Chapter 10

Inductance

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• Objectives– After completing this chapter, the student

should be able to:• Explain the principles of inductance.

• Identify the basic units of inductance.

• Identify different types of inductors.

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• Determine the total inductance in series and parallel circuits.

• Explain L/R time constants and how they relate to inductance.

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• Inductance– The characteristic of an electrical conductor

that opposes a change in current flow.– The symbol for inductance is L.

• Inductor– A device that stores energy in a magnetic field.

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• Once current is moving through a conductor, inductance helps to keep it moving.

• As the magnetic flux lines build up, they create an opposition to the flow of current.

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• Lenz’s law– An induced emf in any circuit is always in a

direction to oppose the effect that produced it. The amount of counter emf is in proportion to the rate of change. The faster the rate of change, the greater the counter emf.

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• Inductance is measured by the henry.– Named after Joseph Henry.– Represented by H.– The amount of inductance required to induce an

emf of 1 volt when the current in a conductor changes at the rate of 1 ampere per second.

– Most commonly used are the millihenry (mH) and the microhenry (H).

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• Inductors– Designed to have a specific inductance.– Consist of a conductor coiled around a core.– Classified by the type of core material,

magnetic or nonmagnetic.

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• Types of inductors– Fixed

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– Variable• Created with adjustable core material.

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– Air-core• Without core material.

• Used for up to 5 millihenries of inductance.

• Wrapped on a ceramic or phenolic core.

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– Iron core• Ferrite or powdered ion cores.

• Used for up to 200 millihenries of inductance.

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– Toroid core• Donut shaped.• Offer a high inductance for a small size.• Magnetic field is contained in the core.

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– Shielded core• Have a shield of magnetic material to protect them

from the influence of other magnetic fields.

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– Laminated core• Used for all large inductors.

• Vary from .1 to 100 henries.

• Sometimes referred to as chokes.

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• Inductors can be connected in series, parallel, or series-parallel.– Connected in series:

• LT = L1 + L2 + L3 . . . + Ln

– Connected in parallel:• 1/LT = 1/L1 + 1/L2 + 1/L3 . . . + 1/Ln

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• Time constants– The time required for current through a

conductor to increase to 63.2% or decrease to 36.8% of the maximum current.

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• L/R is the symbol used for the time constant of an RL circuit.

• Expressed as t = L/R.t = time in seconds.

L = inductance in henries.

R = resistance in ohms.

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• In Summary– Inductance is the ability to store energy in a

magnetic field.– Unit for measuring inductance is the henry (H).– The letter L represents inductance.– Inductors have specific inductances.– Symbol for fixed inductance is:

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– Symbol for a variable inductor is:

– Types of inductors• Air core• Ferrite or powdered ion core• Toroid core• Shielded core• Laminated core

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– Formula for total conductance for inductors connected in a series:

• LT = L1 + L2 + L3 . . . + Ln

– Formula for total conductance for inductors connected in parallel:

• 1/LT = 1/L1 + 1/L2 + 1/L3 . . . + 1/Ln

– Time constant

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– A time constant can be determined by the formula:

• t = L/R

– Five time constants required to fully build up or collapse the magnetic field of an inductor.