FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF INVENTION
An improved external rotor BLDC motor APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 025, Maharashtra, India, an Indian Company
INVENTORS
Joshi Ashish Kashinath, Engineering Group, CG Global R&D Centre, Crompton Greaves Ltd, Kanjur Marg(E), Mumbai - 400042, Maharashtra, India, and Kamble Deepak Gajanan of Engineering Group and Dr Ekram Samsul both of Advanced Motor Design & Technology Centre (AMDTC) Division, CG Global R&D Centre, Crompton Greaves Ltd, Kanjur (East), Mumbai 400 042, Maharashtra, India, all Indian nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed :

FIELD OF INVENTION
This invention relates to an improved external rotor BLDC motor.
This invention also relates to an improved external rotor BLDC motor particularly for use
in a two wheeled vehicle.
BACKGROUND ART
BLDC (Brushless Direct Current) motors are generally internal rotor and external rotor types. External rotor BLDC motors are used for driving mechanical loads typically in automobiles and in various electrical appliances and equipments, for instance, two wheelers, ceiling fans, washing machines or wind turbines. An external rotor BLDC motor comprises a housing rotatably held around a stationary motor shaft using bearings. The motor also comprises a cylindrical stator comprising stator core fitted over a stator holder which in turn is clamped on to the motor shaft. Conductors or windings are inserted in the stator slots provided at the inner circumference of the stator core. The motor further comprises a rotor comprising a plurality of pole magnets mounted around the inner circumference of the stator core with sufficient air gap between the stator core and rotor magnets. The housing is rotatable with the rotor and is provided with connectors for connecting the motor to the mechanical loads to be driven by the motor. The windings in the stator bulge or protrude out at the end faces of the stator core into the spaces in the housing between the end wall at one end and end shield at the opposite end and respective end face(s) of the stator core so as to form an elliptical configuration. When current flows through the windings in the stator, a magnetic or flux field is created
2

or produced by the windings and the flux or magnetic filed created by the stator windings is elliptical in nature because of the elliptical shape of the wound stator windings. The rotor magnets disposed around the inner circumference of the stator core and lying in one long or straight section of the elliptical path are excited causing the rotor and housing to rotate around the motor shaft. Due to rotation of the housing the mechanical load connected to it also rotates. In order to drive mechanical loads of higher capacity and rating, the sizes of the rotor and stator are to be increased so as to deliver higher power output and torque. This will increase the size and weight of the motor and is not normally desirable in the case of appliances or equipments having space constraints. In order to increase the torque and capacity of the motor without increasing its size and weight, a further set of rotor magnets are provided around the outer circumference of the stator core with sufficient air gap with the outer circumference of the stator in addition to the set of rotor magnets provided around the inner circumference of the stator core. The stator comprises conductors or windings inserted in the stator slots both at the inner circumference and outer circumference thereof. The two sets of rotor magnets are so disposed as to lie in the long or straight sections of the elliptical path of the magnetic field created by the windings or conductors inserted in the stator slots both at the inner circumference and outer circumference of the stator core. During flow of current through the windings of the stator, the further set of rotor magnets are also excited thereby increasing the torque and capacity of the motor. BLDC motors comprising two sets of rotor magnets are called external dual rotor BLDC motors.
3

OBJECTS OF INVENTION
An object of the invention is to provide an improved external rotor BLDC motor, which is compact and gives increased torque and power output without any increase in the size thereof.
Another object of the invention is to provide an improved external rotor BLDC motor, which is simple in construction and is economical.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided an improved external rotor BLDC motor comprising a rotor formed by four sets of rotor magnets, the first set of rotor magnets being located around the inner circumference of the cylindrical stator core of the motor describing an air gap with the inner circumference of the stator core and parallel to the motor shaft, the second set of rotor magnets being located around the outer circumference of the stator core describing an air gap with the outer circumference of the stator core and parallel to the motor shaft, the third set of rotor magnets being located around one end face of the stator core describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft and the fourth set of rotor magnets being located around the other end face of the stator core describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft, the first and second sets of rotor magnets being disposed in the long or straight sections of the elliptical path of the magnetic field or flux created by the stator windings during flow of current therethrough and the third and
4

fourth sets of rotor magnets being disposed in the short or conical sections of the elliptical path so as to increase the torque and power out of the motor.
According to the invention there is also provided an improved external rotor BLDC motor for use in a two wheeled vehicle, the motor comprising a housing comprising an inner cylinder and an outer cylinder extending laterally concentrically from an end disc, the housing being provided with connector means for connecting it concentrically with the rear wheel of the two wheeled vehicle and rotatably mounted around a stationary motor shaft extending axially through the housing in association with an end shield fitted at the opposite end of the housing; a cylindrical stator comprising a stator core located within the housing and mounted on the motor shaft over a stator holder, the stator further comprising conductors or windings inserted in the stator slots provided at the inner circumference and outer circumference of the stator core and a rotor comprising four sets of rotor magnets, the first set of rotor magnets being disposed around the inner circumference of the stator core and located around the outer circumference of the inner cylinder describing an air gap with the inner circumference of the stator core and parallel to the motor shaft, the second set of rotor magnets being disposed around the outer circumference of the stator core and located around the inner circumference of the outer cylinder describing an air gap with the outer circumference of the stator core and parallel to the motor shaft, the third set of rotor magnets being disposed around one end face of the stator core and located against the inner surface of the end disc describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft and the fourth set of rotor magnets being disposed around the
5

other end face of the stator core and located against the inner surface of the end shield describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft, the first and second sets of rotor magnets being disposed in the long or straight sections of the elliptical path of the magnetic field or flux created by the stator windings during flow of current therethrough and the third and fourth sets of rotor magnets being disposed in the short or conical sections of the elliptical path so as to.increase the torque and power out of the motor.
The following is a detailed description of the invention with reference to the accompanying drawings, in which
Fig 1 is a cross sectional view of a BLDC motor for use in a two wheeled vehicle according to an embodiment of the invention; and
Figs 2 and 3 are exploded views of the motor housing (without connecting flange), stator, rotor and end shield of the BLDC motor of the invention from the housing side and end shield side, respectively.
The improved external rotor BLDC motor 1 as illustrated in Figs 1 to 3 of the accompanying drawings comprises a housing 2 comprising an inner cylinder 3 and an outer cylinder 4 extending laterally concentrically from an end disc 5. The housing is provided with a connecting flange 6 which is in turn is provided with bolt holes 7,7 for bolting the flange to a corresponding rim (not shown) on the rear wheel (not shown) of a
6

two wheeled vehicle (not shown) with the housing disposed concentrically with the rear wheel. The housing is rotatably mounted around a stationary motor shaft 8 extending axially through the housing in association with an end shield 9 fitted at the opposite end of the housing. 10 is a cylindrical stator comprising a stator core 11 located within the housing and mounted on the motor shaft over a stator holder 12. The stator holder is press fitted onto the motor shaft. The stator core 11 is clamped onto the stator holder by bolts (not shown) engaged in mounting holes 13 provided in lugs 14 extending laterally from the stator holder. The stator further comprises conductors or windings 15 inserted in the stator slots 16 provided at the inner circumference and outer circumference of the stator core. The rotor of the motor comprises four sets of rotor magnets. The first set of rotor magnets 17 is disposed around the inner circumference of the stator core and located around the outer circumference of the inner cylinder with fixing means such as adhesive or screws (not shown) describing an air gap (not marked) with the inner circumference of the stator core and parallel to the motor shaft. The second set of rotor magnets 19 is disposed around the outer circumference of the stator core and located with fixing means such as adhesive or screws (not shown) around the inner circumference of the outer cylinder describing an air gap (not shown) with the outer circumference of the stator core and parallel to the motor shaft. The third set of rotor magnets 20 is disposed around one end face of the stator core and located with fixing means such as adhesive or screws (not shown) against the inner surface of the end disc describing an air gap (not shown) with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft and the fourth set of rotor magnets 21 is disposed around the other end face of the stator core and
7

located with fixing means such as adhesive or screws (not shown) against the inner surface of the end shield describing an air gap (not shown) with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft. During operation of the motor, the stator windings create a magnetic field around them when current is passed through them. Due to the wound stator windings being of elliptical configuration the flux field formed or created around the stator is also elliptical in nature. The first and second sets of rotor magnets are so disposed as to lie in the long or straight sections of the elliptical path 22 of the magnetic field or flux created by the stator windings and the third and fourth sets of rotor magnets are disposed as to lie in the short or conical sections of the elliptical path. Therefore, all the four sets of rotor magnets are excited thereby causing the rotor to rotate at substantially increased torque and power output. The housing rotates with the rotor and causes the rear wheel of the two wheeler vehicle to rotate with it as the housing is concentrically fitted to the rear wheel.
According to the invention the rotor magnets are disposed in all the sections ie two long and two short sections of the elliptical path of the flux field. Therefore, the entire flux field is effectively utilised to increase the power output and torque of the motor without increasing the size of the motor. The motor of the invention is simple in construction and economical.
The above embodiment of the invention is only illustrative of the invention and not limitative of the scope thereof. The basic invention is in the utilization of the entire
8

magnetic flux so as to increase torque and power output of the motor The external rotor BLDC motor of the invention can be used for driving other loads besides rear wheel of two wheeled vehicle. The connecting means and housing configurations can vary. Such variations of the invention are to be construed and understood to be within the scope of the invention.
9

We Claim
1. An improved external rotor BLDC motor comprising a rotor formed by four sets of rotor magnets, the first set of rotor magnets being located around the inner circumference of the cylindrical stator core of the motor describing an air gap with the inner circumference of the stator core and parallel to the motor shaft, the second set of rotor magnets being located around the outer circumference of the stator core describing an air gap with the outer circumference of the stator core and parallel to the motor shaft, the third set of rotor magnets being located around one end face of the stator core describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft and the fourth set of rotor magnets being located around the other end face of the stator core describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft, the first and second sets of rotor magnets being disposed in the long or straight sections of the elliptical path of the magnetic field or flux created by the stator windings during flow of current therethrough and the third and fourth sets of rotor magnets being disposed in the short or conical sections of the elliptical path so as to increase the torque and power out of the motor.
2. An improved external rotor BLDC motor as claimed in claim 1, wherein all the four sets of rotor magnets are fixed in position with fixing means such as adhesive or screws.
10

3. An improved external rotor BLDC motor for use in a two wheeled vehicle, the motor comprising a housing comprising an inner cylinder and an outer cylinder extending laterally concentrically from an end disc, the housing being provided with connector means for connecting it concentrically with the rear wheel of the two wheeled vehicle and rotatably mounted around a stationary motor shaft extending axially through the housing in association with an end shield fitted at the opposite end of the housing; a cylindrical stator comprising a stator core located within the housing and mounted on the motor shaft over a stator holder, the stator further comprising conductors or windings inserted in the stator slots provided at the inner circumference and outer circumference of the stator core and a rotor comprising four sets of rotor magnets, the first set of rotor magnets being disposed around the inner circumference of the stator core and located around the outer circumference of the inner cylinder describing an air gap with the inner circumference of the stator core and parallel to the motor shaft, the second set of rotor magnets being disposed around the outer circumference of the stator core and located around the inner circumference of the outer cylinder describing an air gap with the outer circumference of the stator core and parallel to the motor shaft, the third set of rotor magnets being disposed around one end face of the stator core and located against the inner surface of the end disc describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft and the fourth set of rotor magnets being disposed around the other end face of the stator core and located against the inner surface of the end shield describing an air gap with the respective end face of the stator and windings and perpendicular to the axis of rotation of the motor shaft, the first and second sets of rotor magnets being disposed in the long or straight sections of
11

the elliptical path of the magnetic field or flux created by the stator windings during flow of current therethrough and the third and fourth sets of rotor magnets being disposed in the short or conical sections of the elliptical path so as to increase the torque and power out of the motor.
4. An improved external rotor BLDC motor as claimed in claim 3, wherein all the four sets of rotor magnets are fixed in position with fixing means such as adhesive or screws.
5. An improved external rotor BLDC motor as claimed in claim 3 or 4, wherein the connector means comprises a connecting flange around the housing, the flange being provided with bolt holes for bolting the flange to a corresponding rim on the rear wheel of the two wheeled vehicle with the housing disposed concentrically with the rear wheel.
Dated this 15th day of March 2007
12

ABSTRACT
An improved external rotor BLDC motor (1) comprising a rotor formed by four sets of rotor magnets. The first set of rotor magnets (17) are located around the inner circumference of the cylindrical stator core (11) of the motor and describe an air gap with the inner circumference of the stator core and are parallel to the motor shaft (8). The second set of rotor magnets (19) are located around the outer circumference of the stator core (17) and describe an air gap with the outer circumference of the stator core and are parallel to the motor shaft (8). The third set of rotor magnets (20) are located around one end face (5) of the stator core and describe an air gap with the respective end face of the stator and windings (15) and are perpendicular to the axis of rotation of the motor shaft (8). The fourth set of rotor magnets (22) are located around the other end face (9) of the stator core and describe an air gap with the respective end face of the stator and windings (15) and are perpendicular to the axis of rotation of the motor shaft (8). The first and second sets of rotor magnets are disposed in the long or straight sections of the elliptical path (22) of the magnetic field or flux created by the stator windings during flow of current therethrough and the third and fourth sets of rotor magnets are disposed in the short or conical sections of the elliptical path (22) so as to increase the torque and power out of the motor (Fig
1).