Re: MOT charging mysteries (was: Re: The RAT coil)

["Tesla list" <tesla-at-xxxxxxxxxx>]

Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net> 

Marco and all,

While there may be some microwave transformers that do not require
additional external ballasting, these appear to be quite rare (at least
in the USA). The reason why your particular MOT's do not appear to
require ballasting is actually _considerably_ more interesting! 

The MOT and voltage doubler circuit used in your system does not require
external ballasting since it uses 1 uF caps in series with the high
voltage MOT output legs (See http://www.saunalahti.fi/dncmrc/tank.html).
When the main gap fires, there's never a "true" short circuit placed
directly across your MOT's. In effect, your 1 uF caps are acting much
like a "capacitive" current ballast for your MOT supply, limiting the
short circuit current on the secondary side of the HV transformers!
While inductive ballasting is used on the primary side on most systems,
capacitive ballasting is certainly another viable option. Because of the
capacitive ballasting, it appears that you could actually use a pig with
this circuit WITHOUT the need for external inductive ballasting assuming
your diodes and caps were appropriately rated! 

Assuming the output voltage of your MOT's was about 3 kV RMS, the short
circuit current (shorting ACOUT1 to ACOUT2) is current limited by
capacitive reactance to less than 1 amp, or less than 6 KVA (reactive).
This implies a 25A worst case "load" at the 220 volt 50 Hz input. And,
because the short appears only very briefly when your gap fires, the
average loading will be much less, explaining why you are easily able to
run off a 10A branch circuit. 

Safe coilin' to you!

-- Bert --  
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> 
> Hi there!
> 
> Finn wrote:
> 
> >...the scope picture of the charging
> >waweforms show a odd flat portion which is unaccounted for, other than
> >that it is a MOT driven system. I am going to wind another porkchop to
> >see, if that changes the behaviour into something more readily
> >recognizable.
> 
> I am not suggesting here an explanation for the strange charging waveform
Finn
> measured. On the contrary, I would like to cancel some of the legends
that have
> recently arised about using MOTs.
> 
> 1. Using MOTs you CAN get charging behaviour like with any other
> current-limited
> transformer: check my scope picture at
> 
> http://www.saunalahti.fi/dncmrc/measure.html
> (second picture from up)
> 
> Now, ignore the half-wave rectification of the HV used for the charging
> (the MOT
> voltage doubler produces it, but that is another story). As you can clearly
> see,
> there is nothing special about a MOT supply being there instead of a pig
or an
> NST. There I used the RSG (not synchronous) described in
> 
> http://www.saunalahti.fi/dncmrc/rsg.html
> 
> 2. MOTs do NOT need special ballasting.
> I have been using for almost 2 years my old TC powered by two MOTs
> (primaries in
>  parallel, secondaries feeding a voltage doubler)
> connected as from
> 
> http://www.saunalahti.fi/dncmrc/tank.html
> 
> I had a 10A fused plug powering my TC and I added the third MOT as
ballast only
> because seldom the 10A fuse blew up. A 16A fuse didn't
> need any ballast. This means that 2 MOTs suck about 10A (-at- 230V). Here in
> Finland plain home wall plugs are fed through 10A fuses and
> you surely don't need a separate fuse for each microwave oven in your
kitchen!
> Are MOTs for the US market built differently (apart from
> the 110V rating)? I guess a US MOT can't suck more than 10A (-at-110V).
> All "european" MOTs I checked are HEAVY: no doubt about reduced iron core or
> things like that. The MOT weight do make almost one half
> of the overall microwave oven weight.
> I have never been using any PFC capacitors.
> Note also that my ballast MOT has the secondary shorted: that means that only
> its leakage inductance (pretty low) is acting as a ballast. Not really
> like e.g. a variac or a welding transformer.
> 
> Regards