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Twinsen
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Join Date: Nov 2010
Posts: 3

Inrush Current vs Magnetizing Current

11/18/2010 7:42 AM

Hi all,

I couldn't really find a answer to this question specifically so here goes.

I'm confused as to the difference between 'inrush current' and 'magnetising current' for machines like a power transformer.

I have understood that the inrush current was a current that can be as high as 20 times In on transformer startup due to saturation of the core occuring because of the remanent flux combined with the new flux from the applied voltage.

I have also read that the magnetising current is only a small (maybe 1% or so) of the nominal load current and is there continuously. But other people have told me that this magnetising current is the same as inrush.

If anyone could help me clear this up that would be great.

Thanks

Twinsen
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electricalexpert65
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#1

Re: Inrush Current vs Magnetizing Current

11/18/2010 8:23 AM

One can say that the "in-rush" current is the transient stage of the magnetising current. Actually, the 1% magnetising current you are talking about is the "watt-component" of the magnetising current or the no-load current of the transformer.
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Yuri B.
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#8
In reply to #1

Re: Inrush Current vs Magnetizing Current

11/20/2010 3:36 PM

"Watt-component" of the current drawn by the unloaded transformer is iron losses.
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North of 60
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#2

Re: Inrush Current vs Magnetizing Current

11/18/2010 8:57 AM

My understanding is...

"Inrush" is the current draw present when the inductor is first energized. With no power on the windings, and discounting residual flux levels, the windings appear as almost a pure resistive load. So the current draw is large on startup until the inductive reactance of the winding starts to impede current flow. As the flux rises, the current level drops to the magnetizing level.

I am not sure what level the inrush can reach, that would be a product of the I=Z/R relationship of the resistive load of the winding. Of course as soon as the flux levels in the core start to rise, the load now becomes inductive so the relationship changes.

Therefore... the "magnetizing current" is the demand to maintain the magnetization of the winding regardless of the current flow required to transfer power to the secondaries.

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North of 60
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#3
In reply to #2

Re: Inrush Current vs Magnetizing Current

11/18/2010 9:41 AM

Opps... that last formula should read I= V/Z.

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Yuri B.
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#6
In reply to #2

Re: Inrush Current vs Magnetizing Current

11/20/2010 6:28 AM

With no power on the windings, and discounting residual flux levels, the windings appear as almost a pure resistive load. So the current draw is large on startup until the inductive reactance of the winding starts to impede current flow.

I have stated the same view in one thread on this forum. Then my thought was not liked by many somehow - they argued: the coil does not present resistance at first start up (even at first cycle of the AC wave, it seems. The coil is at all times the inductive reactance, current lagging the voltage at any point).
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North of 60
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#7
In reply to #6

Re: Inrush Current vs Magnetizing Current

11/20/2010 1:22 PM

Yes... but only once the flux starts to rise. Upon energization, the coils will appear to be almost a short circuit, hence the very high inrush. This presents as a resistive load albeit with a very low resistance value.

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JRaef
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#4

Re: Inrush Current vs Magnetizing Current

11/18/2010 2:21 PM

Have a round of GAs on me...

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North of 60
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#5
In reply to #4

Re: Inrush Current vs Magnetizing Current

11/18/2010 2:50 PM

Why thank you kind Sir...

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Yuri B.
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#9

Re: Inrush Current vs Magnetizing Current

11/20/2010 3:44 PM

I would put it not it terms of "difference", but in terms of "relation": the bigger part of the whole current comprises the magnetizing current, the less the inrush (and vice versa.)
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