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| AC Active Soft Starter for Inductive Load with less energy dissipation |
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| MrAl:
--- Quote from: BravoV on January 03, 2019, 05:18:00 am --- --- Quote from: MrAl on January 02, 2019, 11:09:14 pm --- ... Of course this is quite easy to test too. If you change the circuit a little you can get it to switch at zero, but beware the peak could go as high as 2 times normal if the exponential part of the response is slow. I think that is what you were most interested in but if you have any questions no problem. --- End quote --- Hi MrAl, thanks for the reply. What kind of "little change" are you talking about ? Modification for resistance type of load ? I'm curious, and yes, definitely I will ask more. :P --- End quote --- Hi again, Well when i said 'change' i meant that would be JUST for testing, to see the real difference between doing the peak turn on and doing the zero point turn on. I just wanted to be clear about that. The simplest way to test would mean a simple change, and that would be to get rid of the peak turn on circuit. Just make it so that it is 'turned on' all the time. That way when you plug the unit in, the inductance load gets powered by whatever phase the line voltage happens to be at. Now when you monitor the input line with a scope and monitor the current level in the inductor load, you will see what happens at various start phase angles. This allows you to plug in, note the scope display, then unplug, then plug in again, etc. So you would be plugging and unplugging until you see the voltage on the scope start at around zero degrees. For the current display on the scope, you will be looking for the maximum before it damps down. This will look like a sine wave that is above the zero line, then it will work it's way down until it looks like a regular sine wave. However, the point you want to note is the highest peak and that will normally occur at the first peak of the current wave sine. THAT peak could theoretically go up to 2 times the normal run peak. So if the current normally went up to 20 amps, that first peak could go as high as 40 amps. That's what you dont want and that is what makes switching at the peak effective because then the max is 20 amps (which is the normal run current peak). When you do this you could also watch for a time when you happen to plug it in at the positive peak and at the negative peak. You should see a normal sine then. If you happen to plug it in at zero degrees but the input sine is about to go negative, then the first peak will be a maximum negative peak, which following the above example could be as high as -40 amps. It doesnt take long to see the different phase angle responses this way, although it sounds kind of strange to do it this way :-) Hopefully your setup can handle that first 40 amp peak though. If not, you may have to add more resistance in series with your load to keep it lower just for the random plug in tests. Please let me know what happens if you do. |
| duak:
MrAl, the peak current for a zero voltage switched transformer can be quite a bit more than 2X the rated current. A transformer actually has a varying inductance that depends on the currents flowing in the windings. What we see as a single inductance is actually an average of all the instantaneous inductances that are presented throughout a cycle. You probably knwo it's better described as a differential equation, but fortunately I've forgotten anything more than that. I'm at the age now when I've forgotten more than I'll ever remember :o) Here's a link to another explanation with some waveforms: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN I mentioned winding currents above. If the transformer is connected to a resistive load, the current leaving the transformer can prevent the core from saturating during initial connection to the mains. It's counter intuitive but true. The linked paper demonstrates this. Perhaps BravoV, try using an incandescent lamp on the transformer during startup to see if the breaker trips. BravoV, some answers: Q: So this is no different than ordinary contacts bounce right ? Correct. Most any contact bounces or at least has a ragged make and break action. I believe only mercuy wetted contacts get around this. 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! |
| MrAl:
--- Quote from: duak on January 03, 2019, 11:17:39 pm ---MrAl, the peak current for a zero voltage switched transformer can be quite a bit more than 2X the rated current. A transformer actually has a varying inductance that depends on the currents flowing in the windings. What we see as a single inductance is actually an average of all the instantaneous inductances that are presented throughout a cycle. You probably knwo it's better described as a differential equation, but fortunately I've forgotten anything more than that. I'm at the age now when I've forgotten more than I'll ever remember :o) Here's a link to another explanation with some waveforms: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN I mentioned winding currents above. If the transformer is connected to a resistive load, the current leaving the transformer can prevent the core from saturating during initial connection to the mains. It's counter intuitive but true. The linked paper demonstrates this. Perhaps BravoV, try using an incandescent lamp on the transformer during startup to see if the breaker trips. BravoV, some answers: Q: So this is no different than ordinary contacts bounce right ? Correct. Most any contact bounces or at least has a ragged make and break action. I believe only mercuy wetted contacts get around this. 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 --- Hello there and thanks for the comments. Yeah funny what we can forget when we arent even trying :-) As to the 2x current, that was a discussion meant to keep it down to the simplest possible terms which means an ideal situation where we have a perfectly linear inductor and perfectly linear resistance. This is done so that we dont have to use any advanced techniques in order to reason this out. 2x the current is reason enough to use peak switching, and if it is higher than that then so much the better for us if we use peak switching. So just knowing, at least for now, that 2x the current is a possibility even with ideal conditions, is enough to guide our decision on how effective the given circuit will be. So that discussion was not meant to be perfectly complete, just enough to give good reason why the peak switching circuit is a good idea. Note how simple the expressions were even void of any inductance value, and even those as simple as they are were enough. Also, for testing as described i did mention that more resistance may have to be introduced into the circuit to prevent really bad current surges. |
| duak:
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, |
| BravoV:
--- Quote from: MrAl on January 03, 2019, 02:32:11 pm ---Hi again, Well when i said 'change' i meant that would be JUST for testing, to see the real difference between doing the peak turn on and doing the zero point turn on. I just wanted to be clear about that. The simplest way to test would mean a simple change, and that would be to get rid of the peak turn on circuit. ...<snip>... That's what you dont want and that is what makes switching at the peak effective because then the max is 20 amps (which is the normal run current peak). When you do this you could also watch for a time when you happen to plug it in at the positive peak and at the negative peak. You should see a normal sine then. If you happen to plug it in at zero degrees but the input sine is about to go negative, then the first peak will be a maximum negative peak, which following the above example could be as high as -40 amps. It doesnt take long to see the different phase angle responses this way, although it sounds kind of strange to do it this way :-) Hopefully your setup can handle that first 40 amp peak though. If not, you may have to add more resistance in series with your load to keep it lower just for the random plug in tests. Please let me know what happens if you do. --- End quote --- Well, for start, once my prototype is built, I will start with smaller inductive load 1st and later to my 1KVA isolation transformer. I get it, you want me to see & experience what happened with and without the softstarter ? CMIIW Yep, definitely will do that, still waiting for my relay to arrive. :'( Btw, I only have one HV 20Mhz differential probe. |
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