LT3 Flashcards
Which way do flux lines flow?
North to south
What is the formula for magnetic flux?
Magnetic flux =
Magnetomotive force / Reluctance
What is Magnetic Flux
The total number of flux lines in a magnetic circuit
Unit is called the Weber (Wb), equal to 108 flux lines
The symbol for it is “O” with a line through it
What is Magnetomotive Force (MMF)
The force that causes the setting up of flux lines in a magnetic circuit
MMF is equal to the number of coil turns times the current flowing through the coil
symbol for MMF is Fm and the unit is the ampere-turn (A-t)
What is Reluctance
The opposition to the setting up of flux lines in a magnetic circuit
Unit is amper-turn per weber (A-t/Wb)
Reluctance increases as the length of the circuit increases, and decreases as the cross-sectional area of the circuit increases.
What is Permeability?
permeability is the measure of the ease with which flux may be set up in a unit length and cross section of a material.
The better the permeability of a material the easier it becomes magnetized
What is Flux Density
Flux Density is the amount of flux per unit cross-section of the magnetic circuit
The symbol for flux Density looks like a “B”
The Unit of flux density is the tesla which equals Weber’s per square metre.
What is the definition of Saturation?
Once the curve levels out and the magnetic material has been aligned with the magnetic field…the material is said to be saturated
On Hysteresis graph what is “B” and “H”?
B = Flux density
H = Magnetizing Force
Describe the Hysteresis Loop
When the material becomes magnetized, then the current direction is reversed, the graph does not retrace its steps, this is because some of the magnetism is retained by the material even after the current direction has switched…this is called retentivity.
Hysteresis is the lagging of flux density behind the magnetizing force.
What is Residual Magnetism?
The amount of flux density remaining in the magnetic material after the magnetizing force has been removed.
What is Coercive Force?
The amount of magnetizing force required to remove the residual magnetism…to lower the flux density to zero.
What curves are best suited for electromagnets and which ones are better for permanent magnets?
Curves that are tall and narrow are best suited for electromagnets because they are easily magnetized and retain very little residual magnetism
Curves that are wider and relatively flat on top are well suited for use as permanent magnets. They have high residual magnetism and require a strong coercive force to remove this magnetism
What are magnetic cores usually made from?
The cores of magnetic circuits are usually constructed from materials having high permeability and low reluctance…meaning the flux lines are concentrated within the core material
Why are air gaps inportant?
Lots of machinery that utilizes electromagnetism needs air gaps to leave room for clearance
Air Gap increases the total reluctance of the circuit
The flux density in the air gap will be lower than that in the rest of the circuit
If the air gap of an electromagnet is increased, the flux density of the gap will decrease
