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Re: Measuring Current Output of MOT's?



Original poster: David Sharpe <sccr4us-at-erols-dot-com> 

Hi Chris
-----------------------------------
Tesla list wrote:

>Original poster: "Chris Fanjoy" <zappyman-at-hotmail-dot-com>
>   Any ideas on how to do this? I'm sorting through my MOT's and trying to 
> find one that's best suited to building my TC. It didn't take long, to 
> figure out an easy way to measure output AC voltage (power with a variac 
> -at- 10VAC, measure output with a cheap VOM, and multiply the result by 12) 
> but I wasn't so lucky in measuring current output. I tried putting two 
> 190 ohm, 40 watt resistors in series across the output, and measuring the 
> voltage drop across one of them. My theory was something like this: for 
> example, if the transformer could produce 500mA through these resistors 
> (with the Variac set to at least 11 VAC for a 2100V transformer), I 
> should get a voltage drop of of 95 VAC across either resistor. With the 
> Variac at this voltage, the voltage drop across one resistor was nowhere 
> near 95 V. As the Variac was increased, the voltage across the resistor 
> would go way above 95V, but the resistors quickly started smoking a! nd 
> the experiment had to be stopped. My initial conclusion is that I'm 
> either going about this all wrong, or that most MOT's are capable of 
> currents much higher than 300mA, at least for short periods of time.
>   Any thoughts or suggestions?

-----------------------------------
Much hand waving here, but here is a test method that might yield useful 
information with minimal safety (or equipment) risk...
At a minimum, this test will give some indication of magnetic saturation 
point (current wise) in transformer.
1.  Connect 120 VAC through a variac (i.e. 0-120 VAC) to the _high_voltage_ 
connections of the MOT under test.
2.  Apply voltage on HV secondary and measure open circuit ( _no_load_) AC 
voltage on primary.  The ratio of
     Vin AC on HV secondary / Vout AC on LV primary will give a rough 
indication of transformer turns ratio, ignoring
      leakage inductance, shunts, and high frequency parasitics.  A typical 
value would be in the 12-25:1 range.
3.  Turn the AC input off, then SHORT the primary winding through a high 
current AC ammeter.  CAREFULLY adjust
      the Variac 0-120 VAC input upward until a "reasonable" current 
(10-20A) is seen on primary.  Monitor the current and voltage
       in HV windings while measuring primary short circuit current.  Do 
not run transformer in this condition
      longer then necessary to get  voltage and current measurements; the 
transformer will be in thermal overload and could
      burn up if run for an extended time period.
4.   If the primary current is plotted against the secondary excitation 
voltage, the current will be "reasonably" linear (constant
       impedance) until obvious non linearity of current is observed (core 
saturation/magnetic shunting).  This will usually be seen
      at 150 to 250% of nameplate rating of a typical power transformer; 
again ignoring magnetic non linearities.  If you divide
       the primary current measured in step 3 at the non-linear inflection 
point by the turn ratio calculated in step 2 would give
       a ball park estimation of HV load current capacity; prior to saturation.

It ain't perfect, but it is provides transformer characterization data that 
can be repeated at anytime; does not
include arm waving or opinions; and doesn't require powering the 
transformer with HV to get useful performance data... :-)

Regards

Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA