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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:


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