Steve,
I usually use a 10 amp variac same brand and model as you are using,
so I don't think it's the problem. But I suppose a bigger variac might
give some small advantage. I didn't do a comparison.
John
-----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sun, Mar 18, 2018 1:35 pm
Subject: Re: [TCML] VTTC MOT question
Hi Chris,
I wonder if the problem with my VTTC is the variac? My variac is the Superior Electric model 116C. This variac is rated at 10 amps but can be overloaded if the duty cycle is sufficient to allow cooling. Currently I am seeing maximum current pulses of about 15 amps at 140 volts at an interrupter period where I get my longest sparks. I am using this variac because it is small enough to fit into the chassis of my VTTC to make a portable unit. I have bigger variacs but one of those would have to located outside of the main chassis thus decreasing portability. From the information that I see in the Powerstat data sheets, this variac should be able to supply several times its rated current if the duty cycle is sufficiently low so I don't think that this is the problem.
I should note that if I run the VTTC in CW mode, it will draw about 19 amps at 140 volts. As I increase the on-time of the interrupter, the current draw will rise to about 19 amps max but I get my longest sparks at lower interrupter rates where the VTTC draws about 15 amps.
Steve
----- Original Message -----
From: "Chris Reeland" <chrisreeland@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Saturday, March 17, 2018 11:55:58 PM
Subject: Re: [TCML] VTTC MOT question
Hi Steve,
Um, yes I did start to have problems blowing or tripping the circuit
breaker, after 3 times, had to quit and correct this. It was impossible to
fine tune further. Realized I have now run into a poor power factor problem
at the increased power level now. I will give some more details later on
what I did for this to correct this. So currently I have added about 224uF
in PF caps. Amp draw is now at the current state of tuning, going from 10
amps to 16 amps pulsing. Much better now! Can actually do tuning changes
now no problem.
So if we can eventually find out why your coil is not drawing more amps,
you do need more to get the extra spark length, you may run into this also.
I think you said you are currently at about 15 amps, of course no power
factor correction.
I have not done anything Sat on the coil. I worked very early that day. Was
tired after and need to get other things done.
But I can now try some things tomorrow (Sunday). I hope to give some more
details and pictures.
Also want to say, sounds like the better filament transformer change with
meter and small variac is worth the effort and time. Besides no more surge
problem of course, benefit of being able to manually regulate voltage is a
big plus. I have some line voltage drop from the heavy draw of the coil
running, pulls down filament some. Most tube spec sheets want you to hold
unavoidable fluctuations to a minimum. Typically .3-.4 + or - max. Going
low is worse. You will see where to set it on the plus side so it does not
fall to low. Me when under coil running, I like to keep at +.2 for a slight
safety margin for other possible load conditions.
Chris
Sent from my LG V20
On Sat, Mar 17, 2018, 9:44 AM Steve White <steve.white1@xxxxxxxxx> wrote:
> Hi Chris,
>
> Wow! 23 amps at 140 volts. That is 3.2 KW. I am surprised that your 20 amp
> circuit breaker doesn't blow. I guess the slow-blow action of the breaker
> prevents this especially since you are pulsing.
>
> I think my next step will be to replace my home-made filament transformer
> (rewound MOT) with a commercial filament transformer, which I now have, and
> a small variac and voltmeter for filament voltage adjustment. It won't make
> my sparks any longer but it will be much more efficient. My MOT-based
> filament transformer draws about 750 watts! It gets quite warm even with a
> cooling fan. I measured the commercial filament transformer under load and
> it only draws about 120 watts and stays completely cool.
>
> Steve
>
> ----- Original Message -----
> From: "Chris Reeland" <chrisreeland@xxxxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Friday, March 16, 2018 3:30:07 PM
> Subject: Re: [TCML] VTTC MOT question
>
> Hi Steve,
>
> Yes, I am back to my bigger copper tube toroid again. I have said in the
> past recently that I always find myself going larger especially if pulsing.
> I tried 4 other smaller sizes working back up to this. I have several more
> going bigger, but of course they get too physically big for the size of
> this coil, and then I would run into frequency problems again, which I want
> to also say if you are considering larger, you may want to figure out if
> you are going to have a possible frequency problem of pulling it too low.
> This is why I rewound my secondary for a higher starting frequency. I have
> alot more "headroom" to try various changes.
>
> Yeah, having the feedback above a fairly tall primary can have some
> problems. I have mentioned this also in the past. You got lucky you caught
> is soon enough and have taken corrective action, I have seen variations of
> what you said you did to prevent this.
>
> Me, I have stopped a while ago of the "traditional" above the primary. I
> always put it below now. Me, I feel (and some may disagree with me, which
> is ok) that even though in theory, it is only supposed to be affected by
> the primary, it seems to be affected by the higher voltage built up on the
> secondary on the upper end, which happens when you have a taller primary.
> If a short traditional primary, it is ok above.
>
> At the bottom below the primary, you are of course located at the bottom of
> the secondary, which is mostly current of course, and also has the benefit
> of eliminating high voltage short arcs between the feedback and secondary.
> I would also say this is much healthier for the grid wire of the tube to
> eliminate this from happening.
>
> I will have more details on this later, but I "was" peaking during the
> pulses about 23+ amps at "140" 100% variac input.
>
> I have not yet done any changes yet last night or today yet. Kind of taking
> it easy...was tired from work. Going to mess around a bit later I think.
>
> I should be able to give more details and comments and pictures this
> weekend sometime.
>
> Chris
>
> Sent from my LG V20
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