GB2275829A - Cooling commutated electric motors - Google Patents

Abstract

An electric motor (10) has a double fan (34) mounted on an armature stack of the motor and has flange (36) forming a baffle separating two sets of fan blades (38, 40). The first set (40) of the blades draws air over the stack from one end of the motor while the second set (38) draws air over a commutator (26) from the other end of the motor. The blades have a backward curve (65 degrees of arc to the tangent at the inner blade edges 52; 90 degrees of arc to the tangent at the outer blade edges 54). <IMAGE>

Description

ELECTRIC MOTORS This invention relates to electric motors and more particularly to arrangements thereof which provide efficient cooling.

US 4961016 discloses a dynamo-electric machine comprising a casing; a rotor journalled in the casing; and brush rings, a fan and a rotor assembly being mounted on the rotor, the fan having a dividing baffle so that air is drawn from either end of the casing over the brush rings and rotor assembly respectively before being mixed and expelled radially. Where size is not a premium, such an arrangement is satisfactory.

US 5053657 discloses an arrangement in a miniature motor in which space is a problem. Here, the fan is not secured on the rotor shaft but on the rotor assembly.

However, the arrangements described in this patent suffers the disadvantage of inefficient cooling. This is because there are many entrances for cooling air at the brush/commutator end of a motor. On the other hand, the armature is not only a close fit inside the stator field (to improve motor efficiency) and thus offers only limited access for cooling air to flow past, but also the situation is complicated by the fact that the armature is rotating in the stator field and disturbs any axial flow there may tend to be. Consequently, the armature coils do not receive adequate cooling because the cooling air drawn by the fan is drawn mainly from the commutator end of the motor.

Moreover, by reducing the size of the motor to such an extent that the fan is almost outside (in radial terms) the armature, its size becomes so reduced that its efficiency is affected.

Consequently, it is an object of the present invention to provide a motor of compact size and yet with efficient cooling arrangements.

In accordance with the present invention there is provided an electric motor comprising a housing, an armature shaft journalled for rotation in the housing, a commutator mounted on the shaft adjacent an armature stack, a stator field mounted in the housing around the stack, a fan mounted on the stack at its end adjacent the commutator, wherein the fan comprises an annular radial flange, which flange lies in a radial plane with respect to the shaft, a cylindrical flange extending from said radial flange and locating on said stack, a first set of fan blades mounted on said radial flange on the same side as said cylindrical flange but radially outside thereof and adapted to draw air axially between the stack and field and expel said air radially, and a second set of fan blades mounted on the other side of said radial flange and adapted to draw air over said commutator and expel said air radially.

Preferably said housing, one end of said field and said stack define between them a substantially annular space in which said fan is a close fit. Edges of said first and second set of fan blades may lie in a cylindrical surface which is close to said housing and other edges of said first set of blades may lie in a radial plane close to said field.

Preferably said fan blades have a backward curve to further enhance efficiency, particularly a ninety degree backward curve.

Because the cylindrical flange of the fan may be arranged effectively to close against the end of the stack and because the radial flange can be close to the housing, the only available source of air for the first set of blades is past the armature stack and its coils.

Consequently, effective cooling of the coils can be achieved. Moreover, by providing the fan blades with a backward curve, their efficiency, given the constraints of space available, is enhanced.

The invention is further described hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side section through an electric motor according to the present invention; Figures 2a to d are side sections/views of the components of the motor of Figure 1; Figures 3a to d are a front view (arrow a in Figure 3b), a side sectional view, a rear view (arrow c in Figure 3b) and a side view respectively of a fan for the motor of Figure 1.

With reference to the drawings, a motor 10 has a tubular housing 12 mounting a field in the form of two permanent magnets 14. Each end of the housing 12 is closed by end caps 16,18 which together journal an armature 20 for rotation in the housing 12.

The armature 20 comprises a shaft 22 on which a stack of laminations 24 is mounted. A commutator 26 is on the shaft 22 adjacent the stack 24. Coil windings (not shown) are wound on and through the stack 24 in a known manner and connected to commutator hooks 28 on the commutator 26.

Brush carriers 30 are mounted on the end cap 16 and carry brushes (not shown) which bear against the commutator 26 and supply electrical power to the armature coils.

In such a motor there are two primary sources of heat which must be removed. Firstly, electric current arcing inevitably occurs at the brushes/commutator bridge and generates heat in the brushes and commutator.

Secondly, the current through the armature coil generates heat there. With the compact arrangement shown in Figure 1 (where, incidentally, the average extent out of the ends of the lamination stack of the coils is shown at 32) there is no room on the armature shaft to mount a fan.

Consequently, a fan 34 is mounted on the stack 24 at that end thereof adjacent the commutator 26 in an annular space defined by the housing 12 (in fact by the end cap 16 in parts of this region), one end 58 of the field /magnets 14 and by the coils 32.

Referring also to Figure 3, the fan comprises a radial annulus or flange 36 which forms a baffle between first and second sets of fan blades 38,40 disposed on either side of the flange 36.

On one side of the flange 36, intermediate its inner and outer circumferences, there extends a cylindrical flange 42 terminated in a number of tags 44. The rim 46 of the flange 42 is adapted to abut the face of the end of the stack 20 and form a seal therebetween which, of course, is not complete around the circumference of the stack by virtue of the slots (not shown) in the stack in which the coils 32 are received. The tags 44 enter and are located in the slots in the stack. The fan sits on the coils and is held in place by adhesion.

Thus the fan 34 is a double fan, drawing air from two different directions. Blades 40, draw air along the motor through slots 48 in the housing 12 around the armature stack 24, cooling the armature coils. Blades 38, draw air from directly over the commutator 26 where most heat is generated. This air enters the housing 12 through slots 49 in the end cap 16. Only after leaving the fan blades 38,40 do the two airstreams mix prior to exit from the housing through radial slots 50. Thus they do not have the opportunity to interfere with one another and create turbulence within the confines of the fan thereby reducing its efficiency.

Indeed, because the space available for the fan is so limited, the blades 38,40 are arranged as backward curve blades. That is to say, the inner edges 52 of both sets of blades 38,40, ie those edges defining cylindrical surfaces, lie at an angle of about 650 to the tangent of said surfaces. The corresponding outer edges 54 of both sets of blades lie at an angle of about 900 to the respective tangents. This provides the greatest efficiency.

Moreover, edges 56 of the blades 40 which lie in a radial surface are arranged as close as possible to the end 58 of the field 14 while the radial flange 36 extends as close as possible to the housing 12 at 60 (in fact, close to the end cap 16 at 60' where it extends into the housing 12 in this region).

Claims (7)

1. An electric motor comprising a housing, an armature shaft journalled for rotation in the housing, a commutator mounted on the shaft adjacent an armature stack, a stator field mounted in the housing around the stack, a fan mounted on the stack at its end adjacent the commutator, wherein the fan comprises an annular radial flange, which flange lies in a radial plane with respect to the shaft, a cylindrical flange extending from said radial flange and locating on said stack, a first set of fan blades mounted on said radial flange on the same side as said cylindrical flange but radially outside thereof and adapted to draw air axially between the stack and field and expel said air radially, and a second set of fan blades mounted on the other side of said radial flange and adapted to draw air over said commutator and expel said air radially.

2. An electric motor as claimed in claim 1 further comprising tags on said cylindrical flange adapted to engage and locate in slots in said armature stack.

3. An electric motor as claimed in claim 1 or 2, in which said housing, one end of said field and said stack define between them a substantially annular space in which said fan is a close fit.

4. An electric motor as claimed in claim 3, in which edges of said first and second set of fan blades lie in a cylindrical surface which is close to said housing and other edges of said first set of blades lie in a radial plane close to said field.

5. An electric motor as claimed in any preceding claim, in which said fan blades have a backward curve.

6. An electric motor as claimed in claim 5, in which said blades have outer edges defining a cylindrical surface and wherein said blades lie substantially at right angles to the tangent of said surface.

7. An electric motor substantially as hereinbefore described with reference to the accompanying drawings.