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Marko

Wed Dec 05 2007, 01:02PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Guys, I would really like to understand why this happens before building a inrush-limiter circuit for those things :p (The big potentiometer actually works well for now).

The core is sized so that it almost saturates +ve on one half cycle, and almost saturates -ve on the other half cycle. Any less agressive than this and you are wasting iron, or specification.

An unloaded transformer looks like a high value inductor. In the ideal case, you switch on at the peak of the voltage waveform (yes, zero-volt switching is for lamps and interference, not for inductors). In the following 1/4 cycle, the input voltage goes to zero, and the Volt.second area under the voltage quadrant ramps the core flux up to near saturation. The next 1/4 sees a -ve voltage quadrant, which ramps the flux back to zero, and finishes with peak -ve volts input. The next two 1/4 cycles now ramp the flux to near -ve staturation and back in a mirror image of the first two. This is the normal operating action of the transformer.

Now consider the case when you switch on at zero crossing. The volt.second area under the first quadrant ramps the flux up to near saturation. Now the input volts are at peak. The next quadrant, being of the same polarity, continues to ramp the flux up, this time to near 2x saturation - oops.

Ok... I still don't quite understand this

Current generates the flux, and ampere turns on the core simply must not reach value where it saturates.

Now, if I switch at zero cross (bad way), I don't see why would current start rising from zero from 1/4 period, since it wasn't at negative minimum during time 0, but 0..
Am I at least on right track?

Still how exactly does the saturation through several cycles come out from this? How does load affect that? I'd love if someone could bother to draw a crude diagram for me.

These things never got my attention until now.

marko

Steve Conner

Wed Dec 05 2007, 02:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It's like NeilThomas said, the magnetic flux density in the core is controlled by the time integral of the applied voltage. Not the current: current controls MMF, which is a different parameter.

The core is only designed to handle a flux density corresponding to one quarter-cycle worth of volt-seconds, because that's all it needs to handle in the steady state. But if you switch on at a phase of 0 degrees, you're asking it to handle a half-cycle worth: which is twice the saturation flux density.

It will keep on saturating every half-cycle until this unwanted "DC offset" of flux dies away, which it does with a L-R time constant determined by the magnetizing inductance and the winding DC resistance. The load has little effect.

Dr. Slack

Wed Dec 05 2007, 02:30PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Current generates the flux ...

well, yes, up to a point, but current turns out to be the depedant varaible. The applied voltage is the independant variable. The resistance is so low, that the current that flows is determined by the tiny difference between the large applied voltage, and the large back emf being generated by the core flux changing. For a "good" transformer, we can neglect the winding resistance entirely, and simply equate the applied volts and the back emf with negligable error.

The unit of flux is the Weber, or volt.second, which is why the integrated area under the voltage waveform (volts high X seconds across) controls the core flux. So if you start from Vmax, you have a 1/4 cycle area to change the flux from 0 to max, but if you start from v=0, you have a half-cycle's worth of flux to build from 0 to twice max.

The transformer will draw whatever current it needs to (very low winding resistance remember) to keep the flux changing to match the applied voltage. While the core flux is low and its permability is 2000 or so, it needs little current to generate the flux. When it's into saturation, the permaebailioty plummets, and it draws much more. The negligable winding resistance approximation breaks down here, and the resistance will limit the inrush to much less than a perfect inductor would take.

You could do worse than get a SPICE simulator, my favourite is Simetrix, but there are plenty of other free alternatives out there, and do a tranasient simulation of a sine or cosine voltage from time zero with zero initial inductor current. Don't forget a sniff of series resistance to enable the decay of the transient current. Although a linear inductor model will only show a 2x surge, a saturating inductor will draw rather more

Dr. Drone

Wed Dec 05 2007, 03:36PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Marko

Wed Dec 05 2007, 06:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys... I think I'm getting your point!

The first pic is near ideal inductance with little series resistance, takes lots of cycles for current to settle out. Source is started at voltage zero cross (worst case);
As I said, there's no reason why would current go up from 0 at voltage peak, when it was never under zero at all. Red is voltage, blue is current;

This is more realistic case, current gets to nominal value after 2-3 cycles.

I like to look after current, since flux density is B=mi*N*I/l. I don't see anything wrong in considering flux either (B*s), since s is fixed anyway.

Volt/seconds are what gives rise to current though.

So from other point of view, for some time my volt/seconds sum won't be 0 because I have one spare quarter cycle which needs to be averaged out.

Marko

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