Re: Saturable Reactor Ballast for TC from MOT's

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Perhaps Carl can report on this?  Anyway, there is
certainly a rather low limit to how much power a
single MOT pair will tolerate (unless you've got real
beefy MOTs).  Multiple pairs are definitely in order
for ballasting pole transformers!  'course, you could
always sink your MOTs in transformer oil and probably
squeeze a few more amps through them :)  I don't know
why cooling the windings should be such an issue
unless you're really trying to get away with something
(pumping 20, 30, etc. amps through a single MOT).

Alternatively, something I'd really like to see done
is using a relatively HV (e.g., 4160V-480V)
three-phase transformer for a SR:  Use the outer two
HV windings for the control (re-connecting them so the
flux cancels out in the center leg of the core), then
use the middle LV winding for the power winding.
Seems it ought to work, and I think Carl actually
tried this with a lower-voltage three-phase
transformer, but ended up needing too much control
current.  Still seems a 4160V transformer or something
in that neighborhood ought to solve that issue, but
where to find one cheap (and local)?

Regards,
Aaron, N7OE

--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: "Peter Terren"
> <pterren@xxxxxxxxxxxx>
>
> This sounds great but...
> Has anyone tested it at full power or calculated
> dissipation at full power. It strikes me that the
> losses will be effectively resistive (or rather
> heating) with a saturated core and that this
> power will go to heat in the MOT core (unlike a
> perfect inductor which will have no heating
> loss).  I am not clever enough to do the sums
> here but guess that the greatest dissipation
> would occur when there is equal power loss in the
> ballast and load (half power) ie 20 A 55V which
> would give a power loss of less than 1.1kW spread
> over 2 MOT core as the cores are not fully
> saturated. Added to that is the power of the control
> winding (?80V 1 amp).
> I guess that this is OK for a 2kW load but wonder
> about higher loads and perhaps this is why the 4
> MOT setup is proposed.What I am saying is that
> cooling the saturated cores may be an issue but
> is probably lesss so than cooling heat from the
> windings.
>
> Peter
> http://tesladownunder.com/
>
> ----- Original Message ----- From: "Tesla list"
> <tesla@xxxxxxxxxx>
> To: <tesla@xxxxxxxxxx>
> Sent: Saturday, February 25, 2006 1:55 AM
> Subject: Saturable Reactor Ballast for TC from MOT's
>
>
> 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
>
>
>
>
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