TECHNICAL FIELD:
The subject matter described herein in general relates to DC motors and in particular relates to reducing noise in DC motors. BACKGROUD:
A DC motor converts DC electrical energy into mechanical energy. This mechanical energy is delivered via a shaft, which is connected to a rotor of the DC motor. For a given power, a motor having higher number of poles is small in size. However, the noise characteristics of the motor, having lesser number of poles, are better than those having higher number of poles. Due to this reason motors having two poles are preferred in quiet environments, even though the size of the two pole motor is large. Further, as the number of poles is increased, harshness in sound becomes a prominent factor.
Cogging torque is an important factor and it needs to be reduced to prevent the wastage of energy. Cogging torque is the torque due to the interaction between the permanent magnets and the stator slots. Also termed as detent or 'no-current' torque, cogging torque is an undesirable for the operation of a motor. It is especially prominent at lower speeds, with the symptoms of jerkiness. In other words, cogging torque is the variation in torque and speed of an electric motor due to variations in magnetic flux, as rotor poles move past stator poles. Reduction in cogging torque results in reduction in noise and improvement in the efficiency of the motor.
Nowadays, the world is moving towards nanotechnology related products. Compactness is the buzzword in the industry today. Manufacturers, particularly in automobile industry, are constantly devising new ways for reducing the size of existing products, so as to make optimal use of the space available in the vehicle. Consumer's demand for more and more gadgets, within the available space, needs to be fulfilled and anybody who is able to satisfy such demands is amply lewarded.
The challenge therefore is to provide a low n_oise motor having more than two poles, having a small frame size, and having high power output. SUMMARY:
The subject matter described herein is directed to a multiple pole motor assembly that satisfies the need. The assembly includes a rotor, a stator and one or more spherical bearings. The rotor has a core and the core has a plurality of slots. The stator is fixedly positioned relative to the rotor. The stator includes a plurality of poles divided into north poles and south poles. Spherical bearings rotatably support the core. Each of the bearings includes a radial outer surface and a radial inner surfa-ce. The radial inner surface defines a through passage, such that, the passage commences from a first surface and ceases at a second surface. An axial centre of the core is axially offset to a diagonal centre of the poles of the stator, such that, the offset is disposed towards an impeller, thereby reducing an axial thrust of said impeller. The slots of the core are inclined to an axial plane of the rotor. An angular span of the north poles is greater than an angular span of the south poles, thereby reducing a cogging effect during commutation. A first and a second chamfer are provided between the radial inner surface and the first surface, such that the first and the second chamfer are contiguous. A third and a fourth chamfer are provided between the radial inner surface and the second surface, such that the third chamfer and the fourth chamfer are contiguous.
The advantage of the presently described subject matter is that the noise level of the motor is very low and consequently the motor can be used in quiet environments.
Further, for a given power, the frame size of the motor is less as compared to a motor having a lesser number of poles.
These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor it is intended to be used to limit the scope of the claimed subject matter.
BRIEF DESCRIPTION OF DRAWINGS:
The above and other features, aspects, and advantages of the subject matter will become better understood withi regard to the following description, appended claims, and accompanying drawings where:
Fig.l shows an exploded view of a motor having four poles according to one embodiment of the present subject matter.
Fig.2 shows a side view of the stator and rotor, and an isolated view of magnets.
Fig.3 shows a sectional view of the rotor positioned inside the stator.
Fig.4 shows a perspective view of the drive end bracket assembly.
Fig.5 shows a perspective view of the rear end bracket assembly.
Fig.6 shows a perspective view of the brush box assembly.
Fig.7 shows a perspective view of the rotor.
DETAILED DESCRIPTION:
The subject matter, described herein, relates to a multiple pole DC motor which includes a rotor, a stator and one or more spherical bearings. The rotor has slots, which aire inclined to the axis of the rotor. The stator has poles with a variable thickness and variable angular span. The spherical bearings are provided with two chamfers on each side. In spite of utilization of more than two poles, the motor has better noise clharacteristies. Due to this reason, the motor can be used in quiet environments. Also, the size of the motor is reduced for the same power, as compared to a motor having lesser n.umber of poles.
Fig.l shows an exploded view of a motor having four poles according to one e-mbodiment of the present subject matter. The motor includes a stator 100, a drive end bracket assembly 200, a rear end bracket assembly 300, a brush assembly 400 and a rotor 500. Drive end bracket assembly 200 is disposed towards the drive end side of the motor and rear end bracket assembly 300 is disposed towards the rear end side of the motor.
Fig.2 shows a side view of the stator and rotor, and an isolated view of magnets. The stator consists of a yoke 110 and four magnets 112, of which 114 are North Pole magnets and 116 are South Pole magnets. Magnets 112 are attached to the yoke 110 by imeans of adhesives. The rotor 500 includes multiple teeth 520. The yoke 110 has a circular inner surface and the magnets 112 are attached to this inner surface. The profile ®f the magnets 112 is such that they have non-uniform thickness and angular span of morth pole magnets 114 is different than that of the south pole magnets 116. Thickness of imagnets 112 at the centre is greater than thickness at the edges 123. In the present •embodiment, the thickness of the magnets at the centre 122 is approximately 3 mm to 6 mm and thickness at the edges 123 is approximately 2 mm to 4mm. Angular span of the magnets 112 is unequal. North Pole magnets 114 have a greater angular magnetic span as compared to the South Pole magnets 116. The angular span of the North Pole CC ranges
from approximately 50° to 70° while that of South Pole DD ranges from approximately 40° to 60°. The magnets 112 are manufactured by the tri-arc method. This method of manufacturing permanent magnets comprises using two different circle radii to ensure that there are at least two points of contact between the magnet and the frame wall. This design of the magnets 112 produces a variable magnetic flux inside the stator and this flux is utilized to reduce the cogging torque effect.
As the teeth 520 of rotor 500 move past the magnets 112, an uneven magnetic pull is experienced by the teeth 520, because the flux varies from the edges to the centre of magnet 112. This uneven pull results in the generation of a harmonic noise. To reduce the cogging torque, a variable magnetic thickness is provided. This Teduces the wastage of power consumed by the cogging torque, thereby enhancing the efficiency of the motor.
Fig.3 shows a sectional view of the rotor 500 positioned inside the stator 100. The figure shows a yoke 110, poles 112 and a rotor 500. An isolated 'view of the magnet 112 is also shown in the figure. Magnet 112 is having a diagonal centre 116. The axial centre 114 of rotor 500 is axially offset to the diagonal centre 116 of the magnet 112, in such a way that the diagonal centre 116 is disposed towards the rear end side of the motor. The offset distance L ranges from approximately 0.5mm to 2mm.
Fig.4 shows a perspective view of the drive end bracket assembly 200. The drive end bracket assembly 200 has a drive end bracket 210, a spherical Bearing 220, a felt pad 230 and a bearing retainer 240. The spherical bearing 220 includes a radial inner surface 222 and a radial outer surface 224. The spherical bearing 220 has two chamfers, referred to as a first chamfer 226 and a second chamfer 228, on both the sides. The chamfers are provided in such a manner that they are contiguous to each other. The length of the
second chamfer LI on both sides of the spherical bearing ranges from approximately 2 to 3 mm. The angle of the second cha_mfer A1 ranges from approximately 2° to 4°. The above said chamfering of spherical bearings helps in reducing the friction between rotor and spherical and reducing the noise level of the motor.
Fig.5 shows a perspective view of the rear end bracket assembly 300. The rear end bracket assembly 300 has a rear end bracket 310, a spherical bearing 320, a felt pad 330 and a bearing retainer 340. The spherical bearing 320 includes a radial inner surface 322 and a radial outer surface 324. The spherical bearing 320 has two chamfers, referred to as a third chamfer 326 and a fourth chamfer 328, on both the sides, he spherical bearing 320 of the drive end bracket assembly 300 has similar construction as that of the spherical bearing 220 of the drive en-d bracket assembly 200.
Fig.6 shows a perspective view of the brush box assembly 400. The brush box assembly 400 consists of brush holders 410, brushes 420, and a plug molding assembly 430. The brush holders 410 are rectangular in shape having two vertical walls and a top surface. The brushes, when inserted into the brush holders 410, are operatively urged out of each box by a ribbon spring 450. The ribbon spring 450, disposed in brush retainer 460, provides a nearly constant fore e on each brush 420 against the commutator 540 (not shown in the fig.). Brushes 420 are generally rectangular in shape. The position of brushes 420 with respect to the commutator 540 is critical for noise reduction. Brushes 420 are ground to almost exact dimensions as those of the commutator 540, to prevent uneven wear and tear of brushes 42® as well as the commutator 540. The brush contour is such that the brush 420 conforms to the commutator 540. This also helps in reducing vibrations. The smooth and efficient operation of motor is dependant upon the placement clearance between the brush 420 and the brush holder 410 is an important factor. If the clearance between the brush 420 and brush holder 410 is more, then there is a high probability that a chattering noise will occur. To overcome this problem, the clearance between the brush 420 and brush holder 410 is made in the range of approximately 0.05 mm to 0.1mm. Copper content of the brush material plays an important role in noise reduction and it ranges from approximately 30% to 50%.
Fig.7 shows a perspective view of the rotor 500. The rotor 500 comprises a shaft 510, a lamination pack 520, a winding wire 530 and a commutator 540. The lamination pack 520 has a plurality of slots, which are inclined to the axis of the shaft 510. The commutator 540 serves to periodically reverse the flow of DC current supplied to rotor 500 (not shown in the fig.), resulting in a unidirectional torque of the rotor 500. The slots are inclined at an angle S of approximately 10° to 20° with an axial plane which passes through the axis 550.
In general, the angular span of North Pole magnets 114 should be within 0.6 to 0.8 of the pole pitch, the pole pitch being equal to 360° divided by the number of poles. The angular span of South Pole magnets 116 should be lesser than the magnetic enclosure angle of North Pole magnets 114 by approximately 5% to 20%.
The previously described versions of the subject matter and its equivalent thereof have many advantages, including those which are described below. The noise level of the multiple pole motor described herein is low, due to which it can be used in quiet environments. Further, for a given power output, the size of the motor is smaller as compared to a motor having two poles. Smaller frame size is beneficial particularly in applications with space constraints. For example, in HVAC units the motor is placed inside the passenger compartment. In such a case this motor is useful, as it generates less noise. Further, it occupies less space and helps in space utilization.
Although the subject matter has been described ia considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.

I/We claim:
1. A low noise multiple pole motor assembly comprising: a rotor comprisimg a core, wherein
said core has a plurality of slots; a stator fixedly positioned relative to said rotor, wherein
said stator includes a plurality of poles divided into north poles and south poles;
one or more spherical bearings rotatably supporting said core, each of said spherical bearin_gs comprises a radial outer surface, and
a radial inner surface, wherein said radial inner surface defines a through passage, an