Hi,
I am putting together a high current supply and wondering about inrush current.
The situation is a center-tapped 110V 3kVA transformer, feeding a bridge rectifier and a large, high-ripple current capacitance:
. . -------- +70V . ----| |-------o------> 20A rms >--- .|| / | AC + | | | .|| / 55 | | ----- 20,000uF [LOAD] .|| / | | ----- | .|| /-- | | | | .|| / | | | | .|| / 55 | | | | .|| / | AC - | | | .|| /----| |-------o------< 20A rms
Energy storage in 20K uF at 140 V is 196 Joules (i.e., watt-sec), which means that charging the cap is probably not going to blow a thermal breaker rated at 30 amps or more (which is what you'll need for your 2.8 kW load). You can't count on leakage inductance to limit the current much, either, because the initial core state can include a flux offset at the moment that you energize the circuit. A common way to limit this type of inrush is to add a series power resistor and a time delay relay. The relay contacts short out the resistor after a delay. The relay can be electromechanical or SSR type. Paul Mathews
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The good solution is a resistor + triac. The resistor limits the inrush current to a reasonable value. After that the resistor gets shorted by a triac.
The substation will survive; the diode bridge and the power switch will burn out eventually.
I wouldn't rely on that.
True. The thermistor limiter is a simple solution for the low power.
VLV
A much bigger problem is the switch on transients of the big transformer (occasional core saturation). Spice does not model this. Two switch on's out of five maybe OK, the other 3 can (try to) draw many thousands of amps . If it's a one off design, I'd be inclined to fit a small, current limit series resistor in one of the supply leads, which is shorted out by a relay, 1/4 second delayed (say) after power up. You then kill 2 birds with one stone. For production, a softly ramped up triac in place of the resistor, would be better
"John Devereux"
** See:...... Phil
might be a good idea to use a time delay on set of contacts that would bride a power resistor for a soft start.
-- "I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
Yes (bridge) a power resistor. You should also turn the load during this process to avoid the potential of welding the contacts. I guess you could use a SPDT relay if there is no electronic way to turn the load off.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
I've used the disc-type thermistor inrush limiters at the kilowatt level, two units, one in series with each of the two primary windings of a 120/240 power supply. They seemed to work fine, even when I teased the power switch various ways; don't quite know why.
John
That's what they do for power factor correction caps.
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ If God is perfect then why did He create discontinuous functions?
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