Magnetism

A-3 LT 1-4, A1 LT 19-21

Magnetic Circuit

refers to the path along which the flux lines follow as they form their complete loops. The magnetic flux in a circuit is directly proportional to the magnetomotive force and is inversely proportional to the reluctance of the magnetic circuit.

Magnetic Flux

Is the total number of flux lines in a magnetic circuit. It is the counterpart of current in an electric circuit.

Magnetomotive Force

is equal to the number coil turns in the circuit times the current through those coils.

Magnetizing Force

is the measure of the magnetomotive force per unit length in a magnetic circuit.

Reluctance

is the opposition to the setting up of flux lines in a magnetic circuit. It is the counterpart of resistance in an electric circuit.

Flux Density

is the measure of the amount of flux per unit cross-section of the magnetic circuit.

Permeability

is the measure of ease with which flux may be set up in a unit length and cross-section of a material. It is a measure of flux density per magnetizing force.

- B-H Graph

Sometimes referred to as a Magnetization Curve. A graph of flux density with respect to magnetizing intensity are plotted for a magnetic material.

Current Density

refers to the amount of current flowing per cross-section of the conductor.

Voltage Gradient

is the measure of electromotive force per unit length in an electric circuit or field. It is the measure of the voltage between two points divided by the distance between the same two points.

Resistivity

is the resistance of a unit length and cross-section of a certain material.

Conductivity is the reciprocal relationship.

Saturation

when all of the domains in the magnetic material have been aligned with the magnetic field, the gain in flux density for a given increase in magnetizing force is no more than would occur in air for the same increase.

Theoretical Saturation

occurs at the point where all of the domains in the magnetic material have aligned themselves with the magnetic field.

Practical Saturation

occurs when it is no longer practical to try to increase the flux density. That is, the gain in flux density for an increase in magnetizing force is so small that it is not worth it.

Hysteresis

is the lagging of the flux density behind the magnetizing force.

Hysteresis Loop

is a material's magnetization curve plotted to show what happens to flux density as the magnetizing force is increased, decreased, increased in the opposite direction, and then returned to zero.

Residual Magnetism

is the amount of flux density remaining in the magnetic material after the magnetizing force has been removed. To remove Residual magnetism, a magnetizing force must be applied in the reverse direction.

Coercive force

is the amount of magnetizing force required to remove the Residual magnetism.

Active Length of a Conductor

is that portion of the conductor which actually cuts 1-1 through the lines of flux as the conductor moves. The normal way of increasing the active length of the conductor is by adding more loops of conductor in the magnetic field.

Velocity

of the conductor is normally controlled by adjusting the speed of the prime mover.

Prime Mover

The device that supplies energy to the generator, such as diesel engines, turbines and windmills.

Left hand Rule

an easier way to determine the direction of the current resulting from induced voltage is to use Fleming's Left Hand Rule. Where the index finger of the left hand points to the. direction of magnetic flux (field), the thumb points in the direction of conductor-motion relative to the magnetic field, and the middle finger points in the direction of the induced current. Another helpful way to remember this is:

Thumb

Thrust

First

finger Flux direction

Middle finger

Motion of electrons (induced current)

Eddy Currents

when flux lines cut across an iron core, the core is a conductor and will therefore have currents induced in it. These currents called "Eddy Currents" circulate in loops within the core. When the core is made up of laminated steel, this helps to reduce eddy currents.

Air Gaps

are placed in magnetic circuits to allow clearance of moving parts. Since all of the magnetic flux lines must pass through the air gap, the air gap is in series with the circuit. The air gap increases the total reluctance of the circuit.

Len's Law

"The direction of the induced EMF (electromotive force) must be such that any current resulting from it will develop a flux which will oppose any change in the original flux"

Faraday's Law of Electromagnetic Induction

"The magnitude of voltage induced in a tum of wire is proportional to the rate of change of flux passing through (or linked with) that tum". Expressed in the following formula: e = BIv, Where: e = voltage generated. B = flux density of the field in teslas .. 1 = active length of the conductor in the field in meters, v = relative velocity of the conductor at right angles to the magnetic field, in meters per second.

Cycle

the distance from one point on a waveform to where it starts to repeat itself.

Frequency

the number of complete cycles that occur in one second.

Magnetomotive Force (mmf)

F_m = N x I, ampere-turn

Permeability

µ = β / H, webers per ampere-turn

Flux

Φ = F_m / R_m, weber

Reluctance

R_m = F_m / Φ, ampere-turns per weber

Flux Density

β = Φ / A, Tesla = webers per square metre, T

Magnetizing Force

H = F_m / L, ampere-turns per metre

Effective Value

0.707

Average Value

0.637

Frequency

f=\frac{P\cdot N}{120}

Voltage Generated in Volts

E = βlv