Transformer soft start with Triac and uC
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From: blisca on 18 Apr 2010 07:15 "Paul E. Schoen" <paul(a)pstech-inc.com> ha scritto nel messaggio news:BLyyn.13146$Db6.9360(a)newsfe05.iad... > > "blisca" <bliscachiocciolinatiscalipuntoit> wrote in message > news:4bca1e61$0$1122$4fafbaef(a)reader3.news.tin.it... > > Hi,please be patient and read the description of the situation: > > > > A friend is having lot of problems trying to switch primary winding of a > > 400VA toroidal transformer by a triac driven by a zero crossing optotriac. > > Having a bit of measuring instruments i tried to help him. > > The net voltage is 220V,the secondary winding is left open,no load at all. > > Measuring the current peaks by means of a current clamp probe and a cheap > > digital oscilloscope i measured current spikes over 80A. > > > > After a quick research on web i have plenty of explainations of that > > fact.This is a very simple one > > http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm > > The last image should rapresent the situation i'm dealing with. > > It looks like at the zero voltage starting the current from zero,rather > > from > > the negative peak,this current is able to generate enough flux to saturate > > the core,with expected effects. > > > > I 'm trying to implement a soft start using a random phase opto triac > > instead of a zero-crossing one. > > I arranged a zero crossing detector to trigger a microcontroller,and i can > > fire the optotriac and the triac in every moment during each semi-period. > > > > The first soft start attempt consists in this > > 1)detect the zero > > 2)wait until 0.1 ms before next zero(end of semiperiod) and turn on the > > Triac > > 3)detect next zero,Triac turns off a bit later,i guess,when the current > > reaches zero > > 4)wait until 0.2 ms before next zero and turn on the Triac,current flows > > now in opposite direction than in 2) > > 5)same as 3) > > and so on > > > > When i'm close to 90% of the semiperiods i turn off the Triac for a couple > > of seconds,i dont want to remain in ON state so to measure current spikes > > only during the "soft start" > > > > Sadly ,big current spikes are still detected. > > > > Could anyone please suggest me how a correct algorhithm should be done? > > > > I always had helps in this NG,thanks in advance > > I designed several firing circuits for high power circuit breaker test sets, > some of which use toroidal cores with a total of 10 kVA and output currents > of 20 kA or more. We used dual SCRs, and the most recent firing circuit uses > a PIC18F2420. We also had inrush current problems that were worse for the > toroids. Here are some things we discovered and techniques we tried. > > NTC thermistors were effective, but they caused waveform distortion at the > initial firing, which was unacceptable for our purpose of high current > primary injection testing of circuit breakers. They needed a clean starting > waveform with minimal DC offset. Also, when the thermistors heated up, they > were no longer effective until they cooled down, which could take several > minutes. > > We learned that the ideal initial firing angle was somewhat less than 90 > degrees, and closer to 70 degrees, because the load was partially resistive. > We used a variable initial phase angle control and adjusted it for equal > peak amplitudes of all half-cycles. > > We found that the transformer core would be magnetized if there were an > unequal number of positive and negative half-cycles. So we designed our > controller to produce even numbers of half-cycles. Actually we programmed it > to drive the gates for about 4.7 cycles when we wanted 5, because the SCRs > would remain in conduction until current reached zero. But if we used a time > corresponding to exactly five cycles, sometimes the inductance of the load > would carry into another half-cycle. > > But sometimes we could not control how long current flowed, because the > breaker under test would trip depending on the current and a net DC > component would exist, which magnetized the core. We found that, under those > circumstances, reversing polarity of the next pulse train did not produce > the high instantaneous current we had otherwise. On a 480 VAC 200 amp > service, this was sometimes enough to trip the main breaker for the entire > building, and we estimated well over 2000 amps. We could hear the conductors > slap against the conduit, and loose cables to the test set would jump. Really impressive to me,that i never dealed with such currents > > We found that it is necessary to keep current on the gates at all times, > even on the SCRs that were not normally conducting. This was because of the > reactive load, where current and voltage are out of phase. Otherwise we > often saw waveform distortion, especially at the crossover points. > > A proposed modification to the design was to apply a series of diminishing > phase-delayed pulses after the breaker tripped, to demagnetize the core. > This was never implemented, but we proved its possible benefits by adjusting > the test set to a lower output, initiating a pulse, and then returning to > the higher setting where otherwise it would have pulled a huge current. > > Another problem we had was unintentional half-wave triggering of the SCR > when primary power was switched on and off at the source. We could not > tolerate a large R-C snubber across the SCR because it caused leakage > currents which were excessive and potentially unsafe. So we used a large > snubber of perhaps 30 uF and 100 ohms across the load side of the circuit, > which included a tap switch that was the main culprit. This reduced the > random firing to some extent, and also improved the power factor of the > load, but it also caused the output to remain on for a while after the SCRs > were turned off because of the LC network. Ther series resistor was > necessary to reduce the "Q" and minimize this effect, but the exact values > of the components had to be adjusted for optimal overall performance, with > some trade-offs. > > I hope that helps answer your questions and provides guidance for a > successful implementation. If you are interested in more details, or perhaps > obtain one of our firing circuits for your own modification and use, please > contact me. > > Paul E. Schoen > www.pstech-inc.com Wow,this deserves to be read twice or more Thanks for sharing your team's experiences,Paul >
From: blisca on 18 Apr 2010 07:16 "Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> ha scritto nel messaggio news:omryn.106037$NH1.90787(a)newsfe14.iad... > blisca wrote: > > > Hi,please be patient and read the description of the situation: > > > > A friend is having lot of problems trying to switch primary winding of a > > 400VA toroidal transformer by a triac driven by a zero crossing optotriac. > > Having a bit of measuring instruments i tried to help him. > > The net voltage is 220V,the secondary winding is left open,no load at all. > > Measuring the current peaks by means of a current clamp probe and a cheap > > digital oscilloscope i measured current spikes over 80A. > > > > After a quick research on web i have plenty of explainations of that > > fact.This is a very simple one > > http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm > > The last image should rapresent the situation i'm dealing with. > > It looks like at the zero voltage starting the current from zero,rather from > > the negative peak,this current is able to generate enough flux to saturate > > the core,with expected effects. > > > > I 'm trying to implement a soft start using a random phase opto triac > > instead of a zero-crossing one. > > I arranged a zero crossing detector to trigger a microcontroller,and i can > > fire the optotriac and the triac in every moment during each semi-period. > > > > The first soft start attempt consists in this > > 1)detect the zero > > 2)wait until 0.1 ms before next zero(end of semiperiod) and turn on the > > Triac > > 3)detect next zero,Triac turns off a bit later,i guess,when the current > > reaches zero > > 4)wait until 0.2 ms before next zero and turn on the Triac,current flows > > now in opposite direction than in 2) > > 5)same as 3) > > and so on > > > > When i'm close to 90% of the semiperiods i turn off the Triac for a couple > > of seconds,i dont want to remain in ON state so to measure current spikes > > only during the "soft start" > > > > Sadly ,big current spikes are still detected. > > > > Could anyone please suggest me how a correct algorhithm should be done? > > > > I always had helps in this NG,thanks in advance > > > > Diego > > Italy > > > > First of all, you need to start at the peak or falling of the peak > wave form. Personally, I would sync lock a few cycles to make sure we have > a clean wave form and not static from contact closure.. This should > always be running in the Uc so if you do receive noise interruptions on > the line, you can at least allow it to keep using the > same timing cycle until your verification loop expires, in which case > you should stop any further triggers on the Triacs. In my last circuit > i used 12 cycles before I determine I had a good signal. This has to > match your line system. 50hz = 10ms, 8.3ms for US. etc. > > > Have 2 timing algorithms operating, one to sync lock with the > service coming in, and the second to phase delay ~ 7 ms before > sending a trigger to the triac. This assumes you're on a 50hz system.. > > You have a counter in there where you can decrement the delay shift > back to lets say 1 or 2 ms to then get the primary to turn on at just > above the base or maybe keep it on? > > Remember to turn off the output when ever you see the input reference > drop while soft starting, other wise, you'll be kicking in the next > cycle before you can complete the soft start incase you want to have a > ramp up > effect. I'll check it,thanks > > I suppose you can soft start on a single cycle once you have confirmed > you have proper line reference and are locked in but it might be a plus > to allow the connected load on the secondary side to see a few cycles > come up soft. This may not be desired and that is your decision. That > being the case, then sync lock the line for a solid reference then > perform a initial ~ 7ms pulse. (that's 7 ms after the base of a > reference coming in gets at least 5% or more in level. > > Using a bridge type circuit into a compression scheme to get a > rather square wave from your line voltage should give you what you need as a > reference for you to work with. Make sure you pass this through a > resistor or optical coupling so you don't get a phase shift issue, other > wise, you'll have more problems to content with. > > P.S. > Yes, I've made things like this before with lots of power delivery on > 3 phase systems. > > > > > > > > > > >
From: blisca on 18 Apr 2010 07:30 Thanks to you all(Phil A. too),for spending time helping me describing detailed experiences,or even just for reading my post. I read carefully every answer . If i would solve i will post the adopted solution,hoping it could be of some interest to other newbies as i am; Please forgive my English. Have a good Sunday. Diego Milan,Italy
From: Nico Coesel on 18 Apr 2010 10:05 "blisca" <bliscachiocciolinatiscalipuntoit> wrote: >Hi,please be patient and read the description of the situation: > >A friend is having lot of problems trying to switch primary winding of a >400VA toroidal transformer by a triac driven by a zero crossing optotriac. >Having a bit of measuring instruments i tried to help him. >The net voltage is 220V,the secondary winding is left open,no load at all. >Measuring the current peaks by means of a current clamp probe and a cheap >digital oscilloscope i measured current spikes over 80A. >Could anyone please suggest me how a correct algorhithm should be done? There are special NTCs for this purpose. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico(a)nctdevpuntnl (punt=.) --------------------------------------------------------------
From: Baron on 18 Apr 2010 10:11
"blisca" <bliscachiocciolinatiscalipuntoit> Inscribed thus: >> Just stuff a thermistor in series, the type used for inrush current >> protection. >> >> -- > Thanks Baron, > I tried NTCs,and they do have a limiting effect,but in case of > repetitive ON the NTC becomes useless Yes that is a problem with them, they take a while to cool down before reuse. Would a saturatable reactor in series work. DC from the rectified transformer output could be used to control the ramp up. -- Best Regards: Baron. |