Transformer soft start with Triac and uC [Design]

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From: Martin Riddle on 17 Apr 2010 23:52

From: whit3rd on 17 Apr 2010 23:56

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,

If you must, for isolation, use a transformer to drive the triac,
consider
replacing it with a relay. It's more power-efficient, anyhow.

From: Phil Allison on 18 Apr 2010 01:45

"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

From: Paul E. Schoen on 18 Apr 2010 03:44

"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.

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

From: blisca on 18 Apr 2010 04:53

"Phil Allison" <phil_a(a)tpg.com.au> ha scritto nel messaggio
news:82um97F5rcU1(a)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

> > 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.

No load ,the problem appears even with no load
>
> > Diego
> > Italy
>
> ** You said it - pal.

>
> .... Phil
>
Thanks Phil

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