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MOT for Tesla Coil and Magnetic Shunts

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LAN_403

thedatastream

Fri Aug 31 2007, 03:33PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

I'm going to build a small SISG coil using a single MOT to power it (Terry's Pirahna design) and I was wondering whether or not to de-shunt my MOT.

I've looked on the web (unfortunately Pupman is blocked on the work computer) and what I can find suggests that removing these increases potential output current by removing the partial flux path formed by these bits of iron.

Anyone have any idea what sort of current / voltage output increase can be gained by de-shunting a MOT?
Is this conventional practice for MOTs used for Tesla Coils?

Thanks
James

CT2

Fri Aug 31 2007, 06:48PM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

I don't think you should remove the shunts. A mot will run untill it burns itself up/blows fuse. So I don't think you can squeez any more power out without killing it.

Garry Neeley

Fri Aug 31 2007, 09:36PM

Registered Member #976 Joined: Thu Aug 30 2007, 05:20PM
Location: Huntsville, Al
Posts: 6

CT2 is right. A MOT is not a current limited transformer. If you short the secondary on a MOT it will continue to draw more and more current until the breaker flips. I would suggest you leave the shunts in place as well.

sparky

Sat Sept 01 2007, 02:46AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

No ,Garry
MOTs are current limited by nature. I've done tests to prove that a 1000 watt MOT will draw 1700 watts when run in a short circuit operation. Remove those shunts and the transformer will burn up - and the
secondary voltage will climb radically.
So if you are wanting to run higher power you might check the local repair shop and see if you can get one of those 2500 watt - 1900V rated industral MOTs. These guys weigh in at 20 lbs and can short over 3 amps! They do run 240V....

thedatastream

Sat Sept 01 2007, 08:07AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Now I'm at home, I've found some Pupman stuff on this subject

- OK to remove shunts so long as you ballast

- OK if ballasted

- OK if ballasted

The question is, do the 100R series resistors in the secondary of Terry's Pirahna

provide enough ballasting for me to remove the shunts?

Thanks
James

Steve Conner

Sat Sept 01 2007, 09:15AM

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

Hi guys,

I believe the shunts are there to improve the power factor and deliver a less peaky current waveform to the magnetron. When I was simulating the supply for my Tesla-2, the power factor was much better with the shunts than without. It delivered plenty of power even with the shunts: I was blowing 5 amp fuses on the 240V line when I got my bigger tank capacitors.

In newer MOTs, I doubt they are big enough to ballast the transformer into a short circuit. The cross-sectional area of them is much smaller than the cross-section of the core, so they are bound to saturate quite early on.

Older MOTs, and industrial MOTs, were built much more heavily than the ones you get at the kerbside nowadays, and may have had more generous shunting. Sparky probably tested one of these. If nobody believes me I will test some modern MOTs for you and post my own results here.

As for the 100 ohm resistors, ask Terry

sparky

Sat Sept 01 2007, 05:05PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

If you want large easily obtained MOTs, try looking for old 1980's early 90's Kenmore, Sanyo or Amana microwaves (800-1000watt units) The MOT units that where placed in these guys weigh about 10-12 lbs and can short up to 23A with shunts! Secondary voltage tends to be 2300-2400V AC.

Terry Fritz

Sun Sept 02 2007, 04:15AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

PIRANHA uses standard MOTs "as is" without modification. It is a complex voltage tripling resonant thing where the circuit "plays" with the shunt's limiting... Maybe a better circuit could be made without shunts... but for now. leave the shunt in for sure

The value of the resistors is indeed "100 ohms"... Lots of modeling there... Those resisters prevent shorting of the whole gap and all... I did a lot of work on that since it killed my coil at first... So until some "Einstein" comes along to knock me out... The resistors need to be 100 ohms...

Cheers,

Terry

thedatastream

Sun Sept 02 2007, 08:45AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Thanks Terry, I'll definitely take your advice!
FWIW, my resistors are 101.1 ohms (oh no it won't work!)

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