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Determining Inductance of a MOT Stack

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Kyle

Sun Feb 14 2010, 03:27AM

Registered Member #2325 Joined: Sat Aug 29 2009, 10:08PM
Location: Southern California
Posts: 11

I have an eight MOT stack and a nice little LCR meter. ;P How would I go about calculating the inductance (in henries) of the stackâ€™s secondary windings? Iâ€™m trying to find out exactly how much capacitance I would need to achieve near-perfect resonance on the output.
Thanks,
Kyle

radiotech

Sun Feb 14 2010, 05:25AM

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

By stack do you mean, hooking the secondaries of 8 mots in series, and then powering the 8 primaries in parallel of a single power line?

Russell Haley

Sun Feb 14 2010, 06:12AM

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

I'm confused as to the exact configuration of your MOT stack. Because they're designed to operate with the core and one end of the secondary and primary grounded, the insulation between primary and the inner end of the secondary is not so good. You can connect the secondaries in antiphase series, with the midpoint grounded, but more than that and you're pushing it. Oil is necessary but perhaps not sufficient.

Anyway, because the mains is a low impedance voltage source, you want to use the leakage inductance of the MOTs in your calculations. To measure it, short the two ends of the primary together and measure the inductance of the secondary winding.

Kyle

Sun Feb 14 2010, 07:01AM

Registered Member #2325 Joined: Sat Aug 29 2009, 10:08PM
Location: Southern California
Posts: 11

All eight MOTs are in series. There is two 120V phases that supply power to the whole setup. (Runs off of a 100A 240V dual-poled breaker) All of the transformers are submerged under mineral oil, unballasted, and everything runs fine. (No arc overs, and everything runs very cool under the oil.) The six transformers that have additional voltage stress are "modified", and have their secondary to core leads removed, as well as the secondaries being coated with additional epoxy. Plus, the lights dim in my house + my neighbors house when I draw an arc. =P
Here's a schematic I drew up quickly for reference:

Anyhow, will I need to measure the leakage inductance individually on each transformer? (I would hate to have to take the whole darn thing out of the oil) Or, could I treat the whole stack as one "big" transformer and short the inputs and measure the outputs?

Steve Conner

Sun Feb 14 2010, 06:25PM

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

wrote ...
could I treat the whole stack as one "big" transformer and short the inputs and measure the outputs?

Yes, that should give the right answer for your resonant capacitor value.

If it was scary before, the resonant capacitor will make it terrifying. :D

Kyle

Sun Feb 14 2010, 11:14PM

Registered Member #2325 Joined: Sat Aug 29 2009, 10:08PM
Location: Southern California
Posts: 11

Steve McConner wrote ...

wrote ...
could I treat the whole stack as one "big" transformer and short the inputs and measure the outputs?

Yes, that should give the right answer for your resonant capacitor value.

If it was scary before, the resonant capacitor will make it terrifying. :D

Alright, thanks :P - I'll be sure to get this on video once I have the capacitor bank setup. 5-6' Discharges = FTW.

radiotech

Sun Feb 14 2010, 11:17PM

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

My first step would be to decide on a rated current(s) in the primary and then short the entire secondary. Then, with a variac, find out what % of rated voltage gives you what current. btw, glass edison based fuses (plug fuses)give faster surer protection than your house breaker can.

Dimming, or flickering of the neighbors lights, especially with a load capable of casting inductive var's back is chancy.

Modern house wiring is stiff, if it is modern, but if old, esp knob and tube, unexpected things can result.

I have blown out every lamp in a house, all fuses, and even wiped out my TV set in an old k&t house i was renting. Every one of the lamps was blown in the same way-a blob of metal at one end of the filament. I was working on a 5kV power supply at the time.

Nowadays a power bump can destroy the processor in someone's electric stove, about $500. damage.

Steve Conner

Sun Feb 14 2010, 11:39PM

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

radiotech wrote ...

Dimming, or flickering of the neighbors lights, especially with a load capable of casting inductive var's back is chancy.
...
I have blown out every lamp in a house, all fuses, and even wiped out my TV set in an old k&t house i was renting. Every one of the lamps was blown in the same way-a blob of metal at one end of the filament. I was working on a 5kV power supply at the time.

Yes, the resonant cap will also increase the line current from scary to terrifying.

Tesla wrote in his Colorado Springs Notes about experiments he performed that made his line voltage increase and his lights get brighter instead of dimming. I guessed his apparatus was putting a capacitive load onto the power lines that tuned out some of the leakage inductance of the lines and transformers. Maybe that's how Radiotech blew his house up.

Or maybe the 5kv supply just shorted internally and dumped the 5kv filter caps back into the line.

The Edison based fuses are cute, but we don't have them in the UK. My house has the old rewireable fuses. Each fuse is a carrier with a piece of fuse wire inside, and when it blows you can pull it out of the panel and fit a new bit of fuse wire. Or in best 4hv.org style, a nail or .22 bullet

radiotech

Mon Feb 15 2010, 02:20AM

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

Thing tend to be bigger than .22 in the Colonies. Would this do?

HazzWold 1993

Mon Feb 15 2010, 09:21AM

Registered Member #2563 Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280

Look forward to a mean arc video of this :P

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