voltage vs magnetic field effects

[Kanner]

I have an experiment which I am creating a step response into an electro-magnet. The principle frequency is 60hz. Core is hyperco 50 inductance of the coil is about 25mH. My output is measured with a gauss meter. When I change the drive amplitude, I am getting the amplitude change in the field. Additionally, I am getting a DC shift which will drift back to zero after about 6 cycles. This same pattern can be seen with other frequencies. My question is: Would this be expected by the result of the impulse or could it be inherent to the Gauss probe.

The data file attached shows the phenomena.

Thanks in advance.

 

[dgallup]

Looks like you have some remanance which is getting demagged by the 60 Hz sine wave. Did you anneal the core?

 

[BrianE22]

I was thinking it might be getting magnetized also - but I would expect to see some distortion in both the current and the measured field. Do your calculations show that the metal is close to saturation? Is there by chance a low pass filter on the probe output?

 

[BrianE22]

...and, is your scope AC or DC coupled?

 

[Kanner]

Thank you for your replies.

The cores were annealed.

My calculations show that we are not at saturation at these levels. I am assisting someone on this, I will get back with the filter and coupling information shortly.

 

[FeX32]

I don't think the core has remanence flux. When you apply a sinus voltage and then suddenly switch the amplitude you can sometimes create this DC offset inherently.
It can be due to the lag in current flow coupled with the amp switch. (for eg. if you have a + voltage and a + current when you lower the amplitude there is not enough potential to fully discharge the + to - fast enough). I know its a crude analogy, but it was my first thought.
You much have a large inductance.
Try it at a very very low frequency and it should go away.

I wouldn't have expected it to be so dramatic though. It is dynamic, but it's "momentum" seems to be very high.


Fe

 

[Kanner]

I have some more information. The gauss meter that is used has a voltage out port. The output of the Gauss meter is then sent to a Data acquisition channel which has impedance specs of 10Gohm in parallel with 100pF.

If the test is tried at higher frequencies, the phenomena is more pronounced.

 

[FeX32]

try low freq


Fe

 

[Skogsgurra]

What you see is the well-known DC component that you get when you apply a sine to any coil - with or without iron.

If you change amplitude at 90 (or 270) degrees phase angle, the DC component will be zero and you will not see the recovery from DC offset in the magnetic field.

Same thing as closing AC voltage close to zero crossing. That causes the same DC component and sometimes saturates cores so that breakers trip or fuses melt.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

Yes, I think Gunnar and FeX32 nailed it.

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(2B)+(2B)' ?