WO2006064354A1

WO2006064354A1 - Rotating electric machine

Info

Publication number: WO2006064354A1
Application number: PCT/IB2005/003797
Authority: WO
Inventors: Thomas Mahon Shaw; Tony Muldowney-Colston
Original assignee: Dolphin Electric Holdings Inc; Flint, Michael, John
Priority date: 2004-12-17
Filing date: 2005-12-16
Publication date: 2006-06-22
Also published as: EP1869750A1; JP2008524979A

Abstract

A rotating electrical machine (30) has housing, an output shaft mounted rotatably within the housing for rotation at a first speed Nl, a rotor fixed to the shaft and providing a magnetic field, a stator winding positioned about the rotor and a rotational commutator (31) for allowing current in alternating directions through the winding. A drive means (24, 25, 26) rotates the commutator (31) at a second speed N2, where N2 = n x N1 and n is an integer greater than 1.

Description

Rotating Electric Machine

The present invention relates to Rotating Electric Machines that operate as either motors or generators.

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 high cost of the drive and control electronics required to operate the brushless motor. Too often these costs force 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 a wide variety of applications.

Applicant's earlier patent applications, 'Commutators', published as WO2004/86571 Al on 7^th October 2004, and 'Commutator and Method of Commutating Current in an Electric Motor' filed as PCT/IB2005/002278 on August 2^nd 2005, the contents of which are considered included herein, describe a cylindrical commutator for a rotating electrical machine commutator for rotation with or by the shaft of a rotating electrical machine and allowing current to flow from an electrical supply through a winding of the machine, including: a positive slip-ring and a negative slip-ring each having a continuous conducting circular perimeter, a positive electrically conductive commutator segment electrically connected to the positive slip-ring and a negative electrically conductive commutator segment electrically connected to the negative slip-ring, and first and second freewheeling segments electrically isolated from the commutator segments and interspersed at respective positions between the commutator segments, and a first diode between the negative commutator segment and the first freewheeling segment for allowing current flow only from the negative commutator segment to the first freewheeling segment, and a second diode between the positive commutator segment and the second freewheeling segment for allowing current flow only from the second freewheeling segment to the positive commutator segment.

Preferably, the commutator further includes three brushes spatially located 120 degrees apart about the commutator and connected to windings of a motor, wherein during rotation of the commutator the commutator segments and freewheeling segments alternately engage with the three brushes connected to the windings.

The arrangement described above has been shown to provide good performance and very minimal arcing. However, the factor governing the minimum usable size of such a commutator is the contact area of the brushes. It will thus be clear to those skilled in the art that such a commutator when employed in high-power multi-pole machines can become quite large and complex to manufacture thus reducing some of the advantages in overall size, cost and simplicity that the system is designed to confer.

It is an object of the present invention to reduce the size of the commutator in higher-power multi-pole machines. It is a further object of the invention to improve commutation and control of such machines .

According to a first aspect of the invention there is provided a rotating electrical machine including: a housing, an output shaft mounted rotatably within the housing for rotation at a first speed Nl, a rotor fixed to the shaft and providing a magnetic field, a stator winding positioned about the rotor, a rotational commutator for allowing current in alternating directions through the winding, and drive means for rotating the commutator at a second speed N2 , wherein N2 = n x Nl and n is an integer greater than 1.

Preferably, the drive means includes a first gear fixed to the output shaft and a second gear fixed to a commutator shaft and meshed with the first gear.

Preferably, the drive means includes a first gear fixed to the output shaft, a second gear fixed to a commutator shaft and a chain or belt transmitting rotary motion from the first gear to the second gear.

Preferably, the machine further includes idler rollers engaging the chain or belt .

Preferably, the drive means includes a motor coupled to a shaft of the commutator and arranged to operate at the second speed N2. Preferably, the motor is one of a mechanically commutated DC motor, a permanent magnet DC motor or a brushless DC motor.

Preferably, the machine is one of a synchronous DC motor, an asynchronous AC motor or a synchronous AC reluctance motor.

Preferably, the machine further includes an electronic chopper for one or more of regulating speed, improving commutation or affecting phase advance.

According to a second aspect of the invention there is provided a rotating electrical machine including: a housing, an output shaft mounted rotatably within the housing for rotation at a first speed Nl, a rotor fixed to the shaft and providing a magnetic field, a stator winding positioned about the rotor, a rotational commutator for selectively allowing current in alternating directions through the winding, drive means for rotating the commutator at a second speed N2 , wherein N2 = n x Nl and n is an integer, a rectifier for selectively allowing current in alternating directions through the winding, a switch connecting to the rotational commutator and the rectifier for selecting which of the rotational commutator or the rectifier allows current through the winding.

Preferably, the switch is also connected to the winding.

Preferably, n is an integer greater than 1.

Preferably, the machine is an automotive alternator.

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 an exploded view of the mechanical commutation device described above and below.

Figure 2 is a view of a machine with a commutator mechanically connected by a gear drive. Figure 3 is a view of a machine with a commutator mechanically connected by a chain or belt drive.

Figure 4 is a view of a machine with a commutator mechanically connected by a chain or belt drive, with the addition of movable idler rollers to effect phase advance for greater control of the machine.

Figure 5 is a view of a machine comprising a driven motor, a separate driving motor, and the mechanical commutation device.

Figure 6 is a view of a second embodiment of a machine comprising a driven motor, a separate driving motor, and the mechanical commutation device.

Figure 7 is a view of a further embodiment of a machine comprising a driven motor, a separate driving motor, and the mechanical commutation device.

Figure 8 is a waveform graph showing how variations in the voltage of a chopper circuit could improve commutation. Figure 9 is a waveform graph showing how variations in the voltage of the chopper circuit, or the speed of the driving motor, may effect phase changes.

The central insight governing the invention is that a 2- or 4-pole commutator can drive a machine with a much larger number of poles, provided it can be made to rotate at an appropriate multiple of the speed of the driven motor. (For example, if a 2-pole commutator is to drive an 8-pole machine, the commutator must rotate at four times the speed of the driven machine.) The various aspects of the invention describe various means of achieving this, both by mechanically connecting the commutator to the motor, and by the use of a second, smaller motor to drive the commutator. In the first case, the result is a radical reduction in the minimum size, complexity and cost of the commutator for multi-pole machines, as compared to previous embodiments. In the second case, there may be additional benefits derived from mechanically separating the commutator from the driven motor; for example the machine may be employed in such applications as automotive fuel pumps, in which the alcohol in the fuel mixture tends to corrode mechanical commutators; and the opportunity for increased control of the machine at much lower cost than with brushless DC motors. Additionally, a single commutator may be used to drive two or more driven motors, either synchronous (DC) or asynchronous (AC) . This may have a number of applications in industry (e.g. conveyors) .

Figure 1 illustrates individual components of a basic 2- pole mechanical commutation device, comprising: A commutator base 1, a positive slip-ring 2 and a negative slip-ring 3, each having a continuous conducting circular perimeter 46, 47, a positive electrically conductive commutator segment 48 electrically connected to the positive slip-ring 2 and a negative electrically conductive commutator segment 49 electrically connected to the negative slip-ring 3, and first and second freewheeling segments 4, 5 electrically isolated from the commutator segments 48, 49 and interspersed at respective positions between the commutator segments, and a first diode 6 between the negative commutator segment 49 and the first freewheeling segment for allowing current flow only from the negative commutator segment 49 to the first freewheeling segment 4, and a second diode 7 between the positive commutator segment 48 and the second freewheeling segment 5 for allowing current flow only from the second freewheeling segment 5 to the positive commutator segment 48. The commutation elements and freewheeling segments 4, 5 are separated by insulation rings 8, 9. The commutator components are fixed to the base 1 by two fixing screws 12, 13. The fixing screws 12, 13 are located within isolating sleeves 14, 15 to prevent short-circuit of the commutator components.

Preferably, the commutator further includes three brushes (not illustrated) spatially located 120 degrees apart about the commutator and connected to windings of a motor, wherein during rotation of the commutator the commutator segments and freewheeling segments alternately engage with the three brushes connected to the windings.

Figure 2 illustrates an embodiment of the machine according to the first aspect of the invention. The motor 14 has a gear 15 mounted on its shaft. This meshes with a second gear 16 which is connected by a separate shaft to the mechanical commutation device 17, which in turn is connected electrically by wires 18 to the motor 14. Alternatively, the gear 15 may employ axial rather than radial gearing, and thus may drive a shaft instead of another gear. The mechanical commutation device 17 may be mounted at the end of the shaft . The shaft may be either straight or flexible. Figure 3 shows another embodiment of the machine according to the first aspect of the invention. The motor 18 has a gear 19 mounted on its shaft. This is connected to a second gear 20 by a chain or belt 21 which is connected by a separate shaft to the mechanical commutation device 22, which in turn is connected electrically by wires to the motor 18.

Figure 4 shows another embodiment of the machine according to the first aspect of the invention. The motor 23 has a gear 24 mounted on its shaft. This is connected to a second gear 25 by a chain or belt 26 which is connected by a separate shaft to the mechanical commutation device 27, which in turn is connected electrically by wires to the motor 23. Two movable idler rollers 28, 29 are positioned on either side of the belt or chain drive 26 such that moving them may alter the phase relationship between gears 24 and 25. This enables the phase relationship between the motor 23 and the mechanical commutation device 27 to be altered, thus enabling phase variations without the need to employ electronics.

Figure 5 illustrates an embodiment of the machine according to the second aspect of the invention. The driven motor 30, which may be either a DC synchronous, AC induction or synchronous AC reluctance machine is electrically connected to the mechanical commutation device 31. This is in turn driven by the drive motor 32, which may be either a PMDC machine, brushless DC machine, or a machine employing a mechanical commutator device as described above. It should be noted that the use of a brushless DC machine to drive this arrangement may be very cost-effective if the driven motor is of a high power rating. The cost of power electronics for brushless DC motors rises exponentially as the motor's power rating is increased. Therefore the use of a much smaller brushless motor, which only needs to effect rotation of the mechanical commutator at the desired speed, will provide all the control advantages associatd with brushless DC motors at substantially reduced cost.

Figure 6 illustrates another machine according to the second aspect of the invention. This machine is a combined motor-generator, of a type suited to automotive starter- alternators.

Applicant's previous patent application PCT/lB/2005/002277 filed on August 2^nd 2005, the contents of which are considered included herein, describes a three-phase wound field motor-generator, incorporating a mechanical commutation device as described above, mounted on the shaft of the machine. Although this machine has been prototyped and operates very well, there are two problems inherent in its design. Firstly, as standard automotive alternators are 12- or 16-pole machines, the commutator for such a machine is of necessity large, complex, and relatively expensive. Additionally, in automotive applications, space is at a premium, and the mounting of a large and complex commutator may not be desirable in such applications.

The machine described in figure 5 would seem to ameliorate this problem. However, there is a further issue with automotive alternators which needs to be considered. This is that alternators are usually geared up to run at 2.5-3 times the speed of the engine. For short bursts of acceleration, engine speeds of up to 6,000-7,000 rpm are not uncommon. This leads to an alternator speed of over 20,000 rpm, which if connected to the mechanical commutation device would result in reduced lifespan due to frictional wear. The machine shown in figure 6 ameliorates this problem.

A driven motor-alternator 34 is electrically connected to a start control 35, and a rectifier 36. A rectifier, a device familiar to those skilled in the art, consists of 6 diodes, 2 per phase, connected so as to reverse the voltage during the negative half cycle of the AC waveform so that a DC output is obtained. These 6 diodes are mounted on heat sinks at the non-drive end of the alternator and are cooled by the air entering the alternator due to internal fans on the rotor.

Recently synchronous rectification has been proposed where 3 MOSFETs and 3 Diodes are used. This increases alternator generated output at low speeds by advancing the phase connection each cycle. This also increases the cost of the power electronics to an intermediate level . Where an inverter is used for hybrid operation, the benefit of synchronous rectification is obviously obtained. It may be possible to obtain the benefit of synchronous rectification during low speed generation by advancing the phase of the commutator 38 in the manner described above.

An auxiliary drive motor 37 is mechanically connected to the mechanical commutation device 38 described above. Separate supplies, 39, 40 are connected to the drive motor 37 and the mechanical commutator 38 respectively. When the device is operating as a motor, i.e during engine starting and boosting acceleration in hybrid mode, the supplies 39, 40 are energised, thus enabling the machine to operate as a motor. However, when the machine is operating as a generator, or, say, when the speed of the driven machine exceeds 6,000 rpm, the supplies 39, 40 are disengaged. This enables the driven machine 34 to operate as a generator, employing the rectifier 36, without the mechanical commutation device 38 being driven, thus increasing its lifespan.

Referring to figure 7, an alternative method of connecting either the mechanical commutator 38 to the rectifier 36, the 3 phase stator windings may be permanently connected to both the mechanical commutator 38 brushes and to the rectifier 36 but may use a switch 42 to switch the DC input from the battery 41 either to the mechanical commutator 44 input brushes on the slip rings or to the rectifier 45 output. Using a common earth, this may only involve switching from 1 contact to another. However, it would be necessary to ensure that 2 phase brushes could not connect to one commutator bar so, depending on brush width, the arc of a commutator bar would need to be less than 120 degrees - about 100 degrees and the arc of the neutral would be increased from 60 degrees to 80 degrees. This compromise may not be acceptable - in which case the 3 phase switching arrangement described above may be required or possibly a simpler arrangement without the need to isolate the 3 phase connections to the rectifier 36 by opening the DC connection from the rectifier 36 to the battery. In the above arrangements, when in motoring mode, the speed and torque of the machine is controlled by a chopper circuit 43.

The auxiliary motor 37 may be stopped (or run at low speed in stand by mode) whenever the starter-alternator was operating as a generator or alternatively whenever the alternator speed exceeded say 6000 rpm. The latter would allow the mechanical commutator 38 to control phase advance and obtain improved generated output at lower speeds when required.

In all embodiments of the invention in which there is a need to provide variable speed the dc supply includes a chopper (power electronic chopper) . These devices are familiar to those skilled in the art. Such devices are much cheaper than inverters used in standard brushless motors, because they have fewer power devices (1 instead of 6) although the single device has a higher current rating than each of the 6 devices. The volt amp rating will be l/3^rd or less and only 1 device has to be mounted on a heat sink. (There is only 1 freewheeling diode compared with 6 for the inverters) . The power electronic chopper may also provide improved commutation by switching off the voltage (and, hence, reducing the current) at or just before each switching event of the mechanical commutator. This may further reduce arcing, which is minimal to begin with in this design, thus further increasing the lifespan of the machine. Figure 8 illustrates the effect of switching off the voltage at the point of commutation.

A position sensor, such as a Hall effect device, familiar to those skilled in the art, may be used to define the switching positions or the onset of commutation may be detected by a voltage/current sensor in the DC supply or in each phase.

Another possible embodiment of the auxiliary motor driven commutator may provide phase advance. The auxiliary motor position may be adjusted to provide phase advance because it is driven independently of the main motor. A separate voltage supply to the auxiliary motor may adjust the speed and, hence, position of the motor and the mechanical commutation device which may advance the position at which switching takes place. As the auxiliary motor is much smaller than the main motor, its response will be much more rapid, allowing it to be speeded up and down for each phase in turn.

Alternatively, the Power Electronic Chopper may be used to adjust the voltage to each phase in turn, providing a much more rapid adjustment. This may provide a higher voltage at the start of each phase i.e. giving phase advance in effect. Figure 9 illustrates the waveform effect of applying a higher voltage to each phase in turn.

Either rotor position sensors, such as Hall effect devices, may be used to provide main rotor position - or at lower cost - a voltage signal from each phase may provide position signals.

Where in the foregoing description has been made to integers of elements having known equivalents then such are included as if individually set forth herein.

Embodiments of the invention have been described, however it is understood that variations, improvement or modifications can take place. For example, in alternative embodiments other topological commentator arrangements are used. The brush assembly could be mounted inside the circumference of the commutator, rather than outside, thus reducing the overall volume of the device. Or alternatively, the whole device can be configured as a disc, with the brushes arranged perpendicular to the surface of the disc. It should also be noted that while the above description and illustration describes a 2-pole commutator a commutator of n poles where n = 2, 4, 6, 8 etc. can be constructed by- increasing the number of positive, negative and freewheeling segments.

Claims

Claims :

1. A rotating electrical machine including: a housing, an output shaft mounted rotatably within the housing for rotation at a first speed Nl, a rotor fixed to the shaft and providing a magnetic field, a^' stator winding positioned about the rotor, a rotational commutator for allowing current in alternating directions through the winding, and drive means for rotating the commutator at a second speed N2, wherein N2 = n x Nl and n is an integer greater than 1.

2. The machine of claim 1 wherein the drive means includes a first gear fixed to the output shaft and a second gear fixed to a commutator shaft and meshed with the first gear.

3. The machine of claim 1 wherein the drive means includes a first gear fixed to the output shaft, a second gear fixed to a commutator shaft and a chain or toothed belt transmitting rotary motion from the first gear to the second gear.

4. The machine of claim 3 further including idler rollers engaging the chain or toothed belt.

5. The machine of claim 1 wherein the drive means includes a motor coupled to a shaft of the commutator and arranged to operate at the second speed N2.

6. The machine of claim 5 wherein the motor is one of a mechanically commutated DC motor, a permanent magnet DC motor or a brushless DC motor.

7. The machine of any preceding claim wherein the machine is one of a synchronous DC motor, an asynchronous AC motor or a synchronous AC reluctance motor.

8. The machine of any preceding claim further including an electronic chopper for one or more of regulating speed, improving commutation or affecting phase advance.

9. A rotating electrical machine including: a housing, an output shaft mounted rotatably within the housing for rotation at a first speed Nl, a rotor fixed to the shaft and providing a magnetic field, a stator winding positioned about the rotor, a rotational commutator for selectively allowing current in alternating directions through the winding, drive means for rotating the commutator at a second speed N2 , wherein N2 = n x Nl and n is an integer, a rectifier for selectively allowing current in alternating directions through the winding, a switch connecting to the rotational commutator and the rectifier for selecting which of the rotational commutator or the rectifier allows current through the winding.

10. The machine of claim 9 wherein the switch is also connected to the winding.

11. The machine of claim 9 wherein n is an integer greater than 1.

12. The machine of claim 9 wherein the machine is an automotive alternator.

13. A machine as herein described with reference to figures 2 to 5, or 6 and 7.