Magnetic vector potential (Electrical Machine)

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Potentials and formulations

The Maxwell equations represent the physical properties of the fields. To solve them, mainly the differential form of the equations and mathematical functions, the potentials, satisfying the Maxwell equations, are used. The proper choice of a potential depends on the type of field problem. In this section, the various scalar and vector potentials are introduced.

The electric vector potential for the displacement current density will not be introduced here, because it is only important for the calculation of fields in charge-free and current-free regions such as hollow wave-guides or in surrounding fields of antennas.

Various potential formulations are possible for the different field types. Their appropriate definition ensures the accurate transition of the field problem between continuous and discrete space.

Using these artificial field quantities reduces the number of differential equations. Considering a problem described by n differential equations, a potential is chosen in such a way that one of the differential equations is fulfilled. This potential is substituted in all other differential equations, the resulting system of differential equations reduces to n-l equations. It is distinguished between magnetic and electric vector respectively scalar potentials:

Table 4.1. Definition of the potentials.

By using the vector identity and applying eq.(iii), (3.10), introduces the magnetic vector potential A. The magnetic flux density is derived as the curl of another vector field: The magnetic vector potential is suitable in regions with and without conducting currents. The vector field A is assigned right handed to the direction of the magnetic field B (Fig. 4.1). Fig. 4.1. Geometrical assignment of the vector potential A with the magnetic field vector B.

A static magnetic problem is described by By using the magnetic vector potential A, the system of differential equations is reduced to Applying the vector calculus to eq.(4.4) yields: with and by assuming a constant permeability leads to the A-formulation of a magneto-static field, a Poisson equation: To consider quasi-stationary fields, for example necessary for eddy current calculations, the magneto-dynamic formulations have to be employed. In addition to Ampere’s law, the Faraday law (i), (3.26) has to be considered to evaluate the contribution to the field by the eddy currents: Now employing Ohm’s law to calculate the eddy currents Je yields: Ampere’s law can now be rewritten, yielding the A-formulation for the quasi-stationary magnetic field in the time domain: Substituting again and assuming results in a similar A-formulation in the time domain for the transient magnetic field: Assuming sinusoidal excitation currents with an angular frequency and thus substituting yields the A-formulation in the frequency domain to solve eddy current problems. This equation is the A-formulation to describe time-harmonic problems. The time dependent components of the vector potential are expressed by: The current is expressed in analogy in its complex representation.