Re: Dimmer as a Variac.

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

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Marc
I''ve started modeling a totally new design power electronic circuit that
will emulate a variac, with major improvements of dynamic current limiting
(hopefully), limited voltage regulation, and instantaneous over current trip
protection in the event of something downstream going BURAPPPT!!!
Circuit costs seems about right too, ~$200 for a 60A controller.  Will build
a low power unit and verify operation before scaling up.  This unit will
PWM the entire AC waveform, and the HV transformer's primary winding
"rebuilds" the waveform.  This is radically different and IMHO superior
to a SCR controller.

Problem with an SCR controller is once the SCR is ON, it stays on until
a current '0' occurs, or you force commutate the SCR OFF.
So if you get a fault (short circuit) with a SCR ON, it has to swallow
all of the current until over current protection trips,  a current '0' occurs,
or the SCR dies.  This can be as long as a half a cycle (8.33ms) at full power.
Another problem with SCR's are they do have a cycle life.  As they age in
service they get slower (experience from working on inverter grade SCR's
at 1-5kHz/200kW). I don't know if that effect is an artifact of thermal
cycling,
repetitive cathode current pulses in operation degrading gate/cathode gain,
or the phase of the moon, but it is (or was) a common problem in mid to
late 1970's.  Maybe processing has gotten better, and integral gate drive
structures have been improved to handle higher repetitive surge currents
without cumulative damage.

Compare to a PWM controller which is operating at 10kHz, the cycle
time is 100 us. Let through energy in a fault condition is a function of i^2t.
So ignoring current (which with a HF system will have a considerably
smaller cycle di/dt), let through energy is reduced by reduction in time
(83.3X in this case).  On a high power system that could protect a MMC,
spark gap, transformer, or YOU from a catastrophic OH SH..

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA

Tesla list wrote:

> Original poster: "Metlicka Marc by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>
>
> dave,
> i was hoping the 1.5kv scr's would be good up to 480v, thanks.
> i want to duplicate the unit i have but beefier with the bigger scr's,
> but i don't have enough electronics experience to beef the other
> components along with it? i don't think i just double the size of
> everything would i?
> i'm trying to talk the geek group into starting to build these units for
> sale, i had my 30 amp unit hooked to my pt system for a while and it
> worked good as a power controller for that tesla system at the time. i
> now use an isolation transformer setup to drive it, i didn't want to
> blow the vectrol unit up?
> the vectrol unit runs motors, transformers and lights very well,
> allowing speed control for ac motors at a very inexpensive alternative
> to an ac drive of the same current.
> i thank you and all for the input,
> marc