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Re: [TCML] Treadmill Motor Question

I've seen DC rectifiers blow repeatedly on a museum coil.
My take on this is that there is likely to be differences in potential between the RF earth, HV feed and the mains feed to the ARSG motor. This can be simply due to capacitance from the HV (the spark gap is very close to the motor), or voltage drop during spark events giving potentials across the small inductance leading to the RF earth for example. It will only take a 500V transient to blow the diodes. So, how to prevent it? Not so easy.
My suggestions:
1 Completely shield the ARSG motor supply ie shield the mains cable to the motor and earth the shield to mains earth. I have used adhesive aluminum tape over the motor mains cable for mine. 2 Use a microwave oven phi filter after the diodes. Cheap (free) and meant for AC but has caps, MOVs and inductors and will allow stray stuff to go to mains earth.
Maybe just the latter would be sufficient too.


-----Original Message----- From: Joe Mastroianni
Sent: Wednesday, September 11, 2013 4:42 AM

I used to run my ARSG with a G10 wheel mounted on a Pacific Scientific
90V DC treadmill motor.  The input to the motor was the simple
unfiltered output from a monolithic full-wave 120v rectifier (one of
those square bricks about 1" per side by 1/4" thick - don't remember the
pn - but they're available everywhere).  I fed the input to the
rectifier from a 15A variac.   As someone else on the list said, the
inertia of all the components involved would keep the wheel moving
smoothly irrespective of the ripples on the rectifier output.

There were several others here on the list who used the exact same setup
years ago  - i.e.  I didn't get the idea to do that myself ;)

The downside to the approach is that inevitably the monolithic rectifier
would fail.   When it failed, it would sometimes but not always short
the AC input, and thus the fuse on the variac would blow.  In fact, once
I started running reasonably high power - like around 5kva from my pole
pig, the system would fail every time after about 30 seconds.  I could
always trace it back to a fried full-wave rectifier.

I tried all sorts of things to alleviate that problem.  Filter caps on
both input and output.  MOVs.  Shielding the rectifier from the RF.
Etc.  I could never stop the rectifier from frying and the arcs never
(visibly) hit the rectifier, the input cables, or anything in the
ARSG/powersupply circuit.  But clearly some sort of transient was
whacking the rectifier.



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