Point mirroring method (Electrical Machine) Part 1

By

advantages

disadvantages

relatively fast

non-linearities only considered by constant

factors

3D fields

special geometries only

special boundary conditions have to be assumed

To demonstrate the strength and shortcomings of this method, on the other hand, two examples are worked out. In the first example a ferromagnetic circuit is calculated. The main limitations in this example are seen in the assumption that the iron circuit is piecewise constant saturated. In contrast to this shortcoming, the geometry of the disc-type motor is rather complicated and requires a three-dimensional calculation of the field. The assumption of a plane mirror surface can be seen as a very strong boundary condition thus limiting the application range. Therefore, no slots are allowed in this type of model. An air gap winding is recommended in order to be able to compute the air gap field in this machine.

The second example, a high voltage transmission line, is due to the slag of the line, an inherently three-dimensional field problem as well, but in this case linear. The assumption of a plane mirror surface, the ground plane below the line, is here the strong limitation. As a consequence, the field can not be calculated in a hilly neighbourhood. Nevertheless, measurements and computations are in good agreement.

Computation of the field quantities of a disc-type motor

In this section the basic ideas of the method of point mirroring are given. With a relatively small amount of computation time the field quantities of very complicated geometries can be studied. The method permits three-dimensional magnet field calculations. Here, the method is used to calculate the flux distribution of a disc type motor. The rotor of the motor consists of a NdFeB permanent magnet ring. An air gap armature winding is fixed to the stator. Fig. 5.6 shows the construction of the motor.

Construction of the studied disc type motor.

Fig. 5.6. Construction of the studied disc type motor.

Four poles (2p=4) are axially magnetised on both rotor magnet tmp10379_thumb rings. The used magnet material is NdFeB. In the case of rare earth material, the assumption of a straight-lined characteristic of the demagnetisation curve of the magnet material is realistic. In the operating range of the magnet, the magnetisation is almost independent of the demagnetising field strength.

The magnetisation of a volume is the sum of all dipole moments m{ divided by the volume v. Two different computational models, respectively representations for the magnetisation M are possible:

• distributed currents (Fig. 5.7),

• distributed magnetic charge (Fig. 5.8).

The magnetisation is the effect of all elemental currents inside a magnetic medium. The circulating current of one dipole cancels the current of the neighbouring dipole, if the dipoles are parallel and have the same magnitude. If, further, all dipole momentsare uniformly distributed throughout the volume, all volume currentstmp10380_thumbvanish except the current at the surface, the surface current densitytmp10381_thumb(Fig. 5.7). In this model, the distribution of magnetic dipole moments is equivalent to the distribution of currents at the surface of a magnetic medium and within the volume.

With the Maxwell equations the static magnetic field can be expressed as a solenoid field of flux density and the curl of the magnetic field strength:

Magnetic magnetisation M with cunent model.

Fig. 5.7. Magnetic magnetisation M with cunent model.

tmp10385_thumb

Under the assumption of no field exciting currents inside the permanent magnets, it can be written:

tmp10386_thumb

With vanishingtmp10387_thumbformally the magnetic field strength can be described as a gradient field of a scalar potential function.

tmp10389_thumb

Using the demagnetisation characteristic of permanent magnet materialtmp10391_thumbit can be formulated

tmp10394_thumb

yielding

tmp10395_thumb

Formallytmp10396_thumbis appropriate to the Poisson-differential equation of the electrostatic field. Analogous to the electric space-charge density and to the electric surface charge an auxiliary magnetic quantity can be defined.

 

tmp10398_thumb

With known inner magnetisation M of the magnet, the divergence of the magnetisation can be identified as an auxiliary magnetic surface chargetmp10400_thumb

Permanent magnet shape with auxiliary magnetic surface charge.

Fig. 5.8. Permanent magnet shape with auxiliary magnetic surface charge.

As shown in Fie. 5.8 for the upper pole surface, this yields

tmp10404_thumb

and for the lower pole surface

tmp10405_thumb

In this consideration, the laws of electrostatic field can be used to evaluate the scalar potentialtmp10406_thumband magnetic field strength H(P) in a point P. Integration is performed over the surface of the north AN and the south pole surface As-

tmp10409_thumb

To determine the flux density of the ring-formed permanent rotor of the mentioned disc-type motor, the ring is subdivided into trapezoidal single magnet elements (Fig. 5.9).

Permanent magnet ring.

Fig. 5.9. Permanent magnet ring.

The superposition of elementary and simple magnet shapes is performed in order to form a complicated-shape permanent magnet. Here, the superposition of the field components of cubed and triangle magnet elements result in the required trapezoidal magnet as indicated in Fig. 5.10. The magnetic field excited by the permanent magnet ring can now be calculated at every point P outside the magnet volume.

To consider the ferromagnetic back iron inside the real machine, the magnetic surface charges have to be minrored at the boundaries of the air gap. The laws governing the electrostatic field can be used here. A mirror interval of 4 to 5 steps is sufficient to obtain an acceptable accuracy (Walkhoff "’).

Superposition of cube and triangle to obtain a trapezoidal shaped magnet.

Fig. 5.10. Superposition of cube and triangle to obtain a trapezoidal shaped magnet.

With these considerations, the auxiliary configuration for the calculation of the air gap field of the disc-shaped motor, using the point mirroring method, can be constructed (Fig. 5.11).

Auxiliary configuration and co-ordinate system.

Fig. 5.11. Auxiliary configuration and co-ordinate system.

Mirroring the mentioned surface charges is done at the material boundaries having permeabilitiestmp10413_thumb(Fig- 5.11). Saturation is considered by the constant factors.

The z-component of the resulting flux density distribution of a pole pitch of the mentioned disc-type motor can be taken from Fig. 5.12.

Three-dimensional air gap flux density distribution.

Fig. 5.12. Three-dimensional air gap flux density distribution.

Computation of the fields below AC high voltage lines

Overhead transmission lines generate in their vicinity electric and magnetic fields. The source of the magnetic field is the current in the phase conductors. The electric field is caused by the high potential at the phase conductors.

The problem specifies small diameter conductors above a large flat conducting ground plane. The phase conductors are at a time-dependent specified electrical potential and cany a time-dependent current. Due to the slag of the phase conductors, the field problem turns out to be three-dimensional. Only symmetric three-phase voltage and current systems are considered. The ground below the transmission line is a uniform plane.

The field problem may be considered as quasi static. Therefore, the solution can be determined by static techniques. With respect to the slag of the phase conductors, infinitesimally thin, segmented filaments approximate the geometry of a single conductor. Due to the symmetry between two poles, one half of the arrangement is drawn in Fig. 5.13 only. The value of s indicates the slag, / is the distance between the two high voltage poles, the span field length.

Geometric modelling of a conductor.

Fig. 5.13. Geometric modelling of a conductor.

Electric field

The electric field is computed by mirroring single line charges at the assumed to be ideal conducting ground plane below the phase conductors. Each infinitesimally thin filament segment represents in this case a line-charge. A constant line-charge at any position in the original co-ordinate system (x, y, z) is drawn in Fig. 5.14. To evaluate the field quantities of the line-charge, this co-ordinate system has to be transformed into a systemtmp10417_thumbThis transformation is performed in two steps. The first step consists of a parallel shift of the origin into the starting point of the line-charge. In a second step a rotation of this temporary co-ordinate systemtmp10418_thumbaround thetmp10419_thumbaxis is carried out in such a way that the line charge lies in thetmp10420_thumb plane. The last rotation in this step is around thetmp10421_thumbso that the line-charge lies in the x°-axis. In this co-ordinate system the potential cp of the line-charge in the pointtmp10422_thumbis given by:

tmp10429_thumb

 

 

 

 

Co-ordinate transformation of an infinitesimally thin filament segment.

Fig. 5.14. Co-ordinate transformation of an infinitesimally thin filament segment.

To evaluate the field quantities with respect to this boundary condition, the line-charge has to be mirrored with respect to the plane x-y. Superposition of line-charge q and mirror-charge -q, indicated in Fig. 5.14, gives the potential tp at the point ?(x,y,z) inside the global coordinate system.

To consider the slag of the conductors, a quadratic approximation is used. Referring to Fig. 5.13, it can be written as

tmp10431_thumb

With the known complex potentialstmp10432_thumbof the i conductors and transforming eq. (5.15) to compute the coefficient matrix A, a linear set of equations can be formulated.

tmp10435_thumb

The solution determines the chargetmp10436_thumbof each element of the conductors. With these values the components of the electrostatic field strength in the point P(x, y, z) can be computed.

tmp10439_thumb

 

 

Convergence of the simulation with respect to the slag of the high-tension line.

Fig. 5.15. Convergence of the simulation with respect to the slag of the high-tension line.

To illustrate the convergence behaviour of the method, attention should be paid to Fig. 5.15. With an increasing number of infinitesimally thin filament segments for one half of the span field, the electrostatic field strength converges to the correct value. Calculations with 5…7 polygon elements deliver results with a reasonable accuracy at acceptable computational costs.