Permanent magnet motor

Details Permanent magnet motor Permanent magnet motor

2002-03-22

2003-10-14

Ramirez, Nestor (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S049030

Reexamination Certificate

active

06633105

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a permanent magnet motor, and more particularly to an electric motor having a permanent magnet ring rotor capable of reducing cogging torque and increasing a magnetic flux.
2. Description of the Prior Art
A conventionally implemented three-phase hybrid type stepping motor having a permanent magnet ring rotor is discloses in Japanese Patent Application Laid-Open No. 14514/94 and Japanese Patent Application Laid-Open No. 131968/95.
However, no effective manner for reducing cogging torque and increasing a magnetic flux with respect to such three-phase stepping motor has been obtained.
3.Description of Magnetic Flux Analysis
An unexpected magnetic flux flow could be observed according to the two dimensional analysis of the magnetic field with respect to the conventionally implemented three-phase hybrid type stepping motor having a permanent magnet ring motor, and it was found that a shape of the magnetic pole could be improved.
FIG. 1
shows a structure of a permanent magnet type stepping motor of the original design. The stepping motor comprises a stator iron core
1
and a rotor
4
. The stator iron core
1
comprises a cylindrical stator yoke, six magnetic poles
2
extending radially from an inner peripheral surface of the stator yoke, a plurality of small stator teeth
3
formed on a tip end of each of the magnetic poles
2
, and exiting windings (not shown) each wound around each of the magnetic poles
2
. The rotor
4
is arranged concentrically with the stator iron core
1
and comprises a cylindrical permanent magnet
5
and a back yoke
6
of a magnetic material intimately contacted with an inner peripheral surface of the permanent magnet
5
, an outer peripheral surface of the permanent magnet
5
facing with a gap an inner peripheral surface formed of inner peripheral surfaces of the small stator teeth
3
. The permanent magnet
5
has P pieces of N pole and P pieces of S pole arranged alternately in a peripheral direction thereof at equal intervals.
A depth of a groove formed between the small stator teeth
3
is 0.4 mm, and a length of the air gap between the inner peripheral surface of the small stator tooth
3
of the stator iron core
1
and the outer peripheral surface of the rotor
4
is 0.06 mm.
The permanent magnet
5
has 32 pole pieces of radial anisotoropy made of neodymium bond. It is considered that the figure of the permanent magnet
5
, such as a square form or locally cut does not affect on the result of calculation, so that such figure is not considered in order to calculate economically.
For the FEM calculation, only an upper half portion of the magnet is considered to utilize the periodicity, and the air gap is divided by an interval of 0.25° in order to make sure the accuracy of the cogging torque, and the interlinkage magnetic flux passing through the windings and the cogging torque are calculated by rotating until one period of 22.5° with an interval of 0.75°.
FIG.
2
A and
FIG. 2B
show flows of magnetic flux obtained by the analysis. According to these figures, the following results can be obtained.
(1) A relatively large magnetic flux is passed through the groove between the small stator teeth
3
.
(2) There is a magnetic flux closed between the adjacent magnetic poles.
(3) An about 75% of the total interlinkage magnetic flux passing through the windings is entered into the central tooth and about 25% of the total interlinkage magnetic flux is entered into the teeth at the both sides.
(4) The magnetic flux passing through the central tooth is waved while passing through the grooved portion and passed again through the teeth at the both sides.
It has been considered that a magnetic circuit is formed so that the magnetic flux is hardly passed through the grooved potions, but the effective magnetic flux is passed through in each of small stator teeth equally. However, in the actual motor, it is found as stated above that the unexpected flow of the magnetic flux is generated. The value of 3.36 E−5 (Wb) of the interlinkage magnetic flux coincides with the value of 3.18 E−5 (Wb) of the interlinkage magnetic flux calculated from the actually measured value of the induced voltage, so that the validity of the calculation is guaranteed.
Proposed designs as shown in a Table 1 are studied.
TABLE 1
conventional
first
second
third
recommended
Item
design ◯
plan {circle around (1)}
plan {circle around (2)}
plan {circle around (3)}
value
distance of small teeth
 20°
22.5°
21.25°
21.25°
21.25°
width of small tooth
2.0
1.6
1.6
2.2
2.2
depth of groove
0.4
1.0
1.0
1.0
1.0
thickness of back yoke
1.0
1.75
1.75
1.75
1.0
thickness of shoulder
1.1
1.5
1.5
1.5
0.9
of magnetic pole
calculation
interlinkage
3.36E-5 (Wb)
4.74E-5
4.46E-5
4.6E-5
4.6E-5
result
magnetic
flux
cogging
40.5 (gf.cm)
319.3
17.2
22.3
22.0
torque
Here, ◯ is the conventional design, {circle around (1)} is a first plan, {circle around (2)} is a second plan, and {circle around (3)} is a third plan.
In the first plan, the distance between adjacent small stator teeth is varied from the short pitch of 88.9% of the conventional design to the full pitch similar to the magnetic pole period, the width of the small stator tooth and the depth of the groove between the adjacent small stator teeth are so determined that the leakage magnetic flux from the grooved portion is minimized, and the thickness of each of the back yoke and the shoulder portion for connecting the small teeth of the magnetic pole are so determined that the effect of the saturation becomes minimum.
As a result, the interlinkage magnetic flux is increased by 40%, however, the first plan is not favorable because the cogging torque is increased about eight times. The calculation value of the wave form of the cogging torque is shown in FIG.
3
.
It is apparent from the Table 1 that the cogging torque has an oscillation of sixth harmonics as like as that in the other three-phase hybrid type stepping motor. In case of normal m-phase motor, the cogging torque has 2mth harmonics. In order to remove the sixth harmonics, in this case, a deviation angle of (120°/2m/p=1.25°) at which vectors are balanced at (360°/s=120°) in the sixth harmonic plane is selected.
In the second and third plans, a pitch of 360°/p (1−1/2ms)=21.25° is employed. Here, s is a small teeth number per pole with winding, m is a phase number, and p is pole pair number=16. As a result, the cogging torque is similar to that in the conventional design, but the interlinkage magnetic flux can be increased by 30% and more. Accordingly, an output may be increased by more than 30% because the torque is in proportion to the interlinkage magnetic flux. Further, the width of the small stator tooth can be increased to some extent. In this case, a pitch smaller by the deviation angle than the rotor magnetic pole pair pitch 360°/p is selected. However, the same result can be obtained if a pitch 360°/p (1+1/2ms)=23.75° larger by the deviation angle than that is selected.
As stated above, it can be assumed that an optimum value is near the improved second and third plans of the small tooth pitch 21.25°. However, it is necessary to determine the optimum value in consideration of the following points;
(1) a preferable width of small tooth,
(2) a preferable depth of groove,
(3) a preferable thickness of the back yoke, and
(4) a preferable thickness of the shoulder connecting the small teeth of the magnetic pole with winding.
Next, the effects applied on the interlinkage magnetic flux by the small tooth width etc. are studied by the magnetic field analysis. Following results are obtained by calculation the interlinkage magnetic flux by varying the small tooth width etc. on the basis of the design of the best third plan {circle around (3)} in the Table 1.
(Effect of the Small Tooth Width)
FIG. 4
shows calculated values of the interlinkage magnetic flux and the cogging torque in case that the small tooth width is varied from 1.6 mm to 2.4 mm

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