Re: MOTs & Thoughts

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Terry, David,

On 21 Jul 2002, at 18:50, Tesla list wrote:

> Original poster: "David Speck by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dave-at-davidspeckmd-dot-org>
> 
> The simple solution would be to test a transformer before and after you 
> remove the shunts.  All the discussions on using MOT trannys for VTTCs 
> have recommended removing the shunts before applying them.
> 
> I think the opposite would be the case -- that after you remove the 
> shunts, the unloaded current would drop.  I was under the impression 
> that the shunts were designed to limit power output by providing an 
> alternate path for the magnetic flux, thereby diverting it from the 
> secondary windings.  I would expect that removing them would 
> functionally convert the MOT to a potential transformer.  After all, 
> most transformers can accept their rated input voltage unloaded without 
> self destruction.
> 
> Dave
> G2-1170

Consider that the condition is measured with no secondary load - i.e. 
the secondary is effectively not even there. So the issue of flux 
reaching the secondary windings is not even an issue for an unloaded 
transformer. If the shunts are removed, I'd expect the condition to 
worsen somewhat - though not much as there is an airgap in series 
with the shunts. If there is no load on the secondary, most flux will 
appear in the ungapped portion of the core by default (lower 
reluctance path). Upshot - the core has insufficient pole area for 
the applied A-T in the first place.
      I once measured a very distorted primary current waveform 
peaking at around 20A on a transformer designed for use at 230VAC. 
Loading the transformer made little difference.

Regards,
malcolm

> Tesla list wrote:
> 
> >Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> >
> >Hi Steve,
> >
> >I tried this too.  Here are the numbers I got with various input voltages
> >to my single open loaded MOT:
> >
> >VAC	AAC (true rms)
> >30	0.4
> >40	0.5
> >50	0.6
> >60	0.7
> >70	0.8
> >80	1.0
> >90	1.2
> >100	1.7
> >110	2.7
> >120	4.9
> >
> >I "think" (but I don't "know") removing the shunts would tend to drive more
> >magnetic flux into the core and make the saturation worse.  
> >
> >Interesting, I didn't realize they saturated so badly...  I wonder if the
> >core is less likely to saturate if it had a load on the output?  Or, if
> >that makes it worse still...  A PFC cap may also do something "odd"...
> >
> >This saturation is a big concern in some tube coils and other uses where we
> >want just a "nice" HV transformer.
> >
> >Cheers,
> >
> >	Terry
> >
> >
> >At 11:49 PM 7/20/2002 -0600, you wrote:
> >  
> >
> >>I took some no load measurements on a 4 MOT stack that some of you might
> >>find interesting.  All 4 MOT primaries are in parallel, and are powered
by a
> >>120 volt variac.  Here are no load primary volt vs amp measurements:
> >>
> >>Volts                    Amps
> >>below 75              less than .5
> >>80                         1
> >>95                         2
> >>90                         3
> >>95                         4
> >>100                        5.5
> >>105                        7.5
> >>110                      10.5
> >>115                      14.5
> >>118 (max)           17.6
> >>
> >>As you can see, the MOT cores start serious core saturation at around 100
> >>volts, or about 85% of their rated voltage.  The moral of this story is to
> >>design your MOT stacks to produce the output you need without exceeding 85
> >>or 90% of the normal primary voltage.
> >>
> >>Question:  Suppose I knock out the magnetic shunts.  Would the above
> >>measurements be much different??
> >>
> >>Another thing to be aware of is that above 90% of normal input voltage
> >>causes significantly distorted sine waveforms that sometimes have strange
> >>effects on DVMs, causing them to indicate quite inaccurate measurements.
> >>
> >>--Steve Young
> >>
> >
> 
> 
>