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Magnifer & rotary problems
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From: Greg Leyh [SMTP:lod-at-pacbell-dot-net]
Sent: Friday, June 19, 1998 2:30 PM
To: Tesla List
Subject: Re: Magnifer & rotary problems
pacster wrote:
[snip]
> Most of the problems occurred in the rotary gap, the main rotor is made of
> polycarb, this material was the best I had on hand so I used it. It was only
> supposed to expand (by calculation) around 0.1mm at 4500rpm across it's 12"
> diameter, but when I measured it during operation it had expanded almost
> 0.5mm which is worrying! What is the normal rotor material for rotor disks?
> I read a while ago that G10 is a reasonable material, but unfortunately the
> sheet plastic companies in little ol' New Zealand need a polymer name, which
> I didn't have.
Aluminum rotors have always worked well for me, they're cheap,
good conductors. Aluminum has a very high edge speed rating,
and is easy to machine and dynamically balance. An aluminum
rotor also carries the waste heat away from the electrodes.
> The rotary is of a similar design to R Hull's rotary design with pan head
> screws connected in pairs around the radial edge of the disk. the problem I
> had is that after I screwed the M6 screws into the disk, it developed
> cracks. I'm not sure wether this is a result of the glue reacting with the
> polycarb (glue was Arildite) or that the screws pressurised the air in the
> hole when they were screwed into position, as the cracks developed before I
> had even run the diskat any speed.Has any one had this problem and what is a
> good bonding agent for metal to polycarb?
Yikes! What is the max edge speed for polycarb? It's also
important to remember that an electrode assembly mounted on
the edge of a rotor at full speed can experience centripetal
accelerations well in excess of 1500 G's. So a 1lb electrode
assembly must be able to withstand a tensile force of 3000lbs,
for a 2 to 1 safety factor. Also, wouldn't there be an issue
with having a hot metal electrode under extreme phyical stress
against the plastic? A laminate such as G-10 would be better,
as it handles higher temperatures and cracks do not propagate
through it in the same way as they might in polycarb.
> When I was finally brave enough to put the stationary electrodes in position
> (backwards at the moment just in case the rotor turns to dust) I connected
> the rotary up to the primary circuit. The rotary is driven by a servo motor
> with a home made solid state control circuit and a 5amp fuse. As soon as I
> applied power to the primary circuit the servo motor began to change speed
> sporadically, as I increased the neon input voltage to 100V the motor popped
> the fuse, I thought it was odd and so replaced it with a 10 amp fuse and
> reapplied power. At about 100V the controller died, I replaced the broken
> diode and I haven't run it since as I am confused as to why the motor would
> hunt when the primary circuit was energised. I made sure that there was at
> least 2" of clearance between all HV points and the motor housing and frame.
> Is it possible that the high E.M. field given off by the secondary and
> tertiary coils is inducing voltage spikes in the wiring running between the
> motor and the control circuit? If this is the case should I get shielded
> cable instead of normal 3 core wire and shield the control circuit, and then
> connect the shield to mains earth or the RF ground?
When using a solid-state controller for RSG service, it is very
important that _every_ electrical node of the motor drive ckt
have a low-Z path to ground for the RF voltages that are induced
by the E-fields of the secondary. Just think what would happen if
you were to connect a 1pF cap from the toroid to any node in your
motor ctrl ckt! Well, that's what is actually happening.
On my RSG there is a 1uF, 1kV poly cap from _all 4_ motor leads
to power ground. The filter can be seen mounted on the SCR bank.
If I run the coil in low-power single shot mode without these caps,
the o-scope shows that pulses as high as 450V are induced on the
motor (open ckt) by the electric field of the secondary. The caps
reduce this voltage to less than a volt. The 3-phase SCR bank
and homemade phase controller board (TTL) have worked solidly in
this environment, even without ES shielding on the SCR bank.
See http://www.lod-dot-org/electrum/nzscrbank.jpg
The controller board is mounted inside the contactor box,
shown in: http://www.lod-dot-org/electrum/nzcontbox.jpg
--
-GL
www.lod-dot-org