AC control circuit question

[spec]

I seriously doubt those To-247 case 50 amp SCR's are going to hold up to 240 VAC high inductive load service.

The TO-247 case SCRs were chosen for much lower cost and to have a similar electrical specification to the SCRs that were used in the original article and by the OP. It is a shame that you have now decided to find a problem when you could have saved a load of nugatory work by providing this information on day one. The TO247 case is not the best for dissipation compared to stud and minpack etc but, if correctly mounted on a decent heat sink, would not be a mile different. Besides which, the OP has ordered the minpak SCRs anyway.

The problem is when you have a load on your secondary of the transformer the inductive impedance or the primary drops off proportionally plus that impedance is heavily dependent on the relative magnetic field motion and thus its counter EMF effects throughout the whole period of 50 - 60 HZ sine wave.

When you change to a phase angle delay control there are points in the transformers core magnetic field flux transitions where if the core desaturates fast enough due to its inherent magnetic properties plus secondary load that the SCR's may be firing into nothing but the primary coils native resistance due to the counter EMF being extremely low or possibly moving in the same relative direction as the magnetic field the primary coil generates as it starts to conduct.
Given that it's entirely possible that the SCR's could be firing into a less than 1 ohm load at the ~340 volt peak of the 240 volt RMS sine wave.

Basically it's like trying to forcibly hit a nail with a hammer while the nail is moving away from you at the same speed as you swing. Lots of energy involved but nothing to absorb it on the overshoot but your own arm.

That's why the old SP-100's despite only working ana 120 volt 20 amp input rating and the transformers having a ~ 2 - 3 ohm resistance needed to have a 50 - 75 amp capable SCR sets.

Given your unit I would not be trying to run with anything under a 300 amp rated SCR set.

Your 50 amp units might work for a while but the first time you stick your electrode and step on your control peddle I would not be surprised to see them pop.

BTW years ago I used to be a service tech at a local welding supply store and I worked on a lot of units that used SCR based primary control circuits and they always ran with devices that had substantially larger current and voltage ratings compared to their power source volts and amps input numbers. 3 - 4X on voltage and 6 - 8 X on current were common on the larger units.

The heaviest ones I ever regularly worked on were some old Western Arctronics commercial spot welders. They were rated at an input of 240 VAC ~150 amps and only fired for 3 - 10 cycles on a spot weld (but did up to 40 a minute while using a dedicated zero cross detection firing system) and used 1000+ amp 1600 Volt SCR's but blowing one or both to bits 1 - 3 times a year was still common.

That's why I have concerns that your 50 amp units won't take the abuse for very long snubbers or otherwise.

I can only bow to your practical experience about the above.

spec

 

[spec]

A photo of the installation for those interested. And one of the foot pedal. It contains the pot R4 and a microswitch which initiates (or will ) events such as hf start and gas flow.

Shawn,
Putting tcmtech's previous points aside for the moment, the heat sinking for the SCRs is not nearly enough- sorry and your installation is so neat

spec

 

[tcmtech]

It is a shame that you have now decided to find a problem when you could have saved a load of nugatory work by providing this information on day one.

I did.
Post 4 last paragraph mentioning that on the SP-100's system the last ones built were running something like 600 - 800 volt 50 - 75 amp rated SCR's on a machine with a 120 volt 20 amp input rating.

To me when I mentioned the scaling up of that control circuit for 240 VAC 50 amp circuit operation I figured the ratios would have been easily understood as in using something with a 1200+ volt 200 - 250 amp minimum rating.

 

[spec]

I did.
Post 4 last paragraph mentioning that on the SP-100's system the last ones built were running something like 600 - 800 volt 50 - 75 amp rated SCR's on a machine with a 120 volt 20 amp input rating.

To me when I mentioned the scaling up of that control circuit for 240 VAC 50 amp circuit operation I figured the ratios would have been easily understood as in using something with a 1200+ volt 200 - 250 amp minimum rating.

So you did tcm- my apologies. I am just a grumpy old sod.

spec

 

[tcmtech]

Because I am basically impatient,.......I ordered the snubber components as per the SP100 post above and a new SCR so hopefully, it all can be used. If not, another lesson learned....

Unfortunately, that was the whole point of supplying you with a reference how the pros design their control circuits.
To show you the basics of how it's done to make them work effectively and reliably and how and why a proper phase angle firing control circuit is key to that.

 

[tcmtech]

So you did tcm- my apologies. I am just a grumpy old sod.

spec

First step is admitting it. Second is becoming a master of it!

Grumpy old sod in training here.

 

[ShawnR]

spec, I guess I keep posting one incorrect value. I copied and pasted the image but R4, the pot is actually 500K. In the authors research, he found the 250k pot would not reduce the welding current enough so upped the value to 500K. He said this took the welding current down to next to nothing. I went with it. Sorry for that oversight. Thanks for the confirmation on my analysis. I did not go too deep into analysing the circuit. He posted so much info, I went with it...as much as I like the electronics,, this project was a means of using my fading electronics knowledge to get to tig weld aluminum the some equipment I got a deal on. It was also largely an excuse to learn something and that it has done immensely, thanks to you two. And I like building stuff but have a habit of not finishing.Hence the rush to get it done before something else catches my interest....drives my wife nuts. The 5k6 resistors will be easy to add. Nice. Rewiring to reverse the scr's should be easy too as it is all connectors.

tcmtech, I have the mcc56 ordered again.

https://ixapps.ixys.com/DataSheet/MCC56-16io1B.pdf

The forward rms current rating on it is 100 amps (surge currents ridiculously high, if I am understanding the spec sheet correctly) and I will mostly run this unit in the lower 40% of its rated capacity so I am guessing the scrs should see around 10-20 amps when conducting. That gives me a safety of 5...not 8 but pretty good...if it fails, I mail be contacting you for a bucket of parts yet...

I am sitting on this till parts get here. Being Sat tomorrow, I don't think I will be back at it till Monday. Other than looking at rewiring the thyristor.... I may contact the author to ask if it could have been an oversight in the documentation. His unit is working, I believe.

Cheers,
Shawn

 

[spec]

spec, I guess I keep posting one incorrect value. I copied and pasted the image but R4, the pot is actually 500K. In the authors research, he found the 250k pot would not reduce the welding current enough so upped the value to 500K. He said this took the welding current down to next to nothing. I went with it. Sorry for that oversight. Thanks for the confirmation on my analysis. I did not go too deep into analysing the circuit. He posted so much info, I went with it...as much as I like the electronics,, this project was a means of using my fading electronics knowledge to get to tig weld aluminum the some equipment I got a deal on. It was also largely an excuse to learn something and that it has done immensely, thanks to you two. And I like building stuff but have a habit of not finishing.Hence the rush to get it done before something else catches my interest....drives my wife nuts. The 5k6 resistors will be easy to add. Nice. Rewiring to reverse the scr's should be easy too as it is all connectors.

tcmtech, I have the mcc56 ordered again.

https://ixapps.ixys.com/DataSheet/MCC56-16io1B.pdf

The forward rms current rating on it is 100 amps (surge currents ridiculously high, if I am understanding the spec sheet correctly) and I will mostly run this unit in the lower 40% of its rated capacity so I am guessing the scrs should see around 10-20 amps when conducting. That gives me a safety of 5...not 8 but pretty good...if it fails, I mail be contacting you for a bucket of parts yet...

I am sitting on this till parts get here. Being Sat tomorrow, I don't think I will be back at it till Monday. Other than looking at rewiring the thyristor.... I may contact the author to ask if it could have been an oversight in the documentation. His unit is working, I believe.

Cheers,
Shawn

Hope it all turns out OK Shawn and that the SCRs hold out.

Cheers

spec

 

[tcmtech]

Sounds good. Just hope no one else reads this and decides to take me up on my offer.

I really do pack those goodie boxes tight with everything relating to what a person needs and more that I can fit in them!

If they are going flat rate shipping I make those USPS people work for their wages.

 

[ShawnR]

Sent you a pm tcmtech

 

[ShawnR]

Unfortunately, that was the whole point of supplying you with a reference how the pros design their control circuits.
To show you the basics of how it's done to make them work effectively and reliably and how and why a proper phase angle firing control circuit is key to that.

Sorry tcmtech. I guess I was getting a little overwhelmed and frustrated. I don't want you to think I did not listen to you. Actually, the exact opposite. I appreciate the time you and spec have given me. When I first posted, I just wanted an easy answer to protect my next expensive SCR module but not to have to rebuild from scratch what I had already done. I thought you supplied the schematic information to show me the snubber design. I did not realize you intended for me to look at the phase angle firing circuit (which I got a little lost in, despite your attempt to explain it) I have decided to try to figure out a new circuit. I am off on holidays for a couple of weeks (part of the reason I was rushing) so I will get back at it when I return. In the meantime, I may surf around some of the circuits you touched on ie zero detector and the 555 application. I also looked into controlling it with a microcontroller. It might be easier and more versatile for me. It has been documented on the web with Arduino's so the info is available ( I don't use Arduinos but the concepts will all be the same. ) One problem with the microcontroller is the excessive amount of emi in the welder box wreaks havoc on the ucontroller.

So that leads me to a couple of questions (of course ) Recalling from school, does a transformer introduce a lag into a circuit, ie is the secondary slightly behind the primary in terms of phase? or will a transformer center tapped give me a close enough zero point to reference from? In the sp100 circuit, there is a circuit labelled as Arc voltage error amplifier. I am sure this leads to much better arc but I think that machine is based on voltage whereas, if I understand correctly, mine would be considered a current machine. Is an error compensating circuit even necessary in my old transformer welder to add reliability to the integrity of the components, which ultimately, is what I was originally looking for.?

Thanks
Shawn

 

[tcmtech]

It's not a problem. Part of teaching is making the student realize that what they want and what they need is not the same.

Yes there is a slight phase lag between primary and secondary on a transformer but at very light loads it can be under a degree or two which is a negligible concern in this type of system.

Also yes you are correct your circuit is not trying to hold constant voltage so it doesn't need any feedback from the secondary of the welding transformer. Given that for a 555 based phase delay firing circuit all you need is to have a zero crossing detect circuit that reset the 555 every time it crosses zero which is pretty simple circuit.

Basically every time the reference signal drops below a specific voltage it trips the 555 timer to reset and holds it there until the reference voltage goes above that point again.
If you are using a center tapped transformer for your driver circuit you would just be using an extra pair of diodes and a resistor to make a zero crossing reference signal that feeds into the correct pin on the 555 or through a op amp ICl to change states and either set or reset t depending on how you are setting up the 555 to operate.

That should give you some things to read about for a while.

 

[tcmtech]

Another even simpler phase angle firing delay circuit is based around a single Unijunction transistor and only take around 12 - 15 components at most to make a zero crossing reset phase delayed trigger circuit that will work with an optocoupler triac to control the main power SCR banks.

It's basically this circuit modified slightly to run with your potentiometer resistance range an an optocoupler in place of the SCR.

https://www.androiderode.com/ujt-triggering-circuit/

Oldie but goodie bare minimum circuit there!

 

[schmitt trigger]

I worked in a company (Triad Transformer) where we would make among other things, transformers whose primary voltage would be phase controlled by a pair of SCRs.

The key to apply phase control to a transformer primary is volt-seconds symmetry. In plain English this means that the area under the phase-controlled positive sinewave, must be exactly matched with an equal area negative phase-controlled sinewave.
Otherwise, staircase saturation occurs on the transformer's core. Once that the core saturates, the primary becomes almost a short circuit. Immediate failure occurs.

Your triac triggering circuit must ensure this symmetry. As you can observe from the Lincoln Electric's schematic, the trigger circuit is quite sophisticated.

Even so, one can never ensure a 100% perfect match. Therefore the transformer would have a small air gap on its center leg. Air gaps allow steel cores to tolerate higher flux imbalance, at the penalty of a higher exciting (no load) current.

So first things first, use a resistive load to test your circuit and tweak its symmetry across the operating range. Then and only then, connect it to a transformer load.

 

[ShawnR]

I am finally returning to this project. Although I have not been wiring, I have been reading and trying to learn about the 555 based circuit you mentioned and the unijunction circuit. Although I like the unijunction circuit in post 33 (ie simple), they are hard to find. I did pickup some Motorola Programmable Unijunction Transistors (the ON datasheet here ) I also found some tutorials on www.circuitstoday.com using a UJT for scr triggering and on PUT's so doing some tests on the bench to get a handle on it all. The old brain apparently does not absorb stuff fast anymore....;-)

I have attached a diagram (NOT finished, just FYI to show the direction I was heading in for a while. ) I started working on for a 555 circuit but it does seem to be getting complicated. Makes me want to use the PUT

Just an update to let you know I am back at it in hopes of getting it working again. ....and to see if you supporters are still there....;-)

Thanks
Shawn

 

[ShawnR]

In the last post, I realized the circuit was not ready to post so I started on cleaning it up. In working on this project, I found I was jumping back and forth between the two options tcmtech gave me, the 555 based circuit or the unijunction based circuit so was running in large circles (not unusual for me....). Both methods interested me as a solution and an opportunity to learn.

In drawing one up, I started wondering if the attached circuit would be a third option. Since I started with using an op amp zero detector circuit for the 555 circuit, I realized we can vary the value the op amp switches at. Can varying this point via a voltage divider and the pot in the foot pedal be used to initiate the SCR conducting. Know what I mean? I took pieces from the SP100 circuit, as was suggested and other internet resources I could find. (Not trying to take credit for an original design)

I don't have a lot of values on components yet as I am wondering that if the circuit worked, someone would have done it this way already so I am looking forward to why it won't but I can not reason it out so far......

Please anyone with input?
Thanks, Shawn

 

[alec_t]

Your foot-pedal needs to be fed from the bridge output; not from +12V.

 

[ShawnR]

Thanks alec_t. I see what you mean...... and I see why it will not work too.

Once I drew a picture of the waveform and possible trigger point, it becomes obvious. This cct will only allow me to delay the SCR fire point till the peak of the pulsed DC, only allowing me to go to half power. Hence, now I see why I need a time delay rather than a voltage level trigger, so that my fire point can go as low as possible into the period of the pulse, up to about 8.3 msecs if I am understanding correctly. Back to the drawing board. ;-)

Thanks, Shawn

 

[KeepItSimpleStupid]

Comment: Nothing more.

In phase control heating where power is what;s controlled a table of firing angle vs V^2 is used to 0 to 100% of V^2 is the control, Since power is (V^2)/R, V^2 is more proportional to power.

In systems where I used, not made, that operated into a transformer, current feedback was used as was back to back SCR's. I THINK the back to back SCR allowed shorting out the conducting one to turn it off at the current limit. Not sure though.

 

[ShawnR]

Thanks KeepItSimpleStupid.... can we call you KISS for short? ;-)

I have been so wrapped up in trying to get a firing circuit that gets approval ;-) that I have not focused too much on the SCR control part. Back when this thread started, I was directed to a welder circuit, Lincoln SP100, for a reference. It is a reliable complex circuit with feedback. I think tcmtech provided it to illustrate the robust components involved and the snubber circuit, which I did not have in my first prototype (hence a blown SCR module). So you may very well be correct and I will look further at it. I thought I more or less copied it over but may have made some errors (seems to be the theme of this thread but at least I am learning a ton) Thanks for bringing it to my attention.

I have attached another diagram using a programmable unijunction transistor firing an opto-triac into the scr control circuit. Any critique of this part of it would be appreciated. In the meantime, I will review my power control circuit again.