METHOD FOR CONTROLLING SPEED OF NON-SALIENT POLE PERMANENT MAGNET MOTOR (PMM)

FIELD OF INVENTION:
[001] The present subject matter described herein, relates to electrical motors, and, in more particular to, a method and system for controlling speed of non-salient pole permanent magnet motor (PMM) in electric vehicle (EV) applications and industrial applications.
BACKGROUND AND PRIOR ART:
[002] Generally, wide speed range of electric motors is used in the electric vehicle (EV) applications and industry applications. The wider speed range of electric motors is being achieved by using a flux weakening method. An induction machine, a reluctance machine and a permanent magnet motor (PMM) are widely used for larger ratings as wider constant power speed range (CPSR) capability under a flux weakening control (FWC).
[003] The flux weakening control (FWC) is usually carried out by applying demagnetizing current of stator windings such that the demagnetizing current opposes the permanent flux. However, several hybrid excitation machines are also proposed in the literature which use separate field windings in addition to permanent magnets to weaken air gap flux. A self-excited motor without utilization of the permanent magnets is proposed, wherein a magnetizing field is obtained by using the differential frequency between armature field and rotor speed frequencies. A flux reversal machine with dual excitation is proposed for electric vehicle propulsion by using the permanent magnets, DC filed windings and AC armature windings in stator and reluctance rotor. A flux switching machine with dual excitation is proposed for the electric vehicle (EV) application by utilizing five-phase E-core for improving flux regulation and constant power speed range. Multiphase flux switching machines are designed for electric vehicle (EV) application to have advantages of both flux switching machines and multiphase machines.
[004] The flux weakening control (FWC) is achieved by using a mechanical device in an axial flux permanent magnet motor (PMM), wherein the mechanical device depends upon speed and torque. Flux regulation of an axial-?ux permanent magnet automotive alternator built with ironless structure has also been achieved by using electromechanical ?ux weakening. A DC excited dual memory machine is proposed for the electric vehicle (EV) application by using the dual permanent magnet materials and separate field coils to achieve flux weakening through a pole-changing function. A novel self-learning scheme for an embedded type permanent magnet motor (PMM) is proposed for the flux weakening operation. A line-modulation-based flux weakening control is proposed for speed control by maximizing the averaged DC bus voltage utilization and reducing the copper losses. A six-step operation is considered and compared with space vector modulation in flux weakening region of interior permanent magnet synchronous motor. A robust field-weakening algorithm is proposed for a direct torque-controlled reluctance motor, wherein the proposed flux weakening algorithm is robust to variation of d-axis inductance and q-axis inductance in the permanent magnet motor (PMM). The flux weakening control (FWC) with closed loop is proposed for the permanent magnet synchronous motor (PMM) with dual excitation by regulating amplitude of voltage and phase of voltage separately. A current control process for field weakening of wound permanent magnet motor (PMM) is proposed to reduce the copper losses.
[005] In the PCT application number, WO2018080996A1 discloses the closed loop flux weakening apparatus which include a difference circuit that obtains a difference between a q-axis reference voltage and a q-axis voltage, a controller that converts the difference between the q-axis reference voltage and the q-axis voltage into a d-axis current of a stator of the motor, and a summation circuit that obtains a d-axis reference current by adding the d-axis current of the stator of the motor and a feed forward d-axis current of the stator of the motor.
[006] In the European application number, EP 3,346,590 A1 discloses the machine that includes a drive shaft, a PM rotor assembly with multiple PMs arranged around a periphery of the rotor assembly, a first stator assembly including a stator yoke, having stator teeth mounted to the stator core with distal ends proximate the outer periphery of the rotor assembly separated by a first air gap and multiple stator coils mounted between the stator teeth and also a second stator assembly including a stator yoke, having stator teeth mounted to the stator core with distal ends forming closed slots, proximate an inner periphery of the rotor assembly separated by a second air gap and at least one control coil, the a control coil wrapped about a saturable region of the stator teeth thereof, each saturable region is operable to divert magnetic flux of the PMs through the stator teeth.
[007] In US patent number, US 9,762,163 B2 discloses a method for controlling an AC motor, including: receiving a torque command value; generating a command current based on the torque command, and a command voltage by using the generated command current in a current vector controller (CVC) current control mode; switching to a hexagon voltage manipulating controller (HVMC) voltage control mode when the command voltage enters a voltage limit area, and generating a command voltage in the HVMC voltage control mode; and controlling torque of an AC motor by using the command voltage that is generated in the CVC current control mode or the HVMC voltage control mode.
[008] In the European application number, EP 3,163,726 A1 discloses the flux control of permanent magnet electric machine. A permanent magnet electric machine includes a rotor including a plurality of permanent magnets arranged around a central shaft and supported in an outer member and a stator surrounding the rotor and arranged to allow the rotor to turn within in inner diameter of it. The rotor includes one or more flux control elements disposed within the output member and moveably attached to the central shaft that move from an initial position when the rotor is rotating at a first rate and to a second position closer to the permanent magnets when the rotor is operating at a second rate, greater than the first rate.
[009] In US patent number, US 9,614,473 B1 discloses the flux weakening AC motor control by voltage vector angle deflection. A method for controlling a three phase AC motor includes generating an operating point error signal based upon a difference between a reference operating point and an actual operating point. A first control method is used to determine a first motor control vector, whereby magnitude and angle values of the first control vector are determined. A second control method is used to determine a second control vector from the operating point error signal, whereby the second control vector is a voltage vector with a constant magnitude. A pulse width modulated voltage is applied to the AC motor. The pulse width modulated voltage is dependent upon the first motor control vector when the AC motor is operating below voltage output saturation. The pulse width modulated voltage is dependent upon the second motor control vector when the AC motor is operating at voltage output saturation.
[0010] In the present invention has to develop many control strategies and machine configurations to achieve the speed control of electric motors in the electrical vehicle (EV) applications and industrial applications.
OBJECTS OF THE INVENTION:
[0011] The principal objective of the present invention relates to a controller for permanent magnet motor (PMM) to achieve speed control keeping in view of reducing demagnetization effect of permanent magnets and utilization of non-salient pole permanent magnet motors (PMM) for high speed applications. However, the controller can also be used for salient pole permanent magnet motors (PMM) as long as the torque requirement is addressed.
SUMMARY OF THE INVENTION:
[0012] The present subject matter relates to a method for controlling speed of a non-salient pole permanent magnet motor (PMM) in electric vehicle (EV) applications. Though various rotor configurations are invented for achieving high speed range of permanent magnet motor (PMM), the existing controller for high speed operation of all kinds of permanent magnet motors (PMMs) is same and through flux weakening by pumping negative stator d-axis current from base speed onwards.
[0013] According to the aspect of the invention, a controller which carries out the flux weakening by varying the stator d-axis current from positive value to negative value unlike the existing controller which carries the flux weakening by varying stator d-axis current from zero value to negative value.
[0014] When the stator d-axis current is positive in a salient pole permanent magnet motor (PMM), the reluctance torque produced will be opposite to the direction of developed mutual torque unlike in the case of negative d-axis current where the reluctance torque produced will be in same direction as the developed mutual torque. However, the controller of varying stator d-axis current from positive value to negative value can also be applied for the salient pole permanent magnet motor (PMM) also based on the saliency level of rotor and torque demand by load. For instance, the controller will be very useful if the saliency is very low for the given salient pole permanent magnet motor (PMM) and increase in the mutual torque overweighs the opposing reluctance torque during positive stator d-axis current injection.
[0015] Since the reluctance torque in the case of non-salient pole permanent magnet motor (PMM) is zero irrespective of amplitude and polarity of stator d-axis current, the application of controller is being discussed on non-salient pole permanent magnet motor (PMM) in order to avoid the reluctance torque effect. The controller is not being addressed from the load torque requirement perspective as it can be taken care during design of permanent magnet motor (PMM). The controller of varying stator d-axis current from positive value to negative value leads to higher speed range. For instance, consider a non-salient pole permanent magnet motor (PMM) (neglecting saturation effects) is designed for base rated speed for the existing controller which means for zero stator d-axis current at base rated speed. If the non-salient pole permanent magnet motor (PMM) is redesigned for base rated speed for the controller which means for positive rated stator d-axis current at base rated speed. Assuming the rated demagnetizing current is same and torque requirement is met in both the designed machines, the speed range of non-salient permanent magnet motor (PMM) designed for base rated speed for the controller will have two times the speed range with respect to the base rated speed in comparison with designed permanent magnet motor (PMM) for base rated speed for the existing controller.
[0016] In accordance with an embodiment of the present invention, the method comprises the steps of: providing a controller to vary stator d-axis current from positive value to negative value leads to higher speed range of non-salient pole permanent magnet motor (PMM). The controller configured to provide the speed of non-salient pole permanent magnet motor (PMM) is more than base rated speed during flux strengthening, when the positive current is applied to stator d-axis. Further, the controller configured to provide the speed of non-salient pole permanent magnet motor (PMM) is less than the base rated speed during flux weakening, when the negative current is applied to stator d-axis.
[0017] The demagnetizing stator d-axis current for flux strengthening and weakening control (FSWC) is lesser than the demagnetizing stator d-axis current for flux weakening control (FWC) for the same speed range.
[0018] The effects of demagnetization on the permanent magnets is substantially reduced with the flux strengthening and weakening control (FSWC) compared to the demagnetization effects in the flux weakening control (FWC).
[0019] The flux strengthening and weakening control (FSWC) is applicable to low-salient pole permanent magnet motor (PMM) and high-salient pole permanent magnet motor (PMM) also as long as load-torque requirements are met.
[0020] The critical speed is defined as the speed at which the stator d-axis current changes from positive value to negative value in order to differentiate between the flux strengthening and weakening control (FSWC) and the flux weakening control (FWC).
[0021] The speed of non-salient permanent magnet motor (PMM) for flux strengthening and weakening control (FSWC) is greater than the speed of non-salient permanent magnet motor (PMM) for flux weakening control (FWC) when the zero-stator d-axis current is applied to the non-salient permanent magnet motor (PMM).
[0022] The beauty of the controller of varying stator d-axis current from positive value to negative value can also be appreciated from other perspective. For instance, consider the non-salient pole permanent magnet motor (PMM) (neglecting saturation effects) designed for base rated speed for the existing controller which means for zero stator d-axis current at base rated speed for a certain speed range. If the non-salient pole permanent magnet motor (PMM) is redesigned for base rated speed for the controller which means for positive rated stator d-axis current at base rated speed such that it leads to the same speed range as in the earlier case, then the stator demagnetizing field effect on permanent magnets will be substantially reduced.
[0023] In order to further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit scope of the present subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. 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 reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0025] Fig. 1 illustrates a flow diagram of a method for controlling speed of a non-salient permanent magnet motor (PMM), in accordance with an embodiment of the present subject matter;
[0026] Fig. 2 illustrates a functional block diagram of a field-oriented control of permanent magnet motor (PMM), in accordance with an embodiment of the present subject matter;
[0027] Fig. 3 illustrates a graphical representation of stator d-axis current profiles for flux strengthening and weakening control (FSWC) and flux weakening control (FWC), in accordance with the present subject matter;
[0028] Fig. 4a, 4b, and 4c illustrate voltage phasor diagrams for base speed, critical speed, and maximum speed of non-salient pole permanent magnet motor (PMM) for the flux strengthening and weakening control (FSWC), in accordance with an embodiment of the present subject matter;
[0029] Fig. 5 illustrates a reluctance torque profile of salient pole permanent magnet motor (PMM) for the flux strengthening and weakening control (FSWC) and flux weakening control (FWC), in accordance with an embodiment of the present subject matter;
[0030] Fig. 6 illustrates a block diagram of MATLAB simulation, in accordance with an embodiment of the present subject matter;
[0031] Fig. 7 illustrates a graphical representation of a speed and torque plot of non-salient pole permanent magnet motor 2 (PMM-2) for applied positive rated stator d-axis current, in accordance with an embodiment of the present subject matter;
[0032] Fig. 8 illustrates a graphical view of a speed and torque plot of non-salient pole permanent magnet motor 2 (PMM-2) for applied zero stator d-axis current, in accordance with an embodiment of the present subject matter; and
[0033] Fig. 9 illustrates a graphical view of a speed and torque plot of non-salient pole permanent magnet motor 2 (PMM-2) for applied negative rated stator d-axis current, in accordance with an embodiment of the present subject matter.
[0034] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[0035] The present subject matter relates to a method for controlling speed of a non-salient pole permanent magnet motor (PMM) in electric vehicle (EV) applications. The method comprises the steps of: providing a controller to vary stator d-axis current from positive value to negative value leads to higher speed range of non-salient pole permanent magnet motor (PMM). The controller configured to provide the speed of non-salient pole permanent magnet motor (PMM) is more than base rated speed during flux strengthening, when the positive current is applied to stator d-axis. Further, the controller configured to provide the speed of non-salient pole permanent magnet motor (PMM) is less than the base rated speed during flux weakening, when the negative current is applied to stator d-axis.
[0036] The demagnetizing stator d-axis current for flux strengthening and weakening control (FSWC) is lesser than the demagnetizing stator d-axis current for flux weakening control (FWC) for the same speed range.
[0037] The effects of demagnetization on the permanent magnets is substantially reduced with the flux strengthening and weakening control (FSWC) compared to the demagnetization effects in the flux weakening control (FWC).
[0038] The flux strengthening and weakening control (FSWC) is applicable to low-salient pole permanent magnet motor (PMM) and high-salient pole permanent magnet motor (PMM) also as long as load-torque requirements are met.
[0039] The critical speed is defined as the speed at which the stator d-axis current changes from positive value to negative value in order to differentiate between the flux strengthening and weakening control (FSWC) and the flux weakening control (FWC).
[0040] The speed of non-salient permanent magnet motor (PMM) for flux strengthening and weakening control (FSWC) is greater than the speed of non-salient permanent magnet motor (PMM) for flux weakening control (FWC) when the zero-stator d-axis current is applied to the non-salient permanent magnet motor (PMM).
[0041] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art and are to be constructed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.
[0042] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[0043] Fig. 1 illustrates a flow diagram of a method 100 for controlling speed of a non-salient permanent magnet motor (PMM), in accordance with an embodiment of the present subject matter. The comprises the steps of: providing 101 a controller to vary stator d-axis current from positive value to negative value leads to higher speed range of non-salient pole permanent magnet motor (PMM). The controller configured to provide 102 the speed of non-salient pole permanent magnet motor (PMM) is more than base rated speed during flux strengthening, when the positive current is applied to stator d-axis. Further, the controller configured to provide 103 the speed of non-salient pole permanent magnet motor (PMM) is less than the base rated speed during flux weakening, when the negative current is applied to stator d-axis.
[0044] In the flux strengthening and weakening control (FSWC), the flux strengthening is considered in addition to flux weakening for achieving speed control of the non-salient pole salient permanent magnet motor (PMM). The flux strengthening is applied below the base speed while the flux weakening is applied above the base speed. During flux strengthening below the base speed in the flux strengthening and weakening control (FSWC), the positive rated stator d-axis current is applied, and further, during flux weakening above the base speed in the flux strengthening and weakening control (FSWC), the stator d-axis current is reduced from positive rated value to zero value and further to negative rated value.
[0045] Fig. 1 illustrates a functional block diagram for field-oriented control of permanent magnet motor (PMM). The function block is measured between speed input and reference stator d-axis current output achieves the required flux weakening control (FWC).
[0046] Fig. 3 illustrates the stator d-axis current profiles for the flux strengthening and weakening control (FSWC) and flux weakening control (FWC). To say briefly, the flux strengthening and weakening control (FSWC) varies the stator d-axis current from the positive value to negative value. The typical stator d-axis current-speed profile for the flux strengthening and weakening control (FSWC) and compared with profile for the flux weakening control (FWC). The negative rated stator d-axis current (-idmax2prop) of the flux strengthening and weakening control (FSWC) is greater than the negative rated stator d-axis current (-idmaxlexis) of the flux weakening control (FWC) for the same speed range. It means the maximum demagnetizing current of the flux strengthening and weakening control (FSWC) is less than the maximum demagnetizing current of the flux weakening control (FWC) for the same speed range. It clearly indicates that the permanent magnets experience lower risk of demagnetization in the flux strengthening and weakening control (FSWC).
[0047] In order to distinguish more between the flux strengthening and weakening control (FSWC) and flux weakening control (FWC), the critical speed is defined as the speed at which the stator d-axis current becomes negative. It is clear from Fig. 3, that the critical speed (?cprop) of the flux strengthening and weakening control (FSWC) is higher than the critical speed (?Cexis = ?b) in case of the flux weakening control (FWC). It means while the non-salient pole permanent magnet motor (PMM) achieves base speed for zero demagnetizing current under the flux weakening control (FWC) such that the non-salient pole permanent magnet motor (PMM) achieves speed higher than the base speed for zero demagnetizing current under the flux strengthening and weakening control (FSWC).
[0048] The function block to reference the stator d-axis current in Fig. 2 is modelled such that it injects the constant positive maximum current for speed less than or equal to base speed and decreasing current from positive rated current for speed greater than base speed. If idmax1prop and -idmax1prop are maximum positive stator d-axis current and maximum negative stator d-axis current, then the conditions can be mathematically represented in (1)-(3).
id = idmax1prop for ? = ?b (1)
idmax1prop >id = 0 for ?b < ? = ?c1 (2)
0 >id =-idmax2prop for ? > ?c1 (3)
[0049] The flux weakening operation of non-salient pole permanent magnet motor (PMM) can also be understood from voltage phasor diagrams for cases of base speed, critical speed and beyond critical speed shown in Fig. 4.
[0050] Fig. 4a illustrates the non-salient pole permanent magnet motor (PMM) that drives the base speed with maximum positive stator d-axis current.
[0051] Fig. 4b illustrates the non-salient pole permanent magnet motor (PMM) that drives beyond the base speed and drives the critical speed while the stator d-axis current is reduced from maximum positive value to zero value.
[0052] Fig. 4c illustrates the non-salient pole permanent magnet motor (PMM) that drives beyond critical speed and drives the maximum speed while stator d-axis current is reduced from zero value to maximum negative value.
[0053] Fig. 5 illustrates a reluctance torque profile of salient pole permanent magnet motor (PMM) for the flux strengthening and weakening control (FSWC) and flux weakening control (FWC), in accordance with an embodiment of the present subject matter. In the flux strengthening and weakening control (FSWC), the reluctance torque is maximum negative till base speed operation and reluctance torque increases from this maximum negative value beyond base speed operation in case of salient pole permanent magnet motor (PMM) as shown in Fig. 5, whereas the reluctance torque is zero at any speed in case of the non-salient pole permanent magnet motor (PMM).
[0054] As the negative reluctance torque is also produced over certain speed range for the salient pole permanent magnet motor (PMM) in the flux strengthening and weakening control (FSWC) such that the salient pole permanent magnet motor (PMM) with high saliency ratio may not be suitable for the flux strengthening and weakening control (FSWC). The reluctance torque is zero for the non-salient pole permanent magnet motor (PMM) over total speed range and hence the flux strengthening and weakening control (FSWC) process is very useful for the non-salient pole permanent magnet motor (PMM). However, the low-salient pole permanent magnet motor (PMM) as long as the load-torque requirement is taken care while designing the low-salient pole permanent magnet motor (PMM). The reference stator d-axis current needs to be generated based on the measured rotor speed for flux weakening control (FWC) or flux strengthening and weakening control (FSWC) of the non-salient pole permanent magnet motor (PMM). It means the reference stator d-axis current should be expressed a function of rotor speed. If ?b is base speed and ?h is speed higher than base speed, then the relation between ?d and ?m in terms of ?b and ?h is given in (4).
?d = ?b/ ?h ?m (4)
Now the function of reference stator d-axis current in terms of speed for the flux weakening and weakening control (FSWC) of non-salient pole permanent magnet motor (PMM) is given in the below equations (5)-(6).
id = 0 for ? = ?b (5)
id = 1/Ld (1- ?/ ?b) ?m for ? > ?b (6)
[0055] It is important to note that stator d-axis current base speed for the flux strengthening and weakening control (FSWC) of non-salient permanent magnet motor (PMM) is id = idmax1prop. Hence the function of reference stator d-axis current in terms of speed for flux strengthening and weakening control (FSWC) of non-salient pole permanent magnet motor (PMM) becomes as given in the below equations (7)-(8).
id = idmax1prop for ? = ?b (7)
id = 1/Ld (1- ?/ ?b) ?m + idmax1prop for ? > ?b (8)
Now the equation for critical speed of non-salient pole permanent magnet motor (PMM) for the flux strengthening and weakening control (FSWC) can be derived by substituting id = 0 and ? = ?Cprop in equation (8). The equation for critical speed ?Cprop is given in the below equation (9).

If x represents the ratio of maximum stator magnetizing flux to permanent magnet flux, then the equation for idmax1prop is given in the below equation (10).
idmax1prop = ?m / Ld (10)
Using the equations (7), (8) and (10), the function of reference stator d-axis current for flux strengthening and weakening control (FSWC) of the non-salient pole permanent magnet motor (PMM) becomes as given in the below equations (11)-(12).
id = x ?m / Ld for ? = ?b (11)
id = -1/Ld (1- ?b / ?) ?m + x ?m / Ld for ? > ?b (12)
Using the equations (9) and (10), the equation for critical speed of permanent magnet motor (PMM) for the flux strengthening and weakening control (FSWC) is given in the below equations (13).

The equation (13) indicates that the speed of non-salient pole permanent magnet motor (PMM) for the flux strengthening and weakening control (FSWC) is greater than the speed of permanent magnet motor (PMM) for the flux weakening control (FWC) when the zero-stator demagnetizing field is applied.
[0056] Fig. 6 illustrates a block diagram of MATLAB simulation which has been carried out for speed control of the non-salient pole permanent magnet motor (PMM) to verify the controller using sinusoidal pulse width modulation and vector control method under no-load. Two non-salient permanent magnet motors (PMM-1 and PMM-2) with four poles are considered for verification of the controller. Both the non-salient pole permanent magnet motors (PMM-1 and PMM-2) are assumed to be designed with high performance permanent magnets which do not lose the magnetic characteristics for the rated negative stator d-axis current. The non-salient pole permanent magnet motor 1 (PMM-1) is assumed to be designed with the air gap flux density of 0.8T with required thicker permanent magnets and it is capable of getting flux weakened to 0.6T under rated negative stator d-axis current. The non-salient pole permanent magnet motor 1 (PMM-1) is also assumed to be designed such that the air gap flux density is 0.8T and back emf is 300 V at 3000 RPM for zero stator d-axis current. The non-salient pole permanent magnet motor 2 (PMM-2) is assumed to be designed with the air gap flux density of 0.6T with required thicker permanent magnets and it is capable of getting flux weakened to 0.4T under rated negative stator d-axis current. The non-salient pole permanent magnet motor 2 (PMM-2) is also assumed to be designed such that the air gap flux density is 0.8T and back emf is 300V at 3000 RPM for rated positive stator d-axis current wherein the rated positive stator d-axis current and rated negative stator d-axis current are same in amplitude but different in polarity.
[0057] Fig. 7 illustrates a graphical representation of mechanical angular speed in radians per second (first axis) and mechanical torque in N-m (second axis) of the non-salient pole permanent magnet motor 2 (PMM-2) for applied positive stator d-axis current in amperes (third axis) with respect to time in seconds.
[0058] Fig. 8 illustrates a graphical representation of mechanical angular speed in radians per second (first axis) and mechanical torque in N-m (second axis) of the non-salient pole permanent magnet motor 2 (PMM-2) for zero d-axis current in amperes (third axis) with respect to time in seconds.
[0059] Fig. 9 illustrates a graphical representation of mechanical angular speed in radians per second (first axis) and mechanical torque in N-m (second axis) of the non-salient pole permanent magnet motor 2 (PMM-2) for negative stator d-axis current in amperes (third axis) with respect to time in seconds.
[0060] The stator d-axis current in amperes, air gap flux density in tesla and angular speed in radians per second of the non-salient pole permanent magnet motors (PMM-1 and PMM-2) are compared in the below table to appreciate the highspeed range that could be achieved through the controller of varying the stator d-axis current from positive value to negative value.
PMM-1 speed control through existing logic PMM-2 speed control through existing logic
d-axis current Air gap flux density Speed d-axis current Air gap flux density Speed
200 A 0.8 T 314
0 A 0.8 T 314 0 A 0.6 T 418
-200 A 0.6 T 418 -200 A 0.4 T 600

[0061] Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/component of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

We claim:

1. A method for controlling speed of non-salient pole permanent magnet motor (PMM), the method comprising the steps of:
providing a controller to vary stator d- axis current from positive value to negative value leads to higher speed range of non-salient pole permanent magnet motor (PMM);
wherein the controller configured to provide the speed of non-salient pole permanent magnet motor (PMM) is more than base rated speed during flux strengthening when the positive current is applied to stator d-axis.
wherein the controller configured to provide the speed of non-salient pole permanent magnet motor (PMM) is less than the base rated speed during flux weakening when the negative current is applied to stator d-axis.
2. The method as claimed in claim 1, wherein the demagnetizing stator d-axis current for flux strengthening and weakening control (FSWC) is lesser than the demagnetizing stator d-axis current for flux weakening control (FWC) for the same speed range.
3. The method as claimed in claim 1, wherein effects of demagnetization on permanent magnets is substantially reduced with the flux strengthening and weakening control (FSWC) compared to the demagnetization effects in the flux weakening control (FWC).
4. The method as claimed in claim 1, wherein flux strengthening and weakening control (FSWC) is applicable to low-salient pole permanent magnet motor (PMM) and high-salient pole permanent magnet motor (PMM) also as long as load-torque requirements are met.
5. The method as claimed in claim 1, wherein critical speed is defined as the speed at which the stator d-axis current changes from positive value to negative value in order to differentiate between the flux strengthening and weakening control (FSWC) and the flux weakening control (FWC).
6. The method as claimed in claim 1, wherein the speed of non-salient permanent magnet motor (PMM) for flux strengthening and weakening control (FSWC) is greater than the speed of non-salient permanent magnet motor (PMM) for flux weakening control (FWC) when the zero-stator d-axis current is applied to the non-salient permanent magnet motor (PMM).