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Arguyle

Member
Location
michigan
Hi, my name is Bobbie and I'm a 3500 hour apprentice at Chrysler. I have a question about my AC theory class that my teacher has not made clear for me. I do not want to post in the wrong forum, but I reviewed the Education forum and it looked to be mostly code stuff so I will ask here and if I am out of line I will be happy to do as instructed.
We are currently studying inductance and my question is this: If electromagnetic inductance is what is created around a wire when AC current goes through it, could we tap into that magnetic field to apply the voltage to something else at the same time?

Thanks/sorry in advance,
Bobbie
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
What is around a wire with ac is a time varying magnetic field.

Inductance is a measure of the amount of voltage developed across the wire at a given current (and frequency).

'Tapping into' that magnetic field is what a transformer is. It is what induction heating is. etc.

You realize of course that nothing is 'free', tapping into that magnetic field requires an increase in overall power.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151002-0812 EDT

Arguyle:

At what Chrysler plant do you work?

The answer to your question is YES.

If you consider a perfectly staight conductor of electric current with no magnetic material near the conductor, then there are circular magnetic flox lines created around and concentric to the conductor. Do a Google search using the search string
---
magnetic field around a wire
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See the third view of "Images for magnetic field around a wire" for a good view of the concept. Click on the title and many more images are shown.

If the current thru the conductor is a constant value (note an electron beam in a cathode ray tube is a conductor even though there is no wire), then these flux lines remain fixed and constant. If an AC current flows thru the conductor, then the flux lines grow, collapse, grow in the opposite direction, and collapse continuously.

If you have a conductive loop and have changing flux lines thru the interior of the loop, then a voltage is magnetically induced in that loop. An equation that describes this is e = k*N*dphi/dt. k is a scaling constant, N is the number of turns in the coil, and dphi/dt is the rate of change of flux lines relative to time. This rate of change can be obtained by moving the coil thru a constant magnetic field (a DC generator) or by keeping the coil fixed and changing the flux lines coupling the coil (a transformer).

Back to the single straight conductor. The flux lines around the conductor lie in planes perpendicular to the wire. If a circular wire (one turn closed coil) is placed in a plane perpendicular to the conductor, then no current is induced in the coil whether the current in the conductor is changing or not, because no changing flus lines pass thru the coil interior. The flux lines and the coil are in the same plane.

Now rotate this one turn coil 90 degrees so that the conductor is in the plane of the coil. Many of the conductor's flux lines pass thru the coil. No current is induced in the coil if the current in the conductor is constant, but if the conductor current is not constant, cqn be a changing uni-directional current, then there is a current induced in the coil.

By the law of the conservation of energy if electrical energy is coupled from one circuit (the straight conductor) to another circuit (the coil) and consumed in said circuit, then that energy must come from the input circuit.

Suppose you have a wire feeding a precise 1 kW resistive load. Measure the power input to this feeder. It measures exactly 1 kW, assuming that any wire resistance is part of the load. We have available a current transformer and its associated meter. When this current measurement equipment is connect to the circuit to be tested it requires 0.07 W when measuring the 1 kW load.

Without the 1 kW load current passing thru the current transformer the load power reads 1 kW precisely. Pass the load current thru the current transformer, and now input power reads 1000.07 W. Energy was coupled magnetically from the straight wire to the measurement system ( current transformer and its meter).

.
 
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