Sunday, May 14, 2017

8.4 Inductance and Induction

Here's the fourth and final installment of Module 8, Induction, in Navy Basic Electricity and Electronics series from the 1970s. The first three units were:

8.1 Electromagnetism
8.2 Inductors and Flux Density
8.3 Inducing Voltage

1. Inductance is the property of a circuit that opposes any change of the current. Last entry we learned this idea as "Lenz's Law": "The voltage induced in a circuit by changing current always opposes the change causing it" (87). This opposing voltage is sometimes called "counter EMF" or CEMF for short.

This principle reminds me of Newton's third law: "A body in motion wants to stay in motion, and a body at rest wants to stay at rest." So with inductance. When current is decreasing, inductance wants to maintain it. When current is increasing, inductance wants to resist it.

The unit of inductance is the henry. 1 henry is the amount of inductance that yields one volt when the current is changing at the rate of 1 ampere per second. The symbol for inductance is L. The abbreviation for a henry is h, and it often appears in small quantities like the millihenry (1/1000) and the microhenry (1/1000000)

2. Computing inductance is done using the same formulas we used to calculate resistance.
  • In a series circuit, the total inductance is simply the sum of all the individual inductors. 
  • If a group of inductors in parallel all have the same value, then the total inductance is simply the inductance of one inductor divided by the total number of conductors in parallel.
  • If you have two inductors, then the total inductance is L1 x L2/L1 + L2.
  • If you have more than two inductors in parallel with different values, then the total inductance is the reciprocal of the sum of the reciprocals of all the inductors.
3. The amount of inductance for a coil depends on its physical characteristics.
  • The greater the number of turns of the coil increases inductance.
  • The larger the cross-sectional area of the core, the greater the inductance.
  • The greater the permeability of the core, the greater the inductance.
  • The longer the core material, the lower the inductance.
  • The greater the space between coil turns, the lower the inductance.
4. Inductance is the capacity of a coil to oppose a change in current. By contrast, induction is the actual creation of a voltage, which requires motion. Inductance does not require current flow. Induction does. Induction is the action of inducing a voltage when current is changing in a circuit.

So the six factors which affect induction are
  • the number of turns in the coil
  • the cross-sectional area of the core
  • the permeability of the core material
  • the length of the core (inversely)
  • space between the coil turns
  • the rate of change in current flow
4. The changing current in one circuit can induce voltage in another circuit, which is called mutual inductance. This is also measured in henrys. The symbol for mutual inductance is M. Two coils can be positioned next to each other so as to exhibit mutual inductance.

The amount of mutual inductance is affected by their proximity, for this determines the percentage of flux lines of one coil in the turns of the other coil. This percentage is called the coefficient of coupling.

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