Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1996-10-25
2001-04-24
Tamai, Karl (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S089000, C310S257000
Reexamination Certificate
active
06222286
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a PM type of stepping motor and a method of manufacturing a yoke for use in it.
In recent years, there have been greater demands for stepping motors capable of realizing more power savings and higher outputs. In a conventional stepping motor, its stator yoke and its frame yoke are made from pure iron (soft magnetic iron plate, SUY), a cold rolled steel plate (SPC), an electrolytic zinc-coated steel (SEC) or the like. However, it has been found out that any of these materials has a superior direct-current magnetic field characteristic but shows an inferior alternating-current magnetic characteristic during driving of an actual motor. In other words, if such a plate material is subjected to a varying magnetic field, its electrical resistivity becomes low and a large number of eddy currents occur, so that its iron loss becomes remarkably large. As a driving frequency becomes higher, the iron loss becomes more remarkable. This causes a lowering in the efficiency of a stepping motor, and becomes a bottleneck which hinders an improvement in the efficiency which is needed for promoting the battery driving of office automation equipment.
To solve the above conventional problem, various proposals have been made, such as proposals to employ silicon steel or soft ferrite for a stator yoke or a frame yoke (Japanese Utility Mode Laid-Open Nos. 3-104077/1991, 62-135577/1987 and the like) or a proposal to form through-holes in part of a stator yoke (the flow passage of eddy currents) (Japanese Patent Laid-Open No. 3-283049/1991).
However, the above-noted conventional stepping motors respectively have problems which will be described below, and none of the stepping motors can completely solve the problem of efficiency lowering due to eddy currents.
Specifically, the silicon steel is difficult to work by bending (drawing) compared to the aforesaid SUY and SPC, and is further difficult to treat by rust preventive plating. To improve the workability of the silicon steel, the amount of Si to be added may be decreased. However, in this case, the obtained electrical resistivity will lower and an eddy-current decreasing effect will be lost.
The use of soft ferrite can only provide a low saturation magnetic flux density which is not more than one-third of that obtainable from SUY or SPC, so that no sufficient output torque can be obtained. In addition, the soft ferrite cannot be bent and its mechanical strength is week, and, further, it is not suitable for precision machining.
In the arrangement in which through-holes are formed in part of a stator yoke, as the number of through-holes formed increases, the output torque tends to lower, and the number of working steps increases and a cost increase is incurred. In addition, although the through-holes are formed in a portion other than comb-tooth-shaped magnetic poles, no substantial effect can be obtained because of the absence of major variations in magnetic flux or major eddy-current loss in such portions.
The present invention has been made in light of the above-described background, and its object is to provide a stepping motor which is capable of solving the above-described problems and of minimizing eddy currents and providing highly efficient and stable characteristics, and which has good workability, as well as to provide a method of manufacturing a yoke for use in the stepping motor.
To achieve the above object, a stepping motor according to the present invention comprises a rotor made from a permanent magnet magnetized to have multiple magnetic poles, a stator yoke having a plurality of comb-tooth-shaped magnetic poles and opposed coaxially to the rotor, an excitation coil fitted on an external circumference of the comb-tooth-shaped magnetic poles of the stator yoke, and a tube-shaped frame yoke which surrounds the excitation coil and the stator yoke, and at least part of the yoke is formed of an Fe—Cr alloy which essentially consists of Fe and contains 9.0-18.0 wt % Cr and trace additions which are not more than 0.02 wt % C, not more than 0.7 wt % Si, not more than 0.7 wt % Mn, not more than 0.04 wt % P, not more than 0.005 wt % S, not more than 0.5 wt % Ni, not more than 0.02 wt % N, not more than 0.01 wt % 0, not more than 4.0 wt % Al, the Fe—Cr alloy having a ferrite single-phase structure whose F value defined by the following expression is not less than 0 and not more than 8.
F value=Cr+Si+2.1Al−37.0(C+N)−2.0Ni−0.6Mn−10.8
(where the unit of each composition is wt %.)
In another aspect of the invention, at least part of the yoke is formed of an Fe—Cr alloy which essentially consists of Fe and contains 9.0-18.0 wt % Cr and trace additions which are not more than 0.02 wt % C, not more than 0.7 wt % Si, not more than 0.7 wt % Mn, not more than 0.04 wt % P, not more than 0.005 wt % S, not more than 0.5 wt % Ni, not more than 0.02 wt % N, not more than 0.01 wt % 0, not more than 4.0 wt % Al and 0.01-0.4 wt % Ti, the Fe—Cr alloy having a ferrite single-phase structure whose F value defined by the following expression is not less than 0 and not more than 8.
F value=Cr+Si+2.1(Al+Ti)−37.0(C+N)−2.0Ni−0.6Mn−10.8
(where the unit of each composition is wt %.) The F value is an index of the structure stability of ferritic stainless steel. If the F value has a minus sign, no stable ferrite structure is obtained and no good magnetic characteristic is obtained. Therefore, the F value needs to be greater than 0. However, as the F value becomes larger, the magnetic flux density of the ferritic stainless steel becomes lower and a motor characteristic is lowered. An experiment has shown that the upper limit of the F value is 8.0. Therefore, the F value is limited to a range between 0 and 8.
Cr is an element indispensable for ensuring the anticorrosion characteristic required for the stepping motor. If the Cr content is less than 9.0 wt %, the stepping motor cannot have rust preventive performance which is needed in normal use environments. On the other hand, if a large amount of Cr is contained, the magnetic flux density becomes low and the magnetic characteristic is degraded. If the Cr content exceeds 18.0 wt %, no desired motor performance can be obtained. Therefore, the Cr content is limited to not less than 9.0 wt % and not more than 18.0 wt %.
The reason why the respective contents of the trace additions are limited to the aforesaid ranges is as follows. At present, any of the trace additions is an inevitable impurity which cannot be completely eliminated during manufacture of the Fe—Cr alloy. However, if the trace additions can be completely eliminated, as long as the F value is satisfied, the content of a predetermined substance may be zero, i.e., the predetermined substance may not be added.
From among the inevitable impurities, C, P, S, Ni, N and O are elements that degrades magnetic characteristics, and anticorrosion property. Therefore, it is desirable to eliminate those elements. On the basis of the results of various experiments, the upper limit values of the contents of the undesirable elements were determined as described below for desired characteristics suited to the stepping motor.
C easily forms a carbide to degrade the magnetic characteristic and the anticorrosion characteristic The upper limit of the C content is set to 0.02 wt %, because if the C content exceeds 0.02 wt %, the performance required for the stepping motor which is a final product cannot be obtained P is an element which degrades the magnetic characteristic, and if the P content exceeds 0.04 wt %, the performance required for the stepping motor which is a final product cannot be obtained. For this reason, the upper limit of the P content is set to 0.04 wt %. Since S is an impurity element which easily forms a sulphide and degrades the magnetic characteristic, the S content needs to be suppressed. The upper limit of the S content is set to 0.005 wt %, because if the S content exceeds 0.005 wt %, no desired motor perfo
Hirota Ryoji
Nakano Hirofumi
Takemoto Toshihiko
Watanabe Kazuyuki
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Nisshin Steel Co. Ltd.
Tamai Karl
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