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I have an unballasted (in order to draw nicer arcs) MOT which works very well. But the problem is that it consumes a lot of current from the mains (2.5A without load and 20A in short circuit by making arc) and this give rise to some voltage drop. I know if I placed resonant capacitors on the primary, I'd eliminate the magnetizing components of the current (as a result, 150mA without load and 7A in short circuit), which seems more interesting.
As you can see, the reactive power isn't the same when the MOT is loadless and when there is the arc; so this implies 2 values of compensating capacitors.
Here is the question, how can I make an automatic system where the switch 'SW1' get activated (to increase the capacitance so as to compesante the reactive short circuit current) when the current through the primary is >2.5A and gets opened when this currents get back to 2.5A?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
You don't need a system to deal with the loaded and unloaded reactance. Since the current is fine unloaded, you only need the cap to deal with the loaded impedance. I'm not sure how you would find the proper cap for the loaded reactance because the resistance of the windings and wires would play a part in limiting the current when the output is shorted. Maybe try something like 30uF and see what that does.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
How about connecting a TRIAC instead of sw1, Use terminals MT1 & MT2 as the switch. To gate the triac, make a low resistance shunt so 20 amps drops about 2 volts. Connect this in series with the line after the first capacitor. arrange wiring such that MT1 and Gate are across this shunt.
Now when the high current flows, it will gate the triac connecting the capacitor. If you twaek it right, if probably will also regulate the arc.
I have used triacs in this arrangement to replace the centifugal starting switch in single phase motors where a start winding, in series with a capacitor is used.
Old ovens have beefy triacs to switch the MOT primary.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Or you can connect the "resonant" caps in series with the secondary output, two paralelled caps should be fine, you will get bigger arcs and the current draw from mains drops because they also correct the power factor.
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MOT and automatic compensation system
