Magnetic Circuits

MAGNETIC CIRCUITS

The complete closed path followed by the lines of flux is called a magnetic circuit. In low power electrical machines, magnetic field is produced by permanent magnets. But in high-power electrical machinery and transformers, coupling magnetic field is produced by electric current.

• In a magnetic circuit, the magnetic flux is due to the presence of a magnetomotive force same as in an electric circuit, the current is due to the presence of a electromotive force.
• The mmf is created by a current flowing through one or more turns.
  MMF=Current × Number of turns in the coil
  f=MMF = NI (ampere-turns) or (ATs)
• The magnetic flux φ may be defined as the magnetomotive force per unit reluctance.
  φ= MMF/Reluctance
  where reluctance in magnetic circuit is similar as resistance in electric circuit.
• The opposition offered to the magnetic flux is called reluctance,
  Rl= l/μA (AT/wb)
  where,
  l=length of the magnetic path
  A=area of cross-section normal to flux path, m2.
  μ=μ0⋅μr = permeability of the magnetic material
  μr =relative permeability of the magnetic material
  μ0=permeability of free space = 4π × 10^–7 H/m.

The permeance of a magnetic circuit is the reciprocal of its reluctance.

P= 1/Rl

Reluctances in a magnetic circuit obey the same rules as resistances in an electric circuit. The equivalent reluctance of a number of reluctances in series is the sum of the individual reluctances :

R_leq =Rl1+ Rl2+ Rl3+ …..

Permeances in series and parallel obey the same rules as electrical conductances.