8 May
2015
8 May
'15
8:39 a.m.
On 05/08/2015 06:30 AM, Noel Chiappa wrote:
Yup, that's another name. The imaginary comes from the real
and imaginary components of a complex number.
From: Jon
Elson
Well, first, rotary converters draw a LOT of
imaginary power (in other
words, they have an awful power factor) and so the line current can
become MUCH higher than you would expect.
...
We tried to rig up a phase converter scheme to run the motor-generator
set on a 370/145 in a guy's house, and it did NOT go well. he only had
a 60 A 240 V service, and the imaginary current was over 60 A!
Had to Google 'imaginary power'... Not a lot of experience with high-power AC
stuff! :-) ('Imaginary power' is probably not the best term to use, because
there are actual currents involved; I like the 'reactive power' name better.)
The article I read said that in reactive load which is
high in inductance
(which is, I assume, the source of the high reactive load in rotary
convertors - or am I confused - a common happening, I concede :-), judicious
application of capacitance can reduce the reactive load. Why isn't this used
with rotary convertors to reduce their reactive load?
Yes, but it takes a BIG bank
of caps to compensate the
current on a 17 KVA motor. Meanwhile, you are trying to
generate a reactive current (phase shifted) to drive the 3rd
leg of the motor. It gets messy. Also, the cap bank is
going to draw a huge current spike when the relays are
closed, and that could weld contacts in the contactor. it
was just a fool's errand to try this on a house with 60 A
service.
Any idea what the active and reactive powers/currents
were in that attempted
installation?
Well, the MG system in the 370/145 had delta/wye starting.
We had to have a guy HOLD the circuit breaker while in the
wye configuration, so the total current was already over 60
A. Then, when we closed the delta contactor, he could NOT
hold the breaker closed with both hands. That's when we
knew we were totally beat. But, you can compute the line
current for a 17 KVA motor. Line current should be about 40
A with real 3-phase power. If you need to run this off a
single-phase 240 V mains, then the line current just to
satisfy the unity power factor requirement would be closer
to 70 A. Add in bad power factor from the phase shifting
scheme, and you could easily end up over 100 A without a
huge phase compensating cap bank. Now, the crazy thing is
the /145 had a DC power dissipation of 1.9 KW, if I remember
right! 390 A at 4.8 V. (They used +1.3 and -3.5 V I think,
so it put their ECL terminators at ground.) I think they
used the same MG sets for a range of different computer models.
The only sane fix would have been to scrounge up some big
switching power supplies and replace the whole PS system on
the machine. But, then, the guy that had this would have
been able to load microcode and key in programs to type
stuff on the console typewriter. He'd still need a disk
controller and a com controller at the very least.
Jon