TECHNICAL FIELD

[0001] The subject matter as described herein, in general, relates to electrical machines, and particularly but not exclusively to a rotor for an electrical machine and a method of making the rotor.

BACKGROUND

[0002] Generally, electrical machines include motors and generators. Typically, an electrical machine includes sub-assemblies, such as a rotating part known as rotor and a stationary part known as stator. The stator and the rotor, in combination, generate an electric voltage, as in the case of generators, or produce a torque, as in the case of motors.

[0003] Typically, in case the electrical machine is a magneto generator, the stator of the electrical machine includes a stator core and windings, whereas the rotor is provided with a plurality of magnets. During operation of the electrical machine, torque is supplied to the rotor to rotate with reference to the stator. Upon relative motion between the rotor and the stator, the magnetic flux generated by the magnets on the rotor and associated with the stator varies. As a result, an electro-motive force or a voltage is generated in the stator windings, which can be either directly used further or can be stored for future use. On the other hand, during the operation of the electrical machine as a motor, the windings of the stator receive current for operation. During operation of the electrical machine as a motor, the magnetic field generated in the stator, as a result of the current, interacts with the magnetic field of the magnets on the rotor. The interaction of the magnetic fields causes rotation of the rotor.

[0004] Such electrical machines find variety of applications including application in vehicles as an alternator, which, typically, charges a secondary energy storage device of a vehicle and also feeds electrical power to the electrical components of the vehicle. In most of such applications of these electrical machines, the operation of the electrical machines has to 25 be monitored for achieving optimal performance of the machines, as well as for controlling subsidiary operations associated with these electrical machines. For example, in case the electrical machine is coupled to an engine of a vehicle, an engine control unit of the vehicle can monitor the rotational speed of the rotor and the crank angle to control various automotive functions, such as fuel injection and ignition.

[0005] In order to measure the rotational speed of the rotor, a reluctor in the form of a protrusion is provided on the rotor. Further, a sensor coil assembly is mounted on a stationary part, such as the stator or an internal combustion (IC) engine, in proximity of the reluctor.
The sensor coil assembly is capable of sensing the reluctor, as the reluctor passes the sensor coil assembly during operation of the electrical machine. The sensor coil assembly can be replaced by a magneto-resistive sensor, such as a Hall Effect sensor and the reluctor can be replaced by a plurality of magnets. In few other conventional rotors, a toothed wheel is 5 attached to the rotor. The toothed wheel comprises of teeth portions that act as reluctors. Further, the plurality of magnets is fixed to the rotor for position sensing and/or speed detection.

[0006] In case of the electrical machine being employed as an AC generator in a vehicle such as a moped or a motorcycle, the rotor is normally coupled to the crankshaft of the 10 engine. The rotor of the AC generator also functions as a flywheel to store mechanical energy delivered from the crankshaft.

BRIEF DESCRIPTION OF DRAWINGS

[0007] The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the 15 reference number first appears. The same numbers are used throughout the drawings to reference like features and components.

[0008] Figure 1 illustrates a conventional rotor with reluctor protrusions.

[0009] Figure 2a illustrates a rotor having one or more reluctor protrusions, according to an embodiment of the present subject matter.

[0010] Figure 2b illustrates a magnified view of the rotor showing a reluctor protrusion, according to an embodiment of the present subject matter.

[0011] Figure 3 illustrates the rotor having one or more reluctor protrusions, according to another embodiment of the present subject matter.

[0012] Figure 4 illustrates the rotor having one or more reluctor protrusions, according to 25 yet another embodiment of the present subject matter.

[0013] Figure 5 illustrates a method for making the rotor, according to an embodiment of the present subject matter.

DETAILED DESCRIPTION

[0014] The subject matter described herein relates to a rotor for an electrical machine and a method for making the rotor, according to an embodiment of the present subject matter.

[0018] In the above example, the sensor coil assembly and the reluctors, in combination, provide information of rotational speed and crankshaft position to the engine control unit. For example, the sensor coil assembly, in association with the reluctors generates an electrical signal output when the rotor which is coupled to the IC engine is rotated. In order to allow the engine control unit to initiate ignition in the IC engine and for effective operation of the IC engine, an electrical signal output of substantial magnitude has to be generated by the sensor coil assembly. Generation of the electrical signal output of su[0015] Conventionally, electrical machines find variety of applications as motors and generators, including application in vehicles as an alternator. The alternator is operatively coupled to an internal combustion (IC) engine and generates electrical power to be supplied for various purposes, such as ignition of an IC engine of the vehicle, for operation of the various peripheral electrical devices of the vehicle, and for charging the battery of the vehicle. In such applications, the operation of the electrical machines is monitored for monitoring the performance thereof, as well as for controlling subsidiary operations associated with these electrical machines.

[0016] For example, in case the electrical machine is an alternator coupled to a crankshaft of the engine, an engine control unit of the vehicle can monitor and control the rotational speed of the engine or an engine output torque based on the rotational speed of the rotor. In said example, the rotational speed of the rotor can be used to regulate various automotive functions and parameters, such as ignition timing, fuel injection, crankshaft position, rotational speed of the IC engine, etc. As will be appreciated, in said example, the rotational speed and the crankshaft position can be understood to be the basic parameters for controlling the IC engine. For instance, if the rotational speed or the crankshaft position is incorrectly measured, the mechanical power derived from the IC engine may be adversely affected.

[0017] In order to measure the rotational speed of the rotor, a plurality of reluctors and a sensor coil assembly is provided in the electrical machine. The reluctors are formed as protrusions which form part of a magnetic circuit comprising the sensor coil assembly, and are typically provided on an external or an outer circumferential wall of the rotor. The sensor coil assembly is mounted on a stationary part, such as the stator or the IC engine, in proximity of the reluctor. The sensor coil assembly comprises of a permanent magnet, a core made of a magnetic material and a bracket which form parts of the magnetic circuit, and can sense the reluctors as the reluctors passes the sensor coil assembly during operation of the electrical machine.

[0018] In the above example, the sensor coil assembly and the reluctors, in combination, provide information of rotational speed and crankshaft position to the engine control unit. For example, the sensor coil assembly, in association with the reluctors generates an electrical signal output when the rotor which is coupled to the IC engine is rotated. In order to allow the engine control unit to initiate ignition in the IC engine and for effective operation of the IC engine, an electrical signal output of substantial magnitude has to be generated by the sensor coil assembly. Generation of the electrical signal output obstantial magnitude depends on the structure of the protrusion on the rotor.

[0019] Conventionally, a protrusion is formed on the outer circumferential wall of the rotor using a reluctor forming tool, for example, by a semi-piercing process or an embossing process. In said example, the reluctor forming tool can include a punch and a die. The rotor is located in a press tool with the punch. On actuation of the punch, the protrusion is formed on the outer circumferential wall of the rotor with a consequential depression on the inner cylindrical surface. In case, a plurality of protrusions is to be formed, the reluctor forming tool may consist of multiple punches for forming the plurality of protrusions on the rotor. Usually, due to the embossing process on the material of the rotor, the projected height of the protrusions from the rotor surface is always less than the cross-sectional thickness of the rotor circumferential wall. However, if the projected height of the protrusions is further increased, the protrusions will be detached from the rotor due to shearing. Further, the formation of the protrusions by employing high-impact processes, such as embossing, will distort the cylindrical shape of the rotor, and can cause asymmetrical distribution of mass about an axis of rotation of the rotor. Such unbalanced loading will hinder the smooth operation of the electrical machine and will also cause damage to the electrical machine when the rotor is rotated at a high speed. Further, in case of rotors with conventionally formed protrusions, during operation, the reluctors may interfere with main magnetic flux path of the electrical machine, thereby affecting the performance of the electrical machine. For example, in case of a permanent magnet generator, the interference of the reluctors with the main magnetic flux path will cause a reduction in the output of the permanent magnet generator. The output of the permanent magnet generator will further reduce if large number of reluctors is formed by the semi-piercing process or the embossing process.

[0020] Additionally, in the protrusions formed by the conventional methods, the rate of change in magnetic flux is not large enough since the formed protrusions do not have sharp edges. If sharp edges are to be provided to the protrusions of the conventional rotors, then there is a limitation on the projected height of the protrusion from the rotor surface, as increasing the protrusion height beyond the limitation will punch-out the protrusion portion from the rotor. If the protrusion height is to be increased, then the sharp edge of the protrusion needs to be compromised. Thus, in conventional design, at the time of engine starting, the generated electrical signal output from the sensor coil assembly is not of a substantial magnitude for the engine control unit to efficiently initiate ignition in the IC engine. As a result, the startability of the IC engine is adversely affected.

[0021] In certain other conventional rotors, a toothed wheel, having a plurality of teeth formed on the circumferential wall and extending in a radially outward direction, is attached to the rotor, such that the teeth of the toothed wheel act as the reluctor protrusions. In such a case, a separate wheel comprising the plurality of peripheral teeth is fitted on to the rotor using a suitable means. However, in such a case, the cost of the electrical machine increases because of the additional cost of the toothed wheel.

[0022] Further, in rotors made by conventional methods, for a given thickness of a rotor cup, there is no provision for increasing the moment of inertia of the rotor. The moment of inertia of a circular structure, such as the rotor cup can be increased by increasing the mass in the circumferential wall of the rotor cup. However, for a given electric machine, the size of the rotor is limited by the space available in the engine.

[0023] The present subject matter describes a rotor of an electrical machine operatively coupled to an internal combustion (IC) engine and a method for making the rotor, according to an embodiment of the present subject matter. In an implementation, the electrical machine may be a permanent magnet generator. In an example, the permanent magnet generator may be an AC generator. The electrical machine includes sub-assemblies, such as a rotating part known as rotor and a stationary part known as stator. The electrical machine further includes permanent magnets attached to a circumferential surface of the rotor. The circumferential surface may be either an inner wall or an outer wall of the rotor.

[0024] In an example, in case the electrical machine is deployed in a vehicle, the rotor is mounted to a crankshaft of the IC engine of the vehicle. The crankshaft is a mounting shaft where the rotor is typically used in the IC engine. In an embodiment, the rotor may be used in association with the stator, as a prime moving element in an electric motor driven vehicle. In said example, the IC engine comprises of an engine control unit that monitors various engine parameters, such as rotational speed of the rotor, crankshaft position or rotational angle of the crankshaft, and appropriately controls the amount of fuel injected and the timing of ignition. The rotational speed and the crankshaft position can be understood as basic parameters for controlling the ignition operation in an IC engine. According to an aspect of the present subject matter, the engine control unit can monitor the speed of rotation of the rotor of the electrical machine, coupled to the crankshaft of the IC engine, to determine the above mentioned parameters and to further control operation of the IC engine and of the electrical machine.

[0025] In an embodiment, the rotor cup portion of the rotating part is a cylindrical body made from a sheet metal of a magnetic material. The rotor cup has at least one open edge along an axial direction. Further, the rotor may include at least one reluctor in the form of at least one protrusion abutting with the outer circumferential wall of the rotor. The protrusion may be formed by bending a projection provided at the one open edge of the rotor, the projection extending in the axial direction and being bent outwards to abut with the outer circumferential wall of the rotor. In said embodiment, the projection is provided using either a sheet metal notching process or a punching process.

[0026] The rotor also comprises of at least of pair of magnets provided on the inner circumferential wall of the rotor cup. The magnets abut with an inner circumferential wall of the rotor. The magnets may be one of permanent magnets and electromagnets. Further, the magnets may be provided annularly along a circumferential direction with reference to an axis of rotation of the rotor in order of alternating magnetic polarity.

[0027] In one implementation, the reluctors may act as triggering protrusions for a sensor coil assembly. The sensor coil assembly is mounted on a stationary part, such as the stator or the IC engine, in proximity of the reluctors. The sensor coil assembly is capable of sensing the one or more reluctors, as the reluctors pass the sensor coil assembly during operation of the electrical machine.

[0028] In an implementation, the sensor coil assembly comprises of a permanent magnet, a core made of a magnetic material and a bracket which form parts of magnetic circuit. In operation, the sensor coil assembly and the one or more reluctors, in combination, provide information of rotational speed and crankshaft position to the engine control unit. For example, the sensor coil assembly, in association with the reluctors, generates an electrical signal output when the rotor which is operatively coupled to the crankshaft of the IC engine rotates and the reluctors pass within a proximal distance from the sensor coil assembly. The protrusions serve as the reluctors for the sensor coil assembly to detect a position of the mounting shaft and speed of rotation of the rotor.

[0029] Further, according to an implementation, the rotor may be coupled to a boss of suitable mass at a mounting portion of the rotor. In one implementation, the rotor may be coupled to the boss by at least one of a riveting process and a welding process. Further, the boss may be provided with a keyway, the keyway being at a pre-defined angular position relative to the plurality of reluctor protrusions and an axis of rotation of the rotor. The keyway may be prepared by a broaching process.

[0030] During formation of the rotor, a circular blank, with specified number of finger¬like projections located at the circumference of the circular blank and spaced at a pre-defined angular interval, is prepared from a sheet metal of pre-defined thickness. The circular blank may be prepared by a blanking operation. The sheet metal blank thus prepared is subjected to a cup-drawing operation to form a cylindrical cup, also referred to as a rotor cup, such that the finger-like projections are located at the rim of the cylindrical cup, the cylindrical cup subsequently being made part of the rotor. Alternately, the circular blank can be drawn in the shape of a cup with a flange portion and the finger-like protrusions are then provided on the flange portion.

[0031] Further, the rim portion of the cylindrical cup, along with the finger-like projections, is subjected to progressive bending operation to produce a hemmed edge which appears as a u-shaped structure. With the formation of the hemmed edge, each finger-like projection adjoins the outer circumferential wall of the cylindrical rotor cup, thereby forming a plurality of protrusion to serve as the reluctors. The protrusions that serve as the reluctors may be interchangeably referred to as reluctor protrusions. The finger-like projections are formed at the at least one open edge of the rotor cup or on a flange of the rotor cup, say by notching or a punching process. Further, the protrusions are formed by bending the projections extending from the open edge along an axial direction to abut with the outer circumferential wall of the cylindrical rotor cup. Further, width of the projections may be pre¬defined and the projections are formed at pre-defined intervals. The width of the projections and the pre-defined interval between them are indicative of dimensions and spacing between the desired reluctor protrusions, respectively.

[0032] Accordingly, during the formation of the one or more protrusions on the rotor, a height of the circumferential wall of the rotor cup, measured from closed end of the cylindrical cup to the rim along a central longitudinal axis of the rotor, is selected to be higher than the rotor height that is to be finally achieved. For example, if required rotor height is 5 centimeters (cm), the height of the circumferential wall of the rotor cup may be set to 7 cm, so that the extra length is bent to form a hem that comprises the reluctor protrusions. Once the protrusions are formed by bending the finger-like projections on the rim, the height of the rotor is same as the desired height. As a result, during formation of the protrusions, the inner circumferential wall of the rotor cup to which magnets of the electrical machine are attached, is not distorted. Thus, the inner circumferential wall of the rotor cup that forms a part of the magnetic circuit of the electrical machine, offers lesser reluctance to magnetic flux in comparison to conventional rotors with embossed or semi-pierced protrusions.

[0033] According to an aspect of the present subject matter, the above explained structure of the protrusions, i.e., the u-shaped finger-like projections to function as the reluctors, is made by using a flattened hemming process. In said process, a magnetic sheet metal is used for making the rotor, to which the protrusions are integrally formed. According to an aspect, the open end portion of the circumferential wall of the rotor cup is provided with a plurality of projections, produced by a sheet metal blanking operation, a notching operation or a punching process, and the rim of the circumferential wall of the rotor cup is subjected to a bending operation, such as a stretch bending operation, a draw bending operation, a compression bending operation, and the like so as to make the bent portions abut with the outer circumferential wall of the cylindrical cup.

[0034] Since the protrusions are formed by bending of the finger-like projections, which can be produced by the sheet metal blanking operation or the punching process, the edges of the formed protrusions are substantially sharp, for example, in comparison to protrusions of conventional rotors. Thus, when the rotor which is operatively coupled to the IC engine is rotated, the rate of change of magnetic flux due to the passage of reluctor protrusions relative to the sensor coil assembly is large enough to let the sensor coil assembly to generate electrical signal output of substantial magnitude, even if the speed of IC engine is low. Also, the cross-sectional thickness of the protrusions is equal to the cross-sectional thickness of the magnetic sheet metal forming the rest of the rotor. Hence, the change in the reluctance caused by the protrusions at the proximal point to the sensor coil assembly is high.

[0035] Additionally, due to the formation of the protrusions by non-extrusion operations, say by metal cutting and flattened hemming, the overall cylindrical structure of the rotor is not distorted. Such a method of construction of the rotor minimizes rotor cylindricity defects and associated mass unbalance about the axis of rotation of the rotor in comparison to conventional rotors where protrusions are formed by an embossing or a semi-piercing operation. As a result, the operation of the rotor is smooth and efficient, and poses circuit of the electrical machine. As the cross sectional thickness of the rotor cup is uniform throughout the magnet seating area unlike the conventional designs, the reluctance contributed by the rotor cup to the passage of main magnetic flux of the electrical machine is minimal.

[0036] Further, since the protrusions are formed by a bending operation of the rim of the rotor cup, the height of the rotor cup before the bending operation can be suitably increased such that after the bending operation, the excess length of the circumferential wall of the rotor cup is formed into a ring abutting the rotor cup. The excess length of the rotor circumferential wall which is converted into a ring increases the moment of the inertia of the rotor.

[0037] Although the present subject matter has been described with the rotor having the reluctor protrusions abutting with the outer circumferential wall of the rotor and the plurality of pairs of magnets abutting with the inner circumferential wall of the rotor, it will be appreciated that the reluctor protrusions may abut with the inner circumferential wall of the rotor and the plurality of pairs of magnets may abut with the outer circumferential wall of the rotor.

[0038] These and other advantages of the present subject matter would be described in a greater detail in conjunction with the following figures. It should be noted that the description and figures merely illustrate the principles of the present subject matter.

[0039] Figure 1 illustrates a conventional rotor 100 having reluctor protrusions 102, as shown in the figure. Figure 1 also illustrates a sensor coil assembly 104, operatively coupled to the reluctor protrusions 102, also referred to as protrusions 102 or reluctors 102. The protrusions 102 serves as the reluctors 102 for the sensor coil assembly 104 to detect speed of rotation and position of the rotor 100. The protrusions 102 are formed on an outer circumferential wall of the rotor 100 using a reluctor forming tool, which can include a punch and a die, and employ high-impact techniques, such as embossing, cold extrusion or cold forming. The rotor 100 is located in a press tool with the punch. On actuation of the punch, the protrusions 102 are formed on an outer circumferential surface of the rotor 100. Further, the rotor 100 is provided with a plurality of pairs of magnets 106, for operation as part of an electrical machine, such as a motor or a generator or alternator. The magnets 106 can be permanent magnets or electromagnets.

[0040] As shown in the figure, usually, due to the embossing operation of the material of the rotor 100, the projected height of the formed protrusions 102 from the surface of the rotor 100 is always less than the cross-sectional thickness of the wall of the rotor 100. If the projected height of the protrusions 102 is further increased, the protrusions 102 may be detached from the rotor 100 due to shearing. In addition, the protrusions 102 formed on the surface of the rotor 100 by the high-impact techniques as mentioned above, may distort the cylindricity of the rotor 100, and can cause asymmetrical distribution of mass about an axis of rotation of the rotor 100. Such unbalanced loading may hinder the smooth operation of the electrical machine and may also cause damage to the electrical machine.

[0041] The sensor coil assembly 104 and the reluctors 102, in combination, provide information of rotational speed and crankshaft position to engine control unit of an IC engine. For example, during rotation of the rotor 100, the reluctors 102 passes within a proximal distance of the sensor coil assembly 104, so that the density of magnetic flux lines, originating from the sensor coil assembly 104 and passing through the reluctors 102, varies and the sensor coil assembly 104 senses the reluctors 102 as the reluctors 102 passes the sensor coil assembly 104. The sensor coil assembly 104, in association with the reluctors 102 generates an electrical signal output when the rotor 100 which is operatively coupled to the IC engine is rotated.

[0042] Figure 2a illustrates a rotor 200 of an electrical machine, according to an embodiment of the present subject matter, having one or more reluctor protrusions 202. Figure 2b illustrates a magnified view of the rotor 200 showing a reluctor protrusion 202, according to said embodiment of the present subject matter. For the sake of brevity, Figures 2a and 2b are described in conjunction.

[0043] In one example, the rotor 200 can be a rotor of an alternator of a vehicle, operatively coupled to an internal combustion (IC) engine. In said example, the electrical machine may be an alternator in the form of a permanent magnet generator. The electrical machine also comprises of a stator in proximity of the rotor 200. In an embodiment, the electrical machine may be used as a prime moving element in an electric vehicle.

[0044] In an embodiment, the rotor 200 is a cylindrical body made from a sheet metal of a magnetic material. The rotor 200 has at least one open edge along an axial direction. Further, the rotor 200 may include a plurality of reluctors in the form of a plurality of protrusions 202 abutting with the outer circumferential wall of the rotor 200. The protrusions 202 may be formed by bending a plurality of projections provided at the one open edge of the rotor 200, the projections extending in the axial direction and being bent to abut with th outer circumferential wall of the rotor 200. In said embodiment, the projections are provided using one of a sheet metal notching process or a punching process.

[0045] Further, the rotor 200 is provided with one or more pairs of magnets 206, for operation as part of an electrical machine, such as a motor or a generator or alternator. The magnets 206 abut with an inner circumferential wall the rotor 200. The magnets 206 may be one of a permanent magnet or an electromagnet. Further, the magnets 206 may be provided annularly along a circumferential direction with reference to an axis of rotation of the rotor 200 in order of alternating magnetic polarity.

[0046] During formation of the rotor 200, an intermediate structure, in the shape of a cup and referred to as the rotor cup, is formed using a magnetic sheet metal. Further, according to an embodiment, one or more protrusions 202 are formed by forming finger-like projections at a rim of the rotor cup. In an implementation, the finger-like projections of suitable dimensions may be formed at an appropriate location on the rotor 200. In one example, the finger-like projections may be formed at pre-defined intervals, at the rim of the rotor cup. In said implementation, width of the projections and the pre-defined intervals between them are indicative of dimensions and spacing between the desired reluctor protrusions 202, respectively.

[0047] The finger-like projections are then bent to form a u-shaped hemmed portion. With the formation of the u-shaped bend, each finger-like projection adjoins the outer circumferential wall of the rotor cup, thereby forming one or more reluctor protrusions 202. In an implementation, the finger-like projections are made by using a flattened hemming process. In said method, the projections are produced by a sheet metal blanking operation. The protrusions 202 may also be formed by punching out the finger-like projections from periphery of axial length of the circumferential wall of the rotor 200.

[0048] During the formation of the one or more protrusions 202 on the rotor cup, a height of the circumferential wall of the rotor cup, measured from closed end of the cylindrical rotor cup to the rim along a central longitudinal axis of the rotor cup, is selected to be higher than the height of the rotor 200 that is to be finally achieved.

[0049] Once the protrusions 202 are formed by bending the finger-like projections on the rim, the height of the rotor 200 is same as the desired height. As described earlier, the rotor 200 is provided with the magnets 206, for operation as part of an electrical machine. Therefore, during formation of the protrusions 202, the inner circumferential wall of the rotor 200 to which the magnets 206 are attached, is not distorted. Thus, the circumferential wall of the rotor 200 that forms part of the magnetic circuit of the electrical machine offers lesser reluctance to magnetic flux in comparison to conventional rotors. Further, since the magnets 206 are attached to the inner circumferential wall of the rotor 200 or the rotor cup, and the reluctor protrusions 202 are made at the rim of the rotor cup by using the flattened hemming process, the area of the inner circumferential wall of the rotor cup to which the plurality of pairs of magnets 206 are bonded, is enhanced in comparison to a rotor 100 made by the conventional method. As a consequence, the area of bonding between the magnets 206 and the rotor cup is improved, thereby increasing the magnet bonding strength.

[0050] Although, not shown in figure 2a and 2b, the rotor 200 may be attached to a boss of suitable mass at a mounting portion of the rotor 200. In one implementation, the rotor 200 may be coupled to the boss by at least one of a riveting process or a welding process. Further, the boss may be provided with a keyway, the keyway being at an angle relative to an axis of rotation of the rotor 200. The keyway may be prepared by a broaching process.

[0051] The rotor 200 along with the boss serves as an integrated flywheel with a common axis of rotation. Further, a length of the circumferential wall of the rotor 200 which is bent outwards by a flattened hemming process can be suitably increased to improve the moment of inertia of the rotor 200 about the axis of rotation. As mentioned above, the one or more protrusions 202 are formed by forming finger-like projections at the rim of the rotor cup, the height of the rim portion can be appropriately increased to enhance the moment of inertia of the rotor 200 about the axis of rotation. In an implementation, if the one or more protrusions are used for monitoring the rotational speed of the rotor 200, then formation of at least one protrusion at a pre-defined angle relative to the keyway is skipped so that the skipped reluctor protrusion can be used in identifying completion of one rotation of the rotor.

[0052] Figures 2a and 2b also illustrate a sensor assembly 204, operatively coupled to the reluctor protrusions 202 for sensing the engine speed and crankshaft position. In the above implementation, in which the reluctor protrusions 202 are formed bending the projections, the sensor assembly 204 may be a variable reluctance type sensor assembly. Such variable reluctance type sensor assembly is referred to as a sensor coil assembly 204. The sensor coil assembly 204 may include a permanent magnet, a core made of a magnetic material and a bracket which form parts of magnetic circuit. In another implementation, the sensor assembly 204 may be a magneto-resistive type sensor assembly. [0053] Further, in one example, the IC engine deployed in a vehicle may be operably coupled with an engine control unit that monitors various engine parameters using the reluctor and the sensor coil assembly 204, based on which the engine control unit can control fuel injection, ignition timing and other related functions of the engine. Such engine parameters can include rotational speed of the rotor 200, crank angle or rotational angle of the crankshaft. In the above example, the engine control unit may monitor the rotor 200 of the electrical machine, coupled to a mounting shaft of the IC engine, to determine the above mentioned parameters and to further control operation of the IC engine and of the electrical machine. The mounting shaft can be the crankshaft of the IC engine in case the rotor 200 is used in an IC engine.

[0054] In operation, during rotation of the rotor 200, the one or more protrusions 202 passes within a proximal distance of the sensor coil assembly 204, so that sensor coil assembly 204 senses the reluctor protrusions 202. The protrusions 202 serve as the reluctors 202 for the sensor coil assembly 204 to detect a position of the mounting shaft and speed of rotation of the rotor 200. The sensor coil assembly 204 and the plurality of reluctors 202, in combination, provide information of rotational speed and crankshaft position to the engine control unit. For example, the sensor coil assembly 204, in association with the reluctors 202 generates an electrical signal output when the rotor 200 which is coupled to the IC engine rotates and the reluctors 202 passes within a proximal distance from the sensor coil assembly 204.

[0055] Although the present subject matter has been described with the rotor 200 having the reluctor protrusions 202 abutting with the outer circumferential wall of the rotor 200 and the plurality of pairs of magnets 206 abutting with the inner circumferential wall of the rotor 200, it will be appreciated that the reluctor protrusions 202 may be abut with the inner circumferential wall of the rotor 200 and the plurality of pairs of magnets 206 may abut with the outer circumferential wall of the rotor 200.

[0056] As described earlier, the finger-like projections may be produced by the sheet metal blanking operation, therefore the edges of the formed reluctors 202 are substantially sharp. Thus, the rate of change of magnetic flux is large enough to let the sensor coil assembly 204, in association with the reluctors 202 to generate electrical signal output of substantial magnitude when the rotor 200 which is coupled to the IC engine is rotated, even if the speed of IC engine is low. Also, the cross-sectional thickness of the reluctors 202 is equal to the cross-sectional thickness of the magnetic sheet metal forming the rest of the rotor 200. Hence, the change in the reluctance caused by the reluctors 202 at the sensor coil assembly 204 is high.

[0057] Figure 3 illustrates the rotor 200 having one or more protrusions 202, according to another embodiment of the present subject matter. As can be seen in figure 3, a chamfer 302 is provided at each corner where the one or more protrusions 202 join the rim portion of the rotor cup. The chamfers 302 are provided to increase stiffness of the hemmed reluctor protrusions 202.

[0058] Figure 4 illustrates the rotor 200 having one or more protrusions 202, according to yet another embodiment of the present subject matter.

[0059] In one implementation, the one or more protrusions 202 are formed by providing one or more perforations 402 of pre-defined shape at the rim portion of the circumferential wall of the rotor cup. The perforations 402 can be of different shapes that improves the stiffness of the rim portion of the rotor cup. For example, the perforations can be of a circular shape, an oval shape, a rectangular shape, or in the shape of an elongated hole. In said implementation, the one or more protrusions 202 may be formed by a punching process. The one or more protrusions 202 may be formed by punching out the pre-defined shapes from the periphery of the axial length of the circumferential wall of the rotor 200.

[0060] Figure 5 illustrates an exemplary method 500 for making a rotor, in accordance with an embodiment of the present subject matter. The order in which the methods 500 are described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the methods 500, or alternative methods. Additionally, individual blocks may be deleted from the methods without departing from the spirit and scope of the subject matter described herein.

[0061] Referring to Figure 5, at block 502, the method 500 includes forming a cylindrical cup using a circular blank, where the cylindrical cup forms a body of the rotor. The cylindrical cup, also referred to as rotor cup, is made from a magnetic sheet metal of a pre¬defined thickness using a cup-drawing operation. The circular blank is provided with a flange at the rim portion of the cylindrical cup. The cylindrical cup forms a body of the rotor.

[0062] At block 504, the method 500 includes making at least one projection of a pre¬defined shape on a circumferential wall or flange of the cylindrical cup, where the at least one projection extends from a rim of the cylindrical cup. The projections are made using one of a sheet metal notching process and a punching process. The pre-defined shape of the projections may include a circular shape, an oval shape, and a rectangular shape. Further, the projections may be formed at pre-defined intervals. The width of the projections and the pre¬defined intervals between them are indicative of dimensions and spacing between the desired protrusions, respectively.

[0063] At block 506, the method 500 includes bending the at least one projection in a direction of an edge or flange of the cylindrical cup opposite to the rim to form a hemmed edge. The projections may be bent in an outward direction. The projections located at the rim of the cylindrical cup form a u-shaped structure.

[0064] At block 508, the method 500 includes flattening the hemmed edge of the cylindrical cup to abut with an outer surface of the circumferential wall of the cylindrical cup for forming a reluctor protrusion. With the formation of the flattened hemmed edge, the projections adjoin the outer circumferential wall of the cylindrical cup, thereby forming protrusions that serve as reluctors.

[0065] At method 510, the method 500 includes attaching a boss having a keyway to the cylindrical cup at a mounting portion of the cylindrical cup, where the keyway is prepared by a broaching process. The boss may be coupled to the cylindrical cup by at least one of a riveting process and a welding process. Further, the boss is provided with the keyway at a pre-defined angular position relative to the plurality of reluctor protrusions and an axis of rotation of the cylindrical cup.

[0066] Thus, the above described method 500 of construction of the rotor 200 or the rotor cup discloses a method of forming reluctors integral to the rotor cup without resorting to embossing operation practiced in producing rotors by conventional methods. The disclosed method produces reluctor protrusions with projected height equal to the thickness of the sheet metal used to form the rotor cup. Further, the protrusions produced by this process have sharp edges resulting in increased signal output from the sensor coil assembly. In addition, the mass unbalance of the rotor about an axis of rotation is minimized by providing balancing perforations at appropriate locations on the circumferential wall portion of the rotor cup. As a result, the operation of the rotor cup is smooth and efficient, and poses substantially less threat to damaging the electrical machine. Further, the rotor 200 produced by the present method has increased mass at the circumference due to the additional length of the cup height used to form the reluctor projections. This increases the moment of inertia of the rotor. [0067] Although embodiments for structure of rotor and method for making the rotor have been described in a language specific to structural features and/or methods, it is to be understood that the invention is not necessarily limited to the specific features or methods described.

1. A rotor (200) for an electrical machine, the rotor (200) comprising:

a cylindrical body having at least one open edge along an axial direction; and

at least one protrusion (202) abutting with a circumferential wall of the rotor (200) to serve as a reluctor, wherein the at least one protrusion (202) is formed by bending a projection provided at the at least one open edge of the rotor (200), the projection extending in the axial direction and being bent to abut with the circumferential wall of the rotor (200).

2. The rotor (200) as claimed in claim 1, wherein the projection is bent outwardly to abut with an outer circumferential wall of the rotor (200).

3. The rotor (200) as claimed in claim 1, wherein the projection is bent inwardly to abut with an inner circumferential wall of the rotor (200).

4. The rotor (200) as claimed in claim 1, wherein the rotor (200) further comprises at least one pair of magnets (206) provided annularly on a circumferential wall of the rotor (200) opposite to the circumferential wall of the rotor (200) abutting the at least one protrusion (202), for the rotor (200) to function as an electrical machine in association with a proximally placed stator.

5. The rotor (200) as claimed in claim 4, wherein the at least one pair of magnets (206) is one of a permanent magnet and an electromagnet.

6. The rotor (200) as claimed in claim 4, wherein the at least one pair of magnets (206) are provided annularly along a circumferential direction with reference to an axis of rotation of the rotor (200) in order of alternating magnetic polarity.

7. The rotor (200) as claimed in claim 1, wherein the rotor (200) is operatively coupled to a crankshaft of an internal combustion (IC) engine.

8. The rotor (200) as claimed in claim 1, wherein the projection is provided using one of a sheet metal notching process and a punching process.

9. The rotor (200) as claimed in claim 1, wherein the at least one protrusion (202) is formed by bending the projection using a flattened hemming process.

10. The rotor (200) as claimed in claim 1, wherein the rotor (200) comprises a boss having a keyway, the keyway being located at a pre-defined angle relative to the at least one protrusion (202) and an axis of rotation of the rotor (200), wherein the boss is coupled to the rotor (200) at a mounting portion of the rotor (200).

11. The rotor (200) as claimed in claim 10, wherein the keyway is prepared by a broaching process.

12. The rotor (200) as claimed in claim 10, wherein the rotor (200) is coupled to the boss using at least one of a riveting process and a welding process.

13. The rotor (200) as claimed in claim 1, wherein the at least one protrusion (202) serves as the reluctor for a sensor coil assembly (204) to detect at least one of a position of a mounting shaft and speed of rotation of the rotor (200).

14. A method for making a rotor (200), the method comprising:

forming a cylindrical cup using a circular blank, wherein the cylindrical cup forms a body of the rotor (200);
making at least one projection of a pre-defined shape on a circumferential wall of the cylindrical cup,

wherein the at least one projection extends from a rim of the cylindrical cup in an axial direction;
bending the at least one projection in a direction of an edge of the cylindrical cup opposite to the rim to form a hemmed edge; and

flattening the hemmed edge of the cylindrical cup to abut with the circumferential wall of the cylindrical cup for forming a protrusion (202).

15. The method as claimed in claim 14, wherein the flattening comprises flattening the the hemmed edge of the cylindrical cup to abut with an outer circumferential wall of the cylindrical cup for forming the protrusion (202).

16. The method as claimed in claim 14, wherein the flattening comprises flattening the the hemmed edge of the cylindrical cup to abut with an inner circumferential wall of the cylindrical cup for forming the protrusion (202).

17. The method as claimed in claim 14, wherein the circular blank is produced from a magnetic sheet metal.

18. The method as claimed in claim 14, wherein the making comprises forming a plurality of projections on the rim of the cylindrical cup at pre-defined intervals.

19. The method as claimed in claim 14, wherein the cylindrical cup is formed using a cup-drawing operation.

20. The method as claimed in claim 14, wherein the pre-defined shape includes one of a circular shape, an oval shape, and a rectangular shape.

21. An electrical machine comprising:

a rotor (200), the rotor (200) comprising:

a cylindrical body having at least one open edge along an axial direction;

at least one protrusion (202) abutting with a circumferential wall of the rotor (200), wherein the at least one protrusion (202) is formed by bending a projection provided at the at least one open edge of the rotor (200), the projection extending in the axial direction and being bent to abut with the circumferential wall of the rotor (200); and

at least one pair of magnets (206) provided on a circumferential wall of the rotor (200) for the rotor to function in association with a stator as the electrical machine.

a stator in proximity of the rotor (200); and

a sensor assembly (204) mounted on one of the stator and a vehicle assembly.

22. The electrical machine as claimed in claim 21, wherein the electrical machine is a permanent magnet generator.

23. The electrical machine as claimed in claim 21, wherein electrical machine is used as a prime moving element in an electric vehicle.

24. The electrical machine as claimed in claim 21, wherein the sensor assembly (204) is one of a variable reluctance type sensor assembly and a magneto-resistive type sensor assembly.

25. The electrical machine as claimed in claim 21, wherein the projection is bent outwardly to abut with an outer circumferential wall of the rotor (200).

26. The electrical machine as claimed in claim 21, wherein the projection is bent inwardly to abut with an inner circumferential wall of the rotor (200).