Track & Wire Inductance

All conductors, such as track and wire, have 3 electrical properties. 

1) Resistance.
2) Inductance.
3) Capacitance.

This section talks about #2 or Wire Inductance.   To understand how this all relates to our layout wiring and track, lets first understand what a inductance consist of.


THE PHYSICAL LAWS OF WIRES:  INDUCTANCE

In a wire, inductance and magnetic fields are directly tied to each other by physics.  The rules are as follows:

A) Changes in magnetic field generates electricity in wires. (Think Generator where a magnetic field is rotated by an external rotation force.)

B) Current flow in wires generates a Magnetic Field.  (Think of your old science lab where you built or used a battery powered Electro-Magnet that you used to pick up nails.)

C) Magnetic Fields have the ability to store energy in them. 

D) The energy of a magnetic field is directly related to the level of current flowing in the wire.   Higher current flow means stronger magnetic field and visa versa.

E) Inductance is the electrical effect of the presence of a magnetic field.

F) Inductance opposes any changes in the current level flowing in the wire by using the magnetic field as the energy to do it with.

G) The Inductance value in the wire is directly proportional to the length of the wire involved.  Stated another way, it proportional to the length of the magnetic field.

So we have a 3 way relationship between magnetic fields around the wire, inductance in the wire and the current flow down the wire.  That is part of the reason why the topic is complex.


HOW COME THESE PROPERTIES, INDUCTANCE AND CAPACITANCE, WERE NEVER BROUGHT UP WHEN TRAINS RAN ON DC?

When layouts used plain old DC power, we never has to worry about these physical properties of inductance and capacitance.  It was always there, but these propertied never made itself known until DCC came along.  Why?  Because inductance and capacitance make themselves known when we talk about AC power or signals.   DC is not AC, so inductance and capacitance were never important properties to worry about.   DCC is a form of AC power and as such these two properties now enter into equation when dealing with long wiring.


WHY IS INDUCTANCE A BAD THING FOR DCC?

Inductance and it associated magnetic field can be potentially destructive if the high current flowing in a wire is suddenly broken (instant decrease in current) at any time.  The inductance's forcibly generates a independent voltage in the wire that will rise to ANY VOLTAGE NECESSARY to re-establish the broken high current flow.  The new voltage has nothing to do with the original voltage that was there.  It rides on top of it.   Inductance can instantly generate 1000's of volts in a fraction of second which will continue to rise up until the current level is re-established.  We see this in the form of a voltage spike.  The weakest insulating device will be where the voltage will arc over and breakdown the insulation.  But with DCC, we have electronics in our decoders that are a lot more sensitive to electrical votlage spikes.  A voltage spike that is strong enough will destroy electronic parts exposed to it instantly.


DETAILS OF INDUCTANCE IN ACTION

Change in current flow is "AC" no matter how long the change in current takes or last (time/frequency).  This opposition to changes in current flow (increase or decrease) is called AC resistance or otherwise known as impedance.  Inductance can do this by using the magnetic fields energy at its will.  It will take energy away from an increasing current in an attempt to hold it constant by storing it in the magnetic field making the field stronger.  It opposes the increasing current flow by generating an opposing voltage in the wire!  The opposite is true.  If the current decreases, it will take energy back from the magnetic field to boost the current flow in the wire again in an attempt to keep it constant.  It does this by raising the voltage in the wire!  This raising or lowering the voltage is ON TOP OF (relative too) what ever voltage is on the wire initially.