Inductance and Inductors 1&2 (3.11)

• Inductance


  The property of a conductor by which a change in the current flowing through it induces an electromotive force in that conductor, opposing the change in current

  
• Induction fundamentals


  • Electromagnetic induction occurs with relative movement between a conductor and a magnetic field
  • The amount of induced EMF can be increased by increasing the magnetic field strength, increasing the rate of change of flux (or rate of movement of the conductor), or increasing the number of conductor turns that are cut by magnetic flux

  
• Factors affecting coil inductance

  • Number of turns
  • Diameter of coil
  • Length of coil
  • Permeability of core material
• Induction factors


  • Magnetic field strength
  • Rate of change of flux
  • Number of conductor turns

  
• Mutual inductance


  The property of two coils or conductors by which a change in the current in one induces an electromotive force in the other
• Primary current affecting induced voltage

  As the primary current changes, it induces a voltage in the secondary coil

  
• Coefficient of coupling


  A measure of the magnetic coupling between two coils, ranging from 0 (no coupling) to 1 (perfect coupling)
• Magnetic saturation occurs when the core material of an inductor becomes magnetically saturated, limiting the maximum inductance
• Calculating inductance of series and parallel connected inductors

  1. Add inductances for series connection

  2. Reciprocal of sum of reciprocals for parallel connection
• LR time constant


  The time required for the current in an RL circuit to reach 63.2% of its final value when the circuit is energized
• Left-hand rule


  Indicates the direction of the induced current which will generate the induced voltage (CEMF)

  
• After the switch has been opened, the flux field collapses


  Reversal of flux movement causes a reversal in the direction of the induced voltage

  
• The induced voltage then moves in the same direction as the battery voltage
• Self-induced voltage


  Opposes both changes in current - delays the initial build-up of current when the switch is closed, and keeps the current flowing in the same direction when the switch is opened
• Inductance


  • Characteristic of an electrical circuit that opposes the start, stop or change in value of a current

  
• Henry (H)


  Unit of inductance, equal to the inductance required to induce one volt in an inductor by a change of current of one ampere per second
• Inductance has the same effect on current in an electrical circuit as inertia has on the movement of a mechanical object
• Inductors


  Devices used to provide inductance in a circuit, also known as chokes, reactors and coils
• Inductance applications


  • Radio antenna
  • Induction stove
• Toroid choke


  A sudden surge in current is partially choked off or restricted by the CEMF induced when the magnetic flux through the loop suddenly changes
• Inductance can be defined as the property of an electrical circuit which opposes any change in current in that circuit

  
• Coil


  • Increases the property of inductance, as current through one loop produces a magnetic field that encircles the loop and cuts all the other loops
• Inductor core types


  • Air core
  • Soft iron core
• Factors affecting coil inductance

  • Number of turns
  • Diameter of coil
  • Coil length
  • Core material
  • Number of coil layers
• Mutual inductance


  When a conductor's magnetic flux induces voltage in another electrically isolated conductor
• Factors affecting mutual inductance

  • Number of turns in each coil
  • Physical size of each coil
  • Permeability of each coil
  • Position of coils with respect to each other
• Increasing the following elements

  • Magnetic field strength
  • Number of conductor turns
  • Rate of change of flux (increasing frequency)
  • Permeability of core(s)

  
• Increasing the magnetomotive force (MMF)

  Increases the voltage generated in the second conductor
• Increasing the number of turns would also increase the CEMF, reducing the current through the coil
• Mutual induction


  Leads to transformer operating principles
• Transformer operating principles


  1. Supply power to large (primary) wire wrapped around ferromagnetic core

  2. Have corresponding smaller (secondary) wire unpowered wrapped around same core

  3. Change in current in primary induces voltage in secondary
• A changing magnetic field from one coil can induce a voltage in the second coil
• Faraday's finding


  Rate of decreasing magnetic flux affects the induced voltage

  
• Magnetomotive force (MMF)


  Number of turns multiplied by the current

  
• Increase in MMF


  Increases the voltage generated in the second conductor
• Increasing number of turns


  Increases CEMF, reducing current through coil and reducing MMF and induced voltage
• Increasing current flow in primary wire

  Requires larger applied voltage to overcome CEMF
• Transformer connected to AC power source, secondary conductor not connected to circuit, no current flowing
• Adding load to secondary circuit

  Current in secondary creates magnetic field that opposes primary magnetic field, weakening it
• Reduced total flux in primary circuit

  Lowers CEMF, current and flux increase until CEMF and EMF are in balance with voltage and current in secondary