TECHNICAL FIELD
[0001] The present subject matter relates, in general, to rotating
electrical machines and, in particular, to disposition of magnets in components of the rotating electrical machines.
BACKGROUND
[0002] A rotating electrical machine may be an electric motor or an electric generator. The rotating electrical machine includes a stationary member known as stator and a rotating member known as rotor. The rotor and the stator may be coaxially installed in the electrical machine. The stator may include stator windings, and the rotor may include either rotor windings or permanent magnets.
BRIEF DESCRIPTION OF DRAWINGS
[0003] The features, aspects, and advantages of the present subject
matter will be better understood with regard to the following description, and
accompanying figures. The use of the same reference number in different
figures indicates similar or identical features and components.
[0004] Fig. 1 illustrates an electrical machine, in accordance with an
implementation of the present subject matter.
[0005] Fig. 2(a) illustrates a stator tooth, in accordance with an
implementation of the present subject matter.
[0006] Fig. 2(b) illustrates provision of a plurality of grooves on a tooth
surface, in accordance with an implementation of the present subject matter.
[0007] Fig. 3(a) illustrates a first type of locking arrangement used for
securing magnets in a rotor core, in accordance with an implementation of the present subject matter.

[0008] Fig. 3(b) illustrates a second type of locking arrangement used
for securing magnets in a rotor core, in accordance with an implementation
of the present subject matter.
[0009] Fig. 3(c) illustrates a third type of locking arrangement used for
securing magnets in a rotor core, in accordance with an implementation of
the present subject matter.
[0010] Fig. 4 illustrates a comparison of efficiency of an electrical
machine according to the present subject matter and a conventional surface
permanent magnet (SPM) machine.
[0011] Fig. 5 illustrates a comparison of peak phase currents of an
electrical machine according to the present subject matter and a
conventional SPM machine.
[0012] Fig. 6 illustrates a comparison of cogging torques of various
types of electrical machines.
[0013] Fig. 7 illustrates spectrum of generated voltage at a speed of
600 RPM for various types of electrical machines.
[0014] Fig. 8 illustrates a comparison of flux densities along a direction
of magnetization within magnets of rotors in a conventional SPM machine
and in an electrical machine according to the present subject matter.
DETAILED DESCRIPTION
[0015] A rotating electrical machine, hereinafter referred to as an electrical machine, may include permanent magnets on its rotor to generate magnetic field. Such an electrical machine may be referred to as a permanent magnet electrical machine. The permanent magnets may be disposed in a core of the rotor, also referred to as a rotor core, and distributed in a circumferential direction of the rotor core. The permanent magnets may be disposed in the rotor core such that alternate poles are disposed adjacent to each other. Further, the permanent magnets may be disposed so as to face teeth of stator, also referred to as stator teeth. For

instance, if the rotor is provided outside of the stator in the radial direction
of the stator and coaxial to the stator, the permanent magnets may be
mounted circumferentially in an inner wall of the rotor, so that the permanent
magnets can face stator teeth provided on an outer surface of the stator.
[0016] Generally, the arrangement of the permanent magnets in the
rotor core is such that a large volume of permanent magnets is to be used. This is because magnets are provided along almost the entire circumference of the rotor core. For instance, in case of a 40-pole electrical machine, 40 permanent magnets, each spanning a magnet angle of about 9° mechanical angle (equivalent to 180° electrical angle), are distributed circumferentially in the rotor core, so that almost the entire 360° is provided with magnets. Due to excess volume of the permanent magnets used, the cost of the electrical machine is high.
[0017] The present subject matter relates to electrical machines. With
the implementations of the present subject matter, the amount of magnetic
material used in electrical machines can be reduced.
[0018] In an implementation of the present subject matter, an electrical
machine includes a stator and a rotor. The stator incudes a plurality of stator teeth distributed in a circumferential direction of the stator. Further, the rotor includes a rotor core and a plurality of permanent magnets, also referred to as magnets, that are distributed in a circumferential direction of the rotor core. Each magnet may have a magnet angle of 170° electrical angle or lesser. For instance, in case of a 40-pole electrical machine, each magnet may span a magnet angle of 8.5° mechanical angle (electrical angles = no. of poles/2 X mechanical angles) or lesser. In an example, the magnets may be uniformly distributed in the circumferential direction of the rotor core and two adjacent magnets may be separated from each other by a portion of the rotor core.
[0019] The reduction of the magnet angle of the magnets used in the
rotor reduces the amount of magnetic material to be used in manufacturing an electric machine. In an example, the magnet angle of the magnets used

in the rotor may be 126° electrical angle, which is 30% lesser than that used in conventional electrical machines. Therefore, the present subject matter enables achieving a 30% reduction in the amount of magnetic material used in the electrical machines.
[0020] Since the adjacent magnets are separated by a portion of the
rotor core, which includes iron, a reluctance torque is introduced during operation of the electrical machine. This is because, during operation of the electrical machine, stator winding of each stator tooth is supplied with electric current, which cause the stator teeth to act as electromagnet poles. Further, each magnet in the rotor tends to align with an electromagnet pole to reduce the magnetic reluctance, also referred to as reluctance. Accordingly, based on the coil excitation in the stator, rotor tries to align at the minimum reluctance position. The tendency to align with the electromagnet poles of the stator to reduce the reluctance causes generation of a reluctance torque component. As will be understood, the reluctance torque is produced by variation in reluctance across the rotor periphery. The reluctance torque component acts in addition to magnetic torque, which is generated due to interaction of the magnetic field of the rotor and the magnetic field of the stator. The total torque in the electrical machine is the combination of both magnetic and reluctance torque components.
[0021 ] The reduction of the magnet angle of the magnets may give rise
to a cogging torque, which is the torque required to break away from a position of stator teeth and magnet alignment, especially at lower speed of rotation of the electrical machine. To overcome the cogging torque, in accordance with an implementation of the present subject matter, a groove may be provided on a surface of each stator tooth. The surface of the stator tooth, on which the groove is provided, may be a surface that faces a periphery of the rotor and may be referred to as the tooth surface. The grooves in the teeth surfaces generate a torque fluctuation, which acts in counter-phase to the cogging torque. This torque fluctuation, generated by

the grooves, and the cogging torque overlap. Therefore, the cogging torque
and the torque fluctuation may partly or entirely cancel each other out.
[0022] Therefore, the present subject matter enables reducing the
amount of magnetic material used in electrical machines without causing
any detrimental effects, such as increase in the cogging torque.
[0023] The above and other features, aspects, and advantages of the
subject matter will be better explained with regard to the following
description, appended claims, and accompanying figures.
[0024] Fig. 1 illustrates a cross-sectional view of an electrical machine
100, in accordance with an implementation of the present subject matter. The electrical machine 100 may be, for example, an electrical motor or an electrical generator. The electrical machine 100 includes a rotor 102 and a stator 104. The rotor 102 and the stator 104 are mounted coaxially. The common axis for the rotor 102 and the stator 104 is the axis 106, which extends perpendicular to the view illustrated in Fig. 1. In an implementation., the stator 104 and the rotor 102 are substantially cylindrical in shape and have an opening extending around the axis 106. During the operation of the electrical machine 100, the rotor 102 can rotate about the axis 106, while the stator 104 remains stationary. As illustrated, the rotor 102 may be disposed such that it surrounds the stator 104 in the radial direction. In another example, the rotor 102 may be surrounded by the stator 104 in the radial direction of the stator 104.
[0025] The rotor 102 may include a core 108, also referred to as a rotor
core 108. The rotor core 108 may be cylinder-shaped and may have an opening (not marked in Fig. 1) that extends along and around the axis 106. In the opening, the stator 104 may be disposed, so that the rotor 102 surrounds the stator 104 in the radial direction.
[0026] The rotor 102 may also include a plurality of permanent
magnets 112-1, 112-2, ..., also referred to as magnets 112-1, 112-2, .... The magnets 112 may be disposed in the rotor core 108. The magnets 112 may be secured in cavities (not shown in Fig. 1) formed in the rotor core

108. The securing of the magnets 112 in the cavities of the rotor core 108
will be explained in greater detail with reference to Figs. 3(a)-(c).
[0027] The magnets 112 may be uniformly distributed in a
circumferential direction 114 of the rotor core 108. For uniformly distributing the magnets 112 in the circumferential direction 114, the cavities may be formed uniformly in the circumferential direction 114. The magnets 112 may be arranged in the rotor core 108 such that a pole of one polarity is placed between two poles of opposite polarities. For instance, while the magnet 112-2 may be disposed such that its north pole faces the stator 104, the magnets 112-1 and 112-3 may be disposed such that their respective south poles face the stator 104. Accordingly, alternating polarities are adjacent each other in the rotor core 108.
[0028] In an implementation, the magnets 112 may be disposed in the
rotor core 108 such that they face the stator 102. For instance, since an inner surface 116 of the rotor 102 faces the stator 104, the magnets 112 may be attached to the inner surface 116 of the rotor 102. Since the permanent magnets 112 are attached to a surface of the rotor 102, the electrical machine 100 may be referred to as a surface permanent magnet (SPM) machine.
[0029] Similar to the rotor 102, the stator 104 may also have a
cylindrical body. The stator 104 may have a plurality of teeth 118-1, 118-2, ..., also referred to as stator teeth, projecting outside of the cylindrical body. The stator teeth 118 are uniformly distributed in a circumferential direction of the stator 104. The stator teeth 118 may face the inner surface 116 of the rotor. Further, a gap (not visible in Fig. 1) may exist in a radial direction between the stator teeth 118 and the rotor 102. The gap may be referred to as a radial gap.
[0030] The stator teeth 118 may be formed by providing a plurality of
slots 120-1, 120-2, ..., also referred to as stator slots, in the body of the stator 104. Upon formation of the stator teeth 118, windings, referred to as stator windings, may be wound on each stator tooth. For instance, stator

winding 122-1 may be wound on the stator tooth 118-1 and stator winding
122-2 may be wound on the stator tooth 118-2, and so on. The stator
windings 122 may be three-phase windings, so that, in case the electrical
machine 100 is operated as a motor, it can be supplied with 3-phase
voltage, and in case the electrical machine 100 is operated as a generator,
it can supply a 3-phase voltage. The construction of the stator 122 and their
connections will be understood by a person skilled in the art, and is not
explained herein for the sake of brevity.
[0031] Now, the disposition of the magnets 112 in the rotor core 108
will be explained. Each magnet 112 spans a certain length in the
circumferential direction 114. Such a length may be quantified in terms of
an angle formed between two edges of the magnet 112 in the
circumferential direction 114. For instance, consider the magnet 112-n-1,
which has edges 124 and 126 that are displaced from each other in the
circumferential direction 114. The edges 124 and 126 may subtend an angle
of 9 at the axis 106. Such an angle may be referred to as the magnet angle
or magnet width.
[0032] The magnet angle may be represented in terms of mechanical
angle or electrical angle. As will be known, the mechanical angle may refer
to an angle formed by a portion of the rotor 102 relative to the stator 104
during rotation of the rotor 102 relative to the stator 104. For instance, when
the rotor 102 completes one full cycle of revolution about the stator 104, the
portion of the rotor 102 completes 360° of mechanical angle. Further, as will
be known, the electrical angle is related to the mechanical angle by the
below equation:
Electrical angle = No. of poles in an electrical machine X Mechanical angle
2
where the number of poles may be the number of magnets used in the rotor
of electrical machine.

For instance, if the electrical machine 100 has 40 magnets, i.e., if the electrical machine 100 is a 40-pole electrical machine, 18° mechanical angle equals 360° electrical angle.
[0033] The representation of the magnet angle of a magnet in terms of
the mechanical angle and the electrical angle will now be explained. The actual value of the magnet angle may be the same as the mechanical angle. For instance, the actual value of the magnet angle 9 may be the same as its mechanical angle. Accordingly, the electrical angle corresponding to the magnetic angle 9 may be represented as below:
9Eiectricai angle = No. of poles in an electrical machine X 9
2
where 9Eiectricai angle refers to the electrical angle equivalent of the magnetic
angle 9.
[0034] Generally, in conventional electrical machines, magnets
provided in the rotor are sized such that the magnet angle of each magnet
is 180° electrical angle. For instance, in case of a 40-pole electrical
machine, each magnet has a magnet angle of 9° mechanical angle. The
provision of magnets of such a size increases the amount of magnetic
material used for construction of the electrical machines. In accordance with
the present subject matter, the amount of magnetic material used in
construction of electrical machines is reduced, as will be explained below:
[0035] The magnets 112 used in the electrical machine 100 may be
sized such that they have a magnet angle of lesser than 170° electrical
angle. For instance, if the electrical machine 100 is a 40-pole electrical
machine, the magnet angle 9 may be lesser than or equal to 8.5°, i.e., 8.5°
mechanical angle. In an example, the magnet angle may be in a range of
120°-130° electrical angle. For instance, the magnet angle may be 126°
electrical angle, i.e., 30% lesser than that of the conventional electrical
machines. Accordingly, the present subject matter can achieve a 30%
reduction in the amount of magnetic material consumed for construction of
electrical machines.

[0036] Since the magnetic angle of each of the magnets 112 is lesser
than 170° electrical angle and since the magnets 112 are uniformly distributed on the rotor core 108 in the circumferential direction 114, a gap may exist between one magnet and another in the circumferential direction 114. For instance, a gap 128 may exist between the magnet 112-n-2 and the magnet 112-n-1. Such gaps between the magnets 112 may be filled by portions of the rotor core 108. In other words, a portion of the rotor core 108 exists in the gap between the magnet 112-n-2 and the magnet 112-n-1. Accordingly, compared to the conventional electrical machines, the electrical machine 100 has a greater portion of the rotor core 108 exposed to the stator 104. The exposure of a significant portion of the rotor core 108, which includes a significant amount of iron, to the stator 104 causes generation of reluctance torque during operation of the electrical machine 100 as will be explained below:
[0037] During operation of the electrical machine 100 as a motor, stator
winding 122 of each stator tooth 118 is supplied with electric current, causing the stator teeth 118 to act as electromagnet poles. Further, each magnet 112 in the rotor 102 tends to align with an electromagnet pole to reduce the magnetic reluctance, also referred to as reluctance. The tendency to align with the electromagnet poles of the stator causes generation of a reluctance torque to rotate the rotor 102. The reluctance torque supplements the magnetic torque, which is generated due to interaction of the magnetic field of the rotor 102 and the magnetic field of the stator 104.
[0038] The reduction of the magnet angle of the magnets 112 may give
rise to a cogging torque, which is the torque required to break away from a position at which a stator tooth 122 and magnet 112 are aligned, especially at lower speeds of rotation of the electrical machine 100. To overcome the cogging torque, in accordance with an implementation of the present subject matter, a groove may be provided on a surface of each stator tooth 122, as will be explained with reference to Fig. 2(a).

[0039] Fig. 2(a) illustrates the stator tooth 118-3, in accordance with an
implementation of the present subject matter. The stator tooth 118-3 has been zoomed to clearly illustrate various details of the stator tooth 118-3. It will be understood that the other stator teeth will be similar to the stator tooth 118-3 in terms of their respective structures.
[0040] As illustrated in Fig. 2(a), a radial gap 202 exists between the
stator tooth 118-3 and the inner surface 116 of the rotor 102. Further, a
surface 204 of the stator tooth 118-3 is exposed to the inner surface 116.
Accordingly, the radial gap 202 exists between the inner surface 116 and
the surface 204 of the stator tooth 118-3. The surface 204 of the stator tooth
118-3, which is exposed to the inner surface 116, may be referred to as the
tooth surface 204. Similar to the stator tooth 118-3, the other stator teeth
also have a tooth surface exposed to the inner surface 116.
[0041 ] In an implementation, to overcome the cogging torque, a groove
206 may be provided on the tooth surface 204. The groove 206 may be, for example, semi-circle shaped or U-shaped. The groove 206 may extend along the length of the stator tooth 118-3 from the center of the stator tooth 118-3 and parallel to the axis 106. Further, the groove 206 may curve in the semi-circular shape in the circumferential direction 114. Since the groove 206 extends along the center of the tooth surface 204, the groove 206 may be interchangeably referred to as the central groove 206. The presence of the groove 206 on the tooth surface 204 causes a localized enlargement of the radial gap 202 in the region of the tooth surface 204 having the groove 206. Since the other stator teeth are similar in structure to the stator tooth 118-3, the other stator teeth also have the groove on their respective tooth surfaces.
[0042] The reduction of the cogging torque by the groove 206 will now
be explained. During operation of the electrical machine 100, the groove on each tooth surface causes generation of a torque fluctuation. Such a torque fluctuation is in counter-phase to the cogging torque. Also, the torque fluctuation has a similar magnitude as the cogging torque. Therefore, the

torque fluctuations caused by the grooves neutralizes the cogging torque wholly or partially. Thus, the provision of the grooves on the tooth surfaces reduces or cancels the cogging torque caused due to reduction of the magnet angle.
[0043] Although Fig. 2(a) illustrates a single groove on the tooth surface 204, in some cases, a plurality of grooves may be provided on the tooth surface, as will be explained below:
[0044] Fig. 2(b) illustrates provision of a plurality of grooves on the
tooth surface 204, in accordance with an implementation of the present subject matter. In addition to the central groove 206, additional grooves may be provided on the tooth surface 204. Similar to the central groove 206, the additional grooves also serve to reduce or eliminate the impact of the cogging torque. The additional grooves include grooves 208-1, 208-2, 210-1, and 210-2. Similar to the central groove 206, the additional grooves may also be semi-circular in shape. However, the additional grooves may have a smaller diameter as compared to the central groove 206, as illustrated in Fig. 2(b). Further, the additional grooves may extend along the length of the stator tooth 118-3.
[0045] The additional grooves may be displaced from each other and
from the central groove 206 in a circumferential direction of the stator 104.
In an example, the additional grooves may be provided in pairs, where the
grooves of each pair are separated by the central groove 206 and are
equidistant from the central groove 206. For instance, the grooves 208-1
and 208-2 form a first pair of grooves 208 and the grooves 210-1 and 210-
2 form a second pair of grooves 210. Accordingly, the additional grooves
may be said to be disposed symmetrically about the central groove 206.
[0046] The provision of magnets of reduced size in the rotor 102 may
increase the possibility of the magnets flying out of the rotor 102 during operation of the electrical machine 100. The magnets mayfly out due to the failure of adhesive that is used to stick the magnets in the cavities in the rotor core 108. The adhesive failure may happen due to ageing or prolonged

exposure of the electrical machine 100 to high temperatures. To prevent the
magnets 112 from flying out of the rotor 102, the present subject matter
utilizes one or more locking arrangements that secure the magnets 112 in
the cavities of the rotor core 108, as will be explained below:
[0047] Figs. 3(a)-(c) types of locking arrangements used for securing
the magnets 112 in the rotor core 108, in accordance with various implementations of the present subject matter.
[0048] Fig. 3(a) illustrates a first type of locking arrangement. The first
type of locking arrangement involves provision of magnets with rounded corners. For instance, the magnet 112-1 and the other magnets disposed in the rotor core 108 are provided with rounded corners. The rounded corners ensure that the magnets 112 are embedded within cavities of the rotor core 108. Therefore, the rounded corners hold the magnet from flying out of the rotor core 108.
[0049] Fig. 3(b) illustrates a second type of locking arrangement. The
second type of locking arrangement involves the use of one or more circular wires, such as circular wires 302-1 and 302-2. Here, grooves may be formed on lateral walls of the magnets. The grooves may be semi-circular in shape and may be referred to as semi-circular grooves. In addition, the cavities of the rotor core 108 may also have grooves having same dimensions as the grooves on the magnets. Accordingly, the grooves on the cavities may be referred to as complementary semi-circular grooves. Further, the position of the grooves on the cavities may be such that, when a magnet is disposed in a cavity, the grooves of the cavity can interact with the grooves on the lateral surfaces of the magnet. Accordingly, when the magnet is placed in the cavity, the semi-circular groove of the magnet and the complementary semi-circular groove of the magnet interact with each other to form a circular groove. Further, wires may be passed through the circular grooves formed by interaction of the magnets and the cavities of the rotor core 108. For example, circular wires 302-1 and 302-2 are inserted in the groove formed by the magnet 112-1 and the rotor core 108. Such wires increase the

bonding between the magnets and the rotor core 108, thereby securing
them from flying out of the rotor core 108. In an example, a pair of circular
wires, such as the circular wires 302-1 and 302-2, may be provided on
opposite walls of the magnet 112-1.
[0050] Fig. 3(c) illustrates a third type of locking arrangement. Here,
the locking arrangement includes notches 304-1 and 304-2, also referred to
as the pair of notches 304. The pair of notches 304 may be provided on
opposite edges of the magnet 112-1. Each notch may be received in a
recess (not shown in Fig. 3(c)) in the rotor core 108. For instance, the cavity
in which a magnet is to be housed may be shaped such that they have
recesses having a shape that is complementary to the notches. The
complementary shapes of the notch and its corresponding recess enables
securing the magnet 112-1 in an opening of the rotor core 108.
[0051] In addition to a locking arrangement, an adhesive may also be
used to provide additional bondage between the magnets and the rotor core
108.
[0052] The electrical machine 100 can be utilized in a vehicle (not
shown in Figs), such as an electric vehicle or a hybrid vehicle. The vehicle
may be, for example, a two-wheeler or a four-wheeler.
EXAMPLES
[0053] The superiority of the electrical machine 100 over the
conventional SPM machines is elucidated with the help of various
examples. In the below examples, the electrical machine 100 is a 40-
pole/36-slot electrical machine and is compared against a conventional 40-
pole/36-slot SPM machine, hereinafter referred to as the conventional SPM
machine. Both the electrical machine 100 and the conventional SPM
machine have a base speed of 350 revolutions per minute (RPM).
[0054] Fig. 4 illustrates a comparison of efficiency of the electrical
machine 100 and the conventional SPM machine, in accordance with an implementation of the present subject matter. Here, graph 402 represents speed-torque characteristics of the conventional SPM machine and graph

404 represents the speed-torque characteristics of the electrical machine 100. Further, bar 406 represents efficiencies corresponding to different colours shown in the graphs 402 and 404.
[0055] As is clear from the figure, the electrical machine 100 provides
greater efficiency for speeds lesser than the base speed of 350 RPM as
compared to the conventional SPM machine. Further, a relatively higher
efficiency (e.g., about 80% or higher) is achieved for greater speeds in the
electrical machine 100 as compared to the conventional SPM machine.
[0056] Fig. 5 illustrates a comparison of peak phase currents of the
electrical machine 100 and the conventional SPM machine, in accordance
with an implementation of the present subject matter. Here, graph 502
represents speed-torque characteristics of the conventional SPM machine
and graph 504 represents the speed-torque characteristics of the electrical
machine 100. Further, bar 506 represents peak phase currents
corresponding to different colours shown in the graphs 502 and 504. As is
clear from the figure, a region in which a relatively lower current (e.g., about
40 A or lesser) is achieved for a larger area of the speed-torque curve in the
electrical machine 100 as compared to the conventional SPM machine.
[0057] Fig. 6 illustrates a comparison of cogging torques of various
types of electrical machines. Here, the curve 602 represents cogging torque
of the conventional SPM machine. The curve 604 represents cogging torque
of an electrical machine in which the magnets of the rotor have a magnet
angle lesser than 170° electrical angle and in which the stator teeth do not
have grooves on their tooth surfaces. As illustrated, the electrical machine
with lesser magnet angle and no grooves on the stator teeth have a greater
cogging torque as compared to the conventional SPM machine.
[0058] The curve 606 represents the cogging torque of the electrical
machine 100 in a case where the teeth surfaces have the central groove, such as the central groove 206, alone. Further, the curve 608 represents the cogging torque of the electrical machine 100 in a case where the teeth surfaces have both the central groove and the additional grooves, such as

the additional grooves 208-1 and 208-2. As is clear from a comparison of the curves 602, 606, and 608, a considerably lesser amount of cogging torque is generated in the electrical machine 100 has as compared to the conventional SPM machine. Further, it is also clear that the electrical machine 100 with a plurality of grooves on the teeth surfaces have lesser amount of cogging torque as compared to the electrical machine 100 with a single groove.
[0059] Table 1 below illustrates the total harmonic distortion (THD) in
the voltages generated by various types of electrical machine, in accordance with an implementation of the present subject matter.
[0060] As illustrated, the electrical machine 100 has a marginal
increase in the THD as compared to the conventional SPM machine and
the electrical machine having magnets with reduced magnet angle (< 170°
electrical angle) but without grooves on the tooth surface.
[0061] Fig. 7 illustrates spectrum of generated voltage at a speed of
600 RPM for various types of electrical machines. Here, the various harmonic numbers and their contribution to the total generated voltage is illustrated. The curve 702 corresponds to the conventional SPM machine. The curve 704 corresponds to the electrical machine in which the magnets

of the rotor have a magnet angle lesser than 170° electrical angle and in
which the stator teeth do not have grooves on their tooth surfaces. The
curve 706 corresponds to the electrical machine 100 in a case where the
teeth surfaces have the central groove, such as the central groove 206,
alone. Further, the curve 708 corresponds to the electrical machine 100 in
a case where the teeth surfaces have both the central groove and the
additional grooves, such as the additional grooves 208-1 and 208-2.
[0062] Fig. 8 illustrates a comparison of flux densities along the
direction of magnetization within magnets of rotors in the conventional SPM
machine and in the electrical machine 100, in accordance with an
implementation of the present subject matter. Here, the flux density is
measured when the respective electrical machines are delivering a torque
of 98Nm at a speed of 200 RPM. The graph 802 corresponds to the
conventional SPM machine and the graph 804 corresponds to the electrical
machine 100. Further, the bars 806 and 808 represent flux densities
corresponding to different colours shown in the graphs 802 and 804. It can
be seen that the conventional SPM machine has a lower flux density value
(0.38T) when compared to electrical machine 100 (0.48T).
[0063] The present subject matter enables achieving a reduction in the
amount of magnetic material used in the electrical machines. Further, by providing grooves on the outer surfaces of the stator teeth, the reduction of the amount of magnetic material consumed is achieved without causing increasing the cogging torque. Also, the overall efficiency of the electrical machines is increased for a wider speed range. Further, the demagnetization performance of the electrical machines is improved. Thus, the fuel economy of a vehicle utilizing the electrical machines of the present subject matter can be improved.
[0064] The usage of the locking mechanisms for securing the magnets
in the rotor core ensures that the magnets do not fly out of the rotor, even after prolonged usage of the electrical machines. Therefore, the impact of failure of adhesive, which is used to secure the magnets in the rotor, due to

operation of the electrical machine at high temperature is eliminated.
Further, the durability of the electrical machine is increased.
[0065] The electrical machines of the present subject matter can be
used in vehicles, such as electric vehicles and hybrid vehicles. The present subject matter can be used for electrical machines of a wide variety of pole-slot combinations. The present subject matter can also be used in electrical machines in which the stator is skewed to reduce the impact of the cogging torque.
[0066] Although the present subject matter has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the subject matter, will become apparent to persons skilled in the art upon reference to the description of the subject matter.

I/We Claim:
1. A rotating electrical machine, comprising:
a stator comprising:
a plurality of stator teeth distributed in a circumferential direction of the stator, wherein each stator tooth comprises a tooth surface that faces a periphery of a rotor and wherein each stator tooth comprises a groove on the tooth surface; and the rotor disposed coaxially with the stator, the rotor comprising:
a rotor core; and
a plurality of magnets disposed in the rotor core, wherein the plurality of magnets is distributed in the rotor core in a circumferential direction of the rotor core, wherein each magnet has a magnet angle that is lesser than 170° electrical angle, and wherein each magnet is separated from an adjacent magnet by a portion of the rotor core.
2. The rotating electrical machine as claimed in claim 1, wherein each
magnet has a notch disposed on its lateral wall and wherein the notch is
received in a complementary recess in the rotor core to secure the magnet
in the rotor core.
3. The rotating electrical machine as claimed in claim 1, wherein each
magnet has a semi-circular groove disposed on its lateral wall and wherein
the semi-circular groove interacts with a complementary semi-circular
groove in the rotor core to form a circular groove, wherein the rotating
electrical machine comprises a circular wire passing through the circular
groove to secure the magnet in the rotor core.
4. The rotating electrical machine as claimed in claim 1, wherein each
magnet comprises a rounded corner to secure the magnet in the rotor core.

5. The rotating electrical machine as claimed in claim 1, wherein each magnet has a magnet angle of 126° electrical angle.
6. The rotating electrical machine as claimed in claim 1, wherein the groove is semi-circular.
7. The rotating electrical machine as claimed in claim 1, wherein the groove is a central groove that extends along a length of the stator tooth from a centre of the tooth surface.
8. The rotating electrical machine as claimed in claim 7, wherein the
stator tooth further comprises a pair of additional grooves on the tooth
surface, the pair of additional grooves extending along the length of the
stator tooth, wherein the pair of additional grooves is separated by the
central groove in a circumferential direction of the stator.
9. The rotating electrical machine as claimed in claim 1, wherein the rotor surrounds the stator and wherein the tooth surface of each stator tooth faces an inner periphery of the rotor.
10. A vehicle comprising a rotating electrical machine as claimed in any one of claims 1-9.