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Driving a MOT

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Patrick

Thu Sept 30 2010, 12:27AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

you can basically have a digital bridge MOT, theres no problem with that in theory or functionally. however you WILL spend a lot more time, money, and sophistication for little or no gain. if you want this as a expensive complicated device, go for it. i think everyone else on this thread was trying to make you realise there were easier ways to do this. also, for a MOT, 60-200 Hz should be no big deal.

Thomas

Thu Sept 30 2010, 01:02AM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

Alright that sounds good. I'm in the process of designing the inverter.
The only reason why I want one is for my next surge generator and a rail gun power supply. I want the controllers to be fully digital and isolated from the user.
I just like the idea of having close-loop regulation, and the ability to set the voltage to anything with just a remote control.

plus I might use one for a masters program project, so it needs to be very digitized.

I brought this up because I tried making a switching supply and it failed because the core material was knocking out the frequency components in the square wave. I see a lot of this with peoples GDTs in their tesla coil bridges.

Patrick

Thu Sept 30 2010, 01:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Thomas wrote ...

I brought this up because I tried making a switching supply and it failed because the core material was knocking out the frequency components in the square wave. I see a lot of this with peoples GDTs in their tesla coil bridges.

frequency gain ,poles and zeros, frequency distortion, harmonic feed trough... have fun! making a squarewave output is the usual case, sine is possible but less common. you should read the TI/Unitrode pdfs before building your stuff.

Wolfram

Thu Sept 30 2010, 06:21AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

What voltage is your capacitor bank?

Anders M.

Steve Conner

Thu Sept 30 2010, 11:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't see any reason why that wouldn't work. MOTs are so undersized, they might well be happier at 100 or 200Hz than at 50 or 60.

Beware that if you intend to full wave rectify the output, the core will be live with 2kV because one end of the winding is connected to it. You can disconnect it, but the insulation isn't guaranteed to stand the resulting voltage. MOTs aren't meant for full wave rectification.

Proud Mary

Thu Sept 30 2010, 12:25PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

This effort delivers 1.8kV @ 2.4A continuously.

The transformer is an ordinary industrial yellow box mains isolation type. The 800V electrolytics are made by seriesing lower voltage types. The 120K load resistor prevents wild upward excursions of the output voltage under no load conditions, and also bleeds off the charge on the capacitors when the supply is switched off.

Not ripple free, and definitely not for the faint hearted!

Thomas

Thu Sept 30 2010, 11:58PM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

Steve McConner wrote ...

I don't see any reason why that wouldn't work. MOTs are so undersized, they might well be happier at 100 or 200Hz than at 50 or 60.

Beware that if you intend to full wave rectify the output, the core will be live with 2kV because one end of the winding is connected to it. You can disconnect it, but the insulation isn't guaranteed to stand the resulting voltage. MOTs aren't meant for full wave rectification.

Alright, I'll run the MOT at 100Hz and isolate it.
I think the MOT should be okay because I only plan to have the rectified output at around 1802Vdc max. The capacitor bank's max voltage is 1800Vdc.
The MOT's primary winding will be around 100Vrms. Not 100% sure though since I don't know the specs for my MOT (Vin:Vout ratio). I will play around with it first.

Wolfram

Fri Oct 01 2010, 10:41AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

If 1.5kV is enough, this

could be an elegant solution to the problem. These supplies are made for capacitor charging, and they have an adjustable charging voltage limit. No relation to the seller, but I've bought some of them myself.

Anders M.

Russell Haley

Sat Oct 02 2010, 08:02PM

Registered Member #2478 Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47

MOTs are 60 Hz transformers. You may encounter significant losses driving them with a square wave. My suggestion would be a PWM synthesized sine wave. The standard fullbridge gives you three voltage levels. I'm not familiar with the industry standard implementation of a three level inverter, but you could do something like this:

MtQ5w

If you call the signal from the zero cross detector POL, and the signal from the comparator PWM, one possible combinational function is driving left low side switch with (POL & PWM), the left high side with (POL & PWM)', the right low side with (POL' & PWM)', and the right high side with (POL' & PWM). This implementation gives both legs low for the zero voltage state. There probably exists an implementation that allows the use of transformer coupled gate drive by using the both legs high state to keep DC out of the transformers. I'm not certain what it is though. Perhaps XORing each leg's state with the output of a linear feedback shift register?

Edit: after some thought, the XOR with linear feedback shift resister idea wouldn't work. I'm still contemplating a solution, though.

Steve Conner

Sat Oct 02 2010, 08:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Jeez, that looks complicated.

Another possibility is to drive your MOT off the AC line with a phase angle controlled triac. That's basically 60Hz PWM, right? I built a triac/SCR driver a while back that accepts an 0-15V analog input to control the phase shift, so you could just stick that inside your feedback loop.

A PWM synthesised sine wave is usually done with some sort of DSP nowadays. Years ago I built an inverter that used a PIC to read the sine wave out of a lookup table, and sent it out of a DAC, where it modulated an ordinary old-fashioned PWM circuit.

There is no way to get the 60Hz component out of the gate drive so you can send it through GDTs, so you have to use isolated driver ICs like the IR21xx or TLP250.

Schematics for all of this stuff are on my website somewhere.

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