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Saturable Reactor Ballast from MOT's
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