Electric Motor Having an Improved Airflow Cooling System

The present invention relates generally to switched reluctance motors, and more particularly to an improved structure for cooling switched reluctance motors and other electronically commutated motors.

Conventional switched reluctance motors are provided with a rotary position encoder that monitors the position and speed of the rotor. The rotary position encoder is mounted on the motor shaft and typically disposed in proximity to a printed circuit board that controls the motor. The printed circuit board has conventionally been mounted on top of the motor with the motor shaft extending through an opening in the printed circuit board. A drawback of the configuration is that the printed circuit board acts as a barrier to airflow which could otherwise be used to cool the motor and its associated electronic components, including the printed circuit board itself.

The present invention discloses a motor construction having an improved airflow path that overcomes the problems found in the prior art in a convenient and cost-efficient manner.

The present invention is directed to a switched reluctance motor that creates an improved airflow path for cooling motor components in an efficient manner. The motor includes a support frame having an upper and lower frame, a stator having among other components four stator poles formed of multiple laminations, and a rotor having a shaft and four rotor poles formed of multiple laminations. The rotor is mounted for rotation within the stator, which is secured to the support frames. A centrifugal fan mounted to the motor shaft is also provided for circulating air through the motor to keep the motor cool during operation. A rotary position encoder mounted to the shaft beneath the centrifugal fan is further provided for determining the speed, direction, and position of the motor rotor.

In one aspect of the present invention, the electrical functions are divided between an upper and a lower circuit board. The upper circuit board is secured to the upper frame in one horizontal plane perpendicular to the rotor, and the lower circuit board is secured to the upper frame in another horizontal plane. The centrifugal fan is mounted within an opening formed in the upper circuit board and is disposed above the lower circuit board. This construction allows the airflow to pass across both the upper and lower circuit boards to thereby dissipate any heat that may be generated by these devices. The airflow is also directed across sidewalls which are part of the support frame. These sidewalls act as a heat sink for electronic power devices secured to the sidewalls and connected to the upper circuit board. The airflow assists in removing heat generated by the electronic power devices.

The present invention also provides an additional pair of centrifugal fans that are disposed above and below the rotor laminations. These devices direct the airflow across the rotor, stator and stator coils thereby cooling these devices and expelling the air out of ports formed in the support frame. The centrifugal fan disposed above the rotor laminations expels the air from the flow path created by the centrifugal fan mounted within the opening in the upper circuit board. The centrifugal fan disposed below the rotor laminations creates an independent flow path which passes in through and out through ports in the lower frame.

In another aspect of the present invention, a method for creating an airflow path through an electric motor is provided. The method includes the step of drawing air into the motor by rotating a fan which is connected to the rotor. The airflow is then directed across a pair of circuit boards or other planar surfaces, thereby dissipating any heat generated by these devices. The airflow is then directed across a heat sink area to which electronic power devices have been attached, thereby dissipating any heat generated by these devices. Finally, the airflow is expelled out of the motor through multiple ports formed in the motor support frame.

Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1 is a cross sectional view of a switched reluctance motor embodying the present invention.

FIG. 2 is a perspective view of the switched reluctance motor of FIG. 1, illustrating the airflow path of the present invention, with the electrical power devices and spring clamps removed for clarity.

FIG. 3 is a top perspective view of the upper frame of the switched reluctance motor according to the present invention without the printed circuit boards attached, illustrating the airflow path of the present invention.

FIG. 4 is a perspective view of the rotor assembly and attached centrifugal fans according to the present invention.

FIG. 5 is a bottom perspective view of the top-most centrifugal fan and the rotary position encoder attached to the motor shaft.

Turning now to the drawings and referring initially to FIG. 1, a switched reluctance motor 1 according to the present invention is shown. The motor 1 includes a pair of support frames 2 comprising an upper frame 3 and a lower frame 4. The upper frame 3 and the lower frame 4 are attached to one another with fastening means, such as screws 5, as shown in FIGS. 2 and 3. The frame 2 is provided with upper and lower bearing retaining housings 6 and 7, formed in the upper and lower frames 3 and 4, respectively. The upper and lower bearing retaining housings 6 and 7 house bearings 8 and 9, which are provided to support a motor shaft 10 in the frame 2 and allow the shaft 10 to rotate relative to the frame. Part of a vacuum housing 52 and a impeller 59 which are connected to motor 1 are also visible from the perspective of FIG. 1.

A rotor 11 having four equally-spaced rotor poles, which is formed of a series of similarly constructed stacked laminations 12, is mounted to the motor shaft 10, as best seen in FIG. 4. The rotor 11 rotates within a stator core 13, which is also formed of a series of similarly constructed stacked laminations. Stator coils 15 are wound around the stator core 13, as shown in FIG. 1. When energized by an external power source, the stator coils 15 induce an electromagnetic force that causes the rotor 11 to rotate, as is known to those of ordinary skill in the art.

The motor 1 further includes centrifugal fans 17 and 19, which are mounted to the rotating motor shaft 10. The centrifugal fans 17 and 19 are mounted above and below the rotor laminations 12, respectively, as best seen in FIG. 4. These devices force heated air out of the motor 1, as shown in FIGS. 1 and 2, thereby cooling the rotor 11, stator 13 and stator coils 15. A centrifugal fan 21 is also provided, and is mounted to the upper end of the rotating motor shaft 10, as shown in FIGS. 1, 2 and 4. As the centrifugal fan 21 rotates with the shaft 10, it forces cool air into the motor 1, as shown in FIGS. 1 and 2. The centrifugal fan 21 cooperates with the centrifugal fan 17 to generate an airflow path through the motor 1 that cools the motor in accordance with the present invention, as further described below.

In accordance with the design of the present invention, the control circuitry is divided between two separate circuit boards, an upper printed circuit board 23 and a lower printed circuit board 25, which are interconnected by wires or separable connectors. The upper circuit board 23 contains a plurality of devices including the electronic power devices 27 which generate substantial amounts of heat during operation. The electronic power devices 27 are secured in close thermal contact to the upper frame 3 at sidewalls 31 with spring clamps 51, as shown in FIG. 1. The sidewalls 31 act as heat sinks dissipating the substantial amount of heat generated by the electronic power devices 27 away from the devices, thus improving their electrical characteristics and reliability. The sidewalls 31 may also be corrugated, for example, with cooling fins 57 (best seen in FIG. 3), as is consistent with typical heat sink designs. The sidewalls 31 are cooled by the circulating air generated by the improved cooling design according to the present invention.

The lower circuit board 25 contains the control portion of the motor circuitry, such as integrated circuits and related parts which do not generate enough heat to require attachment to a heat sink.

The upper circuit board 23 is supported on the top of the upper frame 3 by frame supports 33. The lower circuit board 25 is disposed below the upper circuit board 23 and is supported on the upper frame 3 by cylindrical frame supports 35. The upper and lower circuit boards 23 and 25 are secured to the frame supports 33 and 35, respectively, by suitable fastening means, such as screws 37, as shown in FIG. 1. The ends of supports 33 and supports 35 terminate at different heights so that the circuit boards 23 and 25, when attached, will appear in different planes within the upper frame 3. FIG. 3 shows the frame supports 33 and 35 with their associated circuit boards removed for clarity.

A circular opening 39 is formed in the upper circuit board 23 to accommodate the centrifugal fan 21, as shown in FIG. 2. The opening 39 also provides an inlet for cool air to flow into the motor 1. A smaller opening 41 is provided in the lower circuit board 25 to accommodate the rotating motor shaft 10, as shown in FIG. 1.

A rotary position encoder 43 is mounted beneath the centrifugal fan 21, as shown in FIG. 5. The rotary position encoder 43 is spaced in close proximity to the lower circuit board 25 to monitor the position and speed of the rotor 11 relative to the stator 13. Encoding is known to those of skill in the art and is only briefly described here. First, copper pads are etched onto lower circuit board 25 which have traces connected to a motor controller integrated circuit (not shown) also mounted on board 25. Rotary position encoder 43 is similarly formed of copper pads, as shown in FIG. 5, fixed on a suitable substrate such as a circuit board. When the rotary position encoder 43 rotates in close proximity to the lower circuit board 25, a signal is capacitively coupled between the copper pads on the rotary position encoder 43 and the copper pads on the lower circuit board 25, the capacitance being greater when the pads are aligned, and lower when the pads are staggered. Thus, when the rotary position encoder 43 rotates over the lower circuit board 25, an oscillating capacitively induced signal is produced which is interpreted by the motor controller integrated circuit to determine rotor position and speed. One of ordinary skill will realize that a similar encoding arrangement could be adapted using optical, magnetic, or inductive sensing means, all of which are well known in the art. It should also be noted that rotary position encoder 43 could consist of a conductive pattern integral with the centrifugal fan 21, such as a hot stamped foil or printed pattern.

Motor 1 is preferably assembled according to the general outline that follows. One of ordinary skill will generally know the specifics of how to assemble a motor 1 such as that disclosed herein, and furthermore, will realize that many of the assembly steps as disclosed may be performed in a different order or in slightly different ways. First, the rotor assembly as shown in FIG. 4 is assembled (at this point, without centrifugal fan 21). Next, the stator assembly, including winding the stator coils 15 around the stator core 13, is formed. Then, the circuit boards 23 and 25 are screw-connected to the upper frame 3. Next, bearings 9 are inserted into the lower frame 4, and the completed rotor assembly is slipped therethrough. A nut 61 fastens the shaft 10 (and, for example, impeller 59) to the underside of the lower frame 4. Next, the completed stator assembly is slipped into place around the rotor assembly and into lower frame 4. Then, bearings 8 are inserted in the upper frame 3, the upper portion of the rotor assembly is fed therethrough. Next, the upper frame 3, lower frame 4, and the stator assembly are fastened together with screws 5, and the stator coils 15 are connected to the appropriate leads on the upper circuit board 23. Next, the centrifugal fan 21, complete with rotary position encoder 43, is slipped though hole 39 in the upper circuit board 23 (the hole 39 should be made just slightly larger than the diameter of the fan 21), and is press-fitted onto the rotor shaft 10. Additionally, a spring clamp can be used to further secure the fan 21 (preferably plastic) onto the rotor shaft 10.

The foregoing construction contains several advantages over the prior art. First, the rotary position encoder 43 is connected directly to the bottom of a centrifugal cooling fan 21. This advantageously combines the functions of cooling and encoding into one part. Second, the separation of the printed circuit board into upper and lower circuit boards 23 and 25 creates an airflow path through which air can be guided by the centrifugal fans 17 and 21 through the motor 1. This not only facilitates cooling the internal components of the motor 1 but it also facilitates cooling the circuit boards 23 and 25 themselves. Most importantly, the power devices 27 attached to the upper circuit board 23 are also cooled by the air flow path. Also, one of ordinary skill will realize that other structures could be used in place of circuit boards 23 and 25 to provide the air cooling path described. For example, metallic or plastic plates or any other suitable member could be used. Such plates could be planar or non-planar depending on the shape and direction of the desired airflow path within the motor 1.

Referring to FIG. 1, the airflow path A according to the present invention is shown. The rotation of shaft 10 causes the centrifugal fan 21 to turn, thus drawing air into the motor 1 through the circular opening 39 in the upper circuit board 23. As the air flows radially outward from the centrifugal fan 21, it passes along the underside of the upper circuit board 23. This flow cools the upper circuit board 23 and thus prevents it from overheating. As the air flows in this direction, it also passes over the upper surface of the lower circuit board 25 thus cooling the lower circuit board. Next, and most importantly, the air flows along the sidewalls 31 thereby cooling the sidewalls and keeping the electronic power devices 27 from overheating. The centrifugal fan 17 draws the airflow back toward the center of the rotor 11 and redirects it across the upper portion of the rotor 11, stator 13, and stator coils 15, thereby cooling these components, and then out ports 45 formed in the side of support frame 2, as shown in FIGS. 1 and 2.

To further assist in cooling the motor components, the centrifugal fan 19, mounted below the rotor laminations 12, creates an independent airflow path B. As shown in FIGS. 1 and 2, airflow path B enters the lower frame 4 at side ports 50 and is directed across the lower portion of the rotor 11, stator 13, and stator coils 15, thereby cooling these components. Airflow path B exits the lower frame 4 at ports 47. By the use of these two air paths, more than just the electronic power devices 27 and upper and lower circuit board 23 and 25 are cooled. The rotor 11, stator 13, and the stator coils 39 are also cooled, thereby improving their performance and reliability.

To form an efficient airflow path A, it is necessary to patch any "leaks" which would otherwise exist in the path. To accomplish this, a film 53 or some other sealing means is installed between the upper circuit board 23, and the sidewalls 31, as shown in FIG. 1. Preferably, the film 53 is a polyimide (e.g., Kapton.TM.) film which insulates the power devices 27 from the heat sink and provides a suitable airflow seal. Because the electronic power devices 27 are mounted on only two sides of the upper frame 3, a suitable foam or insulation paper (not shown) can be used to seal the gaps between the upper circuit board 23 and the upper frame 3 on the other two sides (i.e., the non-heat sink sides) of the upper frame 3.

Those skilled in the art who now have the benefit of the present disclosure will appreciate that the present invention may take many forms and embodiments and have many uses. For example, the present invention can be used on other electronically commutated motors as well. It is intended that the embodiments described herein should be illustrative only, and not limiting of the present invention. Rather, it is intended that the invention cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.