In message
<blupr02mb1162ba09cc137bdb35ca50ac1...@blupr02mb116.namprd02.prod.outlook
.com>, dated Sun, 29 Mar 2015, Brian Oconnell <oconne...@tamuracorp.com>
writes:
nrush peak for 50/60Hz transformers mostly from magnetizing current ->
core saturation and residual flux, and of course input V. As the
saturation curve does not extend past the pi/2 inrush peak, any further
inrush past a few mSec is typically from filling up the coulomb buckets
on the secondary side. This is a tau-based thing, so both DCR and Z
would be used to determine the current-interrupt component's expected I2T.
I did many tests on inrush to determine the requirements in IEC/EN
61000-3-3. The worst case is when the supply was switched off with the
core flux at maximum, which then decays to the remanent point (H_r,
B_r), because the core is a closed magnetic circuit.
If the supply is then switched on at a voltage zero, with the voltage
rising in the direction to (try to) increase the core flux, the core
saturates hard, since Br with modern materials is nearly equal to B_sat,
and the voltage is thus 'trying' to reach nearly 2B_sat. The
permeability drops to 1, so the inductance drops to a very low value,
and the only other significant impedance in the circuit is the winding
resistance.
In most cases, the very high inrush current lasts for half a cycle (we
are considering RMS values, not peak, so our values are nominally
averaged over a whole cycle), but the currents in the following
half-cycles don't decay to the steady-state value for several more
cycles, and the current in the second half-cycle may be large enough to
need taking into account in looking at the necessary I^2t value of the
protective device.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
When I turn my back on the sun, it's to look for a rainbow
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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