On 2/15/2015 5:34 PM, Chris Jones wrote:
On 16/02/2015 04:55, mike wrote:
On 2/15/2015 3:46 AM, Phil Allison wrote:
mike wrote:
if you say so.
** Wot a smug prick you are.
Seems to be your opinion of everyone.
As long as you stay out of saturation,
** No chance - you are all wet right now.
the main component of the
primary current is due to the shorted secondary.
** The secondary is not shorted.
Once again, your signature condescending tone declares that
the other guy is always wrong.
Once again, you nitpick instead of attempting to understand.
Communication is difficult enough, even when you try.
When you try NOT to understand, you're just being you.
I'm sure you win a lot of arguments when the other guy
just gives up trying to influence your thinking.
My presence in this thread is to help others get the most
out of their MOT welder. I gave up trying to influence you
long ago. Just trying to give some balance to the view.
I think I'll give my SCR the benefit of switching on when there's zero
voltage.
** More fool you.
There it is again.
... Phil
Although he is rude, Phil is usually right, and this time is no exception.
I suggest you get a hall-effect current transducer connected to a DSO
been there done that
AEMC MR461 current probe.
TEK TDS540 scope.
I designed my first production forward converter ~40 years ago.
so
that you can measure inrush current, and try out switching the
transformer on at both the peak of the mains voltage, and also at the
zero-crossing,
with the secondary heavily resistively loaded,
and look at the current waveforms. When you run out of
working triacs you could also google it.
The key to understanding this is to realise that the magnetic flux is
proportional to the time-integral of the applied voltage, and that in
continuous operation the flux is normally close to zero when the voltage
is close to maximum, and the flux is close to maximum when the voltage
is close to zero. Have a look at the first link he
Are we talking about inductors or transformers with load resistors
that cause a steady state primary current 2X their design rating?
http://www.te.com/commerce/DocumentD...=CS&DocLang=EN
http://sound.westhost.com/articles/inrush.htm
http://en.wikipedia.org/wiki/Inrush_...t#Transformers
Chris
Thanks for the links. I like to learn new stuff.
I'm still trying to get my head around why the graphs in the first
link are reversed in time, but if I stand on my head, it looks
like the drive signal is optimized to maximize inrush current.
I don't have any argument with that. You can certainly manage
the drive so the core saturates.
My attempts were to arrange the drive signal to MITIGATE inrush
current.
The key point is in the wikipedia link:
"Worst case inrush happens when the primary winding is connected at an
instant around the zero-crossing of the primary voltage, (which for a
pure inductance would be the current maximum in the AC cycle) and if the
polarity of the voltage half cycle has the same polarity as the remnance
in the iron core has. (The magnetic remanence was left high from a
preceding half cycle)."
end quote
If you always turn off the current at the current zero crossing with
a positive voltage slope, then always turn on the next weld pulse
at zero voltage on the positive voltage slope, doesn't that leave
you in a remanence position to avoid saturation at the next turn on?
If not, why not?
The SSR is gonna turn off near zero current. About all I can control
is the slope of the voltage sinewave when I give the command.
To turn it on it's far easier to sense the zero crossing of the line
voltage than the peak. Isn't a major portion of the primary current
in phase with the primary voltage due to the resistive secondary load?
Isn't it the leakage inductance that causes the phase shift?
Under the control conditions described above where we control both
ends of the waveform to manage remanence and have a very low value
resistive load, how much would I gain by waiting for the peak line
voltage at turn on?
I'd go look, but it's stored behind a bunch of junk in the garage.
As I recall, I didn't make many measurements without load. But
with the secondary (almost) shorted in the weld mode, I don't
remember any horrible input current transients.
I do know that synchronization with the line made a major improvement
in the repeatability of the welds.
I'm up for some education.
My thinking was that, if not for saturation, the SCR would be less
stressed if I turned it on at zero voltage when the primary current was
zero. And from the unpowered state, the voltage and current can't
be anything but zero.
And that, if I could arrange the resting place on the B-H curve
from the previous pulse such that the first half-cycle wouldn't
saturate the core, that's the best I could do easily.
Measurements didn't show any horrible first cycle inrush.
Welds got more repeatable.
Let me say the same thing in different words.
If the load is linear resistive, the transformer current and voltage
will be approximately in phase. If the SCR shuts off at zero
current, the voltage will also be near zero volts (plus whatever the
leakage inductance allows).
Case 1, you start the next pulse in a nanosecond.
Isn't the initial current still pretty near zero?
Isn't the point on the B-H curve still about the same?
Case 2, you start the next pulse next week at the zero crossing
of the input voltage headed in the same direction.
What's the initial current?
What's the initial point on the B-H curve?
How is restarting it synchronously significantly different from
just leaving it running?
I'm not disputing the articles you posted.
I'm not saying anything about the general (worst) case.
I'm suggesting that this is how you engineer a spot welder using a MOT.
Where did my thinking go wrong?