Transformer soft start with Triac and uC
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From: Jamie on 18 Apr 2010 07:20 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 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: John Larkin on 17 Apr 2010 19:48 On Sat, 17 Apr 2010 22:47:23 +0200, "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. > >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 > > It's tricky using a triac in series with a transformer primary. It can be both hard to keep them on, and hard to turn them off. RC snubbing is critical, and gate drive may have to be sustained, not just pulsed. NTC inrush limiters work well. They look like big black disk ceramic capacitors. Simple. You can also switch in a series resistor with one triac or relay, to let things charge up/demagnetize, then use another for the direct connection. But that's hard on resistors. Toroidal power transformers can be tricky that way. We had one amplifier that, when you switched it on, you could hear the wires in the wall jump. We were peaking around 1000 amps sometimes. These transformers should have an auxiliary primary winding for startup. Maybe 20% extra primary turns but lots of copper resistance. John
From: Joerg on 17 Apr 2010 20:57 John Larkin wrote: > On Sat, 17 Apr 2010 22:47:23 +0200, "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. >> >> 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 >> >> > > It's tricky using a triac in series with a transformer primary. It can > be both hard to keep them on, and hard to turn them off. RC snubbing > is critical, and gate drive may have to be sustained, not just pulsed. > > NTC inrush limiters work well. They look like big black disk ceramic > capacitors. Simple. > They are good but they won't work in quick-sequence brownout situations. Happens here on occasion. Power goes off, tries to come back on in rapid-fire bursts. Sometimes stays on, sometimes not. > You can also switch in a series resistor with one triac or relay, to > let things charge up/demagnetize, then use another for the direct > connection. But that's hard on resistors. > > Toroidal power transformers can be tricky that way. We had one > amplifier that, when you switched it on, you could hear the wires in > the wall jump. We were peaking around 1000 amps sometimes. > But not all are that way. I've got one here in the office. 1000W, medical grade, the good stuff, I believe made in Sweden. It's on a 15A circuit and neither the lights flicker when turning it on nor did it ever trip the breaker. The only weird thing is that it breaks into a faint growl once in a blue moon. Some sort of resonance with the chassis. > These transformers should have an auxiliary primary winding for > startup. Maybe 20% extra primary turns but lots of copper resistance. > Then you might as well put a proper start-up circuit in front, it's smaller. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 17 Apr 2010 21:08 On Sat, 17 Apr 2010 17:57:36 -0700, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> On Sat, 17 Apr 2010 22:47:23 +0200, "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. >>> >>> 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 >>> >>> >> >> It's tricky using a triac in series with a transformer primary. It can >> be both hard to keep them on, and hard to turn them off. RC snubbing >> is critical, and gate drive may have to be sustained, not just pulsed. >> >> NTC inrush limiters work well. They look like big black disk ceramic >> capacitors. Simple. >> > >They are good but they won't work in quick-sequence brownout situations. >Happens here on occasion. Power goes off, tries to come back on in >rapid-fire bursts. Sometimes stays on, sometimes not. I did a CAMAC crate with a huge power transformer, and used inrush NTCs in the transformer primaries. They seemed to work fine, even for quick off/on cycles. Can't explain why. > >> You can also switch in a series resistor with one triac or relay, to >> let things charge up/demagnetize, then use another for the direct >> connection. But that's hard on resistors. >> >> Toroidal power transformers can be tricky that way. We had one >> amplifier that, when you switched it on, you could hear the wires in >> the wall jump. We were peaking around 1000 amps sometimes. >> > >But not all are that way. I've got one here in the office. 1000W, >medical grade, the good stuff, I believe made in Sweden. It's on a 15A >circuit and neither the lights flicker when turning it on nor did it >ever trip the breaker. > >The only weird thing is that it breaks into a faint growl once in a blue >moon. Some sort of resonance with the chassis. > > >> These transformers should have an auxiliary primary winding for >> startup. Maybe 20% extra primary turns but lots of copper resistance. >> > >Then you might as well put a proper start-up circuit in front, it's smaller. Got any suggestions? A couple of triacs and a BIG resistor will work, but that's a hassle. And resistors don't like this longterm. John
From: Phil Allison on 17 Apr 2010 23:17
"Leo Baumann" From the OP: " 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. " > ... i have solved that problem with a big switch-on inductance and a relay > by full load. It wasn't to solve within the first sin-waves. ** No real need exists for any soft start circuit with a 400 VA toroidal - the in-rush surge currents are manageable by use of slow acting fuses. The larger sizes may need one - starting at about 600 VA, particularly if the load is a big bank of filter caps. The OP is barking mad trying to use uPs and opto-coupled triacs. ...... Phil |