Re: MOT's as Charging inductors in DC Coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mike Knowlton" <amdx@xxxxxxxxxxx>

Hi Ted L,
 I'm not sure how this affects your your situation but;
MOTs are built with magnetic shunts installed these
help protect the magnetron from overloads.
In building a HV power supply these are driven out
with a hammer and chisel.
  This info in from an article in Jan/Feb 1998 QEX,
which can also be seen at
http://groups.yahoo.com/group/rfamplifiers/

In the files section of the group... skipp placed a copy of the Microwave
Oven QEX article.  There's also some pictures in the photos section of the
same group.  The group is open to everyone...

                    Hope this helps
                                 Mike K

----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, August 23, 2005 7:53 PM
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
>
>
>
>