reactor saturation time

[alehman]

When an iron core reactor or transformer is driven into saturation, is there a finite amount of time required for saturation to occur?
Textbooks always discuss in terms of B-H curve, but time is not generally a parameter.

Alan
The engineer's first problem in any design situation is to discover what the problem really is. Unk.

 

[Gr8blu]

alehman: The slope of a B-H curve defines the relationship between B (magnetic flux density) and H (magnetic field intensity). For SI units, B will be measured in Tesla, and H in Ampere/metre). The permeability of the medium (ferrous or otherwise) is defined by the symbol "mu" (that funny little lower case u), which has the units of Henry/metre. The actual equation is: B = mu * H.

The relationship is nonlinear in ferrous materials, hence the "flowing" look to a hysteresis curve. Saturation occurs when it take more current to produce the same step-change in magnetic flux. This makes the curve look "flatter" at higher currents, and steeper at lower currents. In practice, the flux density changes at the same rate as the current changes.

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[alehman]

Thanks Gr8blu. I understand the B-H relationship and saturation in general.
My question is: is there a time factor between B and H? Say you apply a certain field H as a step function in time sufficient to cause saturation. Does the iron actually saturate instantaneously or does it take a certain amount of time? If so, I would assume it would depend on the residual flux in the core.

Alan
The engineer's first problem in any design situation is to discover what the problem really is. Unk.

 

[waross]

That would be the related to the time constant of the overall circuit.
You are applying a voltage to an XR circuit.
That circuit will have a time constant which will describe the rate of rise of the current.
The actual rise time will depend on the time constant of the circuit and the applied voltage.
There are two curves under consideration.
The first curve describes the characteristics of the inductor as an iron core inductor.
At saturation, further increases in current result in a curve describing the characteristics of the inductor as an air core inductor.
The higher the applied voltage, the faster the current will rise and the sooner the core will reach saturation.

--------------------
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[itsmoked]

Certainly, as you suspect, there is a finite time however it's likely far too fast to affect most things you'll probably care about. I suspect that it will be low microseconds but that'll be masked by the rapidly changing current being changed in turn by the change in H.

I believe the harder the magnetic structure is driven into saturation the longer it will take to back out of it.

Curious, why are you asking?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[prc]

My understanding is, it must be instantaneous as otherwise, the voltage developed in the secondary of a transformer should develop slowly; inrush current on switching should increase slowly. But we know these are instantaneous. I know it takes several days or weeks for the residual flux remaining in the core to disappear after switching off.

 

[itsmoked]

It's all being driven by the magnetic domains physically twisting. That takes time. Admittedly not much.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alehman]

Thanks all.
It was suggested that saturation may not be a significant concern in a current limiting reactor because the protective device (fuse or circuit breaker) would operate before saturation can occur. I said that doesn't seem right to me, but I would like to have something to back that up.

To take this a step further, one manufacturer (not the person referenced above) is proposing to furnish large (3000-amp continuous) steel-core current limiting reactors for my application with a design utilizing "opposing coils" and "gapped core" in their design. Is anyone familiar with this type of design? They have provided no details, but furnished some test reports from a reputable test lab that seems to support the claim. Other manufacturers have said air core is the only way to accomplish this.

Alan
The engineer's first problem in any design situation is to discover what the problem really is. Unk.