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Exercise N 7. Magnetic field simulation in the air gap of DC electric motor

Draw the lines of magnetic field in DC electric motor in the non-ferromagnetic region. Calculate pole dissipation coefficients for two formulations. First - the coil is defined by current distribution, the magnetic permeability of steel is finite. Second - simplified formulation: steel assumed to be with infinite permeability and the coil is modeled by current layer.

Problem type
Liner plane-parallel magnetostatic problem.

Air region is defined by contour abcdefg. Dimensions: ab=2 mm, ag=32o, cd=68 mm, bc=45o, de=30o, ef=55 mm. Action of other parts of the motor is modeled by boundary conditions.>

model of dc motor model of the equivalent conducting sheet

Given data
Relative magnetic permeability of steel of rotor and stator μ=1000.
Relative magnetic permeability of air μ= 1.
Current density in the coil j=1000000 A/m2.
Electric power of motor 45 kW.

Simplified model (laboratory unit)
Coil is replaced by the current layer ef, magnetic permeability of steel assumed to be infinite. Problem field is replaced by the sheet of electroconducting paper of proper shape (abcdefg). Obtaining the lines of magnetic field is replaced by the lines of equal electric potential in the conducting sheet. This replacement is correct, as the picture of the field H in the air and field U in the sheet looks the same. Current flows in at line ef and goes out from line ab. Lines of equal electric potential are measured by digital voltmeter.


  • Model of real motor: due to symmetry at line Od Ht=0, at line Oa A=0.
  • Simplified model (laboratory unit): Ht=0 at the steel surface (lines defga and bc) and axis of symmetry Od, at the line Oa A=0.


Flux density in DC electric motor:

Model type

Leakage flux

Full flux


Realistic model

1.11 mWb

52.93 mWb


Simplified model (laboratory unit)

3.39 mWb

78.87 mWb


Download Please, see problems lab7.zip and lab7exp.zip