DC current causing inductor to saturate?

[PhaseShift]

I have heard that DC current can cause an inductor/reactor to saturate quickly. Does anyone know why this is?

Does it saturate the inductor faster than then the same amount of AC current?

 

[winnie]

For low frequencies (including 60Hz for most applications), the amount of core saturation is exactly the same for a given _instantaneous_ current value. So if you have DC of 1A, you will have essentially the same saturation as at the peak of an AC current that peaks at 1A.

Since AC current is usually measures with RMS, this means that a DC current of 1A will have _less_ saturation than at the peaks of an AC current with an RMS of 1A.

But I think that you actually want to ask a different question. Your words said AC current and DC current, but I think you really want to ask about AC voltage and DC voltage.

The current flow in an inductor _changes_ at a rate that is defined by the inductance and the applied voltage. When you apply an AC voltage to an inductor, the current cycles up and down, and an AC current flows. When you apply a DC voltage to an inductor, the current just goes up and up and up, until it gets limited by the resistance of the inductor (or the source). This means that a very low DC _voltage_ will _eventually_ supply enough current to saturate the inductor.

-Jon

 

[Besoeker]

I have heard that DC current can cause an inductor/reactor to saturate quickly. Does anyone know why this is?

Does it saturate the inductor faster than then the same amount of AC current?

The flux in a core is essentially the product of voltage times time. In an ac circuit that product limited. You can still saturate the core if the ac voltage is beyond design limits but in normal operation the flux reverses every half cycle and so does the flux before it gets to saturation level.
DC doesn't reverse. The voltage time integral is infinite.

You can get inductors or chokes that are designed not to saturate with DC. They are used, for example, in the DC link section of a variable frequency inverter. Generally there is an air gap in the core to avoid saturation in normal operation.

 

[SG-1]

Here is a related example, case study, from Fluke.

http://support.fluke.com/find-sales/download/asset/2103547_a_w.pdf

 

[Electric-Light]

It doesn't take much second harmonic(which causes asymmetry) to cause transformer to get pissed at all. There is a pretty big transformer inside computer UPS and if you use any appliance, like a torchiere light that uses half-wave diode to dim or reduce power, the hum will get loud, just like the Fluke case study.

Aside from these half-wave rectified loads, it also happens when a diode blows in a bridge.

 

[gar]

100630-0845 EST

A useful reference is "Electric and Magnetic Fields", by Steven S. Attwood, 1949, John Wiley.

Part III is titled "The Ferromagnetic Field".

With ferromagnetic materials you dealing with hysteresis curves.

A definition of saturation is given by Attwood on page 333. "B (flux density) therefore reaches a definite saturation value Bs when the magnetized material has made its maximum contribution, whereas B continues to increase with H indefinitely."

Although flux density is related to the volt-time integral any practical inductor has internal resistance and thus is current limited and therefore the voltage across the pure inductive portion becomes 0 after a period of time.

A DC voltage (current) applied to a real world ferromagnetic inductor may or may not force the inductor into saturation, but it will cause a displacement on and of the hysteresis curve. As you change the magnitude of the DC component to a ferromagnetic inductor you change the inductance of the inductor.

Power transformers are designed to reach flux densities that take the core material near or into saturation. This causes peak current to occur at the maximum volt-time integral. This occurs twice per AC cycle, once + and once - . If a DC bias current is applied to the core either via the secondary or primary the hysteresis curve becomes unbalanced and depending upon on the magnitude of unbalance high peak currents may occur. This causes extra mechanical noise and heating.

.

 

[mull982]

Although flux density is related to the volt-time integral any practical inductor has internal resistance and thus is current limited and therefore the voltage across the pure inductive portion becomes 0 after a period of time.

Since you are saying that flux density is related to the volt-time integral can it be stated that the reason DC current saturates inductors is because the volt-time integral is always positive and therefore constantly increasing with time to some value of saturation? As opposed to an AC voltage which will have a postive and negative integral value for each cycle?

I dont see what you are saying about the voltage across an inductor becoming 0 after a period of time?

Is part of the reason DC saturates an inductor due to the fact there is no reactance and thus the DC current will be much larger due to the fact that it is only the small resistance of the inductor that is limiting the current?

 

[gar]

100701-0704 EST

mull982:

First, a DC current in a ferromagnetic inductor does not necessarily cause the core to saturate. The magnitude of the DC in combination with the AC will determine whether saturation occurs.

From Attwood:
"The total amount of the magnetization, as represented by the number of flux lines, depends on the magnitude of the current (or current-turns), on the shape of the circuit carrying the current, and, to a marked degree, on the magnetic characteristics of the medium as represented by the permeability u."

With a ferromagnetic material as the flux density varies the permeability varies, and thus the inductance varies. The permeability varies with a hysteresis effect, and therefore the hysteresis curve for the particular material must be considered. Note: an air core coil has no saturation point or effect, and no hysteresis curve other than a straight line, no loop.

I suggest that you try to find a book similar to Attwood's and study it for a better understanding of magnetic circuits than what I can provide here.

.

 

[gar]

100701-0824 EST

mull982:

I dont see what you are saying about the voltage across an inductor becoming 0 after a period of time?

When you create the equivalent circuit of a real world inductor it is drawn as an ideal inductor (no internal resistance) and a resistor. To study the transient or steady-state operation of the circuit you write the differential equations for what you want to study and then solve these.

By observation if you connect a DC voltage, such as a battery, to the series RL circuit you see that the t=0+ i = 0, and for t = infinity i = V/R. Assuming zero initial energy in the inductor. From the differential equation it is found that the current between 0 and infinity is related to an exponential curve.

Clearly at infinity all the voltage drop is across the resistor and none across the inductor.

Most books on differential equations will describe some simple electrical circuits. Most slightly advanced electrical circuit theory books will also describe differential equation analysis of transient conditions and maybe steady-state as well.

What a DC current does in addition to an AC input to a ferromagnetic inductor is to bias the hysteresis curve into an unbalanced condition. The hysteresis curve will have an unsymmetrical shape causing a greater peaking of the current waveform on one half of the AC cycle.

An experiment:
I used a Signal Transformer 241-6-20, a 1N5625 diode, a 330 ohm current sense resistor, a Variac, and a dual channel oscilloscope. The resistor and diode are connected in series with the primary. One end of the resistor needs to connect to the common (neutral) of the Variac. Use a differential input to one scope channel from the resistor. Connect the other channel to the transformer secondary. The secondary voltage is the rate of change of flux.

Adjust the the source voltage from 0 to 120 and observe the change in the waveforms. Do not exceed the full rated primary RMS current for a great amount of time.

The effects of saturation cause an increase in peaking of the primary current and a change in the distortion of the secondary voltage.

.