Transformer soft start with Triac and uC
blisca
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
Phil Allison
"blisca"
> 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.
** Only a total fuckwit used one of them with any kind of mains transformer.
Cos it guarantees a MAXIMUM surge every, single time !!!
> 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.
** Yep - that is what you get.
> 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.
** Huh ????
This wog imbecile must be on some kind of mind altering drugs.
> 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
** Huh ????
This wog imbecile must be on some kind of mind altering drugs.
> 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.
** No fooling ????????????????????????????
>> Could anyone please suggest me how a correct algorhithm should be done?
** Switching on at voltage PEAKS is one option.
But as mains transformers tend to have capacitive loads attached to them -
even this does not solve the problem.
> Diego
> Italy
** You said it - pal.
.... Phil
Sjouke Burry
Then short the resistor.
Or in other words: An inrush limiting relay.
Baron
Just stuff a thermistor in series, the type used for inrush current
protection.
--
Best Regards:
Baron.
Leo Baumann
by full load. It wasn't to solve within the first sin-waves.
mfG Ingenieurbüro Baumann
Jamie
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.
John Larkin
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
Joerg
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.
John Larkin
wrote:
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
Phil Allison
"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
Martin Riddle
"blisca" <bliscachiocciolinatiscalipuntoit> wrote in message
news:4bca1e61$0$1122$4faf...@reader3.news.tin.it...
Toroids like to hold their magnetic state longer that EI cores. You
might be turning on the core in the same direction that it was last
magnetized. An inrush current limiter should help, long enough for the
core to walk back to a balanced state.
EI cores have a natural gap that help eliminate this effect.
In larger equipment that is pulsed, such as X-ray, there is usually some
sort of means to record the last magnetized state of the primary
transformer. So that the core does not saturate and put stress on the
SCR's.
Cheers
whit3rd
> 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.
Yep, that's why you shouldn't ever use zero crossing for the first
cycle
of turnon of an inductive load. Turn on at 90 degrees (voltage peak)
instead, and KEEP it on by continuous gate drive on the triac,
If you must, for isolation, use a transformer to drive the triac,
consider
replacing it with a relay. It's more power-efficient, anyhow.
Phil Allison
"Martin Riddle"
>
> Toroids like to hold their magnetic state longer that EI cores. You might
> be turning on the core in the same direction that it was last magnetized.
** Toroidal transformer cores are not left in a magnetised state by the
simple act of switching off the AC supply.
A core may become magnetised if it is switched onto the AC supply for only a
half or one cycle and so does not settle down - happens when the AC fuse
blows or breaker trips at switch on.
> EI cores have a natural gap that help eliminate this effect.
** But not by any great amount.
E-cores of 800VA or more need soft start circuits too.
.... Phil
Paul E. Schoen
"blisca" <bliscachiocciolinatiscalipuntoit> wrote in message
news:4bca1e61$0$1122$4faf...@reader3.news.tin.it...
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.
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
blisca
"Phil Allison" <phi...@tpg.com.au> ha scritto nel messaggio
news:82um97...@mid.individual.net...
>
> "blisca"
>
> > 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.
>
> ** Only a total fuckwit used one of them with any kind of mains
transformer.
> Cos it guarantees a MAXIMUM surge every, single time !!!
Tnanks,at first
Yes,the guy that designed it is mainly a firmwarist,i guess that to him
turning on a big inductive load looks easy as changing a bit from 0 to 1
>
>
> > 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.
>
> ** Yep - that is what you get.
>
>
> > I 'm trying to implement a soft start using a random phase opto triac
> > instead of a zero-crossing one.
>
>
> ** Huh ????
> This wog imbecile must be on some kind of mind altering drugs.
>
Please dont use words that you are used to hear ,i did not choose that name
http://www.alldatasheet.com/datasheet-pdf/pdf/53865/FAIRCHILD/MOC3051-M.html
No load ,the problem appears even with no load
>
> > Diego
> > Italy
>
> ** You said it - pal.
>
> .... Phil
>
Thanks Phil
blisca
"Sjouke Burry" <burrynu...@ppllaanneett.nnll> ha scritto nel messaggio
news:4bca23a5$0$14132$703f...@textnews.kpn.nl...
Thanks,probably this will be an unavoidable option,but very expansive in
terms of space
If possible I would like to solve with "intelligent" start ,without adding
bulky components.
blisca
"Baron" <baron....@linuxmaniac.nospam.net> ha scritto nel messaggio
news:hqdan9$nfs$2...@news.eternal-september.org...
I tried NTCs,and they do have a limiting effect,but in case of repetitive
ON the NTC becomes useless
blisca
"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> ha scritto nel
messaggio news:kkhks59bovn5bsr8l...@4ax.com...
Phil Allison
"Phil Allison"
> "blisca"
>>
>> > 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.
>>
>> ** Only a total fuckwit used one of them with any kind of mains
> transformer.
>>
>> Cos it guarantees a MAXIMUM surge every, single time !!!
>
> Tnanks,at first
** Wot an idiot.
> Yes,the guy that designed it is mainly a firmwarist,i guess that to him
> turning on a big inductive load looks easy as changing a bit from 0 to 1
** Wot an idiot.
>> >> Could anyone please suggest me how a correct algorhithm should be
>> >> done?
>
>> ** Switching on at voltage PEAKS is one option.
>>
>> But as mains transformers tend to have capacitive loads attached to
>> them -
>> even this does not solve the problem.
>
> No load ,the problem appears even with no load
** Hey fuckwit !!
Try learning to fucking READ !!
There is NO need for a soft start with a 400VA mains toroidal.
.... Phil
Phil Allison
"blisca"
> If possible I would like to solve with "intelligent" start ..
** Be a very good idea for YOU to START art with some intelligence of
your own !!!!
Define the real problem !!!
Define the application properly too -
cos we have no fucking idea what wacky shit you dagos are up to.
..... Phil
blisca
"Martin Riddle" <marti...@verizon.net> ha scritto nel messaggio
news:hqdvlc$7n2$1...@news.eternal-september.org...
> magnetized. An inrush current limiter should help, long enough for the
> core to walk back to a balanced state.
Thanks,Martin
I hoped that feeding the primary with a growing "time slice" of current
,inverted in direction at every semiperiod was equivalent to alternatively
start by a codition of alternatively growing induction,i.e. at semiperiod T1
B=1,then B=-2;then B=+3 and so on.
My impression is that rather i start every time by B=0;but i'm not shure
blisca
"whit3rd" <whi...@gmail.com> ha scritto nel messaggio
news:81a21783-2454-4d1e...@j21g2000yqh.googlegroups.com...
On Apr 17, 1:47 pm, "blisca" <bliscachiocciolinatiscalipuntoit> wrote:
> A friend is having lot of problems trying to switch primary winding of a
> 400VA toroidal transformer by a triac driven by a zero crossing optotriac.
Yep, that's why you shouldn't ever use zero crossing for the first
cycle
of turnon of an inductive load. Turn on at 90 degrees (voltage peak)
instead, and KEEP it on by continuous gate drive on the triac,
Thanks,
Probably i'm wrong,but powering at 90 degree i still have peaks,but let me
try again
blisca
"Paul E. Schoen" <pa...@pstech-inc.com> ha scritto nel messaggio
news:BLyyn.13146$Db6....@newsfe05.iad...
Really impressive to me,that i never dealed with such currents
Wow,this deserves to be read twice or more
Thanks for sharing your team's experiences,Paul
>
blisca
"Jamie" <jamie_ka1lpa_not_v...@charter.net> ha scritto nel
messaggio news:omryn.106037$NH1....@newsfe14.iad...
I'll check it,thanks
blisca
"Phil Allison" <phi...@tpg.com.au> ha scritto nel messaggio
news:830475...@mid.individual.net...
Phil,unless i would need a model to paint an as..h.le portrait
blisca
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
Phil Allison
" Ignorance is blisca = code scribbling dago cunt "
>> > If possible I would like to solve with "intelligent" start ..
>>
>>
>> ** Be a very good idea for YOU to START with some intelligence of
>> your own !!!!
>>
>> Define the real problem !!!
>>
>> Define the application properly too -
>>
>> cos we have no fucking idea what wacky shit you dagos are up to.
>>
>
> you (i mean YOU) dont need to know anything more,
** Is that so ??
You a member of the local Mafiosa too ??
Sure are pig ignorant and stupid enough to be one.
> and i dont need your help ...
** Listen here - you fucking retarded dago SCUMBAG !!
YOU have NO FUCKING IDEA what or how much help you need.
Cos you are a LYING CODE SCRIBBLING IMBECILE.
Makes you a TROLL on this NG.
So FUUUCK OFFF !!!!!!!!!!!!!!
.... Phil
Nico Coesel
>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@nctdevpuntnl (punt=.)
--------------------------------------------------------------
Baron
>> 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.
Jamie
What he needs is something simple like this.
http://www.grainger.com/Grainger/items/1DTK1?Pid=search
Jamie.
Joerg
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
Jim Thompson
wrote:
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
Don Lancaster
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: d...@tinaja.com
Please visit my GURU's LAIR web site at http://www.tinaja.com
Paul E. Schoen
"Don Lancaster" <d...@tinaja.com> wrote in message
news:837ila...@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