Re: [CAD_CAM_EDM_DRO] Re: Power supply softstarts

Graham Stabler wrote:

>Jon, an you explain exactly what you mean when you say "intergrates
>the field"
>
>

This is the heart of any inductor, and why it causes the current to lag
the voltage
by 90 degrees. As voltage is applied to the winding, the building of
magnetic
field lines in the core resists the flow of current. The longer the
voltage is applied,
the higher the field becomes, until it eventually saturates the magnetic
material.
At that point, the inductance falls dramatically, and the current rises
rapidly.
A transformer is designed so the normal effect of the line voltage at
the nominal
frequency never causes saturation. However, for cost reasons, only enough
cross-sectional area of the core is used to keep below saturation.

One of the important observations of this 90 degree phase shift is that
the magnetization
of the core is always at it's peak when the line voltage crosses through
zero. Also,
the magnetization is exactly zero when the line voltage peaks. It takes
the complete
rising quarter cycle (from zero voltage to the peak) to reduce the
magnetization from
the LAST half cycle, and so only as the voltage is at the peak will it
start increasing the
magnetization again on this half cycle. The magnetization is always
increasing as the
voltage drops from the peak, and is always falling as the voltage rises
to the peak.
(The magnetization IS in phase with the current.)

Mathematically, the current is the integral of the voltage. If you take
the integral of
a sine wave, you indeed get another sine wave that lags the voltage by
90 degrees.

Now, you can see if the circuit is turned on at the voltage zero
crossing, that the
core starts at near zero magnetization, instead of having that reversed
magnetization
for this half cycle to de-integrate. The integral of the applied
voltage times time
is now TWICE what it would have been had the current and flux started
out at the
reverse peak at that moment. That's what causes even unloaded
transformers to
draw a huge current surge when turned on abruptly.

Jon