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Rewinding MOT

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LAN_403

klugesmith

Thu Oct 26 2017, 06:15PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

the_anomaly wrote ...
... I've measured the secondary output as 257.5 VAC at 2.01A when connected to the kiln... somethings not right here. Perhaps I should include the inductance and calculate the complex impedance since the kanthal is coiled like an inductor and has significant length.

I bet you will find the inductance of the heater coil to be totally negligible in a 60 Hz circuit.

The measured inductance of a transformer winding, all alone (on the closed core), isn't particularly relevant here -- if the transformer were ideal then that inductance would be infinite. If you are equipped to measure such inductances, it would be educational to see the change after inserting a thin spacer (e.g. a sheet of paper, or even aluminum foil) between the core's E-block and I-block. Then maybe if you make the mating surfaces more flat & smooth than they came from the factory, and omit the spacer, you can assemble the core to be better than when it was new. Like hot rodders and karters making engines work better than new.

Our breath is bated, until you resolve the huge discrepancy between heater resistance measurement and heater volt/amp measurement.

If the latter are correct, then your project power requirement is much less than I'd imagined. Not even close to the max from a 120 volt wall outlet. The cleanest solution would be to get a heavier-gauge, lower-resistance Kanthal coil, better suited to 120 volts over the length that's used in your kiln. Or (duh!) cut your Kanthal coil in half, and connect the sections in parallel. Then if you need 150 V instead of 120 V, rewind your MOT to be a 30 volt transformer & connect it as a booster. We can guide you to an easy solution where the transformer will hardly get warm, running all day. Or cut the heating sections shorter, to get the same current (but less heating power) at 120 V.

If you cut the heating coil into three sections, all in parallel, then the working voltage would be less than 120 V. Now controllable with solid-state dimmers, or various passive ballast devices in series.

Or consider Radiotech's solution -- 240 V with series diode. That reduces RMS voltage by a factor of sqrt(2), to 170 V RMS, and draws a DC current which is abusive to the power-pole transformer. Remedy: run the two halves of your heater on separate diodes, conducting in opposite phases of the power cycle. Multimeters whose AC modes aren't "true RMS" can give very misleading readings in circuits like that. In fact, so will typical True RMS meters, unless you measure V or I in both DC and AC modes, and then combine the results. You could back off from 170 V by adding ballast resistance in series, or using a buck transformer, or an appropriate solid-state dimmer.

If you end up using a boost or buck transformer, and want adjustable power without tying up your variac, construct the transformer secondary with multiple voltage taps.

Before turning your attention to heating things with the kiln, make sure the electric power connections are less ephemeral than alligator clips. As hinted in the schematic I posted above, there are subtle hazards. For example, what if boost/buck primary is not connected, while its low-voltage secondary remains in a series circuit with the power line and the load?

the_anomaly

Fri Oct 27 2017, 03:53PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

Connect a diode in series with it, and run the system at 240 volts.

You will get ~ 1000 watts of heat.

Despite measuring 26.7ohms of DC resistance, it's not drawing 1000W of power at 240 Vac. Hence the kanthal is not getting nearly hot enough.

No, it's not saturating at half an amp, it's saturating at an input voltage of 100v

Understood!

I believe I have solved my issue! My variac has a no load output adjustable from 0-148 Vac. With a couple amp load, the top end voltage drops to 140 V. I've been running the MOT at 140V in boost configuration to reach the ~260 V I wanted. Both primary and secondary get super hot in minutes. So this solution does not work.

As it currently is, with no load on the secondary, the primary gets very hot anyway. I measured it to 130 C after a few minutes and it still seems to be climbing. Thinking about the 100V where its saturating, it makes sense that the primary would get hot the current draw with no load on the secondary increases exponentially with voltage. I dropped the input voltage from 140 to 120 and the temperature of the primary drops to 90 C which I think is acceptable. This is how the transformer was designed to operate, at 120 Vac, and I've been pushing it well past this point. But now the output voltage is not nearly high enough...

After some further research and thinking, I decided to run two (120V) MOTs in series from 240 Vac mains. Each transformer got 30 turns of multi-strand 18 AWG (silicone insulation) wire for its secondary. I connected both secondaries in series and then configured the whole system as a boost transformer as shown:

Secondaries remain cool, primaries hold steady at 90 C, and I get 295 Vac output under load!!! This is perfect! Kanthal is getting much hotter and glowing much brighter. I'm going to add a fan to help the MOTs stay cool and will definetely change to crimp connections before I do any more work. Thanks all for the suggestions and discussion!

radiotech

Sat Oct 28 2017, 12:57AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

An accurate measure of power involving resistance can be had with a thermal ammeter.
These can be found cheaply. The other thing necessary is the actual resistance at
the temperature involved.

Since these values chase their tails, the only other thing is a THERMAL CONVERTER.

Which may be found in the Hobbs of Hell.

30 mV = 1 k W

1509152219 2463 FT180737 Thermal Converter I

1509152219 2463 FT180737 Thermal Converter Calibration Board

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