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Electromagnetics ENGR 367 Inductance

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Introduction Question: What physical parameters determine how much inductance a conductor or component will have in a circuit? Question: What physical parameters determine how much inductance a conductor or component will have in a circuit? Answer: It all depends on current and flux linkages! Answer: It all depends on current and flux linkages!

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Flux Linkage Definition: Definition: the magnetic flux generated by a current that passes through one or more conducting loops of its own or another separate circuit Mathematical Expression: Mathematical Expression:

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Types of Inductance Self-Inductance (L): Self-Inductance (L): whenever the flux linkage of a conductor or circuit couples with itself Mutual Inductance (M): Mutual Inductance (M): if the flux linkage of a conductor or circuit couples with another separate one

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Self-Inductance Formula by Definition Formula by Definition Applies to linear magnetic materials only Applies to linear magnetic materials only Units: Units:

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Inductance of Coaxial Cable Magnetic Flux Magnetic Flux Inductance Inductance (as commonly used in transmission line theory)

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Inductance of Toroid Magnetic Flux Density Magnetic Flux Density Magnetic Flux Magnetic Flux If core small vs. toroid If core small vs. toroid

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Inductance of Toroid Inductance Inductance Result assumes that no flux escapes through gaps in the windings (actual L may be less) Result assumes that no flux escapes through gaps in the windings (actual L may be less) In practice, empirical formulas are often used to adjust the basic formula for factors such as winding (density) and pitch (angle) of the wiring around the core In practice, empirical formulas are often used to adjust the basic formula for factors such as winding (density) and pitch (angle) of the wiring around the core

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Alternative Approaches Self-inductance in terms of Self-inductance in terms of Energy Energy Vector magnetic potential (A) Vector magnetic potential (A) Estimate by Curvilinear Square Field Map method Estimate by Curvilinear Square Field Map method

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Inductance of a Long Straight Solenoid Energy Approach Energy Approach Inductance Inductance

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Internal Inductance of a Long Straight Wire Significance: an especially important issue for HF circuits since Significance: an especially important issue for HF circuits since Energy approach (for wire of radius a) Energy approach (for wire of radius a)

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Internal Inductance of a Long Straight Wire Expressing Inductance in terms of energy Expressing Inductance in terms of energy Note: this result for a straight piece of wire implies an important rule of thumb for HF discrete component circuit design: Note: this result for a straight piece of wire implies an important rule of thumb for HF discrete component circuit design: keep all lead lengths as short as possible

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Example of Calculating Self-Inductance Exercise 1 (D9.12, Hayt & Buck, 7 th edition, p. 298) Exercise 1 (D9.12, Hayt & Buck, 7 th edition, p. 298) Find: the self-inductance of a) a 3.5 m length of coax cable with a = 0.8 mm and b = 4 mm, filled with a material for which r = 50. r = 50.

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Example of Calculating Self-Inductance Exercise 1 (continued) Exercise 1 (continued) Find: the self-inductance of b) a toroidal coil of 500 turns, wound on a fiberglass form having a 2.5 x 2.5 cm square cross section and an inner radius of 2.0 cm

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Example of Calculating Self-Inductance Exercise 1 (continued) Exercise 1 (continued) Find: the self-inductance of c) a solenoid having a length of 50 cm and 500 turns about a cylindrical core of 2.0 cm radius in which r = 50 for 0 < < 0.5 cm and r = 1 for 0.5 < < 2.0 cm

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Example of Estimating Inductance: Structure with Irregular Geometry Exercise 2: Approximate the inductance per unit length of the irregular coax by the curvilinear square method Exercise 2: Approximate the inductance per unit length of the irregular coax by the curvilinear square method

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Mutual Inductance Significant when current in one conductor produces a flux that links through the path of a 2 nd separate one and vice versa Significant when current in one conductor produces a flux that links through the path of a 2 nd separate one and vice versa Defined in terms of magnetic flux ( m ) Defined in terms of magnetic flux ( m )

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Mutual Inductance Expressed in terms of energy Expressed in terms of energy Thus, mutual inductances between conductors are reciprocal Thus, mutual inductances between conductors are reciprocal

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Example of Calculating Mutual Inductance Exercise 3 (D9.12, Hayt & Buck, 7/e, p. 298) Exercise 3 (D9.12, Hayt & Buck, 7/e, p. 298) Given: 2 coaxial solenoids, each l = 50 cm long Given: 2 coaxial solenoids, each l = 50 cm long 1 st : dia. D 1 = 2 cm, N 1 =1500 turns, core r =75 2 nd : dia. D 2 =3 cm, N 2 =1200 turns, outside 1 st Find:a) L 1 =? for the inner solenoid Find:a) L 1 =? for the inner solenoid

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Example of Calculating Mutual Inductance Exercise 3 (continued) Exercise 3 (continued) Find: b) L 2 = ? for the outer solenoid Find: b) L 2 = ? for the outer solenoid Note: this solenoid has inner core and outer air filled regions as in Exercise 1 part c), so it may be treated the same way! Note: this solenoid has inner core and outer air filled regions as in Exercise 1 part c), so it may be treated the same way!

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Example of Calculating Mutual Inductance Exercise 3 (continued) Exercise 3 (continued) Find: M = ? between the two solenoids Find: M = ? between the two solenoids

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Summary Inductance results from magnetic flux ( m ) generated by electric current in a conductor Inductance results from magnetic flux ( m ) generated by electric current in a conductor Self-inductance (L) occurs if it links with itself Self-inductance (L) occurs if it links with itself Mutual inductance (M) occurs if it links with another separate conductor Mutual inductance (M) occurs if it links with another separate conductor The amount of inductance depends on The amount of inductance depends on How much magnetic flux links How much magnetic flux links How many loops the flux passes through How many loops the flux passes through The amount of current that generated the flux The amount of current that generated the flux

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Summary Inductance formulas may be derived from Inductance formulas may be derived from Direct application of the definition Direct application of the definition Energy approach Energy approach Vector Potential Method Vector Potential Method The self-inductance of some common structures with sufficient symmetry have an analytical result The self-inductance of some common structures with sufficient symmetry have an analytical result Coaxial cable Coaxial cable Long straight solenoid Long straight solenoid Toroid Toroid Internal Inductance of a long straight wire Internal Inductance of a long straight wire

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Summary Numerical inductance may be evaluated by Numerical inductance may be evaluated by Calculation by an analytical formula if sufficient information is known about electric current, dimensions and permeability of material Calculation by an analytical formula if sufficient information is known about electric current, dimensions and permeability of material Approximation based on a curvilinear square method if axial symmetry exists (uniform cross section) and a magnetic field map is drawn Approximation based on a curvilinear square method if axial symmetry exists (uniform cross section) and a magnetic field map is drawn

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References Hayt & Buck, Engineering Electromagnetics, 7/e, McGraw Hill: Bangkok, Hayt & Buck, Engineering Electromagnetics, 7/e, McGraw Hill: Bangkok, Kraus & Fleisch, Electromagnetics with Applications, 5/e, McGraw Hill: Bangkok, Kraus & Fleisch, Electromagnetics with Applications, 5/e, McGraw Hill: Bangkok, Wentworth, Fundamentals of Electromagnetics with Engineering Applications, John Wiley & Sons, Wentworth, Fundamentals of Electromagnetics with Engineering Applications, John Wiley & Sons, 2005.

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