RE: MOT's as Charging inductors in DC Coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>


Ted:

My 2 KW Coil uses (4) MOTS with a Full Bridge Rectifier configuration
and filter caps on the output.  Current limiting inductor on the input.

My target charging reactor inductance was/is 31H.  Tank Cap 40 nF.  RSG
speed 420-460 BPS.

I originally used (2) MOT's in series(~ 18H each measured by V/I @ 60
Hz)for the charging reactor.  They were mounted on polycarbonate and the
secondary ground lead separated from the cores.  After some months of
operation the secondary in one of the charging reactor MOT's arced
internally and the secondary became open.  Note:  This has been
discussed previously on the list.  There are a number of others who have
not experienced this problem and are successful with MOT's as charging
reactors.  I did not investigate the result of applying a DC current and
measuring inductance as you did.  During charging(during breaks) the CR
resonates with the Tank Cap.  You must be concerned about the CR
behaviour during firing of the gap, but at that time the CR becomes a
current limiter for about 40 uS while the tank cap resonates with the
Tesla coil primary.

For my charging reactor solution, I got a hold of (2) Ferrite U-100
cores and wound a 7500 T coil from #30 gage wire to drop onto the U-100
cores.  I then gap the cores to get the required inductance(31 H).  I
recall with no gap the unit had about 168 H inductance(using V/I @ 60
Hz).  Don't recall the gap I'm using to get 31 H, but could look it up
if important.

Mark



Original poster: tesla <tesla@xxxxxxxxxxxxxxx>

Team

Being interested in making a medium size (8kVA) DC resonant charge coil
I've been looking at suitable inductors. Naturally MOT arrays are on the
agenda as other options are costly or difficult to find.

I've been testing MOT secondary inductance with DC superimposed and
found:

Most MOT secondaries measure 20Hy to 40Hy with an inductance meter or
using V/I from AC excitation.

What is important for DC coil is the effective L with DC in the
secondary.

I set up a configuration to inject DC into a secondary via a large L and
then measured the impedance to AC applied onto the secondary (removing
the effect of the DC excitation cct which sank 3mA at my chosen AC
excitation voltage of 40v rms). For a typical 600W MOT the following was
measured (I've ignored the contribution of the resistive component as it
is small WRT to Z so error is not particularly large, a more rigorous
calc could easily account for the error in the Z triangle) The 230v
primary was O.C. The AC current being used to measure the Z of the
secondary was quite small ranging from 5.6mA at 0 DC in the secondary to
195mA with 400mA DC flowing thru the secondary. (want to ensure AC
current modest WRT to DC current to ensure max Ipk not excessive)

Idc in Sec (mA)        Sec L (Hy)
0                                42
50                              7.5
100                            3.0
150                            1.7
200                            1.26
250                            0.98
300                            0.8
350                            0.7
400                            0.58

The results suggest to me that the initial permeabilty of the core is
quite high and drops off very quickly as the DC magnetising force rises.

Clearly the implications are significant for use as a charging inductor
where the L will drop to a low value during the DC charging which is in
the 400mA range. It's pretty clear to me MOT's are not designed for DC
in the windings.

The results leave me unclear on the operation in practice and the best
way of making the inductor array to get the target 30Hy.

My intention is to use 6 MOT stack as the supply for +/- 8kV DC, 90nF
Cp, 30Hy charging inductor , BPS up to 190.

Has anybody experience in MOT arrays as inductors in similar situation ?

Thanks
Ted L in NZ