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Re: Saturable Reactor Ballast from MOT's
Original poster: Finn Hammer <f-h@xxxx>
Carl,
Congratulations, this may be the most important discovery in quite some time.
I may be wrong, but from the schematic, it would
appear that you have the secondaries wired in
parallel pairs of opposing series, as you describe.
However, since the primaries are wired in pairs
of opposing parallel, it would appear to me, that
the effect is canseled, and you would in fact get
voltage on the secondaries/controll windings.
Therefore I suggest that the schematic does not
faithfully record the setup as you describe.
Perhaps this is more what is intended?
http://home5.inet.tele.dk/f-hammer/satur.jpeg
However, a very clever idea. I have never seen
anyone taking the controll winding out on 2 separate cores.
Cheers, Finn Hammer
Tesla list wrote:
Original poster: "Carl Litton" <Carl_Litton@xxxxxxxxxx>
In our research into different types of ballast
to control current demand on various projects,
we found that it is often useful to be able to
vary the current independently of the voltage if
a single power supply is to be used for multiple
projects with different V and I requirements. In
the process, we ran across the concept of the
Saturable Core Reactor. The idea is
simple. Introduction of a small variable DC
voltage into a separate winding on an iron frame
inductor will bring the core to saturation,
opposing the inductance of the power
winding. The closer to saturation the core
becomes, the lower the inductance of the reactor
and the larger the current that is allowed to
flow. We find this concept intriguing because
it offers infinitely variable control of large
currents by way of a low power control
circuit. We have conducted several experiments
on this subject and will publish a comprehensive
article when all of the data is in. However,
our most recent experimental configuration has
given such remarkable results that we find it
worthy of being reported separately.
One of the major drawbacks to creating a
saturable reactor from scratch is the
requirement that the control winding consist of
10-100 times the number of turns as the power
winding in order to permit control of the power
winding with low current DC. If the power and
control windings have the same number of turns,
then it will require 100 Amps in the control
winding to regulate 100 Amps in the power
winding. This is hardly efficient. With 10
times the number of turns, control of 100 Amps
would require only 10 Amps DC and with 100 times
the number of turns, only 1 Amp would be
necessary. The winding of several thousand
turns on a transformer is daunting to say the
least. We have therefore been looking into the
use of transformers with configurations that
would require the least amount of
modification. In the process, we have worked
with several core types: round, EI, figure 8,
etc. A recent post to the HV list by Aaron
Holmes suggested the possibility of using two separate transformers.
Having a huge supply of MOT's many of which are
identical in brand and model number, we decided
to test this concept. We are pleased to report a very successful result.
Two pairs of MOT's were selected. Each MOT was
of the older stouter design type, weighing
around 15 lbs. and possessing heavy gauge
primary windings. For each pair, the primaries
were wired together in parallel. The
secondaries were placed in series by connecting
the HV tab of each unit and connecting a wire to
the frame of each by means of a bolt run through
one of the mounting hotels in the frame. These
output wires were connected to the HV side of a
125:1 NST to which a DMM was connected to the LV
side. 0-145 VAC was introduced into the
parallel MOT primaries while monitoring the DMM
for voltage. If no voltage registered, the DMM
was moved to the HV side of the NST and the
procedure was repeated. A value of 30 Volts or
less indicated a successful series connection in
the 'opposing' sense and confirmed that the
transformers chosen were close enough to
identical to proceed. If the first test had
indicated significant high voltage output, one
pair of wires in the parallel primary connection
was swapped and the test repeated to confirm
that the seriesed secondaries no longer registered significant voltage.
Direct measurement of the inductance of the
paralleled primaries was then performed with an
ammeter in series with the input supply circuit
set at 35 VAC. The ammeter registered about ½
Amp, indicating a baseline inductive reactance
of around 60 Ohms. The ends of the seriesed
secondary circuit were the wires attached to the
frame of each transformer. This series forms
the DC control winding. These wires were
attached to the rectified output of a small
Variac. The introduction of 0-82 VDC into the
control caused the reading on the ammeter to
increase smoothly over the range to a final
value of 16.9 Amps. We did not push this
further due to the 20 Amp limitation of the
ammeter, but this corresponds to an inductive
reactance of slightly over 2 Ohms, making the
test a resounding success. With cooling, this
pair could reasonably be expected to handle 40
or 50 Amps as ballast and the other pair gave a very similar test result.
The question then became whether the two pairs
could be successfully paralleled for higher
current handling capability. To this end, shunt
wires were run to connect two sets of paralleled
primaries. Then, the two sets of seriesed
secondaries were connected in parallel with
respect to each other. A brief power test was
performed just to insure that no voltage was
induced into the control. At this point, the
inductance/saturation testing was repeated on
the combination of all 4 MOTS. The testing was
also very successful and the results very
similar to those from the tests of the
individual pairs with a couple of exceptions,
which are as follows. First, the baseline
reactance was reduced to about ½ of the value
measured on the individual pairs - 30 Ohms
instead of 60. This was to be expected pursuant
to the law of parallel inductors. Second and
more surprising, there was only required a total
of 28 VDC in the control to reduce this value to
2 Ohms. It would seem to follow that more pairs
could be added with a corresponding increase in
current capability and decrease in baseline
reactance. The high end reactance drop should
not resent a problem since the useful range of
inductive reactance for most of our project work is about 2-8 Ohms.
An admittedly poor but serviceable photo of the
4-MOT reactor stack has been placed here:
http://hvgroup.dawntreader.net/srmots.jpg
We'd love to repeat this experiment with a pair
of identical transformers removed from 5 or 10
kVA pole pigs, but alas, they are not a plentiful as MOT's around here.
Questions/comments are welcome.
Carl Litton
Memphis HV Group