31 Oct
2000
31 Oct
'00
3:44 p.m.
On October 31, Don Maslin wrote:
180 degrees, if your feed is 220v 2-phase. If you have 3-phase power,
you'll likely be feeding a breaker box from two of the three phases,
which would then be 120 degrees apart. That would, I believe,
technically be 208v, not 220v.
It wouldn't really be 220v center-tapped, though, at least not the
way I'm used to thinking about it.
-Dave McGuire
The 5360, as
well as the 5340 (S/34), are 220 volt single phase (actually
2-phase if you want to be exact). The way I hook mine up is to open the
Oh? What is the phase shift between the "2-phase"s? Isn't it really
just 220 volt center tapped? 
31 Oct
31 Oct
5:31 p.m.
It wouldn't really be 220v center-tapped,
though, at least not the
way I'm used to thinking about it.
US Home wiring is usualy "split-phase". You are fed from a single phase
of the distribution system. You're on the secondary of a transformer,
which is 240V center-tapped. You get two hots and a neutral. Relative
to the neutral, the hots are 180 degress apart.
All of your 120V circuits are fed one hot and a neutral.
This is one reason why the ancient practice of tying an appliance chassis
to neutral has been disapproved for a long time. Suppose you have two
of these appliances close together, but fed from different outlets which
happen to be on the two different hots. If the outlets and the appliances
are wired correctly, you can touch both because they're both on the neutral.
But if anything's wired wrong, you'll get 120V or 240V through you.
And the neutral being safe is potentially a lie anyhow. It's a current
carrying conductor, so even though it's grounded at your service entrance,
it have a non-trivial voltage elsewhere.
Anyhow, your typical US residental 240V outlets, for ranges and clothes
driers, usually only give you the two hots and ground, but NO neutral.
The motors, heating elements, etc. get 240V and don't really have any
use for a neutral. But some equipment may need 240V split phase with
neutral. In this case the plug will have four pins (one for ground).
There are different NEMA standard plugs and receptacles for the various
configurations, with and without neutral.
Three-phase is more complicated. Low-end three-phase stuff is usually
208V three-phase delta, so you'd have four pins (one for ground).
This is fine for three-phase delta motors. But some things need
three-phase wye (Y), which adds a neutral for a total of five pins. A
lot of minicomputers that use three-phase power are wired this way. The
advantage is that you can put a 120V device between any phase and
neutral.
Older equipment (or receptacles) may not have a ground. This is bad,
especially if the equipment is not double-insulated.
I'd advise anyone who is trying to figure out how to run big iron that
doesn't simply plug in to an available outlet to consult an expert. And
I don't claim to be one. But I've spent some time figuring out how
to run PDP-11/70s and KL10s on other than the three-phase power DEC
intended. (Much easier on the PDP-11/70 than on the KL10.)
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eric＠brouhaha.com -
mcguire＠neurotica.com