Saturable Reactor Ballast for TC from MOT's

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Carl Litton" <Carl_Litton@xxxxxxxxxx>

There have been several questions on this list 
recently, regarding variable inductance ballast 
for coiling.  What follows is the report of our 
successful construction of a saturable core 
reactor that allows regulation of current on a 
120 VAC circuit between 4 and 60 Amps with a low 
voltage and current DC control circuit.  The best 
part is that this has been built essentially 
without cost, utilizing 4 microwave oven transformers.

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

The schematic is here:

http://hvgroup.dawntreader.net/4motreactor.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