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

Direct Current (DC) machines will act as either a motor or a generator. If an AC current is input to the stator windings the machine acts as a motor. If mechanical power is applied to rotate the machine shaft it will act as a generator. Thus the machine has potential for use as a combined motor-generator, for example as an automotive starter-alternator.

Current motor-generator designs are usually based around high-power brushless DC motors . However considerable control electronics is required. For motor operation expensive position-sensing and power control electronics are required. In generator operation the output of a permanent magnet machines is controlled entirely by the input speed, output load, and construction of the machine. In practice, costly High-power electronics are required to regulate the output. These High-power electronics are also inefficient because they causes electrical power losses equal to the load current times the voltage drop (P=V?I) required to produce the appropriate voltage. It is an object of the present invention to provide a rotating electric machine that can operate as a motor or as a generator that overcomes or ameliorates the above problems, or at least provides the public with a useful alternative.

According to 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, a rotor winding on the rotor for providing a magnetic field, a stator winding positioned about the rotor, a mechanical commutation device rotatable with or by the shaft and having a first position for allowing current in one direction through the stator winding and a second position for allowing current in an opposite direction through the stator winding, and slip-rings movable by or with the shaft and connected to the rotor winding for allowing a variable current in the rotor winding. Preferably, the mechanical commutation device includes a switch having a first and a second position, the switch mounted with the housing and connected to the stator winding, and a mechanical activator movable with or by the shaft and acting on the switch for moving it between the first and second positions.

Preferably, the mechanical commutation device comprises first and second electrically conductive commutator for electrical engagement with brushes connected to windings, first and second freewheeling segments electrically isolated from the first and second commutator segments and interspersed at respective positions between the first and second commutator segments, and a first diode connected between the first commutator segment and the first freewheeling segment for allowing current flow only from the first commutator segment to the first freewheeling segment, and a second diode connected between the second commutator segment and the second freewheeling segment for allowing current flow only from the second freewheeling segment to the second commutator segment.

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 a schematic illustration of a rotating electrical machine according to the invention, and

Figure 2 is an exploded view of one embodiment of a mechanical commutation device for the machine,

Referring to Figure 1, a rotating electrical machine that and be operated as either a motor or a generator includes a housing 1 enclosing rotor 3 mounted on a rotational shaft 2 and surrounded by a stator winding 4. In the illustrated embodiment the stator winding 4 is a multiphase winding including three separate windings connected in a Delta arrangement. Alternatively, the windings could be connected in a Star arrangement. Another alternative, for a much simpler machine, has two windings.

The winding leads are connected to the machine terminals 5, 6 through a mechanical commutation device 7. The mechanical commutation device 7, which will be described in detail below, is rotatable with or by the shaft 2 for allowing a reversal of current flow in the stator windings The rotor 3 is wound with a rotor winding 8 for providing a rotating magnetic field within the stator winding 4. The leads of the rotor winding 8 are connected to slip- ring/brush sets 9, 10 so that a direct current from a regulator 11 can be supplied to the rotor winding 8.

In Figure 1 details of the mechanical commutation device 7 are not shown. In one embodiment the device may be switches and a mechanical activation means. A switch is connected between each of the second ends of the stator windings 4 and to the terminals 5, 6. A mechanical activator is movable with or by the shaft 2 and acts on the switches so as to move them in a sequence between first and second positions for changing the direction of current in the respective windings 4. Such an arrangement is described in applicant's earlier patent application published as WO 04/051839 on 17 June 2004, the contents of which are considered included herein.

Figure 2 depicts a preferred embodiment of the mechanical commutation device 7 consisting of a circular moulded base 12 with central hub 13 for supporting commutator components. Mounted on the hub 13 are first and second complementarily opposed commutator elements each comprising a ring shaped plate 14, 15 having a continuous conducting circular periphery 17, 18 respectively. A conductive commutation segment 19, 20 projects axially from adjacent each periphery 17, 18. On the hub 13 interspersed between the commutation segments 19, 20 are two freewheeling segments 21, 22. The commutation elements and freewheel segments 21, 22 are separated by insulation rings 24. The commutator components are fixed to the hub 13 by two fixing screws 25, 26 the secure within threaded bores 27, 28 in base 12. The fixing screws 25, 26 located within isolating sleeves 29, 30 to prevent short-circuit of the commutator components.

The mechanical commutation device 7 is coupled to or disposed on a motor shaft 2 with brushes engaging the commutator surfaces to transfer electrical current between a DC supply and the motor windings 4. A first supply brush 31 is connected to the positive side 5 of a DC supply and positioned in continuous contact with the conductive slip- ring surface 17 of commutator element 14. A second supply brush 32 is connected to the negative side 6 of the DC supply and positioned in continuous contact with the conductive slip-ring surface 18 of commutator element 15. By this arrangement the first commutation segment 19 is a positive segment and the second commutation segment 20 is a negative segment.

In the case of a three phase motor three winding brushes 33, 34, 35 are spatially located 120 mechanical degrees apart about the commutation device 7. The phase brushes 33, 34, 35 engage alternately with the commutation and freewheeling segments in the rotational order 19, 22, 20, 21 as the commutator rotates. The conventional direction of rotation of the commutation device 7 is clockwise as viewed in the drawings.

Two diodes 36, 37 are electrically connected between the freewheeling segments 21, 22 and commutation segments 19, 20. The first diode 37 is connected for forward bias current flow from the negative commutation segment 20 to the freewheeling segment 22 that is rotationally ahead of it. The second diode 36 is connected for forward biased current flow from the other freewheeling segments 21 to the positive commutation segment 19 that rotationally follows it.

The connections incorporating diodes 36, 37 between the freewheeling segments 21, 22 and the positive and negative commutator segments 19, 20 are provided such that for a given direction of rotation as a brush leaves a commutator segment and makes contact with an adjacent freewheeling segment the freewheeling segment will connect via a diode to the a commutation segment of the opposite polarity. As brush breaks contact with the commutation segment a reverse EMF is produced that forward biases the diode and current is shunted via the forward biased diode from freewheeling segment to the opposite polarity commutation segment. This helps suppress large inductive voltage spikes as the brushes leave the commutator segments, thus eliminating the arcing normally associated with mechanically commutated machines.

The above machine can be used as both a motor and generator and is particularly suitable to use as an automotive starter-alternator. It addresses the above mentioned problems. Firstly, it is mechanically commutated and so no expensive drive electronics are required. Two examples of mechanical commutation have been given. Secondly, as the input current to the rotor winding can be varied, the output voltage can be controlled when the machine is operating as a generator, to ensure that it delivers the appropriate power to the battery for example. Regulating the rotor input is far less complex and costly than regulating the output, as the current loads are much lower, and the electronics therefore less expensive.

Additionally, the machine can be employed as a traction motor, in which the control of the voltage to the rotor minimises the cost of electronics required for regenerative braking.

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 without departure from the spirit of the invention or scope of the appended claims . For example, the device can also be a universal motor that will operate from both AC & DC input power.