Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-03-02
2003-04-22
Tamai, Karl (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S216006
Reexamination Certificate
active
06552451
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a stepping motor from which vibration and noise are reduced and to a print device or a paper feed device and a printer using it.
BACKGROUND ART
Recently, a stepping motor which outputs a mechanical angle corresponding to an input electrical pulse is extensively used as power for paper feeding, head driving, drum driving or the like of a reproducing machine such as a printer, a scanner, a facsimile machine and the like.
Especially, equipment required having high torque and high accuracy conspicuously uses a hybrid stepping motor in which magnetic flux is formed by a permanent magnet without exciting the motor. That is because such a stepping motor has advantages that it is compact and fast but inexpensively provides power for outputting high torque, does not require a position sensor/encoder and can be controlled easily.
For example,
FIG. 29
is a sectional diagram showing a basic structure of a stator and a rotor of a stepping motor. This stepping motor has a stator
2
which has a plurality of stator poles
21
and a rotor
3
which is arranged to oppose these stator poles
21
with a space therebetween. Rotor pole teeth
31
are formed with a given pitch on the rotor
3
, and stator pole teeth
22
are formed on each stator pole
21
to oppose the rotor pole teeth
31
, so that torque is caused in an output axis
5
by exciting a stator coil
4
disposed on each stator pole
21
.
As equipments are being made to operate faster with higher accuracy in recent years, there are disadvantages of vibration and noise caused due to detent torque peculiar to the stepping motor when the motor is running. And, it is an important issue to find a method of reducing the vibration and noise.
To solve the disadvantage, there is conventionally adopted a method of reducing the harmonic contained in the detent torque by setting the pitch of the stator pole teeth to be different from the pitch of the rotor pole teeth.
Generally, it is assumed that the pitch of the rotor pole teeth is PR, the pitch of the stator pole teeth is PS and the number of the stator pole teeth is m, and when they are in the following relation:
PS=PR
{1±1/(
m·&ngr;
)}
it is known that a plurality of harmonics containing the order &ngr; are lowered. And, such a structure is adopted to the stepping motor which is disclosed in, for example, Japanese Patent Publication No. Hei 6-14779 and the like.
When the pitch PS of the stator pole teeth is displaced to be made narrow or wide by {PR/(m·&ngr;)} with respect to the pitch PR of the rotor pole teeth, a phase of detent torque caused in each stator pole tooth is displaced appropriately, and specific harmonics cancel each other out.
For example,
FIG. 30
is an explanatory diagram showing pitch P
1
of the rotor pole teeth
31
and pitch P
2
of the stator pole teeth
22
of a conventional stepping motor.
FIG. 30
shows that each stator pole
21
has six stator pole teeth
22
, and the pitch PS of the stator pole teeth
22
is set to be narrower (or wider) than the pitch PR of the rotor pole teeth
31
by (PR/18). The harmonics decreased by this structure is not only a third harmonic but also the harmonics of the orders of multiples of three, that is, 6, 9 and 12.
Actually, a three-phase motor was formed by using a stator which has six stator poles and a rotor having
38
rotor pole teeth as shown in FIG.
30
. And, its detent torque characteristic was measured. The result shown in
FIG. 31
was obtained.
The same measurement was made on a plurality of motors produced experimentally, and the order of the harmonic with respect to detent torque characteristic was analyzed with the frequency, and obtained the results as shown in FIG.
32
.
A motor was also produced by using a stator having the same pitch of the stator pole teeth with the pitch of the rotor pole teeth and subjected to the frequency analysis. The results as shown in FIG.
33
and
FIG. 34
were obtained.
By comparing the results obtained above, it was confirmed that the sixth harmonic and the 12th harmonic were remarkably decreased when the pitch of the stator pole teeth was determined as shown in FIG.
30
.
In the aforesaid drawings, variations in the first and second harmonics seem to result from a dimensional error of the stator and the rotor, eccentricity of the rotation axis or the like.
It is assumed that the nearer the amplitude of the harmonic of each order of the aforesaid stepping motor becomes 0 gcm, the lower the vibration and the noise become.
The harmonics caused in the respective stator poles with different phases are mutually weakened or strengthened, so that the order of detent torque which particularly becomes a problem is decided according to the number of phases and the number of the stator poles.
With the three-phase motor, the fourth harmonic is canceled by the layout of the three-phase structure, and the sixth harmonic becomes a problem. And the aforesaid pitch of the stator pole teeth is set corresponding to such conditions.
In practice, however, it was very hard to achieve satisfactory quietness by the aforesaid structure.
Specifically, when an effect is to be obtained from the layout between the phases and between the stator poles, the accuracy of motor parts manufacturing and assembling processes is extremely restricted, and as a result, a sufficient effect can not be achieved due to a little dimensional error.
It is apparent from FIG.
31
through
FIG. 34
that the fourth-order harmonic and the like, which must not constitute a matter of concern as compared with the sixth-order harmonic theoretically, appear largely.
Accordingly, in view of the aforesaid problems, it is an object of the invention to provide a stepping motor which efficiently lowers a harmonic of detent torque and provides quieter and smoother rotations.
As described above, when the stepping motor is driven, its undesired vibration and noise are reduced by lowering or removing detent torque.
The detent torque is torque which retains the rotor in a given position by disposing permanent magnets and the like on the rotor without exciting a stator coil and the like and discriminated from static torque indicated by a so-called stiffness characteristic.
For the stator
2
and the rotor
3
shown in
FIG. 29
, permanent magnets are arranged in the key points of the rotor
3
. The detent torque is present between each stator pole tooth
22
of the stator poles
21
and the rotor pole teeth
31
of the rotor
3
and becomes a total of the detent torque, which exist between the individual pole teeth, as the entire motor. In other words, it is configured that the detent torque caused between the stator pole teeth
22
and the rotor pole teeth
31
which have different phases in the circumferential direction are canceled each other.
The detent torque of one stator pole tooth is represented by a sine wave which has the same cycle with the pitch of the rotor pole teeth. Actually, however, a harmonic component is contained therein, so that it has a complex waveform including distortion as shown in FIG.
35
.
Since this harmonic component disturbs the smooth rotations of the rotor, undesired vibration and noise are caused when the motor is running.
Particularly, as disclosed in Japanese Patent Laid-Open Publication No. Hei 9-308213, the stator pole teeth paired mutually are conventionally configured to mutually cancel particular harmonics by properly determining a pitch of the individual pole teeth. But, it was not satisfactory because it was influenced easily by a magnetic imbalance due to eccentricity, falling or the like of the rotor with respect to the stator.
Specifically, the stator pole teeth and the rotor pole teeth usually have a very narrow gap length of tens of &mgr;m. Therefore, the aforesaid mutual cancellation becomes incomplete because a difference is caused in the size of the harmonics of both the pole teeth due to eccentricity, falling or the like of the rotor.
And, the reluctance motor pole shape and torque characteristic by Institute of E
Ikegami Akihiko
Koike Yoshikazu
Kanesaka & Takeuchi
Seiko Epson Corporation
Tamai Karl
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