) and
it isn't the power :-( But maybe by tomorrow the nice man from SG will
swap our boards and get us going.buck@drax.gsfc.nasa.gov (Loren (Buck) Buchanan) (02/14/90)
In article <9002140322.AA05845@physics.phy.duke.edu> rgb@PHY.DUKE.EDU ("Robert G. Brown") writes: Thanks for the description [[[mostly deleted]]]. >The reasons for running 220V lines in this way are to minimize risk -- >unless you touch two lines simultaneously you can only get a 110V >shock ... In a prior life (before college) I was a marine electrician. The ships I worked on had "two phase" 110V lines. 55V on neutral, and 55V on hot. Again this is for the above saftey reason. The only problem with this is the modifications that needed to be made to convert commercial electrical equipment (radios, coffee pots, etc.) safe (well, actually just a bit more sailor proof :-}). B Cing U Buck Loren "Buck" Buchanan | internet: buck@drax.gsfc.nasa.gov | standard disclaimer CSC, 1100 West St. | uucp: ...!ames!dftsrv!drax!buck | "By the horns of a Laurel, MD 20707 | phonenet: (301) 497-2531 or 9898 | sky demon..."
markb@denali.sgi.com (Mark Bradley) (02/15/90)
In article <9002140322.AA05845@physics.phy.duke.edu>, rgb@PHY.DUKE.EDU ("Robert G. Brown") writes: [ Lot's of information about electrical deleted ] > > The bad news, of course, is that our machine doesn't work (still) and > it isn't the power :-( But maybe by tomorrow the nice man from SG will > swap our boards and get us going. I hope so. Thanks very much for the informative posting. As I recall, the info I posted referred to the power distribution unit receptacles in the rear of the unit. Hope that was not too confusing. markb -- Mark Bradley "Faster, faster, until the thrill of I/O Subsystems speed overcomes the fear of death." Silicon Graphics Computer Systems Mountain View, CA 94039-7311 ---Hunter S. Thompson ******************************************************************************** * Disclaimer: Anything I say is my opinion. If someone else wants to use it, * * it will cost... * ********************************************************************************
art@lsr-vax.UUCP (Art Hays - PSTAFF) (02/20/90)
"Robert G. Brown" <uunet!phy.duke.edu!rgb> writes: > This wiring uses two out of three legs of a "three phase 220" > circuit. This is typical output of a "Wye" transformer and is common > in Universities and offices. Again, there may or may not be a current > carrying neutral allowing it to be split into three 120V lines. The > potential difference is: > 120V sin(wt) - 120V sin(wt + 2Pi/3) = 207.8V sin(wt + Pi/3) While on the subject of power, another interesting topic is neutral heating due to switching power supplies. The typical three phase wye transformer has the same guage wire for the phases and neutral. The assumption is that if the loads on all the phases are equal, there will be very little current flow in the neutral. Panels have all three phases in them, and the breakers alternate which phase they connect to. The current waveform of a switching power supply is far from a sine wave. It draws current in brief periods near the peak of the voltage waveform. For various reasons (which I dont claim to fully understand) this creates currents in the neutral of the three phase wye. I believe this problem is being addressed in the electrical codes now. There are various derating factors to apply in calculating loads on the distribution transformer to prevent neutral heating when much of the load is from switching power supplies. Even measuring load isnt easy, as I recently discovered. I wanted to measure our current load in the computer room to buy a UPS. The normal clamp on ammeter isnt sufficient. Inexpensive ones read out in RMS current, but measure AVERAGE current. They assume the current waveform is a sine wave, and apply the factor to convert average to rms. If the current waveform is not a sine wave (such as with a switching power supply) this type of meter will read low. One must use a true rms meter with a wide frequency response (I used a Fluke true rms handheld with their widest freq. response current clamp. Amprobe has a computerized meter that works also). What I measured in my computer room was: phase 1: 21 amps phase 2: 17.3 amps phase 3: 17.0 amps neutral: 30 amps Note how the neutral is carrying much more than would be expected from the phase imbalances. An average reading meter would have been off by more than a factor of 2. Art Hays, Nat. Eye Institute, uunet!lsr-vax!art Nat. Institutes of Health, Bethesda, MD (301) 496-7143
henry@utzoo.uucp (Henry Spencer) (02/24/90)
In article <9002192231.AA15320@> art@lsr-vax.UUCP (Art Hays - PSTAFF) writes: >The typical three phase wye transformer has the same guage wire for >the phases and neutral. The assumption is that if the loads on >all the phases are equal, there will be very little current flow in the >neutral... The current waveform of a switching power supply is far from >a sine wave. It draws current in brief periods near the peak of the >voltage waveform. For various reasons (which I dont claim to fully understand) >this creates currents in the neutral... It's not hard to understand. If the current waveforms are sine waves and all lead/lag the voltage waveforms by the same amount so they are 120 degrees apart like the voltages, then the sum of the current waveforms is zero. For example, when one of them is at maximum positive current, the others are mildly negative (one has recently gone negative, the other is about to go positive). However, this doesn't work for an arbitrary waveform. In particular, in the example, if the loads draw current only in the middle of the cycle, then when load #1 is at max positive current, the other two will be at zero instead of being mildly negative. Help is on the way, because the way to get maximum power while staying within a given current limit is to draw current over the entire sine wave rather than just the peaks, and the power-supply manufacturers see a considerable market for supplies that get the most out of an ordinary 15-amp circuit in particular. But for a while this will be premium technology applied only in things that really need it. -- "The N in NFS stands for Not, | Henry Spencer at U of Toronto Zoology or Need, or perhaps Nightmare"| uunet!attcan!utzoo!henry henry@zoo.toronto.edu