. . .

# ELECTROMAGNETS

part III

 As it was shown in the previous section, solutions for the magnetic field are based on Maxwell's equations. Due to their complexity simplifications are made for practical engineering purposes. The solutions are then obtained by manual calculations with acceptable engineering accuracy.     An example for the magnetic circuit depicted in Fig. 1 is given below, to demonstrate the results obtained by FEMM: Example: DC electromagnet system - heat transfer Movie 1 - Heating     Movie 2 - Heating     Movie 3 - Heating Movie 4 - Heating     Movie 5 - Heating     Movie 6 - Heating     Movie 7 - Cooling     Movie 8 - Experiment

It was found out manually that for the magnetic flux density in the air-gap B=1,0T, the magneto-motive force required is Fw=968mA. For the same mmf Fw, the result obtained by FEMM, using magnetic material with the same quality as in Table 1, is B=0,942T. This is an average value. Actually the distribution of the flux density B along the pole face is not uniform, due to the fringing effect. In the central part of the pole B is less than at the pole edges. However the error of the manual calculations does not exceed 6%. In Fig. 2 based on the data taken from the FEMM solution, the picture of the fringing is depicted. The actual distribution of the flux density along the pole face (line C; A; Al; Cl) is presented in Fig. 3. It is worth mentioning that the distance between the corresponding points C; A and Al; Cl is 10 mm.

Table 1

 B[T] 0 0,025 0,25 0,5 1,0 1,5 1,6 1,7 1,8 1,9 H[A/m] 0 10 100 200 400 600 700 800 1200 2000

When the driving mmf is doubled Fw=1936 mA, the result obtained by FEMM is B=1,453T.

Fig. 1 - Illustration of magnetic circuit section's definition.
Fig. 2 - Magnetic field distribution in the magnetic circuit in fig. 1 based on computer-aided analyses by FEMM.
Fig. 3 - Distribution of the magnetic flux density B in the air-gap for the fig. 2 configuration.

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