Will...
Wow...that's a scary paper. But, I just don't understand it. ...And
the calculations don't make sense. For example, the paper insists that
zero voltage switching will cause horrible problems and computes the
current as
I = E/R = 120V / 1.5A = 80 Amps
...which reads to me as 120V not 0V. So where does the 120V come from
if the line is crossing through zero????
Maybe they're talking about phase control SCR's or something. Maybe
they're talking about some extreme power system with very high power
factor. But a zero crossing switch turns on once in a condition with
the transformer at idle and the line a zero volts. There's no residual
magnetism or stray currents running around. There's no DC in the windings.
Also there a reference to switching an SSR at 90 degrees and it shows
some scope pictures with high currents in a small transformer. But 90
degrees amounts to switching the SSR on at the peak of the line cycle,
not a zero volts.
I'm very confused. Do you have any other references to this effect?
jeff, wa1hco
Will Matney wrote:
I thought I ought to give a link to the following PDF file by Potter &
Brumfield about the effects of using a zero-crossover switch with
transformers. From this, I think most will see how and why surge
currents act. Also, see the references (footnotes) on page 2.
Link; http://relays.tycoelectronics.com/app_pdfs/13c3206.pdf
Will Matney
Dennis12Amplify@aol.com wrote:
In a message dated 11/21/04 1:11:18 PM Central Standard Time,
wa1hco@adelphia.net writes:
Dennis...
A transformer has inductance when open circuit, but when something's
connected to its secondary, that impedance get transferred to the
primary by the square of the turns ratio. So, when the transformer
runs open circuit, a nearly infinite impedance gets transferred to
the primary and it draw little current. When a transformer has a a
load such as a resistor or discharged capacitor, then the primary
impedance depends on the winding resistances, turns ratio and
coupling coeficient. For most transformers one can model the coupling
as perfect.
jeff, wa1hco
***Jeff,
***Thanks for the education.
***I still find it very hard to believe that there can be an almost
instantaneous change in the secondary current when the secondary
inductance is probably measured in multiples of HENRYS, and I know
it's been a long time since I worked in that area, but when I did, I
rarely saw 'M' values greater than .85 for a high voltage step up
transformer.
The turns ratio is a big part of the problem in this case, because
the ratio is so high, it is very hard to get a coefficient of
coupling much higher than that. Maybe the technology has changed and
that is no longer the case.
***Now if that same transformer was bifilar wound with a 1 to 1 or
even a 4 to 1 ratio I would understand completely and absolutely
agree with you.
***If what you are saying is true, then wouldn't the reciprocal also
be true and allow me to greatly extend the frequency range of my tube
stereo output transformer by making my speaker load impedance as low
as possible; like paralleling ten 4 ohm speakers for an output load
of only .4 ohms on the secondary of the output transformer? I
understand that the efficiency would be poor unless the turns ratio
was readjusted, but the audio bandwidth should be HUGE!
***I guess the best way to test this concept would be to run a
'significant', (like 1 ampere for a high power linear amplifier
transformer), amount of DC current through the secondary winding,
then short the primary and open the secondary circuit. If what you
say is true, there should be no high voltage spike when the secondary
discharges because of the shorted primary and the perfect coupling.
I, personally, would not want to be holding both ends of those
secondary leads while they were being open circuited......
***Regards,
***Dennis W. Ostrowski
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