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Re: MOT current limiting (fwd)



Original poster: Tesla List Moderator <mod1-at-poodle.pupman-dot-com>



---------- Forwarded message ----------
Date: Tue, 5 Dec 2000 16:09:42 -0500
From: Jason Johnson <hvjjohnson13-at-hotmail-dot-com>
To: Tesla list <tesla-at-pupman-dot-com>
Subject: Re: MOT current limiting

I have five MOTs now (all US brands no fancy european stuff), and all of
them limit themselves just fine (relatively speaking). They will all draw
about 2-2.5 times their rated power when shorted and are damn near
indestructible, provided that you give them adequite cooling in an oil bath
(just like a small pig). The only thing you really need to do is multiply
their power capacity by three and buy a big enough breaker correspondingly.
Also you have to take into account that a 750 watt oven will OUTPUT 750
watts of microwave radiation. This does not mean that it will only draw 750
watts, or that the MOT is rated at 750 watts, the actual power is quite a
bit higher because of the magnetron's ineffieciency. I've got a 750 watt
output oven that says right on the sticker that it draws 12 amps at 120
volts. This means that the MOT actually handles 1440 VA (12*120) and the
oven has an efficiency of 52% (750/1440). This same MOT will handle around
2880 VA in tesla service and will draw up to 3600 VA when shorted. For some
people this is too much current, but I like power. The pig ballast IS very
interesting and would be a good idea to try on a 4800 volt pig or similar.
Also I would like to add that you can run high peak powers on low amp
breakers. I've blown a 30 amp fuse that was on a 15 amp breaker before
(actually several times)

Flame away,
Jason Johnson

----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, December 04, 2000 9:07 PM
Subject: Re: MOT charging mysteries (was: Re: The RAT coil)


> 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
>
>
>
>