WO1998000849A1

WO1998000849A1 - Capacitor for mounting to an electric motor

Info

Publication number: WO1998000849A1
Application number: PCT/US1997/011505
Authority: WO
Inventors: David C. Howe; Roger E. Schaefer
Original assignee: Aerovox, Inc.; Emerson Electric Co.
Priority date: 1996-07-01
Filing date: 1997-06-30
Publication date: 1998-01-08
Also published as: AU3647997A; WO1998000849A8

Abstract

A capacitor of the type mounted to the housing of an electric motor includes a packageless capacitive element (22) (without its own capacitor package) disposed within an internal chamber of a capacitor cover (20) ('doghouse'). Potting material (24) is used to fill the internal chamber to surround and insulate the capacitive element (22). Conductive leads (26) are also connected to the capacitive element (22).

Description

CAPACITOR FOR MOUNTING TO AN ELECTRIC MOTOR

Background of the Invention This invention relates to capacitors for electric motors.

Many single-phase AC motors include (in addition to the main winding) a start winding connected in series to a "start" capacitor. This start capacitor produces an effective rotating magnetic field for generating the torque needed to bring the motor up to speed. After the motor has reached operating speed, the start winding and capacitor are disconnected from the motor.

Permanent split capacitor (PSC) motors include an auxiliary winding and a "run" capacitor which remain permanently connected to the motor and are used during running as well as starting. Run capacitors typically comprise wound metallized plastic films having thin, vacuum-deposited metal layers. Run capacitors of this type generally have a high dielectric strength characteristic and a self-healing function. The wound assembly is typically enclosed within a casing or package having terminals mounted to one or both ends of the casing and connected to the metal layers of the wound assembly. The packaged capacitor is then enclosed within a capacitor cover ("doghouse") which is mounted to the housing of the motor.

Summary of the Invention In general, the invention features a capacitor including a package-less capacitive element (e.g., without its own capacitor package) which is disposed within an internal chamber of a capacitor cover (e.g., "doghouse") mounted to the housing of an alternating current induction electric motor. Potting material (e.g., asphalt) fills the internal chamber to support and insulate the package-less capacitive element. Conductive leads for supplying current are connected to conductive members of the package-less capacitive element. The internal chamber of the capacitor cover is defined by walls together with an open end adapted to face the housing of the motor.

The invention dispenses with the capacitor's own separate housing or package, thereby reducing cost. Cost is reduced not only because the capacitor package is eliminated, but also because potting the capacitive element is generally less expensive than using materials (e.g. , foam rubber) which are needed to support conventionally packaged capacitors within the capacitor cover. In addition, eliminating the capacitor package reduces the overall size of the capacitive element so that the size of the capacitor cover can be reduced.

In some preferred embodiments of the invention, the capacitive element is of the plastic film type including coiled conductive members separated by a plastic film. The capacitive element may be wrapped in an insulative sheet to provide additional electrical isolation between the conductive members of the capacitive element and capacitor cover which may be metal. The conductive leads extend externally from the capacitor cover to connect, for example, to a winding of the electric motor. Alternatively, where access to the internal windings of the electric motor is difficult, the conductive leads may be connected to a terminal board which forms a wall of the internal chamber and includes terminals electrically connected to the conductive leads. With this configuration, the wires or leads from the motor winding are connected to the terminals. The capacitor may include more than one capacitive element and may include other non-capacitive electrical components (e.g. , discharge resistors) disposed within the internal chamber of the capacitor cover, all of which are surrounded by the potting material. In another aspect of the invention, a capacitor, adapted to be mounted to a housing of the electric motor, includes a single cover having an internal chamber and means for mounting the housing; a capacitive element within the internal chamber; and potting material within the internal chamber which insulates the capacitive element.

In preferred embodiments, the potting material surrounds the capacitive element and secures the capacitive element within the chamber. Also, the housing is a motor housing.

These and other features and advantages of the invention will be apparent from the following description of preferred embodiments and from the claims. Brief Description of the Drawings

Fig. 1 is a bottom view of an electric motor having a capacitor, in accordance with the invention, mounted thereto.

Fig. 2 is a cross-sectional top view of the capacitor taken along lines 2-2 of Fig. 1.

Fig. 3 is a cross-sectional side view of the capacitor taken along lines 3-3 of Fig. 2.

Fig. 4 is a cross-sectional side view of an alternate embodiment of a capacitor for mounting to an electric motor.

Fig. 5 is a cross-sectional top view of the capacitor of Fig. 4.

Fig. 6 is a cross-sectional top view of a capacitor for mounting to an electric motor having other electrical components.

Description of the Preferred Embodiments Referring to Fig. 1, an electric motor 10 operated from an alternating current (AC) power source (not shown) is shown enclosed within a motor housing 12. Mounted to a sidewall of the motor housing 12 is a capacitor cover 20, known as a "doghouse", which encloses a capacitor connected to a winding (not shown) of the motor. Screws 21 or other mechanical fasteners are used to attach the capacitor housing 20 to the motor housing.

Referring to Figs. 2 and 3, capacitor cover 20 is formed as an open-sided sheet metal box having a rectangular shape and a size commensurate with the capacitance requirement and voltage rating of the motor. Positioned within the housing is a capacitive element, constructed here as a dry-wrap coiled winding 22. The coiled winding includes one or more metallized films, each having a very thin layer of metal (e.g. , zinc, aluminum, or alloys of either) deposited on a dielectric film (e.g. , polypropylene or polyester) . The film is wound around a plastic core 23. The coiled winding may be wrapped in an insulative sheet to increase the electrical isolation between the coiled windings and the metal capacitor cover and prevent possible arcing. The insulative sheet is particularly useful in applications in which the capacitive winding utilizes substantially the entire internal volume of the capacitor cover leaving little space between the winding an the inner surfaces of the metal capacitor cover. The inner surfaces of the metal capacitor cover may be coated or anodized so that the inner surface is non-conductive. Alternatively, the capacitor cover may be formed from a non-metallic material .

The coiled winding 22, without its own casing or package, is embedded within potting material 24, for example, blown asphalt pitch which may include wax or other additives to change the curing temperature or vary its pouring characteristics. Alternatively, the potting material may be an epoxy resin. Insulated wires 26 are connected to the coiled winding 22 and extend out of the capacitor housing where they connect to the winding in the motor. This approach for connecting the coiled winding to the motor is generally preferred where a larger capacitor is required and, thus, a substantial portion of the internal volume of the housing is required.

In another embodiment, shown in Figs. 4 and 5, a terminal board 30 is positioned within the capacitor cover 20 and includes a pair of bladed terminal connectors 32 which allow quick connection/disconnection of the motor to the coiled winding. Terminal board 30 serves as a stop or barrier between the terminal connectors and the poured asphalt potting material 24. This embodiment is well-suited for applications in which access to the interconnection points of the motor is difficult. Moreover, because the terminal board reduces the volume of the housing, it is generally used where a large capacitor is not necessary.

While preferred embodiments have been described above, other variations and modifications are within the scope of the following claims. For example, in certain applications, a separate start capacitor may be switched in parallel with the run capacitor of a permanent split capacitor motor. The start capacitor is connected only during start-up to provide increased torque. Referring to Fig. 6, a start capacitor 44 is included within capacitor cover 40. The start capacitor is connected through a relay switch 46 and in parallel to a coiled winding 42 which represents a run capacitor 42. The start capacitor is generally required to be packaged if, for example, the capacitor includes an electrolyte. In other embodiments, in which a plastic dielectric film is used, the package of the start capacitor may be eliminated. Moreover, other components, for example a discharge resistor 48 electrically interconnected to the coiled winding 42, may be disposed within the capacitor cover 40. As was the case in the embodiments of Figs. 2- 5, potting material 50 is used to surround and support the components within the cover.

Claims

What is claimed is:

1. A capacitor of the type connected to a winding of an alternating current induction electric motor, the winding enclosed within a housing of the electric motor, the capacitor comprising: a capacitor cover adapted to be mounted to the housing of the motor, the capacitor cover having walls and an open end adapted to face the housing of the motor, the walls and the housing of the motor together defining an internal chamber; a package-less capacitive element including conductive members separated by a dielectric, said capacitive element disposed within the internal chamber of the capacitor cover; potting material disposed within the internal chamber, said potting material electrically insulating and mechanically supporting the capacitive element within the capacitor cover; and conductive leads which connect to the package-less capacitive element.

2. The capacitor of claim l wherein the package- less capacitive element is formed as a coiled winding comprising a plastic film coated with a metallized film.

3. The capacitor of claim 1 wherein the potting material comprises asphalt.

4. The capacitor of claim 1 wherein the capacitive element is wrapped in an insulative sheet.

5. The capacitor of claim 1 wherein the conductive leads extend externally from the capacitor cover.

6. The capacitor of claim 1 wherein the capacitor cover further comprises a terminal board which forms a wall of the internal chamber and includes terminals electrically connected to the conductive leads.

7. The capacitor of claim 1 further comprising a plurality of capacitive elements.

8. The capacitor of claim 1 further comprising non-capacitive electrical components disposed within the internal chamber of the capacitor cover and surrounded by the potting material.

9. A method of providing a capacitor of the type mounted to the housing of an alternating current electric motor, the method comprising: providing a capacitor cover adapted to be mounted to the housing of the electric motor, said cover having walls and an open end adapted to face the housing of the motor, the walls and the housing of the motor together defining an internal chamber; positioning a package-less capacitive element within the internal chamber of the capacitor cover, the capacitive element including conductive members separated by a dielectric; connecting conductive leads to the capacitive element; and filling the internal chamber with potting material to mechanically support and electrically insulate the capacitive element.

10. The method of claim 9 wherein the package- less capacitive element is wound into a coil comprising a plastic film coated with a metallized film.

11. The method of claim 9 wherein the potting material is formed of asphalt.

12. The method of claim 9 further comprising extending the conductive leads externally from the capacitor cover.

13. The method of claim 9 f rther comprising providing a terminal board which forms a wall of the internal chamber and includes terminals; and electrically connecting the conductive leads to the terminals prior to filling the internal chamber with potting material.

14. The method of claim 9 further comprising positioning more than one package-less capacitive element into the internal volume of the capacitor cover.

15. A capacitor adapted to be mounted to a housing of the electric motor, the capacitor comprising: a single cover having an internal chamber and means for mounting the housing; a capacitive element within the internal chamber; and potting material within the internal chamber which insulates the capacitive element.

16. The capacitor of claim 15 wherein the potting material surrounds the capacitive element.

17. The capacitor of claim 16 wherein the potting material secures the capacitive element within the chamber.

18. The capacitor of claim 17 wherein the housing is a motor housing.