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Re: MOT testing

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>


A MOT can deliver multiple amperes of short circuit current. MOT's come
with varying degrees of current limiting, but unlike a NST, most
consumer-grade MOT's are simply NOT built to withstand being directly
short circuited for more than a few seconds without burning up. MOT
current limiting for consumer-grade ovens is designed to limit the fault
current during abnormal transient arcing conditions in the magnetron or
waveguide. This prevents damage to the (expensive) magnetron while
causing the oven's mains fuse to blow, killing the arc. In fact, many
discarded microwave ovens have simply blown a fuse during a transient
arcing event. And, a waveguide arc often "clears" the offending debris
that initiated it - many times simply dust and condensed popcorn oil(!)
- replacing the fuse brings the unit back to life. 

Your MOT is best treated like a plate transformer or utility
distribution transformer ("pig"). You wouldn't think about directly
measuring the short circuit current from a utility transformer, and you
really shouldn't try to directly measure it for your MOT either.
Shorting a pig will take down your mains power (or possibly your feeder
fuse). Shorting the MOT may fry it before your mains breaker has a
chance to opens. The "useable" short circuit current you can expect from
a MOT will be limited by the EXTERNAL ballast you put in series with it,
or by  capacitive reactance if you use the MOT pulse doubling circuit
originated by Marco Denicolai for a twin MOT supply. See Greg Hunter's
web site for more details on the latter:  

Be very careful with that MOT - it can fry you just as easily as a
distribution transformer.

Best regards,

-- Bert --


Tesla list wrote:
> Original poster: "PotLuck by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> Hey Dale,
> A MOT can output much more than 30ma.
> Let's suppose a particular MOT can output 1/3 amp.
> .33 amps X 1000 ohms = 330 volts
> 330 volts X ..33 amps = 109 watts
> If a MOT can output more than 333 ma then overheating will occur. Granted,
> the test will be run a very short time but I have the resistors.
> I simply want to know if the shunts work indentically as NSTs in that the
> output can be heavily loaded on a MOT without damage. I was using the 15/30
> NST as an example as to how I've been testing NSTs. Now I'd like to test the
> MOTs I have for current capabilities.
> Rick W.
> Salt Lake City
> ----- Original Message -----
> From: Tesla list <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Sunday, May 26, 2002 10:47 PM
> Subject: Re: MOT testing
> > Original poster: "Dale Nassar by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <dnassar-at-i-55-dot-com>
> >
> > Why are you increasing the wattage--Is it for voltage reasons--It cant be
> > because a 15/30 NST is overheating a 100W 1kOhm resistor.  I have tested
> > 15/30 NST's with a TINY 1/4 watt resistor!!!!
> >
> >  From standard formulas:
> > 30mA -at- 1000 Ohms (I*I*R) dissipates only 0.9 watt--a watt 1 resistor holds
> > here!
> > a 15/60 NST imposes 3.6 Watts.
> >
> > Looks like you need to go DOWN to 5W  >>not UP to 200W<<
> > --dale nassar
> > ===================================================
> >
> > At 09:10 PM 5/26/02 -0600, you wrote:
> > >Original poster: "PotLuck by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> > ><potluck-at-xmission-dot-com>
> > >
> > >Hi all,
> > >
> > >Can a MOT be tested for current the same way an NST is tested?
> > >I've tested my NSTs (15/60s and 15/30s) by connecting a 1000 ohm 100watt
> > >resistor to one secondary then ramping up the primary voltage with a
> variac
> > >while monitoring the voltage drop across the resistor. In this case 1
> volt
> > >equals 1 ma.
> > >
> > >Maybe put two 1000ohm 100watt resistors in parallel to handle the
> wattage? 1
> > >volt should equal 2 ma.
> > >
> > >Rick W.
> > >Salt Lake City
> > >
> > >
> >
> >
> >
> >

Bert Hickman
Stoneridge Engineering
Coins Shrunk Electromagnetically!