[AMPS] resonated input chokes

[Tom Rauch]

> from an old parts list.  Then there was a 4 uF shunt C. In the final
> design I followed the first section with a second series L, about 3.5 H
> (small and inexpensive) and another shunt 4 uF. This gave excellent ripple
> rejection for even 720 Hz, and also was very well regulated due to the
> resonant choke. It was modelled to prevent any resonances at power line
> multiples, or at 50 or 60 Hz, in case a rectifier failed open, and allowed
> half wave operation.

In 1970, I was almost killed by a resonant choke supply.

I built a resonant filter driven by a full wave bridge and a 4160 volt 
RMS 10 kVA pole pig. All parts had a large amount of headroom, I 
used power line power factor correction caps in the supply. 

I made the choke EXACTLY resonant by following the guidelines in 
Bill Orr's Handbook.

When I powered-up the PA, the 3-500's arced from anode to grid 
and the breakers dumped. Removing the tune anode connection, 
on the next power-up a 20  kV bypass capacitor exploded.

Suspecting too much HV, I connected a meter through a 20 kV 
rated lead to the power supply and powered it up. The 20 KV lead 
arced through the insulation and blew in half. The wild end of the 
lead came flying up of the cabinet and attached itself to my wrist.

I fell backwards and broke the arc between my hand and the front 
panel (I was on a rubber blanket like line crews use so the only 
ground path was via my hand, the same one that the wire attached 
to since I kept my other hand "in my pocket" as is customary 
when working on live HV).

The sensation was almost like having my arm between the coupler 
on a freight train as the cars were coupled.  

I remember watching the second hand on the clock, and trying to 
recall from biology class how long the brain would function after the 
blood supply was removed. After three sweeps of the second hand, 
I decided it was time to go get help. 

 I wound up with a quarter sized hole in my wrist and first, second, 
and third degree burns on my arm.

If I was doing a HV resonant choke filter, I would be damn sure the 
choke wasn't resonant at the ripple frequency. I also would include 
spark gaps or some other kind of peak limiting across the choke in 
case something did bring the choke into exact resonance.

By the way, resonant choke filters make crummy filters for time-
varying loads. They have a tendency to "bounce" and "overshoot" 
when load current is rapidly varied. Unless you are very careful with 
the choke's magnetic path design and recovery time, you can wind 
up with horrid HV dynamic regulation.

IMO, resonant chokes are not worth the headaches for most 
applications. You often need to follow them with a BUNCH of 
capacitance to improve dynamic regulation if the load current varies 
rapidly.


73, Tom W8JI
w8ji@contesting.com

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