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MOT Testing

Original poster: "Paul B. Brodie" <pbbrodie@xxxxxxxxxxxxx>

Mark,
OK, I have tested a couple of the MOT's as you suggested. The first one is one of the smaller ones. I connected my variac to the primary and at 10 V in I got 200 V out, at 20 V in I got 400 V out, and at 30 V in I got 600 out. So I feel like I can be sure it has a 1 to 20 turns ratio. At 120 V in, this MOT should have 2400 V out. BTW, I have a "true RMS" DMM. When I first checked this MOT, I connected it in reverse in order to deal with much lower and safer voltages. Checking it this way with power going in to the secondary, it checked as if it has a 25 to 1 turns ratio. I put 100 V on the secondary and got 4 V on the primary. I checked again with 50 V and got 2 V out. I then checked with 25 V and got 1 V out. I don't understand why I get different results when checking it in reverse. Anybody have an explanation for this?

I then checked the large MOT that is labeled 4000 Vac. At 10 V in I get 185 V out, 30 V in 550 V out. This gives a turns ratio of approximately .055. So 120 V / .055 = 2180 Vac out. I measured the impedance of the primary by shorting the secondary and measuring the amps on the primary while feeding first 10 volts and then 20 volts. At 10 V I measured app. 2.1 A and at 20 V I measured app. 4.2 A. Z=V/I and Z=20/4.2 or Z=4.76 ohms. With no current limiting and a short on the secondary it should pull 120V/4.76ohms=25.2 A. So evidently it is at least current limited a little. It shouldn't require too much current limiting to get it down to a more workable 15A.

I'm somewhat fuzzy on current limiting. How do I determine the size of my current limiting inductor? I guess using resistance to limit the current would be highly inefficient and generate too much heat?

If any of my math is wrong or if I'm using incorrect equations, will you please straighten me out? As if it is possible to straighten me out!!! {:-)
Thanks.
Paul
Think Positive
PS Is it possible to have too many MOT's??!!




----- Original Message -----
From: "Tesla list" <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
To: <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
Sent: Friday, April 22, 2005 10:02 AM
Subject: RE: Expensive hobby

> Original poster: "Mark Dunn" <<mailto:mdunn@xxxxxxxxxxxx>mdunn@xxxxxxxxxxxx>
>
>
> Paul:
>
> Short the Secondary and put 5 - 10 VAC @ 60 HZ on the primary. Make
> sure your variac or power supply is good for min 5 amps for this test(Z
> will likely be 3 to 4 Ohms so at 10 VAC you will draw 2.5 to 3.5 amps).
> If not go with even lower voltage or put a resistor in series to drop
> the voltage down. You can work in the mV range and still get good
> results. Then measure current and voltage across the primary and you
> can compute impedence as Z=V/I. You can then choose your current
> limiting based on this.
>
> If you want to do full tranformer analysis, you can measure secondary
> current as well. Then you need to repeat the testing with the secondary
> open(Obviously, sec I = 0) for this. >From all this data you can figure
> leakage inductance, k, etc.
>
> I have approx 20 MOT's and none are current limited by the shunts even
> though the shunts are in place. I have tested MOT's momemtarily across
> 120 volt mains. They will pull 30 to 40 amps easy with the secondary
> shorted. If you try be prepared to weld the breaker contacts or weld
> the plug into the wall.
>
> Mark
>
>
>
> Original poster: "Paul B. Brodie" <<mailto:pbbrodie@xxxxxxxxxxxxx>pbbrodie@xxxxxxxxxxxxx>
>
> Mark,
> Thanks for the info. How are you going about measuring the impedance of
> the
> primary? Do your MOT's have shunts? All of mine do. If so, are there
> just
> not enough to limit current like the NST's do? Thanks.
> Paul
> Think Positive
>
>
>
> ----- Original Message -----
> From: "Tesla list" <<<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> To: <<<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> Sent: Thursday, April 21, 2005 1:56 PM
> Subject: RE: Expensive hobby
>
> > Original poster: "Mark Dunn"
> <<<mailto:mdunn@xxxxxxxxxxxx>mdunn@xxxxxxxxxxxx>mailto:mdunn@xxxxxxxxxxxx>mdunn@xxxxxxxxxxxx>
> >
> >
> > Paul:
> >
> > I have many MOT's. A number are marked 4000 Volt. They are not.
> All > of my MOT's have a ratio between 16:1 and 20:1. Most of my MOT's
> have > an impedence with the secondary shorted of about 3 to 4 Ohms.
> Thus they > will pull 30 to 40 amps from 120 Volt mains with the
> secondary shorted > -Don't try that. You test at 10 VAC with the
> secondary open I have done > many times to verify ratio. Note you will
> be reading 160 to 200 VAC on > the open secondary. Hook up meter
> before applying power so you avoid > shock risk. > > Mark > > >
> Original poster: "Paul B. Brodie"
> <<<mailto:pbbrodie@xxxxxxxxxxxxx>pbbrodie@xxxxxxxxxxxxx>mailto:pbbrodie@xxxxxxxxxxxxx>pbbrodie@xxxxxxxxxxxxx>
> >
> > Mike,
> > I doubt it because this MOT is substantially larger than the other
> coils > > and it has a lot more turns on the secondary. Also, the 4000
> V is > labeled > right on the transformer with the manufacturer's
> data. Since the > manufacturer doesn't know how the end user is going
> to wire the > transformer, they wouldn't put the 4000 V assuming it is
> going to be > driving a voltage doubler or anything else, for that
> matter. > > I'm curious, where did you get the 1650 vac figure? Almost
> everything > I've > read on this list and on countless web sites say
> that the majority of > MOT's > are 2000 vac and the heavy duty ones
> 4000 vac. I am going to drive them > with my variac set to 10 vac and
> measure the output from the secondary. > Then, I can extrapolate the
> output at 100 vac on the primary. Anyone > have a > better idea of
> determining the secondary voltage on MOT's?? > > Paul > Think
> Positive > > ----- Original Message ----- > From: "Tesla list"
> <<<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> > To: <<<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> > Sent: Wednesday, April 20, 2005 6:31 PM
> > Subject: Re: Expensive hobby
> >
> > > Original poster: "Mike"
> <<<mailto:mike.marcum@xxxxxxxxxxxx>mike.marcum@xxxxxxxxxxxx>mailto:mike.marcum@xxxxxxxxxxxx>mike.marcum@xxxxxxxxxxxx>
> > >
> > > Odds are the 4000v is dc after the 1650vac or so from the mot is
> > rectified > and doubled under the load of the magnetron. > > Mike
> > >
>
>