I have not performed the measurements you describe. I have measured the
leakage inductance in the past and it is very important and significant
with magnetically shunted MOT's. All MOT's are mag shunted originally and
operated at resonance in most MWO circuits I think, hence the specific value
of the capacitor in the MWO.
From memory I recall it was some 6Hy/MOT which will impact, naturally
transformed into a lower value in the primary. In AC designs with multiple
MOT's there is resonance potential at mains frequencies with big values of
Cp IMHO.
I am unsure what you may be meaning about their "strayfield" properties and
how this could be a problem. You may be referring to the leakage inductance,
this has been somewhat overlooked IMHO when trying to use secondary
capacitive current limiting in standard AC designs. The effect of that
leakage inductance when trying to deliver short current pulses into DC
filters is also an aspect I do not fully understand but believe it will tend
to severely limit the ability of a MOT stack to deliver short current pulses
at the peak of the AC cycle when charging capacitors and this may be a
factor in your regulation.
I did once place a purely capacitive load on an unmodified MOT and
progressively increased it to look for resonant rise. Forgotten the exact
figures but it did exhibit a low Q resonance at the expected value of
secondary C. I also did careful measurements to find out the saturation
effects of DC flowing in the secondary to use them as inductors and found it
was dramatic and gapping to about 0.3mm was essential in maintaining Lsec
under DC conditions and with that gap L holds up well for Idc in the
secondary of 0.5A. The value of Lsec drops like a stone in an un-gapped MOT.
A few hundred mA DC has a major impact.
I de-shunted most of the MOT's in my 6 MOT pack which seems to me to be
appropriate since I do not need the current limiting or resonance feature
they provide thinking that leakage inductance just gets in the way of
regulation and peak performance.
Values in the DC Pi filter are 2uF /10Hy no DC in the core using a 4 MOT
array/2uF . I did not consider stiffness in the primary AC supply to be
particularly important. I am not surprised to read the DC voltage under load
in your 3 phase system drops to 11kV it "feels" reasonable to me considering
the design and MOT's used. If I had 3 phase a DC system using lots of MOT's
would be the only way to go, sadly domestic 3 phase is not available to me
in NZ.
I have assume your MOT's have their shunts IN. I would suggest getting a
couple of similar MOT's and de-shunt one, then place a resistive load on
them to see what the intrinsic regulation is under resistive load
conditions. You may find it informative to progressively place a capacitive
load on them to replicate my resonant rise checks.
Rgds
ted in NZ
--------------------------------------------------------------------------------
-----Original Message-----
From: Teslalabor
Sent: Sunday, April 12, 2015 6:53 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Inductive Ballast for MOT Power Supply
Hi Ted,
have you ever checked / measured the voltage drop of the MOT's under load
conditions? Could be a problem because of their strayfield properties.
In my 12kVA DC system I use a heavy duty 3-phase transformer beast and even
there is a significant voltage drop under load. I'm using an accurate DC-HV
meter on the DC output, so I know exactly what is going on there :-) DC
voltage without load is 13,3kV, which drops to nearly 11kV under load,
although I even use a 5µF smoothing cap after the 6-pulse bridge. I expect
this to be an even larger problem when using MOT's...
Regards,
Stefan
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