Re: MOT's as Charging inductors in DC Coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Paul Benham" <paulb@xxxxxxxxxxxxxxxxxxxx>

Hi Ted,

Interesting results.  I wonder if it will be as bad as this if the I is
removed and the air gap is increased, or even if the test carried out
without the I's.

I use MOT's as a DC charging reactor.  I have used four in series with 2mm
plastic spacers in between the I's and E's with good results, and also used
two in series in parallel with another two in series, again with additional
plastic to increase the air gap.  Running up to 10kV DC caused one of the
MOT's to fail.

I am now running three in series without I's at the moment.  I did end up
with power arcing in my rotary at slow speeds, but I hope to solve this by
using less electrodes on the rotary and spinning it faster.  I am also going
to increase my charging inductors inductance.

I still mean to test my charging inductor current against applied AC
voltage, but your test sounds much better to simulate what happens in
practice.

Cheers,

Paul.

----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, August 24, 2005 1:53 AM
Subject: MOT's as Charging inductors in DC Coil


> 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
>
>
>
>
> This message has been scanned for viruses by MailController -
www.MailController.altohiway.com
>