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Main > Application > Sample problems

Exercise N 2. Force of interaction of two cylindrical coaxial coils

Task
Find the force applied to the coils with current with and without the shield between them.

Experiment
Forces are measured by use of digital balance on which one of coils is installed (see the right picture below). Tests are performed with currents varying, and different conductors and ferromagnetics are placed between coils.

Geometry
Due to symmetry of the formulation only upper half of the problem (above ab line) is defined, and at the axis of symmetry (line ab) the boundary conditions are set.

model of the two cylindrical coaxial coils

Given data
Current density in the coil j=100000 A/m2.
Relative magnetic permeability of air, aluminum and copper coils μ= 1.
Relative magnetic permeability of the steel shield μ= 1000.
Electrical conductivity of steel σ= 10000000 Sm/m.
Electrical conductivity of aluminum σ=37000000 Sm/m.
Coils are wound by insulated wire, so cross-section conductivity in coils σ= 0 Sm/m.

Boundary conditions
Along the horizontal symmetry axis (line ab) Bn =0.
Equation B=rot A in cylindrical coordinate system leads to A=const at the axis ab. The field fades at the infinity, so due to the condition of continuity of the potential A=0 at the line ab.

Results

Field distribution around coils:

  • Time-harmonic magnetic field, f=50 Hz.

    Shield type

    Interaction force, mN

    Problem

    No shield

    0.533

    lab2.pbm

    Steel

    0.098

    lab2_Fe.pbm

    Aluminum

    0.282

    lab2_Al.pbm

  • DC magnetic field

    Shield type

    Interaction force, mN

    Problem

    No shield

    0.533

    lab2c.pbm

    Steel

    0.138

    lab2c_Fe.pbm

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