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RE: About MOTs..

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>


I've worked with aprroximately (20) MOT's.  All of my units require
current limiting.  I have heard(I think from this list) that some units
have shunts installed(ie a stack of metal strips between the primary and
secondary windings).  This type supposedly are current limited.  Beware
that I have two like this and they still need current limiting.

I test all my MOT's with 12VAC on the primary with the secondary open
and shorted.  This way I can calculate the characteristics of the
transformer before I use it.  It also means that I can calculate how
large a current limiting inductor I for it.

My MOT stack is (4) MOT's with the secondaries in series and grounded
between the middle two.  The primaries are in parallel, but the pairs on
either side of the secondary ground are anti-parallel.  This gives two
out of phase AC currents on the secondary side.  For this set up, I use
a home-made 15 mH inductor(gapped U-Cores so I can vary inductance)
which current limits at about 20 amps with the secondary shorted.  I
feed the inductor 120 VAC @ 20 amps.  The MOT's see about 90 VAC(I have
pushed to 100 VAC by increasing the gap).  On the MOT secondary side I
get (2) 3500 Volt out-of-phase legs and a whopping 250mA of available
current.  In my case, I run a DC TC set-up so this feeds a full-wave
bridge rectifier with filter caps, so my DC voltage winds up at 10 KV.
By using a home-made 30H charging reactor(again gapped U-cores), the
capacitor charges to 20 KV.  This produces easily 1.5 to 1.75 KW of
output power.  Power supply runs cool.  It is mounted on polycarbonate.
I have not submerged in oil.  You do need to disconnect the MOT
secondaries so that they are not connected to the core.

Last week, there was discussion about poor power factor for the MOT's.
I have not observed this.  In my case, the power factor is caused by the
15 mH inductor that I use for current limiting and does create a power
factor of about .75.  I know the current on the MOT secondary side
because I have a CT on the ground leg between the MOT secondaries.

If you don't current limit the MOT's the primary current will be
outrageous.  I have drawn 75 amps instaneously with two MOT's not
current limited powering a Jacob's Ladder.  Be careful if you try
experimenting in this area.  Whatever you are using for a contactor will
likely fuse and you'll be unable to break the circuit and circuit
breakers will start popping.


-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Friday, April 08, 2005 6:11 PM
To: tesla@xxxxxxxxxx
Subject: Re: About MOTs..

Original poster: "Mercurus2000" <mercurus2000@xxxxxxx>

Hmm, how can you tell when a MOT doesn't require current limiting? Adam
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, April 08, 2005 7:47 AM
Subject: RE: About MOTs..

> Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>
> Adam:
> The Primary side of the large MOT is not insulated well enough to
handle > the ~2000 volts that you will try to run it at. It will arc
to the > core, arc across the windings, etc. Most MOT's need current
limiting. > There are a few that don't. I think in your special
application you > will require it. I have built current limiting
inductors for my MOT's > and can give you guidance. My typical MOT
Primaries draw 50 to 75 amps > at 120 VAC with the secondaries shorted.
> > You would be better served to use the 1000 watt MOT alone and build
a > voltage multiplier for the HV side. You will still need current >
limiting for this configuration. > > If you are trying to build a
power supply for a TC I/We can point you to > a number of websites that
have used MOT's. > > Mark > > > -----Original Message----- > From:
Tesla list [mailto:tesla@xxxxxxxxxx] > Sent: Thursday, April 07, 2005
6:30 PM > To: tesla@xxxxxxxxxx > Subject: About MOTs.. > > >
Original poster: "Mercurus2000" <mercurus2000@xxxxxxx> > > This
question has probably been asked before, I want to connect the high >
> voltage output of a small like 600 watt MOT I have to the 120 side of
a > really large MOT like 1000w to get about 20-30KV out, I'll vacuum
pot > the > MOTs in oil and cut the center ground on the large MOT to
prevent > arcing, > does anyone see a problem with this? Will I have
to use some sort of > current limiting even tho I'm using a little 600
watt MOT for the > primary > transformer? > Adam > > > > > -- >
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