Re: Three phase conversion In Rush current Concern.

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Original poster: David Dean <deano@xxxxxxxxxxxxxxxx>

Hi Jim

On Friday 29 September 2006 11:50, you wrote:
> Original poster: "Jim Mora" <jmora@xxxxxxxxxxx>
>
> Hello All,
>
> Here is the schematic of my proposed three phase converter to power my
> Raytheon 3 phase 24 KV DC power supply (I also have a generator in
> progress). The capacitors are mid-range based on other phase converters
> documentation and are subject to trail and error, to achieve balanced
> output.
>
> http://hot-streamer.com/temp/JimMora/3PhaseStaticIdlerMotor.pdf
>
OK. I looked at your schematic.
I assume (correct me if I am wrong) that the three squiggly lines indicating
coils represent the windings of the motor, and that the unterminated wire
coming out of the motor and labled with an "N" is the connection to the
center of the wye or star configuration of the windings. If there really is a
wire there then that is a good thing, as it will provide a reference point to
measure the voltage across each individual winding which will be helpfull in
getting your delta balanced.
I also see two banks of run caps. one from L1 to T3, and one from L2 to T3.
Then in between it says 270 MFD but I don't know if you mean each or total or
if you mean they are both the same or what. I have seen some schematics that
show having some caps between the line that is not connected to the
capacitors (in your case that would be the bank between L1 and T2) and have
read that sometimes it is necessary to do so to balance the phases. I have
never seen that in any commercial static converter I have worked on or seen
the inside of. Nor have I seen that done on any of the few homemade rotary
converters I have seen. I have never worked on a rotary converter other than
the two which I made for my own use. One was a motor generator set with a 5
HP 1PH motor turning a 1.5 HP 3PH motor via a belt and two 5 MFD caps going
from  L1 to T1 and L2 to T2 just to tickle it. The other was just a 1HP 1PH
motor with the start winding coming out to male fake L3. It made a squished
delta, 240 165 180, but it was a delta and worked. Got my old South Bend
lathe by for 4 or 5 years.
A couple of years ago some guys from another city came by on a Saturday said
they had a static converter that did not do the job for them and ask me to
build them a rotary converter from an old 3PH motor, they could get one from
the Green Guy for cheep, and would I be willing to do that for them. Anyway I
went, I looked. I saw, I was amazed. There on the floor was a new $2400
static converter with a nameplate boasting a range of HP it would run and the
one they were using marked as factory preset, and an efficiency rating of
85%. I had always heard/read that 70% was the best you could get from a
static, 85% is as good as a motor generater set.   So I just opened it up,
read the documentation,  moved some taps and had the motor purring in a few
minutes. I mension this because the "secret" to this thing was the multitap
autotransformer.  You step up the voltage feeding the cap banks to evercome
the voltage drop across the caps. When adjusted properly for the motor the
delta is balanced to within 5%, better than some of the motors I see all the
time running on 240 delta systems.

> If anyone has experience in this arena I could use some help.
>
> The motor will run at ~3425 rpm which has the benefit of handling sudden
> change in current draw (think spark gap) and the disadvantage of inrush
> current to start it up.
A "rule of thumb" used to size transformers for motors,  calculate proper size
for overcurrent protection, etc. is 600% FLA for the inrush current. You will
be starting with no load, so I don't think you will ever see anywhere near
that. In fact I would not be surprised if you find the motor will start
without using any start caps. Think PSC like the blower motor in your furnace
or the condenser fan motor in the A/C condensing unit outside. Many A/C
compressors do not have start caps or potential relays, just a run cap of
approx. 20 MFD/HP (another rule of thumb) between common and start.
>
> The motor will draw around 49 amp in static state load conditions. The LR
> rating @ 240v is 290 amps, yikes. I have seen a few schematics of "soft
> start" with triac's or SCR's.
>
Soft start.  THIS IS NOT A SUGGESTION. But I can't help myself, I just have to
tell you this story.  When I first bought my lathe I got a quote for a static
converter of $1700 to run a 2 HP motor. The power co. would set a pole and do
the primary work and put the drop for no charge, but I had to buy the
transformers at a cost of $1500, plus I would have had to build a new three
phase service at a cost of more than $1000 just for the parts. My uncle told
my dad about a guy and Dad took me to see him. He had made his own rotary
converter from a three phase motor and no capacitors at all.  He had a small
1PH motor hooked up to a large 3PH motor with a kinda loose belt and some
missaligned pulleys.  He demonstrated the operation for me. First the small
motor was started and the belt slipped till the big motor caught up with it.
Then 1PH power was supplied to the big motor, and as it took over, it would
throw the belt. Then the small motor was turned off and the big motor idled
along on 1PH power generating the third phase all the while. It worked. The
guy claimed it had been working like that for years. It probably worked
better than the 1PH motor with the start winding brought out I made and used,
but the belt thing just did not tickle my fancy. But it goes to show that
there are probably as many ways to do something as there are people to try
it.
> I think  my neighbors to the right will see a burnout as will their
> computers as we are a distance from the pig and the service wire is 1950's
> # 4 I think. I have upgraded to 100 amp service. Beef connected to turkey
> at the Mast head.
>
Very common. #4 aluminum triplex is used for services up to 200A all the time
as long as the drop is not so long that there has to be another pole in the
middle to support the span. If the insulation looks cracked or has moss
growing on it you might ask the electric company to replace it. They should
at least send someone to look. What the will or won't do is up to them of
course.
> Any suggestions would be appreciated, I am a novice in poly phase, yet know
> the complexities of a seeming simple circuit.
>
> Thanks Much,
> Jim Mora
Good luck.
later
deano