Transformer soft start with Triac and uC
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From: Jamie on 18 Apr 2010 22:32 Baron wrote: > "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. > What he needs is something simple like this. http://www.grainger.com/Grainger/items/1DTK1?Pid=search Jamie.
From: Joerg on 18 Apr 2010 12:44 John Larkin wrote: > 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. > We had that problem with ultrasound machines, back in the days when digital circuits were extreme guzzlers. NTCs didn't work in situations where power could flicker. Nothing bad happened in our machines but breakers popped and in a hospital that's a real nuisance. >>> 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. > You have to add a hold-off circuit that, for example, says "enough already" after the x-th start attempt and puts in a pause interval so nothing gets hot. If you want to get by with the smallest possible resistor you need to set the number of starts per interval to one but that may not work well in areas with shaky power distribution, like this one possibly is: http://imghost1.indiamart.com/data2/YR/AI/MY-1696986/power-distribution-250x250.jpg -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 18 Apr 2010 13:23 On Sun, 18 Apr 2010 09:44:12 -0700, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> 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. >> > >We had that problem with ultrasound machines, back in the days when >digital circuits were extreme guzzlers. NTCs didn't work in situations >where power could flicker. Nothing bad happened in our machines but >breakers popped and in a hospital that's a real nuisance. > > >>>> 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. >> > >You have to add a hold-off circuit that, for example, says "enough >already" after the x-th start attempt and puts in a pause interval so >nothing gets hot. If you want to get by with the smallest possible >resistor you need to set the number of starts per interval to one but >that may not work well in areas with shaky power distribution, like this >one possibly is: > >http://imghost1.indiamart.com/data2/YR/AI/MY-1696986/power-distribution-250x250.jpg Around 1980, during my disco music "career", I was designing boom boxes, and all kinds of dancing and dimming lights (frequency and amplitude sensitive). To dim neon, I used integer-cycle switching, plus a DC safety sensor. Never croaked a neon transformer :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Don Lancaster on 21 Apr 2010 02:01 On 4/17/2010 1:47 PM, 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 > > > > > > > > > > > > The trend these days when driving transformers is to go reverse phase control. In which the output is turned on immediately and off later in the cycle. This requires a true linear switching device, rather than a triac or scr. Advantages are ridiculously less RFI (you switch slowly...) and better control of transformers and fluorescents. LSI/CSI has parts. -- Many thanks, Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss: http://www.tinaja.com/whtnu.xml email: don(a)tinaja.com Please visit my GURU's LAIR web site at http://www.tinaja.com
From: Paul E. Schoen on 22 Apr 2010 03:42
"Don Lancaster" <don(a)tinaja.com> wrote in message news:837ilaF9ndU1(a)mid.individual.net... > > The trend these days when driving transformers is to go reverse phase > control. In which the output is turned on immediately and off later in > the cycle. This requires a true linear switching device, rather than a > triac or scr. > > Advantages are ridiculously less RFI (you switch slowly...) and better > control of transformers and fluorescents. > > LSI/CSI has parts. I suppose that means an IGBT or power MOSFET bridge. That would be an option, and the device could probably be fairly small if used in conjunction with an electromechanical relay as I saw in some "hybrid" configurations on the Crydom website. The problem with solid state controls is the pesky forward diode drop or On resistance that make for rather high power dissipation during extended power on conditions. Electromechanical contactors for high voltage, high current, and inductive loads must often be made larger than required for just current carrying capacity and breakdown voltage because of switching transients and arc control. If you can make initial connection to power and final disconnection by means of a solid state control, the contactor need only handle the current and be able to provide safe voltage breakdown for open contacts. Motor control is now moving toward inexpensive small three phase induction types driven by cheap PWM modules, even for items as small as food processors and blenders, and especially for things like washing machines. Paul |