Interior rotor for an electric machine

The bipolar permanent field interior rotor for an electric machine comprises a multi-sided shaft which can be rotated about an axis of rotation, a multitude of sheet metal disks disposed on the multi-sided shaft, and a number of permanent magnet slabs. Each disk has a central hole formed by central edges and consists of at least two segments. The number of magnet slabs is smaller than the number of shaft sides. The magnet slabs directly engage respectively the sides of the shaft. The remainder of the shaft sides are engaged by central edges of the disks. The axes of magnetization of the slabs are arranged radially with respect to the axis of rotation of the shaft.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an interior rotor for an electric machine. In 
particular, this invention relates to a bipolar permanent magnetic field 
exited interior rotor for an electric machine. More particularly, this 
invention relates to an interior rotor for an electric machine having 
sheet metal segments disposed on a shaft, the shaft being shaped 
many-sided (as opposed to round) at least in that area where the segments 
are disposed thereon. 
2. Description of the Prior Art 
German utility model No. 77 26 439 discloses a bipolar permanent-field 
interior rotor for an electric machine in which a shaft is used the middle 
section of which is shaped many-sided. The magnetic poles of the rotor are 
formed by sheet metal segments which are arranged on the many-sided 
section of the shaft. Between the segments and the shaft are inserted 
permanent magnet slabs which abut the sides of the shaft. The axes of 
magnetization of the magnet slabs are arranged radially with regard to the 
axis of rotation. 
In the previously known interior rotor a shaft having six sides is used. 
Each side of the shaft is covered by one of the permanent magnet slabs. In 
other words, the number of permanent magnet slabs equals the number of 
sides of the shaft. 
In case that there shall be used rare earth magnets (containing elements of 
the lathanide series) for the permanent magnet slabs in such a rotor, the 
following problem will arise. 
Because of the good magnetic properties of the rare earth magnets only a 
relatively small magnet volume is required for the generation of a 
sufficiently strong magnetic field. The magnetic slabs, therefore, can be 
designed to be thin, for instance, 2 mm. Due to the brittleness of the 
rare earth magnets, however, and in order to obtain sufficient mechanical 
strength or stability, the thickness should not be less than a certain 
minimum value, for instance, 3 to 4 mm. By virtue of this greater 
thickness of the magnets, an improvement of the magnetic conditions of the 
electric machine cannot be obtained. On the contrary, expensive magnetic 
material will be used without any effect. 
SUMMARY OF THE INVENTION 
Objects 
It is an object of this invention to provide a permanent field excited 
interior rotor for an electric machine which provides a sufficiently large 
magnetic field by application of only few permanent magnets. 
It is another object of this invention to provide a permanent field excited 
interior rotor for an electric machine which contains permanent magnets 
the thickness of which may be selected great enough to overcome or 
compensate for their brittleness, while any magnet volume which would be 
superfluous with regard to the magnetic requirements of the machine must 
not be provided. 
It is still another object of this invention to provide a permanent field 
excited interior rotor for an electric machine in which rare earth magnets 
can be used the total volume of which can be kept comparatively small. 
It is still another object of this invention to provide a bipolar permanent 
field excited interior rotor for a synchronous motor which is of small 
size, but of large power. 
Summary 
According to this invention, a bipolar permanent magnetic field interior 
rotor for an electric machine contains a many-sided shaft, a plurality of 
parallel sheet metal disks containing segments for forming magnetic poles, 
and a number of permanent magnetic slabs. The disk has a certain number of 
central edges, and the shaft has a given number of sides, for instance, 6. 
The sheet metal segments are disposed on the shaft. The number of 
permanent magnetic slabs is smaller than the given number of sides of the 
shaft. The magnetic slabs are inserted between only some sides of the 
shaft and the segments. At the remaining sides of the shaft, the sheet 
metal segments abut directly the shaft. The axes of magnetization of the 
slabs are arranged radially with respect to the axis of rotation of the 
shaft. 
In other words, according to the invention, the shaft is completely 
enclosed by and in close contact with some of the central edges of each 
segment and with the magnet slabs. 
Since only a part of the total surface of the shaft is covered by the 
permanent magnet slabs, the slabs must be designed by well known criteria 
so as to have a sufficient thickness to obtain the required magnetic 
volume. Therefore, the thickness of the permanent magnetic slabs can be 
chosen in accordance with the mechanical requirements. In spite of the 
greater thickness, the total magnetic volume will be proportioned only 
according to the electric requirements of the machine. 
A specially favorable mechanical design of the internal rotor will be 
achieved if a shaft is used which has an even number of sides. In this 
case, the permanent magnet slabs will be inserted only between each of one 
half of the shaft sides and the sheet metal segments. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of a 
preferred embodiment of the invention, as illustrated in the accompanying 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the drawing, the cross-section of a bi-polar interior 
rotor 1 of a synchronous motor is illustrated. The interior rotor 1 has a 
stack of parallel disks one of which is shown in the drawing. Each disk 
contains a central hole having central edges and comprises two laminations 
or sheet metal segments 2 and 3, respectively. The segments 2 and 3 of 
each disk are punched in one piece. The rotor 1 further contains a 
magnetic shaft 4, at least a section of which is shaped multi-sided, here 
having six sides. The sheet segments 2 and 3 are disposed centrally on the 
shaft 4. In the rim of the sheet metal segments 2 and 3, there are 
provided holes 5 for the conductor bars of a squirrel cage. 
Three sides of the shaft 4 are designated by 6, 7 and 8, respectively. 
These three sides 6, 7 and 8 are located next to each other. Three 
permanent magnetic slabs 9 are inserted between the central edge of the 
sheet metal segment 3 and the sides 6, 7 and 8 of the shaft 4. The 
magnetic slabs 9 directly engage the sides 6, 7 and 8, respectively. The 
magnet slab 9 which is arranged in the middle between the two other slabs, 
has a width which equals the width of each of the sides 6, 7 and 8, 
whereas the two side slabs 9 have a greater width. The magnet slabs 9 are 
rare earth magnets, that is they contain an element or elements of the 
lanthanide series. The magnetic slabs 9 each have an axis of magnetization 
which is arranged radially with regard to the axis of rotation of the 
shaft 4. The axis of rotation is located perpendicularly to the plane of 
the present drawing. As can be seen in the drawing, the other sheet metal 
segment 2 directly engages the three other sides of the shaft 4. 
In order to avoid a magnetic short circuit between the segments 2 and 3, a 
non-magnetic material as is well known is inserted into the separation 
area 10 between the two sheet metal segments 2 and 3. This material 
connects the two sheet metal segments 2 and 3 mechanically. 
As can be seen, the shaft 4 is completely enclosed by and directly 
contacted by the first segment 2 on three sides and by the three magnet 
slabs 9 on the other three sides 6, 7 and 8. 
As mentioned above, each of the three sides 6 to 8 of the six-sided shaft 4 
is covered with one of the three permanent magnet slabs 9. Therefore, it 
is possible to design these three permanent magnet slabs 9 with a greater 
thickness without exceeding thereby the magnet volume necessary for the 
electrical requirements of the machine. Thus, the permanent magnet slabs 9 
have at least the minimum thickness which is mandatory for their 
mechanical strength or stability. 
There has thus been shown and described a novel bi-polar permanent field 
interior rotor for an electric machine which fulfills all the objects and 
advantages sought therefore. Many changes, modifications, variations and 
other uses and applications of the subject invention, will, however, 
become apparent to those skilled in the art after considering the 
specification and the accompanying drawing which disclose a preferred 
embodiment thereof. All such changes, modifications variations and other 
uses and applications which do not depart from the spirit and scope of the 
invention are deemed to be covered by the invention which is limited only 
by the claims which follow.