Magnetic Terms Glossary

Term Glossary

AL Value (nH/N2): The inductance rating of a core in nanohenries (10-9 henries) per turn squared based on a peak AC flux density of 10 gauss (1 millitesla) at a frequency of 10 kHz. Note: 35.0 nH/N2 = 350 μH for 100 turns = 35.0 mH for 1000 turns.

Butt-Gap: The gapping of E Cores by equally spacing all three legs of the cores rather than introducing a gap in the center-leg only. Twice as much center-leg gap is required to electrically duplicate a given butt-gap.

Choke: Another term for an inductor which is intended to filter or choke out signals. 

Common-Mode Noise: Electrical interference that is common to both lines in relation to earth ground. 

Copper Loss (watts): The power loss (I2R) or heat generated by current (I) flowing in a winding with resistance (R). 

Core Loss (watts): The power loss or heat generated by a magnetic material subjected to an alternating magnetic field. 

Cross-Sectional Area (A): The effective cross-sectional area (cm2) of a core available for magnetic flux. The cross sectional area listed for toroidal cores is based on bare core dimensions with a 5% radius correction. 

Differential-Mode Noise: Electrical interference that is not common to both lines but is present between both lines. This is also known as normal-mode noise. 

Energy Storage (1/2 LI2): The amount of energy stored in microjoules (10-6 joules) is the product of one-half the inductance (L) in microhenries (10-6 henries) times the current (I) squared in amperes. 

Full Winding: A winding for toroidal cores which will result in 45% of the core’s inside diameter remaining. A winding for E Cores which will result in a full bobbin. The type of insulation, tightness of winding, and coil winding equipment limitations will all introduce variations. 

Initial Permeability (μo): That value of permeability at a peak AC flux density of 10 gauss (1 millitesla). μ=B/H. The permeability listed for each material is for reference only. The cores are manufactured to the listed AL values.

Magnetizing Force (H): The magnetic field strength which produces magnetic flux. 1 oersted = 79.58 A/m = .7958 A/c


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Mean Magnetic Path Length (l ): The effective magnetic length of a core structure (cm).

MLT (cm): The mean length per turn of wire for a core.

Peak AC Flux Density (Bpk): The number of lines of flux per unit of cross-sectional area generated by an alternating magnetic field (from zero or a net DC). In general: (1 gauss = 10-4 tesla) 

Peak to Peak Flux Density (∆B): In an alternating magnetic field, it is assumed that the peak to peak flux density is twice the value of peak AC flux density. ∆B = 2 Bpk. 

Percent Initial Permeability (%μo): Represents the percent change in permeability from the initial value. Since the cores are manufactured to the AL value rather than to the listed reference permeability, this can also be considered Percent AL Value. 

Percent Ripple: The percentage of ripple or AC flux to total flux; or in an inductor, the percentage of alternating current to average current. 

Percent Saturation: This is equal to 100% – Percent Initial Permeability. ie: 20% saturation = 80% of initial permeability. 

Simple Winding: A winding for toroidal cores which will result in 78% of the core’s inside diameter remaining. Often times this will produce a single-layer winding. 

Single-Layer Winding: A winding for a toroidal core which will result in the full utilization of the inside circumference of the core without the overlapping of turns. The thickness of insulation and tightness of winding will affect results. 

Swing: A term used to describe how inductance responds to changes in current. Example: A 2:1 swing corresponds to an inductor which exhibits 2 times more inductance at very low current than it does at its maximum rated current. This would also correspond to the core operating at 50% of initial permeability (also 50% saturation) at maximum current. 

Surface Area (cm2): The effective surface area of a typical wound core available to dissipate heat.

Temperature Rise (∆T): The increase in surface temperature of a component in free-standing air due to the total power dissipation (both copper and core loss).

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