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Magnetizing current?

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Dago

Sun Dec 23 2007, 03:42PM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

I have never really grasped the concept of magnetizing current. What is it and how can I reduce it in an SSTC?

uzzors2k

Sun Dec 23 2007, 04:13PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

A TC primary is a transformer primary and an inductor, so in addition to the current which the secondary pulls some current will travel through it as though it were a stand-alone inductor. To reduce this current one simply increases primary inductance. There's not much more to it than that. See Richie's page for more info on it ->

While on the topic, is there any reason not to go for maximum primary inductance in a regular transformer?

Finn Hammer

Sun Dec 23 2007, 04:24PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Uzzors wrote ...

While on the topic, is there any reason not to go for maximum primary inductance in a regular transformer?

Absolutely.
Maximum inductance would be acheived by filling the whole winding window with extremely thin wire
First trade off is to reduce the winding to fill half the window, next is to increase wire thickness.

You increase thickness untill you have *just* enough inductance to keep the transformer from drawing excessive current, when it is connected to the power source with unloaded secondary.

Cheers, Finn Hammer

Marko

Sun Dec 23 2007, 04:35PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

A TC primary is a transformer primary and an inductor, so in addition to the current which the secondary pulls some current will travel through it as though it were a stand-alone inductor. To reduce this current one simply increases primary inductance. There's not much more to it than that. See Richie's page for more info on it ->

More primary turns will mean lower stepup ratio and less power transferred to secondary, and too few will result in too high magnetizing current.

If we increase coupling larger primary inductance can be used for same power throughput but flashover usually puts a limit there.

Lowest current that can ever flow in your primary is idle current, which is U/2pifL of your primary - when secondary is coupled, it will always be higher.

At full load circulating current in your primary circuit can be several times larger thanDC input current in this case.

So take care in designing your primary properly.

All these problems could be eliminated by use of a ferrite transformer. But that's something else now.

Marko

Steve Ward

Sun Dec 23 2007, 06:32PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

All these problems could be eliminated by use of a ferrite transformer. But that's something else now.

Re-read what Finn said about designing a *practical* transformer. You will still have a healthy amount of magnetizing current present. I really dont see a problem with the way SSTCs are designed typically right now. Adding a huge ferrite transformer may very well cost more money than it saves in silicon costs. Also, you would need a fairly high output voltage (assuming you are just base feeding) which would be difficult to design without lowering the coupling of the transformer and putting you back in the same place you started without the transformer :P.

Marko

Sun Dec 23 2007, 06:39PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I really don't see a problem with the way SSTCs are designed typically right now.

Why then nobody made some of those wanted multi-kilowatt audio modulated SSTC sparks when it is so easy?

Surely in most cases transformer is not worth effort over just buying bigger silicon and taking the magnetizing current. As I said, it's another story.

Steve Conner

Sun Dec 23 2007, 07:25PM

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

I think Steve Ward already made a multi-kilowatt audio modulated SSTC.

...

Mon Dec 24 2007, 03:09AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

It is quite easy to make a multi-kilowatt sstc, you just use big fets and live with the high currents (increasing the voltage into the bridge also helps a lot). See my SSTC

for example. I have since added more primary turns to reduce the power to about 1500w to be able to run withough blowing breakers.

Marko

Mon Dec 24 2007, 12:49PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'm talking about direct audiomodulation (PWM or FM) to CW SSTC's, not DRSSTC's.

I don't remember anyone having gotten out of range of few hundred watts and realtone audio modulated arcs bigger than like 10cm. Not DRSSTC's.

I assumed this is simply because of large losses from continuous hard switching.

I'd be happy to see 2 foot singing streamers from a CW SSTC right now.

Anyway, I believe use of ferrite transformer would allow much easier hard-switching and thus greatly decrease losses on the silicon. It's expensive but only solution to the problem I see.

Why am I met with such skepticism on that part?

Didn't want to get too much OT, sorry..

Marko

...

Mon Dec 24 2007, 04:55PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The coil linked in the previous post uses quasi-fm modulation to produce 'hi-fi' audio on the output. Unfortunately the output is still only about 6" due to problems with the streamers not propagating properly. The are just extremely hot, thick, and coiled up. If I bring my sound meter anywhere close to the coil it pegs at like 120db presumably due to interfearence, but the audio was loud enough that it could be easliy be heard in a room with 35 people chatting and playing video games in the corner.

That was running at about 1500w with a 5ms exposure.

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