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Registered Member #19
Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168
Hello all,
I need a power supply capable of generating ~280-300V ac/dc at least 3amps to power a small kiln I built. Since I'm on a bit of a budget, I tried rewinding a MOT I have. I think the primary is roughly 120turns so I made the secondary about 240turns of 20awg wire. I power it from a variac. It works fine with nothing connected but as soon as I put a load on it the output drops considerably. 150vac in gives about 190vac out. The kiln uses kanthal wire, approx 26.7ohms resistance. Any ideas what I might be doing wrong?
Also, any other ideas to generate this voltage aside from using a MOT? I have 120/240vac as input. With 240 it just does not quite get hot enough hence the desire to bump the voltage up a bit
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Are the magnetic shunts still in place? Little stacks of laminations that span the core window between primary and secondary winding places. They give MOT's a current-limiting behavior similar to that of a NST. Looking more closely, they create a deliberately large leakage inductance -- unless removed.
Your application sounds like a good use for a buck/boost transformer. That'll be much more efficient, size and weight-wise, since you only need to transform the power associated with the amount that voltage is boosted. It'll cost less if you use a regular power transformer instead of one billed as a buck/boost transformer.
For example, suppose you had a 240 to 48 volt transformer with a 3 amp rating. 144 VA (0.144 kVA) will give you an idea of the size and weight. Connect primary to your 240 volt source. Put the secondary in series with the hot side of your 240 V source, to get 288 or 192 volts (depending on which way you connect it) at an honest 3 amps. You can probably get one brand new for less than $50. Transformers with 24 V output might be easier to scrounge, and if one doesn't boost your voltage enough for satisfaction, add another one.
Or reduce the number of turns in your rewound MOT secondary to, for example, 60 volts worth. The voltage sag (in percent) at a given output current would be about like the sag (in percent) of previous winding at 1/4 as much current. Suppose you run that from a small variac, and use it to buck or boost an unvarying 240 volt supply. After adding a 4-terminal reversing switch, you could have a low range (180 to 240 V) and a high range (240 to 300 V). Neither MOT nor variac would be subjected to more than 180 volt-amps, if you keep the MOT primary voltage below its nominal value. (they are designed to run very close to saturation, and need forced-air cooling even with no load).
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Judging by the original post, the starting voltage is 120, not 240.
The Boost transformer is a good idea though, would only have to be half the size, and if you can find a 120-120V isolation transformer then it would do the job perfectly of doubling the voltage.
ESP gives some diagrams and explanation here:
Another option if you want DC is to make a voltage doubler rectifier (like you find in old non-PFC PC PSUs). Would need 2 decent electrolytic caps and a couple of diodes, and would give you ~325V DC.
Registered Member #19
Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168
Are the magnetic shunts still in place? Little stacks of laminations that span the core window between primary and secondary winding places. They give MOT's a current-limiting behavior similar to that of a NST. Looking more closely, they create a deliberately large leakage inductance -- unless removed.
I thought they were just to take up some interstitial space so I pulled them out and tossed them. I suppose that was not a bad move?
I'll try the buck/boost out today. Looks like the same principle used in an ignition coil.
Another option if you want DC is to make a voltage doubler rectifier (like you find in old non-PFC PC PSUs). Would need 2 decent electrolytic caps and a couple of diodes, and would give you ~325V DC.
I tried that but I used film caps (15nF) and with any significant load the output dropped dramatically.. I thought maybe I needed more capacitance but all the resources I could find only talked about number of stages vs. final voltage.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
I think some readers aren't getting it. OP says 240 V is available but he wants to get 280 V, with at least 3 amps to a resistive load. Something is amiss, because the stated resistance of the Kanthal heating element is much too low to draw only 3 amps. Yes, the wire resistance will be higher when hot, but only by five percent at 1400 °C.
The point is, he can do that with a transformer that delivers just 40 volts at 3 amps, with either 120 or 240 volt primary. Connected properly, it would be called a boost transformer. It only needs to be rated for 1/7 of the load power. The other 6/7 of the load power comes from the 240 volt wallplug without any transforming.
Buck and boost transformers have no semiconductors, and nothing but 60 Hz sinusoidal voltages. Not much in common with the switch-mode (DC to DC) topologies called buck and boost converters, whose wire-wound components are single-coil inductors on ferrite cores.
Registered Member #19
Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168
OP says 240 V is available but he wants to get 280 V, with at least 3 amps to a resistive load. Something is amiss, because the stated resistance of the Kanthal heating element is much too low to draw only 3 amps. Yes, the wire resistance will be higher when hot, but only by five percent at 1400 °C.
My mistake, I was typing this out after a long day. I should expect a few more amps draw at 280 V.
The boosted option worked! I managed to get 260Vac and the kanthal wires were glowing bright! But now the transformer gets really hot. I shut it off when I measured the secondary windings at 140C. I cut a coil form out of a block of HDPE and although I could not smell anything, I believe I could hear it melting..
Here are some pics of my setup:
The cable on top is from my mig welder. I was about to weld it back together in this pic.
Here is the whole system:
I was a bit worried that using alligator clips would cause conduction problems from limited surface contact but it seemed to work ok.
Finally the kiln:
The bottom coil is not connected.
Any thoughts on the heat issue? Perhaps I should rewind the secondary so that under load it only produces 40 V. With less windings I would expect less I2R losses but the secondary is only about 1.8ohms and the inductance is 197mH..
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Nice work there!
Are you using the original MOT primary at its nominal voltage or higher? How hot does the transformer get, if you run it for half an hour with no load on the secondary?
It might be instructional to measure and chart the no-load primary current with respect to the primary voltage, as you ramp up the voltage with your variac. Typically it curves up sharply (hockey stick shape) when the core begins to saturate. With ordinary power transformers, that "knee" appears at some voltage higher than nominal. With MOT's, you are well into the curve when you reach nominal primary voltage. Not because it's useful in the cooker application, but to save core weight and cost. They can get away with it because there is a noisy fan blowing air over the transformer whenever it's energized.
On this forum, people have recommended extending the primary winding with 10% to 20% extra turns. That reduces the volts per turn, so the core is not so close to saturation. Of course the extra primary turns leave less room for secondary turns. And they require more secondary turns just to get the same output voltage.
[edit] The current in that brightly glowing heater coil is around 10 amps, right? What does that work out to, for the I-squared-R power loss in your secondary winding? What's the current density in amperes per square mm of copper wire, or per square mm of bobbin space? Rewinding with fewer turns, for lower voltage, brings a double benefit. Less resistance because of fewer turns, and less resistance because you can use thicker wire and still have it all fit through the core. And less current in the primary winding, for the same amount of secondary current.
Is your only stock of magnet wire the size we see in the pictures? 20 AWG? Suppose you took the same length of the same wire, and cut it in half. Not the long way. Put the two halves side by side, touching, and wind them as if they were a single wire. That's called bifilar winding. The bobbin will fill up just as much as the original. But if the two strands are connected in parallel, you now have half as many turns as the original, with wire that has twice the cross-sectional area. Resistance is 1/4 of the original resistance. Another way to look at it: current density in A/mm2 is halved, which reduces the copper-loss heating (watts per cm^3? Watts per pound?) by a factor of four. The same arithmetic extends to multifilar windings (more than two strands traveling together), or to windings with heavier gauge wire.
Registered Member #19
Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168
I took a bunch of data as suggested. Interestingly I am getting much lower current values than expected. I do trust my current meter although non of my measuring instruments are calibrated. Just to be sure tomorrow Ill measure current through a known resistance as a sanity check.
I definitely see the knee in the non loaded output. So I see it begins to saturate just past half an amp.
I'm not opposed to trying a bifilar winding but for ease of construction I would probably just order a larger gauge like 16.. plus I've maxed the coil form so that a few more turns and I could not fit it in the 'E'. 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 found a bunch of old 250V 680uF caps so I decided to give the voltage doubler rectifier a shot. Interestingly it increased the voltage significantly but there was almost no current draw, even at high voltages +100DC. However the input current draw was comparable to the MOT. I thought maybe the output impedance might have been high so I assembled a full wave rectifier and cap but I got the same result. Almost no current draw at a high DC voltage. I thought ac/dc would not matter to kanthal but perhaps it does?! Looks like I'll be sticking with ac going forward.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
No, it's not saturating at half an amp, it's saturating at an input voltage of 100v, even though they are the same point on the graph. It's important to think the right way round for transformers. It's the volts that control saturation, even if it's the amps that do the magnetising.
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Rewinding MOT
Put the two halves side by side, touching, and wind them as if they were a single wire. That's called bifilar winding. The bobbin will fill up just as much as the original. But if the two strands are connected in parallel, you now have half as many turns as the original, with wire that has twice the cross-sectional area. Resistance is 1/4 of the original resistance. Another way to look at it: current density in A/mm2 is halved, which reduces the copper-loss heating (watts per cm^3? Watts per pound?) by a factor of four.
