RE: John Freau's SRSG motor controller question

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Jeff,

Interesting.  There are two situations that worry me.

First is that if you switch off the power to the circuit at a peak of the 
AC cycle, the cap will store the voltage at that level and 1/2 cycle latter 
there will be double the voltage across the switch.  That works out to:

120 x SQRT(2) x 2 = 340 volts across the switch.

Not a big deal, but the switch should be rated for 240VAC instead of 
120VAC.  That would apply to fuses and such too.  But 240VAC stuff is 
common so no worries.

But what is a real problem is that if you switch the thing on when the 
voltage is at a peak of 170 volts directly across a zero volt cap.  That 
will cause a very big current spike and could theoretically hit the variac 
brushes with:

1/2 x 270uF x 170^2 = 4 Joules.

That will probably cause a bit of high temperature arcing at the brushes 
which "might" do visible damage in time.  It is just like discharging the 
cap right into the variac brush.  If one were having a bad day and you 
"chattered" the switch, so it dropped out for 1/2 cycle, one could hit the 
brushes with double the voltage and 16 Joules which is getting pretty 
serious.  After the motor controller had been switched on and off many 
times, there may be damage apparent where the brush sits during turn 
on.  Another problem is that these spikes may blow the fuses for really no 
good reason.  The damage could also appear at the on-off switch contacts.

I am looking into the use of Zero Voltage Crossing electronic relays which 
are simply a little box that solves that problem.  The relay's logic simply 
eliminates these spikes all together.  Big 240VAC ZVC relays run about $50 
(I just ordered two ;-))) but they may solve many problems with the basic 
circuit at least for "purists" ;-))  By stopping the current spikes, all 
kinds of fuse, switch, and brush problems just neatly go away...

This may not be a big problem, but I think it can be fixed easily.  In the 
mean time, you may want to check the brush contact areas and switch 
contacts for excessive wear.  In your situation, having an unexpected 
switch or brush failure stop your dwell controller could be a big pain.

I cannot "predict" some of the dynamic behavior of the circuit without 
really trying it, but the above are obvious problems right off.

I think I can calculate the cap value needed accurately with a Kill-A-Watt 
meter on the motor too.  Thus one would not have to trial and error the 
caps to find the right size.  You could just order them right off, and they 
would work.

ZVC relays could also do some vary interesting coil control 
functions.  Probably mostly of a scientific rather than practical 
nature.  They could easily control the number of bangs so you could say 
fire a coil for 7 bangs or something like that.  Might be neat of streamer 
growth studies.  Skeldon mentioned this in a paper in 96 but he never did 
anything with it...  They could also "soft switch" a coil which might help 
with RFI...

Much more testing to do here...  Stay tuned...

Cheers,

         Terry



At 12:06 PM 5/30/2003 -0700, you wrote:
>Terry,
>
>I start the 3/4hp motor on the 9J with the circuit engaged. The variac
>growls but I don't see any heating effect and the start cycle is only
>about 5 seconds. We're using 5 x 55uf caps and a W20 variac. We mounted
>the phase control into our latest controller (24 kVA continuous!) and
>have it calibrated so that the travel of the knob matches the phase
>adjust (thanks Brian Basura). The new controller uses 3 x 1256Ds for
>voltage and 4 x W50 for current and a W20 for the Phase adjust. It
>weighs about 600 lbs. I'll upload photos in a few days.
>
>Jeff
>
>
>
> > -----Original Message-----
> > From: Tesla list [mailto:tesla-at-pupman-dot-com]
> > Sent: Friday, May 30, 2003 11:43 AM
> > To: tesla-at-pupman-dot-com
> > Subject: John Freau's SRSG motor controller question
> >
> >
> > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
> >
> > Hi,
> >
> > I am working on a dwell controller of the John Freau design and I was
> > wondering if there is any problem with starting the motor
> > with the variac
> > in any random position?  I think I heard that it should be
> > started with the
> > variac in the minimum inductance stetting.
> >
> > Also, does the say 45 amp peak starting current hurt the variac?
> >
>http://hot-streamer-dot-com/temp/030513-03.gif
>
>Cheers,
>
>         Terry