Inductance

Below is a discussion about what is a INDUCTOR, the physical rules it follows and how this directly relates to the track we use.  Inductors have electrical properties that "Come Alive" in an electrical circuit when there is a changing current or some form of AC present.   In other words, inductors generate voltage like a battery between it two terminals when presented with AC current regardless of how it has been created. 

1) WHAT IS AN INDUCTOR?
2) WHY WIRES AND RAILS ARE INDUCTORS
3) INDUCTOR POLARITY
4) INDUCTORS GENERATE VOLTAGE
5) INDUCTORS HAVE AN AC RESISTANCE (IMPEDANCE)

One of the key things to understand about indutance is that it is present in EVERY CONDUCTOR MATERIAL.  


1) WHAT IS AN INDUCTOR?

To understand inductance, we need to start at the beginning.

Wire Basics.


A given electrical wire is simply a solid conductive path for current to flow through in moving from point A to point B.    For a complete circuit, a conductive path loop must be establish that allows current to flow from it power source back to it source.   A Battery is a good example of a power source.  A power supply is another.

In terms of conducting current, the conductors shape and material, as long as it is conductive, is not relevant but the round wire is the most common and flexible form. 

Current flow is measured in Amperes which is often abbreviated as simply "Amps".  It is a measure in VOLUME of the "electron flow" down the wire.  The higher the current flow, the higher the amperage in the wire is flowing.   Current flow does physical work and can generate heat and other physical properties when it is flowing.  Voltage is PRESSURE and it just pushes the current down the wire.  Both must exist for anything to happen.


Resistance: Comparison starting point.


As a reference point, I will first talk about resistance and it properties as a comparison point in talking about Inductance since most people can relate to it at some level.

The most common physical property of a conductor is its resistance.  Resistance is something you can measure with a simply Ohm meter.   Ohm is the unit of measure that describes the degree of opposition the wire has to current flow.   The amount of resistance is function of

1) The conductor's length.
2) The conductors cross section/diameter.
3) The material being used.

The resistance value is always the same per unit of length (Inches or feet) regardless of the TYPE of current flow being passed down the wire as in DC or AC current.   The longer the wire gets, the higher the value of resistance the wire has from end to end.For common copper round wire, we have standard wire sizes in fixed diameters, called Wire Gauge, that have corresponding standardized ohms per length values.

Resistance opposes the flow of current.  When current if forced down the wire, it will develop a voltage drop per unit length and a corresponding heat generation per unit length.  The voltage drop and heat generated are considered losses.   With appropriate choices in wire sizes for a given current level, these losses are typically minimal such that we most often ignore them.


Magnetic Field Property.


Like resistance, this is another property of a conductor.  

When current is flowing in a conductor it creates a magnetic field is created around the conductor.  The strength of the magnetic field is proportional to the level of Amps flowing.  

Like resistance, the magnetic field intensity is spread evenly over the length of the wire.  The longer the wire gets, the higher the value of inductance the wire has from end to end.

When you wrap a wire around a rod to make a coil, you can concentrate the magnetic field in a small area.  When each loop, or turn of the wire around the rod, it is placed right next to the next loop of wire and so on down the coil length in a tight formation, the magnetic fields of each loop are NOW right next to each other.  This allows the magnetic field that is normally distributed down the length of wire to become concentrated in a very short distance around the rod.   If the rod is made of material that has magnetic properties, the magnetic field becomes even more effectively concentrated at the ends of the rods. 

A magnetic field is potential energy that can do work.

You may already be aware of this magnetic field and coil concept if you remember your science classes in elementary school where you made a magnet by wrapping wire around a nail connected to a battery or power source.   With current flowing, you could take the nail and pick up any metal that has magnetic properties. 
Here is a website that shows you more about the magnetic field:
Making an ElectroMagnet. Cute.

Going a step furhter, a Motor is based on the coil magnet concept that makes rotational force from a rotating magnetic field that it creates intrnally.  However the magnetic field is not limited to doing something mechanical.  It can effect the electron flow in the wire.

As we have seen,  a magnetic field is created when current flows in a wire.  But the reverse is also true.  An outside magnetic field can force current to flow in a wire that normally does not have current flow.  An example of that is found in what we call a transformer.

A transformer takes the magnetic energy created by the input side coil (Primary) and it is coupled to the output side coil (Secondary) to transfer power.  The two coils are electrically insulated from each other.  However the two coils are magnetically connected to each other buy the transformer's use of a common magnetic core.  Most transformers are made of metal with strong magnetic properties to.  This allow the transformer to support a strong magnetic field which in turn means it can "transfer" large amounts of energy back and forth between the two coils as needed.   But regardless if there is a magnetic core or not, two simple insulated STRAIGHT wires running side by side can magnetically couple to each other current flow just like a transformer.  One wire being the primary and the other wire being the secondary.  The longer the run together side by side the more magnetic coupling they will have between each other.



Inductor Basics:

A inductor is very simple electrical device.   It stores and releases energy in the form of a magnetic field.  An inductor can convert changing current into voltage.  The inductance value is a reflection of the amount of energy the inductor can store for a given current.  Inductors use this ability to store or discharge energy to OPPOSE any change in current passing through it.  From an electrical circuit point of view, inductor can look like light load (Open Circuit) when the current increases through it OR a power source when the current falls through it. When there is no current changing through the inductor, the inductor literally disappears from the electrical circuit.



It is a very simple device that consist of single metal conductors of a given thickness.  A straight conductor forms an inductor with no special construction required.
To make a larger inductor in a smaller space, the conductor is spun into a spiral.  Each 360 degree spin of the conductor is called a "turn".   The conductor must has some form of insulation so that the conductor does not short out to conductor next to it.   All inductors have a magnetic core with the default core simply being air or "AIR CORE"



The electrical symbol for a inductor is:










2) WHY WIRES AND RAILS ARE INDUCTORS

Wire and rails are conductors and by default.  They are inductors since there is nothing special needed to make a inductor other than some kind of conductor.  The amount of inductance they have is a direct function of the length of the wire or rail.

You can figure out the wire inductance using a inductance calculator.

L(wire) = 2*Length*[ln (2*Length/Radius) - 0.75]nH

Where:

Length: Length of the wire in meters.
Radius: Diameter of the wire in meters.


3) INDUCTOR POLARITY

Inductors do not have a polarity since they are simple a wire.


4) INDUCTORS GENERATE VOLTAGE

There is an equation that describes the physics of what is happening.

V = L di/dt.

Where:

V = Voltage: This voltage spike is on TOP of the track voltage.

L = Henrys:  Inductance of the wire. (Function of wire length)

di = Amps:  Change in current flow in the wire.

dt = Time:  Change in time in which the current changed.

Y


5) INDUCTORS HAVE AN AC RESISTANCE (IMPEDANCE)

We know that the lower the resistance gets, the more current can flow.  The same is true with inductors.   The lower the reactance (AC resistance) gets, the more current can FLOW THROUGH the inductor.   Stated another simplistic way, a inductor is a resistor who's ohm value changes with applied frequency.  The higher the frequency, the higher the resistance and visa versa. This is the exact opposite of what a capacitor does.  The equation showing inductor reactance is:




Where:

Xl  = (Ohms) is the reactance value of the indcutor
∏   = 3.141 (Pi) is a mathematic constant.
f    = (Hertz) is the AC frequency of the applied voltage.
L   = (Henry) is the capacitance value of the inductor.

If you explore this equation, you will find that the reactance (ohms) gets HIGHER when:

1) you increase the inductance (size of the inductor value).
2) you increase the frequency content of the applied AC voltage.

or any combination of the above two. 

The term "Impedance" (Z) consist of two parts, series resistance (R) + reactance (X), combined into a calculated single value.  However for our purposes to keep things simple, lets just assume Reactance and Impedance are the same thing since they both use the same units of "Ohms".   In other words, we are not going to worry about the R value.