Where does the magnetic field go???

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haklesup
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Re: Where does the magnetic field go???

Post by haklesup » Mon Mar 14, 2011 12:02 pm

The inductor is the dual of the capacitor, and voltage is like the dual of current.
This is primarily true in the context of the time domain math expressions that describe their action in a circuit but breaks down in various ways when you look at the physical processes that make each work.
This brings up another interesting question. If there are only two parts in a circuit, how can you tell if they are in parallel or in series? Both circuits look the same.
Series and parallel designations are with respect to the source. if there are just two (2 terminal) components connected together, you don't quite have a circuit yet. Once you add the source, it should be obvious.

Bygar
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Re: Where does the magnetic field go???

Post by Bygar » Sat May 28, 2011 10:58 am

Your magnetic field collapses when you remove the current.
In larger inductors, e.g. electro-magnetic brake coils used in industrial apps.
can retain the magnetic field for a considerable length of time.
Sometimes to the detriment of the app., my experience with a brake coil
using 220 Volt D. C. which required a back EMF diode maintained its field
for several seconds. The problem was to collapse the field as quickly as possible.

dyarker
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Re: Where does the magnetic field go???

Post by dyarker » Sat May 28, 2011 11:58 pm

... to collapse the field as quickly as possible
Put a resistor in series with the diode. In the example you mentioned, the resistor should drop about 200V at the max current of the brake coil.

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MrAl
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Re: Where does the magnetic field go???

Post by MrAl » Mon May 30, 2011 4:34 am

Hi,

Yes, the coil energy can be considered as volt seconds so if the discharge voltage is raised the coil discharge can happen much faster. The application has to be able to sustain the reverse voltage however, but that is often ok.
A resistor works because it allows a larger voltage drop during discharge, but of course the high voltage has to be accounted for so that nothing else blows out. A zener is also an idea that is often used because the voltage is more predictable.
LEDs vs Bulbs, LEDs are winning.

bconroy
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Re: Where does the magnetic field go???

Post by bconroy » Mon Jun 06, 2011 12:05 pm

Hi,
The magnetic field exists as a result of electrons flowing through the inductor. As the electron current raises from zero to max electron flow, the magnetic field expands. When the electron flow stablizes at max flow, the field is no longer changing. When the electron flow reduces from max flow to zero flow by opening the circuit, the magnetic field collapses to nothing. So think of the magnetic field as a result of the momentum built up in the movement of the electrons. Only a changing magnetic field can induce a force into a nearby wire or inductor. Electrons flowing at a constant rate will not transfer any power.

Bygar
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Re: Where does the magnetic field go???

Post by Bygar » Fri Aug 12, 2011 1:12 pm

When you disconnect an inductor, the magnetic field which was present
collapses. Thereby inducing a voltage of opposite polarity, the magnitude
of which can be very large.
Note that most designs with DC relays use a reverse diode to dump
the induced voltage., to protect the device used to to control the relay.
If the inductor is of a large value you will get considerable delay of release
of the relay.

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Bob Scott
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Re: Where does the magnetic field go???

Post by Bob Scott » Sun Aug 14, 2011 1:26 am

This is an old thread, but I noticed that the equations for stored energy are missing. I bet the O.P. is long gone. :sad:

For an inductor:
E = 1/2 * L * I^2

For a capacitor:
E = 1/2 * C * V^2

E is energy in Joules (or Watt-Seconds, same thing).
L is inductance in Henries.
C is capacitance in Farads.
I is current in Amps.
V is voltage in Volts.
-=VA7KOR=- My solar system includes Pluto.

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