MOT saturation tests

To: tesla-at-pupman-dot-com
Subject: MOT saturation tests
From: "Bill the arcstarter" <arcstarter-at-hotmail-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Sat, 05 Sep 1998 22:20:11 -0600
Approved: terryf-at-verinet-dot-com

Coilers,
 I finally got around to performing my microwave oven transformer 
saturation tests.  The goal was to characterize the primary winding in 
terms of current - ie - see just how nonlinear this reactance is as a 
function of current.

Setup:
* One MOT (out of a 120vac 60Hz oven)
* This MOT contains about 122 turns of wire on the primary.
* No secondary windings (hacked off the core)
* Shunts undisturbed (left in place).
* The resistance of the primary winding is 0.32 ohms +-0.04 ohms.
*
* Two generic non RMS digital volt/ammeters (Alfa electronics)
*
* Variac.
*
* Fire extinguisher (never hurts...)

Procedure:
Apply various values of 60 Hz voltage.
Record drawn current (as sensed by an inline DVM ammeter).

Results:
V=volts, I=amps, Z=complex ohms
 V   I     Z(approx'd as Z=V/I) (Neglecting the 0.32 ohm primary)
 50  0.26  192.
 60  0.31  194.
 70  0.38  184.
 80  0.50  100.
 90  0.73  123.
100  1.16   86.2
110  2.14   51.4
120  3.86   31.1
130  5.90   22.0

Clearly this core is starting to do "something odd" even for low 
currents near 0.5 amps.  An ideal linear (non-saturating) inductance 
would show no change in Z as the applied voltage was changed.  Perhaps 
this is the action of the shunts?

Interestingly enough - my LCR DVM indicated an inductance of 0.065 H, or 
Z=24.5 complex ohms, which is surprisingly close to the measured value 
at around 120-130 volts!  I wonder how it 'knew' that?

I'm not sure what conclusions to draw from this data, only that using 
MOTs as current limiters will most definitely introduce some odd 
nonlinearities as a function of the current, especially at either higher 
currents or voltages.  (We've used MOTs as current limiters on the 240 v 
side of our pig.)

Possible future directions:
*I suppose I should have measured things at voltages <50V...
*Maybe I'll knock the shunts out tomorrow and redo the measurements.  
*Maybe I'll fire up my Tek Type 543B oscilloboatanchoroscope...
*Running the tests at voltages over 130 volts might be amusing!
*Make measurements using a MOT w/ the secondary shorted-out.

Comments invited.
-Bill the arcstarter
Starting arcs in Cinci, OH
http://www.geocities-dot-com/CapeCanaveral/Hangar/6160


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