Question about magnetic field interaction

[chadj2]

Suppose I had two air core electromagnets wound in a bucking configuration (ie North poles opposing each other) and they are also tightly coupled. Also assume it is 12 volts pushing the current through both electromagnets. If I was to terminate the current to one of the electromagnets very quickly obviously that same electromagnet would induce a massive voltage as part of its inductive reactance. Assuming my components were able to sustain this voltage spike without failing, what would likely happen to the current flow in the other electromagnet? It would seem that the voltage induced by the electomagnet who had its current terminated would also temporarily terminate or substantially reduce the current flow through the other electromagnet. But on the other hand in a field coil excited alternator the exciter current doesnt seem to change in magnitude even though it is in the presence of strong changing and opposing magnetic fields which result from current flow in the pickup coils. Any magnetic experts willing to offer their insights? Or maybe some good resources to check.

 

[electricpete]

"It would seem that the voltage induced by the electomagnet who had its current terminated would also temporarily terminate or substantially reduce the current flow through the other electromagnet"
Yes, I agree with this statement.

Phi1 = L1 i1 - M i2
Phi2 = - M i1 + L1 i1
(the negative sign in front of M is due to the fact that the currents from i1 and i2 drive flux in opposite directions).

Let circuit 1 be the one that is switched off. All you need to know from circuit 1 is the rate of change of i1. In the simplified scenario that the current ramps to 0 linearly, then the di2/dt is a short-duration negative rectangular pulse.

V2 = dPhi2/dt = - M * di1/dt = L2 di2/dt + R2 i2

V2 + M di1/dt = = L2 di2/dt + R2 i2

We know everything about the lefthand side. V2 is a constant. di1/dt is that negative rectangular pulse. We know the initial condition on i2 (I2=V2/R2).

It's an easy problem to solve. Could do with Laplace or with excel or just visualize the result. i2 will decrease during the pulse. Once that short pulse is over, i2 returns again toward the steady state I2=V2/R2 with an L2/R2 time constant.

If it is an air core system with modest wire sizes, L2/R2 may be very short and the current may return to it's steady state I2 before you ever notice.











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[electricpete]

Correction in red:
V2 = dPhi2/dt = - M * di1/dt + L2 di2/dt + R2 i2

V2 + M di1/dt = L2 di2/dt + R2 i2

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[chadj2]

Thank you for your answer electricpete.

 

[electricpete]

The attached spreadsheet provides a simulation of the scenario described:

http://home.comcast.net/~electricpete1/SwitchOffCkt1WatchCkt2.xls

You can vary the parameters in green to see the results.

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