WO2004086571A1

WO2004086571A1 - Commutator

Info

Publication number: WO2004086571A1
Application number: PCT/IB2004/000823
Authority: WO
Inventors: Tony Elijah Muldowney-Colston; Thomas Mahon Shaw
Original assignee: Dolphin Electric Holdings Inc
Current assignee: Dolphin Electric Holdings Inc
Priority date: 2003-03-24
Filing date: 2004-03-22
Publication date: 2004-10-07
Anticipated expiration: 2005-09-24
Also published as: WO2004086571B1; TW200427182A

Abstract

A device for mounting an a shaft of a rotating electrical machine and allowing current from an electrical supply through a winding of the motor includes a pair of electrically isolated commutation elements. Each element includes a rotary contact member having a conductive outer circumferential surface, and at least one commutation member mounted with the rotary contact member. The commutation member has a conductive surface electrically connected to the conductive outer surface of the rotary contact member. A first pair of contact members connect to the electrical supply and communicate with the conductive outer surfaces of the rotary contact member. A second pair of contact members connect with the winding and alternately communicate with the conductive surfaces of the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

Description

Commutator

The present invention relates to commutators, that is to say mechanisms by which the direction of a DC electric current flowing through a coil is reversed back and forth to facilitate rotation in an electric motor. It relates specifically to commutators used in multiphase permanent magnet electric motors.

Multiphase permanent magnet brushless motors are extremely versatile and efficient machines offering superior control and efficiency compared with AC synchronous and PM DC motors in many different applications. In some cases, however, end users require the compactness, high efficiency and long life of a brushless motor but without the level of controllability offered with brushless motors. Too often the significant cost of the drive electronics and position sensing electronics required to operate the brushless motor, regardless of the level of control required, forces these end users to settle for an inferior class of motor.

The ability to use a simple, inexpensive mechanical commutation arrangement in place of an expensive electronic drive may allow end users to specify a multiphase PM motor in low control, cost sensitive applications. DC commutator motors are often used in cases where smooth torque and simple voltage control are prerequisites. However, these motors have a number of disadvantages.

When the brushes require replacement the motor has to be disassembled which takes significant time; the majority of DC commutator motors will have a wound armature which a) restricts the minimum rotor diameter that is practical to manufacture, b) leads to inferior copper slot fill compared with brushless and asynchronous motors and c) makes low inertia rotors difficult to achieve; the large number of commutator segments needed to achieve smooth torque can lead to increased sparking which accelerates brush wear and gives high frequency RF noise.

It is an object of the present invention to provide a device with overcomes or ameliorates the above problems.

According to the first aspect of the invention there is provided a device, for mounting on a shaft of a rotating electrical machine and allowing current from an electrical supply through a winding of the machine, including: a pair of complementary opposed and electrically isolated commutation elements, each element including a rotary contact member for mounting on the shaft and having a conductive outer circumferential surface, and at least one commutation member extending axially from the rotary contact member and having a conductive surface electrically connected to the conductive outer circumferential surface, a first contact members communicating with the conductive outer circumferential surfaces, and a second contact members alternately communicating with the conductive surfaces of the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

According to the second aspect of the invention there is provided a device, for mounting on a shaft of a rotating electrical machine and allowing current from an electrical supply through a winding of the machine, including: a first rotary contact member for mounting on the shaft and having a first conductive outer circumferential surface, a second rotary contact member for mounting on the shaft and having a second conductive outer circumferential surface, a first commutation member having a conductive surface electrically connected to the first conductive outer circumferential surface, a second commutation member having a conductive surface electrically connected to the second conductive outer circumferential surface, first contact members communicating with the first and second conductive outer circumferential surfaces, and second contact members alternately communicating with the conductive surfaces of the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals .

Preferably, the circumferential position of the first and second commutation members and the second contact members are so aligned that current-inducing effects of a magnetic field on the winding are at or near a minimum when the current direction is reversed.

Preferably, the first and second rotary contact members are generally annular or disk shaped with the commutation members projecting axially from the outer circumferential surfaces . Preferably, the contact members are brushes slidably communicating with the conductive surfaces.

Preferably, the contact members are rolling contacts.

Preferably, insulating material is interposed at circumferential positions between the commutation members and has an outer circumferential surface coincident with the conductive surface of the commutation members.

Preferably, a neutral member of non-conducting material with a surface material identical to the conductive surface of the commutation members is interposed at circumferential positions between the commutation members and has an outer circumferential surface coincident with the conductive surface of the commutation members.

According to the third aspect of the invention there is provided a commutation device including: a pair of commutation elements for mounting on a shaft of a rotating electrical machine, each element including a slip ring and at least one commutation electrically connected to the slip ring, a first contacts communicating with the slip ring, and a second contacts alternately communicating with the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

According to the fourth aspect of the invention there is provided a rotating electrical machine comprising: a housing; a shaft mounted rotatably within the housing; a rotor fixed to the shaft and providing a magnetic field; a stator positioned about the rotor within the housing and having a winding; a pair commutation elements for allowing current from an electrical supply through the winding, each element including a slip ring and at least one commutation member electrically connected to the slip ring, first contact members for connecting to the electrical supply and communicating with the slip rings, and second contact members for connecting with the winding and alternately communicating with the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals. Preferably, the circumferential position of the first and second commutation members and the second contact members are so aligned that the conduction periods of the three phases are symmetrically phased so as to produce a three- phase set of balanced 120 electrical degree current square waves and said conduction midpoints of the current square waves are synchronised with the midpoints of rotor field induced emfs of the same phase in the three phase stator windings. This minimizes sparking at the point of commutation.

Further aspects of the invention will become apparent from the following description, which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 is perspective, plan and elevation views of a commutation device according to the invention,

Figure 2 is a rolling contact,

Figure 3 is a second perspective view of the commutation device,

Figure 4 is a section view of the commutation device, Figure 5 is an exploded view of the commutation device, and

Figure 6 is an exploded perspective view of a second embodiment of a commutation device according to the invention.

Referring to the drawings, the basic commutator device for a rotating electrical machine comprises: two complementary opposed and electrically isolated commutation elements 1, an insulating member 2 located between the commutation elements, three minor contacts members/brushes 4 (one for each of three windings) , two major contacts/brushes 3 (one for each terminal of the supply i.e. +/-) , and a contact housing (not sketched and probably comprising several parts) .

Figure 1 illustrates a commutation device for a 4-pole rotating electrical machine. The device commutates the phases assuming 120 electrical degrees on time (60 mech. degrees) and 60 electrical degrees off time (30 mech degrees) . For a 2 -pole motor there is 120 electrical degrees on time (120 mech. degrees) and 60 electrical degrees off time (60 mech degrees) .

Each commutation elements 1 comprises an annulus or disc shaped rotary contact member with a conductive outer circumferential surface with two diametrically opposed commutation members 5 projecting axially from proximate the outer circumferential surface of the rotary contact member. Each diametrically opposed commutation member 5 spans 60 circumferential degree (for a 2 -pole motor one 120 degree axial extrusion) . The commutation members 5 have a conductive outer surface of that is coincident with and electrically connected to conductive outer circumferential surface .

The two commutation elements are arranged facing each other and displaced angularly by 90 degrees (for a 2 -pole motor, 180 degrees) and axially by a few millimeters. The airspace between the plates dictates the geometry of the insulating piece 2. The outside diameter of the insulator 2 is flush with the outside diameter of the commutation members.

The three minor contacts 4 are positioned such that they make alternating sliding contact on the outer circumferential surface of the commutation members. The contacts are circumferentially displaced from each other by 120 mech. degrees and are connected to each of the three phase supply connections.

The two major contacts 3 are positioned such that they make sliding contact on the outer circumferential surface of the rotary contact member. These make up the supply connections to the machine winding.

In this way, one of the rotary contact members 1 is always positive and the other is always negative. The phase contacts 4 switch connection between the commutation members as the commutation device rotates. And each phase does so 120 electrical degrees away from the other two phases . Current passes through the winding and the current direction is reversed at regularly recurring intervals.

Although the accompanying show conventional carbon brush contacts, the device could equally well employ rolling contacts as illustrated in figure 2.

Rolling contacts offer the advantage of longer lifespan than conventional carbon brushes, but the fact that a rolling connector can only make a line contact with the outside diameter of the commutator discs (1) limits its application to low voltage, low power applications.

The commutator and rotating electrical machine according to the invention alleviates problems with the prior art:

1. The commutator and brushes can be housed in a cartridge which is easily assembled onto the shaft and affixed to the exterior face of the motor enabling easy maintenance without disassembling the motor.

2. Because the commutator is external to the motor and outside the bearing landings the commutator does not impose a restriction on the minimum rotor diameter.

3. The windings are on the stator enabling increased space utilisation and improved efficiency.

4. The commutator does not require a large number of segments for smooth operation, which leads to increased brush life and lower RF noise.

5. Speed control of the motor can be effected by simple changes in the supply voltage, whereas conventional brushless motors, as do AC motors, require speed control by changes in the frequency of the supply current. This significantly reduces cost in applications where precise speed control is not of paramount importance . Preferably, the circumferential position of the first and second commutation members 5 and the second contact members 4 are so aligned that the conduction periods of the three phases are symmetrically phased so as to produce a three- phase set of balanced 120 electrical degree current square waves and said conduction midpoints of the current square waves are synchronised with the midpoints of rotor field induced emfs of the same phase in the three phase stator windings. This minimizes sparking at the point of commutation.

Additionally, a neutral member 2 of non-conducting material with a surface material identical to the conductive surface of the commutation members 5 is interposed at circumferential positions between the commutation members 5 and has an outer circumferential surface coincident with the conductive surface of the commutation members 5.

In this case the contacts can all be considered to be slip rings, which have a greatly increased brush life compared to a standard brush/commutator arrangement.

The brush commutated multiphase PM motor offers the low cost, simple control of the DC commutator motor with the high efficiency operation and compact rotor of the brushless DC motor.

Referring to Figure 6, in an alternative embodiment of the invention a neutral member 2 of non-conducting material with a surface material identical to the conductive surface of the commutation members is interposed at circumferential positions between the commutation members and has an outer circumferential surface coincident with the conductive surface of the commutation members 5. This means that the neutral member 2 will wear at the same rate as the outer surface of commutation members 5 reducing maintenance requirements and prolonging life. The outer circumferential surfaces of neutral member 2 are isolated from the commutation elements 1.

Associated with insulating neutral member 2 are a shaft sleeve 6 and a spacer 7. The spacer 7 provides contact surfaces to define the angular displacement between the commutation members 5 and the surfaces of insulating member 2. The shaft sleeve 6 provides the concentricity with the motor shaft and shoulders to define axial displacement between components. The shaft sleeve 6 is blind i.e. the central bore does not go all the way through so that the shaft sleeve can be accurately placed (axially) on the motor shaft. The assembly is held together with Peek (a plastic) screws 8. Alternatively, the assembly is bonded together.

Embodiments of the invention have been described, however it is understood that variations, improvement or modifications can take place without departure from the spirit of the invention or scope of the appended claims.

Claims

Claims :

1. A device, for mounting on a shaft of a rotating electrical machine and allowing current from an electrical supply through a winding of the machine, including: a pair of complementary opposed and electrically isolated commutation elements, each element including a rotary contact member for mounting on the shaft and having a conductive outer circumferential surface, and at least one commutation member extending axially from the rotary contact member and having a conductive surface electrically connected to the conductive outer circumferential surface, a first contact members communicating with the conductive outer circumferential surfaces, and a second contact members alternately communicating with the conductive surfaces of the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

2. A device, for mounting on a shaft of a rotating electrical machine and allowing current from an electrical supply through a winding of the machine, including: a first rotary contact member for mounting on the shaft and having a first conductive outer circumferential surface, a second rotary contact member for mounting on the shaft and having a second conductive outer circumferential surface, a first commutation member having a conductive surface electrically connected to the first conductive outer circumferential surface, a second commutation member having a conductive surface electrically connected to the second conductive outer circumferential surface, first contact members communicating with the first and second conductive outer circumferential surfaces, and second contact members alternately communicating with the conductive surfaces of the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

3. The device of claims 1 or 2 wherein the circumferential position of the first and second commutation members and the second contact members are so aligned that the conduction periods of the three phases are symmetrically phased so as to produce a three-phase set of balanced 120 electrical degree current square waves and said conduction midpoints of the current square waves are synchronised with the midpoints of rotor field induced emfs of the same phase in the three phase stator windings, thus minimsing sparking At the point of commutation.

4. The device of any preceding claim wherein the first and second rotary contact members are generally annular or disk shaped with the commutation members projecting axially from the outer circumferential surfaces.

5. The device of any preceding claim wherein the contact members are brushes slidably communicating with the conductive surfaces.

6. The device of any preceding claim wherein the contact members are rolling contacts.

7. The device of any preceding claim wherein insulating material is interposed at circumferential positions between the commutation members and has an outer circumferential surface coincident with the conductive surface of the commutation members .

8. The device of claim 7 wherein a neutral member of nonconducting material with a surface material identical to the conductive surface of the commutation members is interposed at circumferential positions between the commutation members and has an outer circumferential surface coincident with the conductive surface of the commutation members.

9. A commutation device including: a pair of commutation elements for mounting on a shaft of a rotating electrical machine, each element including a slip ring and at least one commutation electrically connected to the slip ring, a first contacts communicating with the slip ring, and a second contacts alternately communicating with the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

10. A rotating electrical machine comprising: a housing; a shaft mounted rotatably within the housing; a rotor fixed to the shaft and providing a magnetic field; a stator positioned about the rotor within the housing and having a winding; a pair commutation elements for allowing current from an electrical supply through the winding, each element including a slip ring and at least one commutation member electrically connected to the slip ring, first contact communicating with the slip rings, and second contact alternately communicating with the commutation members for allowing current through the winding and reversing the current direction at regularly recurring intervals.

11. The rotating electrical machine of claim 10 wherein the circumferential position of the first and second commutation members and the second contact members are so aligned that the conduction periods of the three phases are symmetrically phased so as to produce a three-phase set of balanced 120 electrical degree current square waves and said conduction midpoints of the current square waves are synchronised with the midpoints of rotor field induced emfs of the same phase in the three phase stator windings, thus minimising sparking at the point of commutation.