[Amps] Re: Step-start calculation

[jeff millar]

Will...Indeed that's what it says. 

But what's all this stuff about magnetizing current in transformers??  
Someone needs to explain that effect.  The paper seems to say that a 
transformer with nothing connected to its seconday may look like a short 
circuit (!!??) because the core needs to magnetize.  But inductance 
resists an increase in current.  That's why people recommend choke input 
power supplies, to resist the current surges.  The energy absorbed by a 
transformer is small because the inductance is large and the current is 
small.  I've never seen a surge when switching on a transformer, yes it 
may hum a bit, and pull a small amount of current, but at a fraction of 
it's rating, not a multiple of it's rating.

Another thing, what's this concept that surges build up over several 
half cycles?  An AC transformer gets 100% magnetically polarized, 
depolarized, and polarized the negative direction on every line cycle.  
There's no build up of magnetic energy unless its ferroresonant or 
something.

On the timing of the switch...maybe in 1974, SSRs switched slowly, but 
today, suppliers focus on precise timing because accurate zero crossing 
means less emitted RFI noise and less filtering required.

I just don't know what the paper means to the type of transformers and 
power supplies hams use.  I Googled for other references to this effect 
and didn't find anying relevant.  The best paper referred to magnetizing 
current as small compared to load current.

jeff, wa1hco

Will Matney wrote:

> Jeff,
>
> I think what they're talking about is the core saturating during the 
> first few 1/2 cycles. When this happens, there is little inductance 
> and thus no impedance except the DC resistance of the winding. I think 
> their example was a primary with a 1.5 ohm DC primary at 120 volts. 
> 120 / 1.5 is 80 amperes for a 150 VA transformer which is pretty darn 
> small. The ZSC SSR would need to be able to withstand this surge for 
> the first few 1/2 cycles until the core settles down to normal. Their 
> thinking is it would be better to apply full line voltage at the peak 
> instead of at zero voltage. below is a quote from the texts;
>
> "A “zero-crossover” SSR does not always turn on at precisely zero 
> voltage.
> It takes perhaps a millisecond or more for the circuitry to react. 
> Therefore,
> the load switch may not be fully on until load voltage is perhaps 15 
> to 20
> volts. In this event, surge current isn’t as great, but it is still 
> potentially
> destructive. Also, a random turn-on SSR may, at times, turn on at or near
> zero cross-over. The best method of turning on transformers and other
> saturable, highly-inductive loads is by use of a peak voltage turn-on 
> device.
> Turn-on at peak voltage results in minimal surge, if indeed any surge is
> present at all."
>
> In other words, because of the time it takes for them to switch, 0 
> current draw can't be acheived. 1/4 cycle ( the time for the wave to 
> peak) is .00417 sec. or 4.17 milliseconds. From this one can see 
> there's not much time for the ZSC SSR to switch on before the voltage 
> rises to a good amount if the time is 1-2 milliseconds. The only way 
> for it to switch 0 current would have a 0 sec. switching time.
>
> In Richs original example, he was using a transformer way larger than 
> the above example. I forget what the primaries DC resistance was but 
> it was very little, 0.14 ohm maybe at 240 volts? At least that's the 
> way I read it.
>
> Best,
>
> Will Matney