[Amps] Re: Step-start calculation

[jeff millar]

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 at aol.com wrote:
>
>> In a message dated 11/21/04 1:11:18 PM Central Standard Time, 
>> wa1hco at 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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>
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