4 Jan
2022
4 Jan
'22
8:03 a.m.
From: Scott Quinn
I have seen some roads where the utility has 2 of the
phases plus
neutral going down them, not true 2-phase power, but 2 phases 120/240
degrees apart with the third phase just not present.
My street has that. The subdivision as a whole has all 3 phases (down the
main road through it), but individual streets off of it have only 1 or 2.
(The whole subdivision is on poles, so it's easy to see.) On the ones with 2,
some houses are connected to one, some to the other.
I guess they figure twice the loads for only one more
wire.
No, because most homes are only connected to one phase. I think the main
reason to do it is that it allows the total load (of the entire subdivision)
to be somewhat balanced across all 3 phases.
Can't remember what it was called but I do
remember seeing in some book
somewhere about a "phantom 3rd leg" or something
My house has something like that; the previous owner wanted '3-phase service'
for machine tools (I think - could have been a compressor, or something) in
his basement workshop, so they sold him a pseudo-3-phase service. I forget
the exact details of how it works, but the 3rd phase is at 170V to neutral,
or something like that. (So I can't power any 110V outlets off the third
phase.)
I think the way it works is that the two 'main' phases are 220V to each
other, 110V to neutral (I think from the usual center-tapped transformer off
one of the three main feed phases, i.e. 180 degrees to each other). The third
'pase' is generated by a second, smaller transformer connected to the other
feed phase in some arcane way I forget the details of. So it's 120 degrees
away from the other two.
Noel
4 Jan
4 Jan
8:52 a.m.
On 1/4/22 10:03 AM, Noel Chiappa via cctalk wrote:
It
should be 208V - see center grounded delta online.? 240 *
sin 60 degrees = 207.8 V.
Jon
From: Scott
Quinn
I have seen some roads where the utility has 2 of
the phases plus
neutral going down them, not true 2-phase power, but 2 phases 120/240
degrees apart with the third phase just not present.
My house has something like that; the previous owner wanted '3-phase service'
for machine tools (I think - could have been a compressor, or something) in
his basement workshop, so they sold him a pseudo-3-phase service. I forget
the exact details of how it works, but the 3rd phase is at 170V to neutral, 
9 a.m.
you can use a vfd drives to get 3phase power from single u just gotta size
them bigger then the load u would normally need
On Tue, Jan 4, 2022 at 10:52 AM Jon Elson via cctalk <cctalk at classiccmp.org>
wrote:
On 1/4/22 10:03 AM, Noel Chiappa via cctalk wrote:
120/240
service'
From:
Scott Quinn
> I have seen some roads where the utility has 2 of the phases plus
> neutral going down them, not true 2-phase power, but 2 phases degrees
apart with the third phase just not present.
My house has something like that; the previous owner wanted '3-phase for machine tools (I think - could have been a
compressor, or something)
in
his basement workshop, so they sold him a
pseudo-3-phase service. I
forget
the exact details of how it works, but the 3rd
phase is at 170V to
neutral,
It should be 208V - see center grounded delta online. 240 *
sin 60 degrees = 207.8 V.
Jon
10:04 a.m.
On Jan 4, 2022, at 12:00 PM, Adrian Stoness via cctalk
<cctalk at classiccmp.org> wrote:
you can use a vfd drives to get 3phase power from single u just gotta size
them bigger then the load u would normally need
I've said that too, and I've been told that this is not a good idea for power
supplies. Something about the waveforms involved in VFDs. I haven't studied the
subject enough to decide whether that's accurate, and the only VFD usage I have is a
lathe -- the sort of application VFDs are designed for.
The other option, typically somewhat more expensive but cheaper than an 11/780, is a
rotary converter. Those are 3 phase motors, sometimes modified a bit, driven from single
phase power that construct the missing phase somewhat like a dynamotor would. Those
things produce proper sine waves so they are good to use even with things that are picky.
Rotary converters can be found in machine tool supply catalogs.
paul
10:35 a.m.
On 1/4/22 10:04 AM, Paul Koning via cctalk wrote:
The other option, typically somewhat more expensive
but cheaper than an 11/780, is a rotary converter. Those are 3 phase motors, sometimes
modified a bit, driven from single phase power that construct the missing phase somewhat
like a dynamotor would. Those things produce proper sine waves so they are good to use
even with things that are picky. Rotary converters can be found in machine tool supply
catalogs.
One has to be careful here. The common "idler" type of rotary converter
uses a 3-phase motor, with only one set of windings fed from the single
phase supply and leading the third phase idle to develop a current
that's +/- 90 degrees from the powered phase. So the output looks like
0,90,180, rather than 0 120 240 degrees. Good enough for powering a
3-phase motor with slight derating, but nothing I'd use to power a
computer power supply.
The second type is similar, but incorporates a dual field arrangement,
with a single-phase field coaxial with a 3-phase one comprising a motor.
You get much better output phase relationships there and one would
probably work fine with a computer.
--Chuck
7:36 p.m.
On 1/4/22 11:04 AM, Paul Koning via cctalk wrote:
I've said that too, and I've been told that
this is not a good idea
for power supplies. Something about the waveforms involved in VFDs.
My limited understanding is that VFDs simulate / emulate various
frequencies by turning the output on and off (at full input voltage)
such that the (sliding) /average/ of the output looks like it's a at a
lower voltage. So when you're early in the typical sine wave (first
45?) the output is on a small percent of the time and off a large
percent of the time. Near the upper portion of the peak (45?-135?) the
output is on a large percent of the time and off a small percent of the
time. Repeat that method on through the sine wave. So, you end up with
very odd shaped signal.
Supposedly motors (or any inductive load) can smooth the pulsations to
behave more like a true sine wave of the simulated / emulated frequency.
Non inductive loads don't smooth / average things nearly as well. Hence
why you probably want to not use a VFD to power computer equipment.
At least that's my understanding.
The other option, typically somewhat more expensive
but cheaper than
an 11/780, is a rotary converter. Those are 3 phase motors, sometimes
modified a bit, driven from single phase power that construct the
missing phase somewhat like a dynamotor would.
Wouldn't that be construct the
/two/ missing phases? You input one
phase between the two lines and get the other two phases between the two
existing lines and the added line.
Those things produce proper sine waves so they are
good to use even
with things that are picky.
Yep.
--
Grant. . . .
unix || die
9:58 p.m.
On Tue, Jan 4, 2022 at 8:36 PM Grant Taylor via cctalk <
cctalk at classiccmp.org> wrote:
My limited understanding is that VFDs simulate /
emulate various
frequencies by turning the output on and off (at full input voltage)
such that the (sliding) /average/ of the output looks like it's a at a
lower voltage.
That's how all buck switching power supplies work. It's just a question of
how much low-pass filtering you do on the output. A switching power supply
to power sensitive electronics that requires good voltage regulation and
low ripple contains electrolytic capacitors on the output. A VFD may or may
not. The specs on the VFD should tell you about the output characteristics.
If you're just going to power a three-phase synchronous motor, then no
filtering is needed. The inductance of the motor windings and the
mechanical characteristics of the motor will filter it just fine.
3:52 p.m.
On Tue, Jan 4, 2022 at 12:00 PM Adrian Stoness via cctalk <
cctalk at classiccmp.org> wrote:
you can use a vfd drives to get 3phase power from
single u just gotta size
them bigger then the load u would normally need
Yes, in principle this is true. In the simplest case, the VFD input is
just a 6
diode full wave rectifier into a capacitor. BUT things like power
factor correction on modern drives means the input circuit is a lot more
complex. Usually the small VFD will be designed for single phase power or
3phase power and the derating is done for you. Using a 3phase drive on
single phase typically won't work because the drive will detect a phase
loss and shut down. I know for one Rockwell drive there was an internal
hack that allowed it to work on single phase so the sales people could plug
into a 120vac outlet.
Chuck D.
853
days inactive
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days old
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