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AC Active Soft Starter for Inductive Load with less energy dissipation

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BravoV:

--- Quote from: duak on January 03, 2019, 11:17:39 pm ---Q: Snubber design

 here's a link to something on snubber design for triacs:  https://www.st.com/content/ccc/resource/technical/document/application_note/38/88/44/b8/2c/26/44/b8/CD00004096.pdf/files/CD00004096.pdf/jcr:content/translations/en.CD00004096.pdf

 I know what you mean - I'd like to just have the answer too.  I've been a hobbyist since the time of slide rules and after college and a career I can now explain why some designs are bad and won't work or work well or work for long :o) 
 I think of snubbers this way: I have a current that's trying to pass thru a high impedance eg., an open switch.  If I let it, I'll see a high voltage across the switch that will probably damage it.  Therefore, I let the current go thru a lower impedance such as a resistor (or transient suppressor).  The series capacitor limits the amount of time that the resistor is across the switch and generally for a circuit like this, the bigger, the better.  However, for repetitive signals, the capacitor will have an impedance that will be in parallel with the switch so if it were really large (microfarads) it'd be a dandy snubber but would also bypass much of the current around the triac.  It could also resonate with the transformer's inductance leading to very high voltages in the circuit.  It could also damage the triac should it be charged with a high voltage when the triac fires.  I'm going to say to start around 100nF.  Good thing is that the triac is not used to control the current, rather just when and only for the length of time needed for the relay to pull in.  Please note that the mains switch actually has to do the heavy switching, especially when interrupting the mains current.

 BravoV, the schematic is exactly what I had in mind.  The original design is quite sound and hopefully mine is not worse.  Please note that some of my changes were to solve problems that may not be there and that I may not be aware of some problems that are.

Best o' Luck!

--- End quote ---

Thanks for the link for the snubber article and confirming your schematic.

Ok, noted, I will start with 100nF and see what will happened later.

BravoV:

--- Quote from: duak on January 04, 2019, 04:47:59 pm ---.... I looked at this again because I have a spot welder at home that I want to build a timer for. 

<snip>

.....  Time for the current probe & scope.

--- End quote ---

Duak, if you happened to build it,  share the result here please.

MrAl:

--- Quote from: duak on January 04, 2019, 04:47:59 pm ---MrAl, I learned something in this exchange that I hadn't had right before.  I encountered this problem many years ago with an induction motor and a solid state relay with a colleague's project and helped solve it by getting rid of the zero voltage switching SSR.  It wasn't my project so I didn't look into the nuts n' bolts of it.  I looked at this again because I have a spot welder at home that I want to build a timer for.  I knew about the general problem but my mental model wasn't accurate - the app note clarified it and showed that your approximation was more correct in predicting the current waveform during the first few cycles.  I had understood that the motor or transformer would saturate on the first cycle but it appears it may not.  Time for the current probe & scope.

Cheers,

--- End quote ---

Hello again and thanks for the reply,

Oh yeah, back when i worked in the industry nothing got shipped before it was tested thoroughly.  That's a major rule.  Something could always be overlooked.  Everything depends highly on other things we dont know for sure sometimes.
You know the old saying, "One test is worth a thousand expert opinions".

Please report back any tests you happen to do and results.  Would be nice to see some real data.

BravoV:
Its here, Omron LY2 AC 10 Amp relay + the socket. Going to assemble/wire up the prototype, and the measurements setup/arrangements, will post update.

MrAl:
Hi again,

Always nice to see pics in the thread that shows what is really going on.  I glimpse of real life.

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