MOT primary as hv inductor / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

MOT primary as hv inductor

1 2 next

Move Thread

LAN_403

Crunchy Frog

Thu Oct 15 2009, 12:42AM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

How much voltage can the primary of a MOT stand? (in air, not in oil)
Also, what's the effect on the inductance of removing the secondary / leaving it / shorting it?

Proud Mary

Thu Oct 15 2009, 04:39AM

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

Crunchy Frog wrote ...

How much voltage can the primary of a MOT stand? (in air, not in oil)

It's probably good to 720V RMS for European 240V mains types, half that for US types.

Crunchy Frog wrote ...

Also, what's the effect on the inductance of removing the secondary / leaving it / shorting it?

If you remove the secondary you will be left with a useful low-frequency choke.
If you leave it floating, it won't take much part in the electromagentic goings on.
If you short circuit the secondary, the transformer will heat up until it is destroyed, as it has no current limiting.

Dr. Slack

Thu Oct 15 2009, 07:37AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Do you want a choke for AC only (say the ballast for a conventional SGTC), or that will have a DC component (say the charging inductor in a DC design)?

If AC, then removing the secondary gives you a not particulaly useful low frequency choke. There is no air gap, so saturation will occur at low current, making very inefficient use of the iron.

If you leave it floating, then that's just a simpler way of "removing it", as it doesn't take much part in the electromagnetics goins on.

If you short circuit the secondary, now you are getting somewhere. The closed copper turns round part of the magnetic circuit in series with the primary resist magnetisation as if you had introduced an air-gap into the core. Now you can tolerate much more current before saturation, the inductance is correspondingly lower of course. This low indutance is too low for continuous operation at mains voltage, and it will get hot, but in 10s of seconds or minutes rather than seconds. So you can short a secondary and measure the primary current at mains voltage for long enough to let your meter stablise, I do this to match different MOTs. In Jacob's Ladder duty, as a primary ballast, it can run for minutes before overheating.

The shorted secondary trick only works for AC, the air-gap action introduced by the secondary depends on it being a transformer. Continuous DC bias on a MOT with shorted secondary will saturate. For an inductor for use with DC components (and even for an AC inductor, because it reduces losses and makes it adjustable) grind off the welds that hold the Is to the Es of the core, and introduce a real airgap. Now it's adjustable, you can set the inductance to what you like, and without the extra losses in the secondary.

Remember that a MOT has been designed to work reliably for years when connected to dirty domestic mains. Dirty in this sense means with occaisional 1500V spikes, and all domestic equipment, or at least its input transformers, is subjected to a flash test of at least this. I am happy about putting up to 2kV terminal to ground on any equipment that is mains rated, as long as it stays within other limitations of the equipment. I've not had a failure yet. Even though US mains is only half the working RMS of the rest of the world, the lines are still as dirty with the same sort of spike level expected, so I'd use that overvoltage for both US and ROW equipment. Terminal to terminal is a little different, you are reliant on the wire insulation alone, rather than the relatively heavy bobbin insulation. Standard stuff is rated at 50v between wires, but I've yet to find undamaged enamelled copper wire that doesn't withstand 250v rms between wires. Look at the primary, and if it's well wound, progressing from one terminal to the other without crossing wires, then count the layers and multiply by 50v per layer if you're ultra-conservative, and 250v per layer if you're feeling lucky.

Steve Conner

Thu Oct 15 2009, 08:57AM

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

I once made a ballast choke by cutting a MOT apart, removing the secondary, and clamping the core back together with a 1/8" piece of plexiglass jammed in to act as an airgap. The inductance was 32mH.

If I had used the secondary instead, the inductance would have been about 3H, much more useful for HV applications. It was a 230V MOT with turns ratio of about 10:1.

Proud Mary

Thu Oct 15 2009, 11:00AM

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

Steve McConner wrote ...

I once made a ballast choke by cutting a MOT apart, removing the secondary, and clamping the core back together with a 1/8" piece of plexiglass jammed in to act as an airgap. The inductance was 32mH.

If I had used the secondary instead, the inductance would have been about 3H, much more useful for HV applications. It was a 230V MOT with turns ratio of about 10:1.

32mH could be very useful as a filament choke in directly heated thermionic applications.

Crunchy Frog

Thu Oct 15 2009, 11:41AM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

I was planning on using it as the inductor in a DC resonant power supply. It would be fed (depending on the design I use) either 5kv dc or dc pulsing between 5-6kv and 10-12kv. So it would need to be in the low H range.

Steve Conner

Thu Oct 15 2009, 12:15PM

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

Should work fine if you use the secondary, leave the primary open, and cut the core to introduce an air gap like I did. If one MOT prepared like this doesn't work, use several in series.

I've seen quite a few DC resonant Tesla coils built like this, and they typically needed about 3 MOTs in series. The ones I saw had the I parts of the cores cut off completely.

Crunchy Frog

Thu Oct 15 2009, 12:23PM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

What's the purpose of the air gap, anyway?

Proud Mary

Thu Oct 15 2009, 02:44PM

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

The usual answer to this question is to say that the air gap is there to prevent core saturation - the state where increasing the current through primary winding causes no further increase in the magnetic field* - but while this is true in certain cases, it is not as generally true as is often thought.

It is more useful to say the presence of the gap reduces permeability and inductance, and so increases the magnetizing current in the primary.

In real world transformer design, it is quite common for transformer manufacturers to use a very narrow variable (at the point of manufacture) gap as a way of tweaking the primary inductance, so that all the transformers in a production run can be brought into spec.

In the case of the Line Output Transformer ("LOPT")- the "flyback" which occupies so many pages of 4HV.org - the role of the air gap is completely different to that of a regular transformer. The "flyback" is a special case of a choke overwound with a high voltage secondary. In this case - unlike the regular iron transformer - the effect of the gap is to see that the residual flux value is closer to zero, which allows a wider working flux density range, and, paradoxically to store most of th field energy.

Strange to relate, almost all of the the field energy in the 'flyback' is stored in that narrow gap of air, rather than in the core material, where you'd expect it to be, but we can leave explaining this interesting oddity for a bit further down the road.

*Once a transformer core is saturated, any increase in the primary current is simply dissipated as heated - i.e. the excess energy beyond saturation must go somewehere and do something, and the easiest thing for it to do is heat up the core, which it does. (Easiest? Yes, never forget that electricity and magnetism will always take the line of least resistance, what is most pasimonious and most directly leads to the equilibrium which is craves. It's a good thing to write Energy Can Neither Be Created Nor Destroyed on your shaving mirror. It will save you from a million misunderstandings, and expose the sometimes quite persuasive arguments of 'free energy' freaks and makers of 'perpetual motion' machines.) Electronics, in my view, is an attempt to overcome the natural laziness of electrons by forcing upon them decisions which they cannot refuse!

I often have a go at answering this sort of question, because it helps me to clarify my own understanding, but it's always a bit difficult if we don't quite know the questioner's level - beginner, intermediate, advanced etc. So if what I have written is either too simple for too difficult, don't be afraid to ask again when you've had a think.

Dr. Slack

Thu Oct 15 2009, 06:45PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

What's the purpose of the air gap, anyway?

It seems paradoxical doesn't it. You spend all that money on carefully engineered low remnance alloy steel laminations, then put an air gap in. However remember that a transformer is not an inductor. What's good for the first (ideally no airgap) is absolutely not good for the second (an appropriately sized airgap).

One thing an inductor must do above all else is to store energy (0.5LI^2), just like a capacitor does (0.5CV^2). The problem is, iron is useless at storing energy, air is much much better. So why not use all air? Unfortunately, to get the inductance you need lots of turns of copper, and as it's lossy you need a large area wire. Once you've got all that, it's physically a long way round the windings for the magnetic field, so the H field (amps * turns / length of magnetic path) is very low, and so is the B field. What the clever inductor manufacturer does is to put iron on 95% or so of that path, to concentrate the H field in a few mm of gap. The air stores the energy, the iron matches the size and shape of the copper winding to the size and shape of the practical airgap.

What happens in an all-iron inductor? You get a large B field for a small H field, so in order to keep B below saturation, H must stay small. As energy is proprotional to the BH product, you store little energy beaucse of low H. In an air inductor, you have large H but small B, same problem with the product. Only in a gapped iron inductor do you get large H *and* B.

Think of it this way. If you wanted to make a mechanical energy store, would you use non-stretchy polypropylene rope (which makes an excellent medium for transforming energy to a different force/distance ratio via a pulley system, just like a transformer changes the volts/amps ratio), or would you use a stretchy rubber rope? Here, energy stored is proportional to force * distance. Polyprop rope minimises the energy stored (little bounce back in your pulleys). If you found that 1m of rubber rope had the right stretch for your energy storage application, but you had 10m between your fixing points, then it would be better to make up the 9m difference with some rope, rather than to use 10m of rubber (like an air-cored inductor which is too stretchy) or 10m of rope (like an iron cored inductor which is not stretchy enough). And the metaphor/model is still going strong, even with all that stretching!

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob