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RE: Saturable Reactor Ballast from MOT's



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


Hello Finn,

Thank you.  I do like your drawing better!

For some reason, I did not get the copy of my own post so I actually sent it in again thinking it a worthy report. Hopefully, Chip caught that second one and deleted it. If not, please accept my apologies for the duplication.

As an update, we now think we can adequate explain the lower DC control voltage required when the 2 control windings were connected in parallel. This appears to be a function of the DC resistance of the windings. By placing the 2 series in parallel, we were essentially doubling the cross sectional area of the control winding and therefore lowering its DC resistance. There seems to be no reason that a series connection could not have been used instead to give a higher and wider control voltage range if that had been desirable.

We are now in the process of taking this project to its logical conclusion. We are adding 4 more MOT's to the mix prior to stress testing the system. Stress testing will be performed by using the 8-MOT reactor as ballast on a 10 kVA pole pig with the HV shorted and on a 90 kVA x-ray transformer to observe the effects of currents greater than 100 Amps on the system.

We will post the results of whether we can smoke 'em or not!

Carl




-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Friday, February 24, 2006 11:54 AM
To: tesla@xxxxxxxxxx
Subject: 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
>
>
>