GB2102216A

GB2102216A - Noise reducing motor fan drive

Info

Publication number: GB2102216A
Application number: GB08213449A
Authority: GB
Inventors: Yasuhiro Sekine; Hiroshi Harada; Masayuki Kuwayama
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1981-05-19
Filing date: 1982-05-10
Publication date: 1983-01-26
Also published as: MX152982A; CH664242A5; AU552562B2; ES512312A0; ES8309037A1; JPS57193951A; GB2102216B; FR2506536B1; AU8385182A; FR2506536A1

Abstract

A blower apparatus for an electric motor comprises an air cooling fan 15 rotatably mounted on a rotor shaft 7 by a bearing 14, a plurality of pairs of magnets 16a, 6b mounted on the rotor or the fan, and a non-ferrous disk 17 mounted on the fan or the rotor and having an annular flange disposed between the magnet pairs. The fan is driven by the interaction between the magnets and eddy currents induced in the disk flange with an increasing degree of slippage as a function of speed, thereby reducing the noise level generated by the fan. Alternative embodiments are described with reference to Figs. 7 to 9 (not shown). <IMAGE>

Description

SPECIFICATION Noise reducing motor fan drive BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a noise reducing cooling fan drive for a rotating machine, such as an electric motor.

Description of the Prior Art A conventional rotating machine, for example, a motor for driving an electric car, is shown in Figure 1, and includes a yoke 1 having an air intake opening 2 and an air exhaust opening 3, brackets 4, 5 at each end of the yoke, and bearings 6, 6 which support a shaft 7. A rotor 8 comprises an iron core 8a, a rotor coil 8b, a commutator 8c, and a spider 9. An air passage 10 bored through both the rotor and the spider communicates with a cooling fan 11 mounted on the shaft. A brush 12 provided on the end bracket 5 contacts the commutator 8c. A stator 13 mounted to the yoke comprises an iron core 1 3a and a stator coil 1 3b.

As the rotor 8 and fan 11 rotate with the shaft 7, an airflow is established from the intake opening 2 to the exhaust opening 3 through both the gap between the rotor and the stator 13 and the air passage 10, as shown by the dotted lines, whereby the heated rotor and stator are effectively cooled.

An electric car motor is operated over a wide speed range from 0--4000 rpm. Since the fan 11 in the conventional motor is fixed to the shaft, the rotational speed of the fan is the same as that of the motor as shown by the dotted line in Fig. 2.

Noise developed by the rotation of the motor thus increases in almost linear proportion to the speed thereof, as shown by the dotted line in Fig. 3.

It is well known that such motor noise primarily results from the airflow caused by the fan 11, and such noise N is represented by the following formula: N = K (70 log D + 50 iog n), where K is a noise constant (0.1 SO.3S), D is the diameter of the fan 11, and n is the rotational speed of the fan.

The conventional motor is thus characterized by the disadvantage that noise developed during its operation increases in almost linear proportion to increases in its rotational speed.

SUMMARY OF THE INVENTION To reduce the above described motor noise, the present invention provides a new and improved blower apparatus for a rotating machine which comprises a fan journaled on the rotor shaft through a bearing, a plurality of pairs of opposed magnets having a predetermined magnetic force mounted on either the rotor or the fan, and a rotating disk made of non-ferrous metal and having an annular flange disposed between the magnet pairs, said disk being mounted on either the fan or the rotor -- whichever is not provided with the magnets.

Another feature of this invention is the alleviation of the aforementioned noise problem without increasing the axial length of the motor.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a cross-sectional view of a conventional rotating machine, for example, an electric motor, Fig. 2 is a graph showing the relation between the rotational speed of a motor and its cooling fan, Fig. 3 is a graph showing the relation between the rotational speed and noise developed by a motor, Fig. 4 is a front sectional view of the blower part of a rotating machine according to a preferred embodiment of this invention.

Fig. 5 is an enlarged sectional end view taken on line V-V of Figure 4, Fig. 6 is a schematic diagram to aid in explaining the basic operation of this invention.

Figs. 7 and 8 are front sectional views of the blower according to alternate embodiments of this invention, and Fig. 9 is a schematic sectional view of a reverse embodiment wherein the magnet pairs are mounted on the rotor and the disk flange is mounted on the fan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 4 through 8, reference numeral 14 designates a bearing provided on the rotor shaft 7. A fan 1 5 is mounted on the outer bearing race so as to be independently rotatable around the shaft, and has an annular cavity 15' having two interior waIls 1 spa, 1 5b facing each other near the bearing 14. Reference numerals 1 6a and 1 6b designate pairs of magnets respectively mounted on the interior walls 1 spa and 1 5b of the fan cavity 15'.

The detailed arrangement of the magnet pairs 1 6a, 1 6b is shown in Fig. 5. A plurality of pairs of magnets each comprising an N pole and an S pole facing each other are arranged circumferentially and alternatingly on the interior walls 1 5a and 1 5b of the fan cavity 15'.

A rotating disk 1 7 made of non-ferrous but electrically conductive metal such as copper or aluminum is mounted on the rotor spider 9, and the disk 17 has an annular flange extending into the annular cavity 1 5' of the fan and disposed between the facing magnet pairs 1 6a, 1 6b with gaps 18, 18 (Fig. 5).

In the embodiment shown in Figs. 4 and 5, and assuming that the disk 1 7 moves in the direction of the arrow as best shown in Fig. 6, the magnetic lines of force 19 flowing from the N poles toward the S poles of the magnets are cut by the annular disk flange, whereby eddy currents are induced in the surface of the disk. The resulting interaction af the flux produced by the fan magnets with the induced eddy currents in the rotor disk flange causes the fan 15 to rotate. That is, the resultants of the eddy currents induced in the disk flange may be visualized as a series of current vectors of alternate polarity perpendicular to both the magnetic lines of force and the direction of disk movement, and this will produce a series of unidirectional force vectors acting on the fan/magnets to rotate the fan in the same direction as the disk.The rotational speed of the driven fan is not directly or linearly proportional to that of the driving disk 17 or the rotor, however, as in the prior art, but instead obeys a flattening or falling off curve as shown by the solid line in Fig. 2 due to an increase in the slippage between the magnets and the disk flange as the rotor/disk speed increases. Such slippage is due, inter alia, to the increasing air resistance as a function of speed, and does not adversely affect the cooling of the motor as the fan draws through an excess amount of air at higher speeds.

As the rotary power to drive the fan is generated by eddy currents which are induced due to the fact that the fan rotates at a slower speed than the disk, the rotational speed of the fan is necessarily less than that of the motor. This results in a reduction of the noise developed by the motor, as shown by the solid line in Fig. 3.

The rotational speed of the fan, and attendantly the noise level, is freely controllable by adjusting the strength and/or the number of pairs of magnets, and/or the magnitude of the air gaps 1 8.

Fig. 7 shows another embodiment of the invention, wherein the flanged disk 1 7 is disposed in an annular recess in the spider 9. The other components and the operation of the blower apparatus are substantially identical to those of the Fig. 4 embodiment. In this embodiment the disk 1 7 and the magnets 1 6a, 1 6b are disposed inside of the spider, thus avoiding any enlargement of the axial length of the motor without diminishing the intended effects of the invention.

Fig. 8 shows another embodiment wherein the rotating disk 1 7 is also disposed in an annular recess of the spider 9 but extends perpendicular to the axis of the motor, and the magnets 1 6a, 1 6b are disposed inside of a radial cavity 15' in the fan, again enabling the axial length of the motor to be decreased in comparison with that of Fig. 4.

In the foregoing embodiments, a blower apparatus has been described in which magnets are mounted in a fan cavity and a flanged disk is mounted on the spider, but a reverse arrangement of those components may equally be used with similar operation and effect, as shown by the embodiment illustrated in Fig. 9.

Test results have confirmed that noise from a motor according to this invention was reduced approximately 8 dB(A) at 1 m at a rotational speed of 2500 rpm as compared with a conventional motor,

Claims

1. A noise reducing fan drive for a rotating electrical machine including a stator mounted to a yoke, and a rotor mounted on a shaft rotatably supported on said yoke, comprising: a) a cooling fan rotatably mounted on said shaft, b) a plurality of pairs of magnets mounted on either the rotor or the fan, and c) a rotating disk formed of a non-ferrous electrically conductive material mounted on either the fan or the rotor, whichever does not carry said plurality of pairs of magnets, to interact with said magnets and to constitute therewith a drive coupling for said fan characterized by an increasing degree of slippage as a function of rotor speed.

2. A rotating machine according to claim 1, wherein said magnets and rotating disk face each other in a substantially parallel relation with said shaft, and said disk includes an annular flange disposed between said magnet pairs.

3. A rotating machine according to claim 1, wherein said magnets and rotating disk face each other and are substantially perpendicular to said shaft, and said disk includes an annular flange disposed between said magnet pairs.

4. A rotating machine according to claims 2 or 3, wherein each pair of magnets comprises an N pole and an S pole disposed facing each other in an annular cavity of said fan.

5. A rotating machine according to claim 4, wherein said plurality of pairs of magnets are circumferentially and alternatingly arranged on interior walls of said cavity.

6. A rotating machine according to claim 4, wherein said rotating disk is disposed in an annular cavity of said rotor.

7. A rotating machine according to claim 1, wherein said rotating disk is formed of either copper or aluminum.

8. A rotating machine substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.

9. A rotating machine substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

10. A rotating machine substantially as hereinbefore described with referenced to Figure 8 of the accompanying drawings.