25 Oct
2000
25 Oct
'00
2:07 p.m.
From: Richard W. Schauer <rws(a)enteract.com>
I've recently acquired an RP06 as part of a
PDP-11/44 system, and I wish
to run it (why not?) I remember reading on this list a while back, that
it could be operated on single-phase power instead of three-phase. I
got
a printset along with the drive, and it's pretty
unclear about what
would
use the three-phase- it's almost like the third
phase is ignored, the
208v
between the first two phases is fed to the transformer,
and the primary
is
sort-of center tapped to feed the 115 volt fans. Is
this right? How
would I go about feeding it single-phase if that isn't right?
Rewire. The reason most of these devices use three phase is to
distribute a load more uniformly over what was a normal power system
in computer rooms then. Also many of these devices have a rather harsh
startup current and slicing it two or three ways makes it much easier
for the sytem (wiring and power circuits) to sustain.
For example a lowly MVII in a BA23 has a startup surge of over 100A
for one cycle (about 16mS). In the second 16mS that might be only 12A
and by the 10th your near nominal of say 3.2A. If you turned ten of
them
on at the same time and same circuit the running current is maybe
32-40A but the starting current could easily exceed 1000A for several
cycles. Why? If they were all on one breaker the whole load would
attempt to bring the caps in the power supply up that first 16mS...
save for the 10,000A surge would pull the 115v down to say 60V
(assuming thick wires and the fuses held!) the next cycle would be
needed to charge those caps more, likely 10 or 15 cycles so the huge
current would take longer to dissapate. Add motors like the RP06 with
their nasty starting currents and you see why power systems were so
robust. Granted the startup condition is pathological as no one would
do that (staggered is also common) but the need to distribute the
sustained load is also important.
If I had a 6000, I'd ahve rewired it already and could tell. Most often
(but not always) three phase power is used as independant legs.
Exceptions would be beasties like VAX9000, PDP10s and other
power hungry older machines.
Allison