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Will this circuit provide 'clean' 240V 50Hz AC???

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Ash Small

Wed Sept 14 2011, 01:22PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Dr. Kilovolt wrote ...

What is the intended purpose of this device?

Edit: I just read your last post, so you want to power a microwave inverter with this? Why? I think it's not a good idea.

Thanks for asking, Dr K. I must admit this thread is 'a bit erratic' . I've had an extremely stressful week.

I shall now attempt to answer your query.

There is another recent thread on the subject of microwave inverter circuits, which explains that during mains power interruptions, etc, the control circuit 'shuts down' the inverter driver in order to 'protect' the IGBT from extreme transients.

I'm devising a circuit that will operate a magnetron, at reduced power, and with much greater control than is required for a microwave oven. I'm currently working through the a 'concept' in order to establish if it is a viable solution.

As you are no doubt aware, a microwave oven inverter powers the magnetron, intermittently, with a pulsed DC supply of around 30kHz.

My 'concept' involves running it with a continuous supply of between a few Hertz and a few MHz. It also involves exploring the possibility of using a single pulse with a period of several seconds or longer (continuous DC.)

This is still in the concept stage.

In order to achieve this I'd need a constant voltage AC supply that isn't interrupted by the control circuitry during any interruptions in the mains supply, hence the idea of using a 'constant voltage ferro-resonant transformer'.

As I require less power than a standard MOT produces, I felt the logical place to start investigating this concept was to consider the possibility of converting a standard MOT into a 1:1 ferro-resonant transformer.

Subsequent research that I have undertaken since my last post in this thread (Your own website was useful, as was Uzzor's, and Terry Fritz even supplied worked mathematical examples in his posts on the Pupman website) that the standard MOT secondary coil does, in fact, resonate with the standard MOC at mains frequency. (Due to manufacturing tolerances, etc, the resonant frequency is only 'approximately' mains frequency.

It would seem that, if I am able to fit , in my case, two windings, each of ~240 turns, along with a third 'resonant' winding onto an MOT, and use a suitable value resonant capacitor, This is, in fact, quite a feasible solution.

This is the basic concept so far. I'm sure it will require further 'tweaking'.

Thanks for your interest. Any comments will be appreciated.

Dr. Dark Current

Wed Sept 14 2011, 01:56PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Ash Small wrote ...

I'm devising a circuit that will operate a magnetron, at reduced power, and with much greater control than is required for a microwave oven. I'm currently working through the a 'concept' in order to establish if it is a viable solution.

What about using a standard MOT circuit, with reduced capacitor size to limit the maximum power (if needed) and control the primary winding with a simple TRIAC "dimmer" (phase angle controller) circuit? Excuse me, maybe I'm still not sure what exactly you are attempting.

The resonant MOT circuit will not transfer frequencies much different from mains frequency.

Ash Small

Wed Sept 14 2011, 05:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Dr. Kilovolt wrote ...

I think I understand your point. I 'may' have gone 'belt and braces' here', it's still a concept.

The ferro-resonant transformer is required purely to provide a 'clean' supply to the inverter switching circuit, for two reasons,

Firstly, I don't have to worry about control circuitry to cut off the gate driver during voltage irregularities, thus interrupting the output of the inverter.

Secondly, to 'ensure' that the inverter is 'always' operational.

It's quite possible that I don't actually require the inverter to 'always' be operational, but if I do, I consider this to be a solution.

(You can't increase the size of the smoothing capacitor that feeds the inverter, or the circuit won't work.)

I've not yet described the part of the circuit that controls the the magnetron itself. It appears I've left out some vital information.

I'm sure there are alternative ways to power this part of the circuit that I'm about to describe, what I've described until now is just one option and quite possibly isn't required, the point of the exercise was to devise a circuit that provided a 'clean' supply so that it's not necessary to interrupt the supply to the inverter, thus simplifying the control circuitry and ensuring no interruptions.

[/quote1316008745]
The resonant MOT circuit will not transfer frequencies much different from mains frequency.
[/quote1316014985]

Before I go any further, are you saying that the CVT won't supply current to a ~30kHz load?

If that is the case, I can see why., so scrap everything and start again.

Now onto the magnetron control circuit. (basically a separate circuit by itself, and it now looks like I'll have to find another way to drive it.).

Quite simple really, but I don't know that it will work.

A capacitor charged to (and maintained at) 4000V. (The idea was to use the inverter to maintan it at 4000V, using a voltage comparator or something to switch the gate drive circuit on when voltage drops, and off when 4000V is reached again.)

I then use a high voltage valve (vacuum tube) to drive the magnetron at much higher frequencies than the inverter can manage, up to the MHz range.

This enables me to control voltage, current, frequency, and pulse duration (PWM), giving much more control over the magnetron than the usual magnetron drive circuits.

It looks like I'll have to scrap the idea of using the CVT, and use another method to protect the IGBT from mains interruptions, though.

I hope this answers your question, and thanks for pointing out that the CVT won't pass 30kHz (if that is what you meant).

EDIT: I still think using the inverter to 'top up' the 4000V capacitor is a good idea, though. I know there are plenty of other capacitor charger circuits I could use. It's just that I have a few MOT's and an inverter 'lying around'. I'll look into it a bit more. Thanks.

radiotech

Thu Sept 15 2011, 12:50AM

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

The CVTs that I am familiar with are voltage regulators over a range of currents.
They have some properties of note.

(1) they are capable of delivering a constant current into a short circuit, this current being
a bit above their design maximum regulated current.
(2) they produce distortion of the input waveform depending on what portion of
the output curve thay operate at. Certain classes of CVTs are called harmonicaly
corrected by virtue of an another tuned circuit on the core.
(3) the output voltage is shifted in phase from the input voltage depending on
load.

property (1) makes any downstream fault protection, breakers of fuse incapable
of responding to bolted faults.
property (3) makes power circuits on the primary and secondary have varying
voltage differences.

Some experience with them showed me that under certain conditions, extremely
high and unpredictable voltages were on the secondary when subjected to high fast
switched loads, such as contacts closing and opening into capacitors )(in motor
systems)
For your problems , perhaps an isolation transformer with an electrostatic shield
between primary and secondary would help to keep high frequencies form crossing either
way due to capacitive coupling.

Use 2 MOTs. forget the secondaries, place the primaries on opposite sides on an E core with the leg/shunt
between. think of a NST, but both windings are 240 volt.

Ash Small

Fri Sept 16 2011, 06:29PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

radiotech wrote ...

Some experience with them showed me that under certain conditions, extremely
high and unpredictable voltages were on the secondary when subjected to high fast
switched loads, such as contacts closing and opening into capacitors )(in motor
systems)

So this rules out using a CVT to power a 30kHz IGBT switched inverter driver, as the transient voltage peaks would destroy the IGBT?

radiotech wrote ...

For your problems , perhaps an isolation transformer with an electrostatic shield
between primary and secondary would help to keep high frequencies form crossing either
way due to capacitive coupling.

Would the electrostatic shield(s) prevent 'all' EMI, etc from getting into the secondary?

Would I still need to protect the circuit from mains interruptions?

(One of the articles linked to in the other thread said that interruptions of, say, 1 1/2 cycles in the mains supply would also result in transients that would destroy the IGBT)

Thankyou for your advice, Radiotech.

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