Re: [TCML] Alternative RSG Motor and shielding

[Greg Peters <greg.j.peters@xxxxxxxxx>]

Hi all,

Thanks everyone for taking the time to reply. I will look at this in more detail tomorrow (it's late here). It sounds like perhaps I need to apply a "star" grounding concept similar to that used for audio amplifier construction. In such a configuration everything needed to be grounded has a dedicated line to the respective system ground and no two grounded objects are directly connected. This minimises ground loops. As per Richard Quick's advice a high RF ground in conjunction with a 50/60hz system ground and star grounding practices makes sense. I'll have to experiment when I get my new gap completed!

Sent from my phone

On 30/08/2013, at 12:06 AM, Jim Lux <jimlux@xxxxxxxxxxxxx> wrote:

> On 8/29/13 12:37 AM, Yurtle Turtle wrote:
>> I have done so, and managed to kill 2 VFDs. I finally found evidence that my electrodes were arcing to the motor base, so that's easy to fix.
>> 
>> Like you, I have a question on the best way to not kill my last remaining VFD. I've read about protecting the three phases, but don't have the specs handy.
>> 
>> I'm currently only running three wires to the motor, and letting the motor housing float.
> 
> Interesting questions..
> 
> So, what you want to do is "not allow spark currents to flow through the motor windings and VFD".
> 
> There's two possible sources of such current:
> 1) Primary, either directly conducted (e.g. spark gap) or induced (the current flowing in the primary loop around C, L, and sparkgap)
> 2) Secondary, either a direct strike or induced (magnetic field from loop consisting of topload, spark, ground return path)
> 
> 
> Look at it systematically:
> 1) Direct contact: make sure you have an adequate insulating path or bushing between your rotary contacts and the motor. If your rotor disk is made of insulating composite (Most are.. G10, FR4, etc.) that might be good enough. Might is the operative word.  You could get creeping discharges along the surface of the disk at a surprisingly low voltage.  Ballpark, I'd assume you need at least 3" of surface insulating distance.
> 2) Induced from magnetic fields from primary.  The key here is to make sure that the wires between primary, tank cap, and spark gap are short, and things are close together.  And, that the motor and all electronics are not near that loop.  Shielding is hard to do effectively for magnetic fields.  The other thing is to keep all the motor wires(especially including the case ground) together, so they don't form a loop. That bundle of 4 wires should go to the box holding the VFD, and the 3 or 4 power wires going to the VFD should be bundled too.  You don't want anything where there is a loop of conductors that has appreciable area.
> 
> 3) Secondary direct strikes:  Put some mesh between the motor and electronics that is connected to the bottom of the secondary (RF ground).  That means the spark will hit the mesh, not the motor or electronics.  Basically you're building a little faraday cage for your motor and electronics.  It does not have to completely cover it (it's not "shielding the motor"; it's more like providing a safe place for the strike to hit.
> 
> 4) Induced transients from secondary.  These are really tricky. Fast HV pulses can destroy things fairly easily by causing internal sparks (ask anyone who has built a Marx bank and fried their power supply charging it).  The key here is to make sure that the loop area is minimized.
> 
> This is the kind of thing where "more grounding" is not necessarily better. Adding random ground wires can create a loop that can intercept the magnetic fields and create voltage differences between points on that loop.
> 
> Here's a way to get bitten by this.
> 
> Wall socket ground and power goes via one power cord to VFD, which then goes to motor case.
> Wall socket ground and power goes to primary side power transformer, with a different cable and a different wall socket.
> Primary side low voltage ground (e.g. NST case) is connected to RF ground (for safety, so that if you touch the RF ground, you won't get a shock).
> 
> So far, no problems..
> Now, you add a wire from primary low voltage side ground (NST case, Pig tank) to the motor.
> 
> You've just created a potentially large loop: Wall ground-motor-new ground jumper-primary ground-wall ground-wire inside wall to ground in other socket.
> 
> And that large loop will have induced currents from the transients in the primary and secondary.
> 
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