CA2073195A1 - Small-sized dc motor - Google Patents

Abstract

ABSTRACT
A small D.C. motor (1) provided with a flat circular or square PTC thermistor (8) between an input terminal (7) and a phosphor bronze plate (6) connected to the motor brush (5). The thermistor has electrodes on both faces and an opening provided substantially in its central portion, and it is fitted on the inside or outside of the motor casing (2). The motor shaft (9) or an extension thereof is passed through the opening in the thermistor in such a manner that the plate surface of the PTC thermistor (8) is nearly perpendicular to the shaft (9).

Description

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DESCRIPTION
S~I~LL,-SIZED DC ~IOTOR
TECHNICAL _IE~D
The present invention relates to a smal.L-si~ed DC mo-tor characterized in -that a PTC (posi-tive temperature coetEicien-t~
thermistor which is applied to a fixed-temperature heating resis-tor, a -temperature sensor! a curren-t limitin~ element, and so on, is incorporatecl into or circumscribed on the motor as the over-load protection element for the motor.
BACKGROUND OF THE INVENTION
Mainly a bimetal, a PTC thermistor, etc. are known as over-current protection elements to be used as a measure counter an overload of a small-sized DC motor, and there is a tendency that the amount of use of those elements increases particularly centering the Eield of car industries.
Of those elements, a bimetal is connected in series mainlY
to a motor having a diameter of 40 mm~ or more, that is, having ~an overload current of about 5A or more, and usually used as the ~;~ motor bu1lt-in -type.
Generally, when an overload current is not larger than 5A, the accuracy of switching operation of a bimetal is low so that ~the bimetal fails to perform stable operation. Accordingly, a :~ :
~ PTC thermistor having a cost lower than that of the bimetal is :
broadly used.
`~ As is known, a PTC thermistor is a resistor element having a positlve temperature coeff~icient, and al-though the resistance value thereof is low at an ordinary temperature, the value sud-denL~ increases to be 104 - 107 times as large as the value at an ordlnary~ temperature if the e`lement reaches or exceeds a pre-:

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-~73~5 determined swi-tching temperature beca~lse of self heating or heat -trans~er from a cer-tain heat source.
A PTC thermistor made of BaTiO3 ceramics is most generall~
known, and a PTC thermistor of ~23 ceramic ? a resin PTC -ther-mistor in which conductive par-ticles such as carbon black or -the like are contained in polyolefin resin, and the liket are known in addition to -the BaTiO3 PTC -thermis-tor.
Conventionally, a PTC thermistor to be used in a small-sized DC motor is made of semiconductor ceramics containing BaTiO3 as the principal component. Because of limita-tion in the material characteristic that the resistivity of the material must be not lower than 8 Q cm and the dielectric strength bust be in a range of 30 50 V/mm, in use, a PTC thermistor is usually connected in series to a motor as a disk-like part provided wi-th a lead wire havin~ a diameter not thinner than 10 - 12 mm ~ . Since a PTC
thermistor cannot be incorporated into a motor because of its shape and its size, however, ~t is general that a separate PTC
thermistor is mounted on the outside a motor through a printed substrate or the like.
Consequently, there has been a problem that it takes a lon~
time to mount a PTC thermistor, and development of a small-sized motor, even a small-sized motor havin~ a diameter smaller than ~0 mm ~ , provided with an overload protection element is desired by a car industrial field, etc., who seek reduction in size and in weight.
In such a background~ the inventors of this appLication previously proposed "A small-sized DC motor" (Japanese Patent ~: :
Appl~cation No. Nei-1^292397) and "A small-sized DC motor (Japanese Patent Application No. Hei-1-292398~. They are those :~ : :

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~?7;~ s structured fundamentally with ma-terials havin~ charac-teris-tics considerably improved than -those of the conventional ~na-terials.
Next, those motors will be described.
The t`irs-t small-sized DC motor disclosed in Japanese Pa-tent Application No. Hei-1-292397 is characteri~ed in that a BaTiO3 ceramic in which the ratio ~VD/ P 25) ~ the dielec-tric s-treng-th VB (V~mm) to an ordinary-temperature resistivity P 25 ( Q cml is not smaller than ~ and not larger than 20 is connected in series to an armature winding and the PTC thermistor is incorpora-ted in a motor casing. The BaTiO3 ceramic is formed oE, as starting raw materials, a mi~ed crystal or a solid solution of BaTiO3, SrTiO3, PbTiO3, and CaTiO3 synthesized through a liquid-phase solution reaction method, and a valency control agent and Mn and Si which are added to the above principal components by predetermined amounts. Further, as the arrangement position of the PTC ther-mistor made of the foregoing materials, places such as a position between a phosphor-bronze plate connected to a brush of the small-sized DC motor and input terminals, a position between -the brush and the phosphor-bronze plate connected to the input termi-nals, etc. have been proposed.
The small-sized DC mo-tor disclosed in Japanese Patent Application No. Hei-1-292398, on the other hand, is characterized in that a resin PTC thermistor is connected in series to an arma-:~ ture winding and incorporated in a casing of the motor. The material of the PTC thermistor a mixture oE polyolefin or halogen-insulating resin and a high conductive material of a carbon~group, the mixture generally having ordinary-tempera-ture resistivity oE l - 2 Q cm and dielectric strength not lower than l00 V/mm.~Further, as the posltion where the PTC thermistor made :::
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2~?7~ '3~i of the foregoing material is -to be provided, a position between a phosphor-bronze plate connec-ted to an input termi.nal and a brush of the small-sized Dc mo-tor, a position between -the brush and the phosphor-bronze plate connected to the input terminal, etc., have been proposed.
Both -the above small-sized DC motors are characterized in ..
that the PTC thermis-tor is formed in-to a small-sized or thin plate-like chip or into a cyLindrical shape or a shape accordin~
the foregoing shape and disposed in the inside of the motor, on the basis of t,he fact that the material characteristic of the PTC
thermistor, particularly, the dielectric strength and the ordinary-temperature resistivity have been considerably irnproved.
Each of the above proposed small-sized DC motors, however, has the following problems, while the PTC khermistor superior in material characteristic is formed like a chip and incorporated into the motor so as to make the motor have an overload protect-ing function itself and be reduced in size as well as in weight.
(1) With respect to the position where the PTC thermistor is incorporated in the small-sized DC motor and -the shape of the PTC
thermis-tor, a plate-like PTC thermistor having electrodes at its opposite surfaces and being arranged between the input terminal and the pho.sphor-bronze plate connected to the brush, a PTC ther-mistor having electrodes at its opposite surf'aces and being ar-ran~ed between the input terminal and the phosphor-bron~e plate connected to the brush, a cylindrical PTC thermistor having elec-trodes at its opposite end surfaces and being arranged on a ter-minal board, etc. have been proposed. In each o~'-the cases, there is a possibility of occurrence of problems that the space for position of arrangement is very small so that the size of the . . - :
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chip-shaped PTC thermistor is limited because of the foregoing limi-tation of -the space, -the moun-tirlg is not easy, the PTC ther-mistor may be allowed -to come off or damaged by vibrations and/or shocks.
(2) In a motor having internal resistance not lar~er than about 4 Q even if the motor has a diameter no-t larger than 40 mm~, the PTC thermistor which matches with -the motor mus-t have a re-sistance value not larger than 1 Q . Assuming that such a PTC
thermistor is formed by using the materials proposed in Japanese Patent Application No. Hei-1-292397, then the size of the ther-mistor is no-t smaller than about ~mm X ~mm in the case of the square chip, and in order to mount the PTC thermistor in the casing with no problem, a considerable device is required in view of space and vibration-resistance.

(3) Although detailed description will be made later with re-spective to the embodiment, a PTC thermistor is a heating resis-tor, and the resistance value of the thermlstor under a predeter-mined condltion i5 determined by a balanFe between the heat gen-eration of the thermistor itself owing to Joule heat and the heat :
radiation to the circumference. That is, the resistance value relates to not only the shape of the thermistor and the circum-ferential h.eat environment but the hea-t capacity of the ther-mistor, that is, the size thereof. In other words, this fact ~m~ans that the smaller the si.ze of the thermistor becomes the smaller:the heat capacity thereof becomes so that thermistor : becomes~more :sensitive against a circumferential influence.

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Consequently, in the case where a thermistor is merely de creased:in size by using a material. having a large ratio of di-electric strength to resistivity and is incorporated in a motor .
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as shown in the preceding proposal~ when an overloaded state occurs in the motor~ h&at ~enera-tion of a motor coil is trans-fer red -to the PTC thermistor through a phosphor-bronze plate or the like by -the heat coupling effect so that the temperature of the PTC thermistor i9 raised easil~. Accordingly, there is a defect that the Joule heat of the PTC thermistor is transferred to the motor -to thereby increase the limitation current value, while the response of the switching operation of the PTC thermistor is made high, and in the current limiting state, on the contrary, the dielectric strength is made high because of such a function as if a radiation plate, is provided. Consequentl~, if the the PTC
thermistor is merely reduced in size, a bad influence is rather generated in view of heat capacity.
~4] The fundamental point of the foregoing proposals is in the material in which the ratio of the d1electr1c strength to the ordinary-temperature resistivity is remarkably improved in com-parison with the conventional one. The material is a BaTiO3 PTC
ceram1c synthesized through the 11guld-phase solution reaction method~, or PTC res1n. Those mater1als are hi~her in cost than B~aT103~ ceramic prepared by the conventional solid phase method, and~therefore such a~PTC thermistor~-that can be incorporated in a~
motor even~if lt 1S mads of ths convsnt1onal material has been desired.
DISCLOSURE OF THE~INVENTION
An~obJsct~of the~present~1nvsnti~on is to prov1de a~small-s1sed DC~motor in wh1oh a PTC thermistor wh1ch can be incorpo rated 1n~thè motor e~en whsn ~1t~is formed of a~conventional mate-ri~al,~which~can be easily mounted attached even if the motor has ;a d1ametsr not~larger~than 40 mm ~ and 1nternal rssistance not of . ~ .

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larger than about ~ Q , which has no problem i.n space and in vibration resistance, and which ls inexpensive.
Three important. factors in configuration of the present invention are as follows.
A first one is a way how to increase -the dielec-tric stxength with the conven-tional material. That is, this is because if the dielectric strength can be increased, the thickness of -the ele-ment can be reduced and the element is decreased in size to -thereby make it possible to be incorpora-ted in or to be circum-scribed on a mo-tor.
Next, the dielectric strength of a PTC ma-terial will be described.
as shown in Fig. 4, The PTC material~ particularly the BaTiO3 ceramic PTC material,~is a semiconductor generally having of such a resistance/tempera-ture characteristic that the tempera-ture gradient has a negative gentle gradient in the vicinit~ of an ordinary temperature ~an NTC region), but the resistance value suddenly increases with a width of change of about 104 107 tiJnes ta PTC region~ when the temperature exceeds a Gurie point A
as shown in Fig. 4. Then, a negative temperature gradient is generated again (NTC region) when the temperature further in-creases to.exceed a point B.
Fig. 5 shows the voltage/curren-t characteristic of the PTG
ceramic material having such .a resistance/temperature character-istic. :In Fig. 5j the region from a point P to a point Q is a fixed resistance region where a current increases in proportion to voltage rising, and the temperature of the element is rai.sed : ~ :
gradually by Joule heat, and the region corresponds to the fixed-resistance region from the ordinary -temperature to the Curie : : :
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point in Fig. ~. The region from the poin-t Q -to a poin-t R in Fig. 5 is a fixed-power re~ion where the current clecreases in inverse propor-tion to voltage rising ancl corresponds to the PTC
region from the poin-t A to the point B in Fi~. 4. In Fig. 5, when the vol-tage i5 further increased~ the temperature o-E -the element further rises to enter the NTC region not lower than the poin-t B of Fig. ~, and -therefore the element is burnou-t broken by the rockless run of -temperature rising. The voltage immedia-tely before the burnout break is called dielectric strength VB of a PTC material.
Thus, unlike a dielectric, the dielectric strength of the PTC ceramic is defined to be a voltage which can endure burnout break due to Joule heat. Unlike the BaTiO3 PTC material, a resin PTC material has no NTC region not lower than the point B in Fig.

4, and it is therefore considered that the dielectric strength of the resin PTC material rather corresponds to the dielectric strength of the resin itself.
Since the dielectric s-trength of a generally widely used P5aTiO3 PTC material is a voltage which can endure burnout break due to Joule heat as described above, it lS said that the dielec-trlc strength varies in accordance with the condition of radia-tion of Joule heat from the element~ That is, the dielectric strength oP the PTC material is not a physical constant but a value synthetically determined in accordance with no-t only the material characteristic of the element bu' the shape thereof, the circumferential heat environment5 and the like. In the case of ~: :
evaluating the dielectric strength of the PTC material itself, therefore, the inventors of this application use measurement values obtained when an annulax disk having a diame-ter of 10 mm~

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and a thickness of 1 mm is set a-t a predetermined temperature and in a atmosphere where no convec-tiorl e~ists. In view ot part design, on tbe o-ther hand, in order to raise -the dielec-tric streng-th, such a measure -that, for example, a heat radiation pla-te is attached onto a PTC thermistor is widely carried out to improve the heat radiation property.
From such a poin-t of view, the inventors of this application have earnestly investigated -the shape oE a PTC -thermistor which can be incorporated in a motor nnd which can improve the dielec-~ric streng-th, and as a result, they have found that the above condition can be satisfied by a so-called dou~hnut-shaped PTC
thermistor which is formed of an annular or square plate-like body or a plate-like body similar to the former, an openin~ por-tion being formed at the center of the plate-like body.
That is, the inventors have found that doughnut-shaped PTC
thermistors have larger dielectric strength than others when thermistors having the same resistance value and the same thick-ness are designed by use of the same PTC material. Conventional-1~, although doughnut-shaped PTC thermistors are seen in materi-als in the trade so as to be well known, there is no knowledde concerning such an improvement of the dielectric strength.
It is considered that in an ordinary annular or square PTC
thermistor or an ordinary PTC thermistor having a shape similar to the former, the heat radlation property of the central portion is p~oorer than that in the circumferential portion, while in such a doughnut-shaped PTC thermistor described above, heat radiation rom the thermistor is made uniform and promoted because the central portion is opened so that the dielectric strength is improved. This discovery is -the first factor of the present :

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invention .
Next~ a se&ond -factor will be described. The second -f'ac-tor is such -that -the opening portion of a doughnut-shaped PTC ther--mistor is inser-ted on-to a shaft of a small-sized motor or an ex-tension thereof so that the thermis-tor is incorporated into or circumscribed on a motor casing so as to ma~e the disk-like sur-face of the thermis-tor be substantially perpendicular to -the shaft~ whereby the thermistor having electrodes at i-ts opposite surfaces is arranged between an input terminal and a phosphor-bronze plate connected to a brush of the motor. When fi~edl preferably, the thermistor is mounted in the motor casing and then sealed with a shielding material such as silicon resin having a high heat-radiation property. If arranged as above, the PTC -thermistor is loca-ted so as to be axis-symmetrical with re-spect to the shaft of the motor and the vibration resistance and -the shock resistance are extremely improved owing to an effect of the elastlcity of resin.
Further, a third factor is a heat coupling effect between the PTC thermistor and the motor coil when the PTC thermistor is incorporated in or circumscribed on the motor. That is, in the method according to -the present invention, (1) there is generated such~an effec-t that when the motor comes into an overloaded state, such a heat coupling effect that heat genera-tion of the motor coil is transferred -to the PTC thermistor through the phosphor-bronze plate is added to an ordinary self heat genera-tion of the PTC due to an overcurrent so that the response pro perty of the switching operation of the PTC thermistoI is further :
improved. (2) Further, when the motor comes into a current limi-tation state, on the other hand~ there is generated such an ef-~ ' ,: - , . - : ~ ,.

fect that the hea-t radiation rate increases so tha-t -the dielec-tric strength of the PTC -thermistor is further improved ~eca-lse the phosphor-bronze plate reversely acts as a heat radiati.on plate Eor the PTC -thermistor. In this case, in -the methocl ac-cording to the present invention, the degree of freedom in de-signing the size of the PTC -thermistor is la;rge in view of the position of arrangement and the manner of moun-ting, and there-fore, unlike the method of the foregoing proposals, the increase in the limitation current value in the current limitation s-tate can be reduced as small as possible if design is made in con-sideration of the heat capacity.
BRIEF DESCRIPTION OF D AWIN&S
Figs. 1(a) and l(b) are explanatory diagrams schematically showing an embodiment of the small-sized DC motor according to the present invention in which a PTC thermistor B is incorporated.
Fig. 2 is an explanatory diagram schematically showing an example of the conventional small-sized DC motor in which a PTC
thermistor A is pro~-ided on a control substrate.
Figs. 3(a) and 3(b) are explanator~ diagrams schematically showing a compari.son example of the small-sized D~ motor in whlch a PTC thermistor C used in the preceding proposal is incorporated.
Fig. 4 is a diagram showing the resistance/temperature cha-racteristic of a ceramic PTC material.
Fig. 5 is a diagram showing the voltage/current character-; : istic of the ceramic PTC materlal havlng the characteristic of : Fig. 4.
:~ Flg. 6 is a diagram showing the relation between -the response :~ ~ time and the limltatlon current for explaining the table 1.
~ Figs. 7(a) and 7(b~ are explana-tory diagrams scheMa-tically :: :
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2~3~35 showing~ another embodimen-t of the small-sized DC motor according to the present invention in which a PTC -the:rmistor is circum-scribed on a motor casing.
BEST MOnE FOR CARRYING OUT THE_rNVEN1'ION
(Example 1) Basic bodies of a PTC thermistor A cons-tituted by an ordi-nary annular disk having a diameter of 10 mm~ and thickness of 0.6 mm and a PTC thermlstor B constituted b~ a doughnu~-shaped disk having an outer diameter of 12 mm~ , and an inner diameter of 6.5 mm ~ , and thickness of 0.6 mm were prepared by using an ordinary BaTiO3 PTC ceramic material in which the ordinary-temperature resistivity value P25 was 8 Q cm, the dielectric strength VB (hereinafter, referred to as "standard dielectric strength") was about 50 V/mm in the condition of a diameter ot` 10 mm ~ and a thickness of 1 mm, and the Curie point was about 100 C. Then, silver electrodes were applied onto opposite surfaces of each of the basic bodies to thereby form PTC thermistors. The thus~formed PTC thermistors have the same disk area and thick-ness, and have the same resistance value of 0.6 Q . Next, a point-contact electrode was set on each of the PTC thermistors, ~and a voltage was gradually applied to each PTC thermistor in a constant-temperature oven of 25 C to thereby measure -the dielec-tric streng$h. The measurement result of the disk B was 40 V, a~though that of the disk A was 30V.
It is found from the measurement results that even in the case of forming PTC thermistors by use of the same material and so as to~have the same area, the same thickness, and the same resistance value, the dielectric s-tren~th of a so-called doughnut-shaped thermistor having an opening at or substantially ' ' ~ . :
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'' - : : ' at its central portion is higher -than o-thers. That is, it is ound that the d:ielec-tric s-trength which is one of the important fac.tors in design o:E parts of a PTC thermis-tor is eurther im-proved, and therefore if the PTC thermis-tor is ormed into a doughnut shape, -the parts may be made thin and -the disk area may be made smaller.
(E~ample 2) Lead wires and a molding agent were fur-ther applied to the disk A formed in Example l to thereb~ form an ordinar~ PTC -ther-mistor A provided with lead wires. A square chip having a size of 6mmX 6mm and having~ thickness of 0.4 mm was prepared by using a material having an ordinary-temperature resistivity valu0 of 5 Q cm, standard dielectric strength ot 90 V~mm and a Curie point of about 100 C as the high~dielectric strength and low-resistivity material disclosed in Japanese Patent Application No.
Hei-1-292387, and then silver electrodes were respec~tively ap-plled onto the opposite surfaces of the square chip to thereby form a square chip-shaped PTC thermistor C. The resistance value of the PTC thermistor C was 0.6 Q and the dielectric s-trength thereof was 40 V.
Thus, the three kinds of PTC thermistors, that is, the PTC
thermistor A ~the diameter was lO mm ~ and the basic body thick-ness was 0.6 mm) provided with lead wires, the doughnut-shaped PTC thermistor B (the outer diameter was 12 mm ~ , the inner dia-meter was 6.5 mm ~, and the basio body thickness was 0.6 mm~, and the square chip-shaped PTC thermistor C ~the siæe was 6mmX 6mm and -the basic body -thickness was 0.4 mm), were formed. Although :~ ~
the shapes and sizes of the three kinds~of PTC thermistors were ; differenf from each other, -the PTC thermistors have the same ~ ~ 13 ;:
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resis-tance value of 0.6 Q .
Ne~-t, three small-sized ~C-12V motors each having internal resistance of about 4 Q and a diameter oY 26 mm~ were prepared and connected in series to the formed PTC thermistors A, B, and C. Next, the respective methods of connection will be descrlbed in de-tail.
The PTC thermistor A is of a usually generally-used ~isk type provided with lead wires. A PTC thermistor A 14 is mounted on a printed substrate 13 entirely separately from a mo-tor 11 and connected in series to the motor 11 as shown in Fig. 2. Further, in Fig. 2, the reference numeral 11 designates a small-sized direct current (DC) motor; 12, input terminals of the motor ll;
and 15; a DC power source. This connection is the most ordinari-ly used conventional system.
The PTC thermistor B is a doughnut-shaped element having silver electrodes at its opposi-te surfaces, according to the present invention. As shown in Fig. 1, an opening of a PTC ther-mis-tor B 8 is inserted onto a shaft 9 o~ the foregoing motor so that *he PTC thermistor B 8 is incorporated into a mo-tor casing 2 so as to be substantially perpendicular to the shaft, and the PTC
thermistor B 8 is electrically connected between an input termi-nal and a phosphor-bronze pla-te 6 connected to a brush 5 of the motor 1. Fur-ther, in Fig. 1, the reference numeral 1 designates a motor body; 3, a field magnet; 4, a coil; 7, input terminals;
8, a PTC thermistor B according to the present invention; and 10 a commutator.
The PTC thermistor C is a square chip-shaped one formed on the basis of the foregoing proposal and has silver electrodes at its opposite surfaces. As shown in Fig. 3, a groove is formed in :: ~

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~73~5 the inside of a motor casing 2 so -tha-t a PTC thermistor 8a is incorporated in the groove. Further, the PTC thermis-tol C is electrically connected between an input terminal and a phosphor-bronze p}.ate connected -to a brush 5 of the mo-to:r 1. Further, in Fig. 3, the reference numerals 1 10 e~cept 8 clesi~nate portions -the same as or corresponding to those of Fig. 3, and -therefore the explanation -thereof is omitted.
The PTC thermistors were set in a constant-temperature oven (not shown) and the response performance the:reof was measured in a state that the motor was locked and under three conditions, that is, the temperature/voltage c.onditions of -30CJlOV, 25C/14V, and 80C/14V. Table l shows the measurement results.
The conditions were set on the estimation of temperature/voltage fluctuations in ordinary cars.
Table 1 ~ - , , , :
I I Response time t~(sec~ I Limit current Io (A) I
, I 1 30C~ I 25C/ I 80C/ 1-30C/ I 25C/ I 80C/ i I I10~ 114V I14V I lOV I 1~- 1 14V
I I I I I I
I PTC I 53 1 lO I 2.5 1 0.24 1 0.14 1 0.12 1 I thermistor A

l PTC I 21 1 5 1 1.6 1 0.26 1 0.15 1 0.12 1 I thermistor B

I PTC ~ 18 1 4 1 1.2 1 0.34 1 0.22 1 0.16 1 I thermistor C

As shown in Fig. 6 5 the response time in Table 1 means time tl(ses) required for limitin~ a circui.t current to lA and the limitation current value in the same table means a stable dark : ~ current Io(A) after current limitation.
: It is found from Table 1 that in each of the conditions, the response time of the PTC thermistor C, that is, -the square chip ~ .
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~3~L~5 formed by us.ing the previously proposed ma-terial of high-dielectric strength and low-resistance is the shortest and ttle response -time of the PTC thermis-tor A is -the lollges-t. This is, because -the volume of the basic body of -the PTC thermistor A is about 30% as large as each o-f -the PTC thermistors B and C and hence the heat cnpaci-ty of the basic body of` the PTC thermis-tor A
is small, i-f the resistance value, that is, the heat value, is the same, the temperature of the PTC thermistor A rises easil~
and the PTC thermistor A is apt to be subjected to heat coupling with the phosphor-bronze plate. Further, this is because -the PTC
thermistor A is mounted on the printed substrate separated ~rom the motor, and there is no e~fect in heat coupling with the motor and the heat environment in the surrounding of the thermistor A
directly contacts with the air, so that the rate of temperature rising of the thermistor A is the slowest~ In the PTC thermistor B, on the other hand, the heat capacity thereo~ is the same as that of the PTC thermistor A, while it takes a value near that of:
the PTC thermistor C because of the heat coupling effect owing to $he incorporation.
Further, the expansion o~ the responss ti.me relative to a fluctuation of temperature/voltage is smaller than that of the PTC thermistor A. This is because an e-ffect due to existence of heat coupling and a difference in heat environment. Reduc-tion o~
expansion of the response time relative to a fluctuation of tem-perature~voltage is one of the: conditions required particularly from the fi ld of car industries.
Further, the limitation current value of the PTC thermistor C is the lnrgest and therefore the power loss is the largest, ~hile that of the PTC thermistor A is the smallest. Tha limita-: 16 ~ ~73~S
-tion current value o the PTC thermistor B is subs-tantially equal to that of the PTC -thermistor A although i-t is slightly higher than that of the PTC thermistor A.
Thus, according -to the method of -the present invention, -there are bo-th effec-ts that the response performance o~ the PTC ther-mis-tor A of the conventionally widely used system is improved, and that the powe.r loss after current limi-tal;ion as seen in the PTC thermistor C according to the previously proposed method ls reduced.
(Example 3) The thermistor formed in Example 1 was circumscribecl onto the motor casing of the motor used in Example 2 as shown in Fig.
7. That is, a PTC thermistor B 8 was circumscribed onto a motor casing 2 so as to be disposed axis-symmetrically with respect to the extension axis of a shaft 9 of the motor 1 and electrically connected between an input terminal 7 and a phosphor-bronze plate 6 connected to a brush of the motor. A~ter the circumscription, the doughnut-shaped PTC thermistor 13 was sealed with silicon resin (not show) having a high heat radiation property so as to stabilize the thermistor. The PTC thermistor circumscribed type motor was formed as described above, and the respQnse time and the limitation current value were measured in the same manner as in Example 2. Table 2 shows the measuremen-t results.
Table 2 I I Response time t~sec) I Limit current Io ~A) I

I l-30C/ 1 25C~ I 80C/ l-30C/ I 26C/ I BQ~C/ I
l I lOV I 14V I 14V I lOV I l~V I l~V

I PTC 1 2~ 1 6 1 1.9 1 0.25 1 0.14 1 0.12 1 I th~rmistor B
-- L I, , ... _.__ I . ... 1._._ .__ I I

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It is found from Table 2 -that although -the response time is slightly long in comparison with the incorporation -type, for e~ample, in the case of the PTC thermistor B of` Example 2, the l.imitation current value is further reduced to -thereby reduce a power loss.
As described above, according -to the present invention, a PTC thermistor is formed into a doughnut shape and an opening thereof is inserted on-to a shaft of a small-si~ed DC motor or an extension thereof so that the PTC thermistor is incorporated in or circumscribed on a motor casin~ so as to make the plate sur-face of the PTC thermistor substantially perpendicular -to -the shaft. Further, the PTC thermistor is provided betwèen input terminals of the motor. There~ore, the improvement of the two characteristics are attained such that the response performance of -the PTC thermi~tor is improved in comparison with the conven-tional case where the PTC thermistor is pro~ided on the control substrate and that the power loss after current limitation is reduced~in'comparison with the conventional case where the pLate-like thermistor is incorporated.

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Claims (2)

1. A small-sized DC motor, characterized in that a doughnut-shaped PTC thermistor having an annular or square plate-like body or a plate-like body similar to the former and an opening portion formed at or substantially at a central portion of said plate-like body is provided in a manner so that a shaft of said small-sized DC motor or extension thereof is passed through said open-ing portion to make said PTC thermistor be incorporated in or be circumscribed on a motor casing with a surface of said plate body made perpendicular to said shaft, whereby said PTC thermistor having electrodes formed on its opposite surfaces is disposed between an input terminal and a phosphor-bronze plate connected to a brush of said motor.

2. A small-sized DC motor according to Claim 1, characterized in that said PTC thermistor is sealed in said motor casing with resin having a high radiation property.