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Title:
ELECTRONICALLY CONTROLLED PERMANENT-MAGNET ELECTRIC MOTOR
Document Type and Number:
WIPO Patent Application WO/2001/045235
Kind Code:
A1
Abstract:
An electronically controlled permanent-magnet electric motor comprises a stator (2, 3) with one or more excitation coils (4, 5) for creating a main magnetic field (F¿1?), a rotor (6) with permanent magnets (10, 11) mounted rotatably inside the stator with a given air gap (T), an electronic circuit (18) for controlling the electrical supply of the coil (4, 5). The stator comprises at least one magnetic element (20) located in the vicinity of the air gap (T) so as to generate a secondary magnetic field (F¿2?) which is angularly offset ($g(a)) with respect to the main magnetic field (F¿1?) so as to ensure start-up of the rotor (6) in a predetermined direction. The air gap (T) has a width (W) which is substantially constant and has a minimum value compatible with rotation of the rotor in any operating condition so as to reduce the magnetic losses. The electronic control circuit comprises first sensor means (24) for detecting the position and angular speed of the rotor (6) and second sensor means (27) associated with the coil (4, 5) for detecting the overcurrents of the circuit, a microprocessor unit (22), and a power unit (23) for driving the excitation coil.

Inventors:
KARR JEAN MICHEL (CH)
Application Number:
PCT/IB2000/001900
Publication Date:
June 21, 2001
Filing Date:
December 18, 2000
Export Citation:
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Assignee:
KARR JEAN MICHEL (CH)
International Classes:
H02K11/04; H02K21/18; H02K29/08; H02P6/22; (IPC1-7): H02K21/14
Domestic Patent References:
WO1997045943A11997-12-04
Foreign References:
US5475277A1995-12-12
EP0455578A21991-11-06
DE19701856A11998-07-23
Attorney, Agent or Firm:
Vannini, Mario (Maroscia & Associati S.r.l, Corso Palladio 42, Vicenza, I-36100, IT)
Download PDF:
Claims:
CLAIMS
1. Electronically controlled permanentmagnet electric motor comprising a stator (2,3) with at least one excitation coil (4,5) for creating a main magnetic field (F1), a rotor (6) with permanent magnets (10,11) rotatably mounted inside said stator (2,3) with a given air gap (T), an electronic circuit (18) for regulating and controlling the electrical supply of said at least one coil (4,5), characterized in that said stator (2,3) comprises at least one magnetic element (20) located in the vicinity of said air gap (T) so as to generate a secondary magnetic field (F2) which is angular offset (a) with respect to the main magnetic field (F1) so as to ensure startup of the rotor (6) in a predetermined direction upon excitation of said at least one coil (4,5).
2. Electric motor according to Claim 1, characterized in that said air gap (T) has a width (W) which is substantially constant and has a minimum value compatible with rotation of the rotor in any operating condition so as to reduce the magnetic losses.
3. Electric motor according to Claim 1, characterized in that said stator (2, 3) comprises at least one pair of pole shoes (7,8) having an axis of symmetry (A) substantially perpendicular to the direction of said main magnetic field (F1).
4. Electric motor according to Claim 1, characterized in that said offset angle (a) of said secondary magnetic field (F2) with respect to said main magnetic field (F,) ranges between 0° and 180°.
5. Electric motor according to Claim 1, characterized in that it comprises first sensor means (24) facing said rotor (6), for detecting the magnetic field in the vicinity of said air gap (T) as well as the position and angular speed of said rotor (6).
6. Electric motor according to Claim 5, characterized in that it envisages second sensor means (27) associated with said at least one coil (4,5) for detecting the overcurrents induced during rotation of the rotor.
7. Electric motor according to Claim 6, characterized in that said electronic circuit (18) comprises a microprocessor unit (22) designed to process electrical signals supplied by said first sensor means (24) and said second sensor means (27) in order to drive a power unit (23) intended to supply said at least one excitation coil (4,5).
8. Electric motor according to Claim 7, characterized in that said microprocessor unit (22) comprises a microprocessor (33) in which a program for sampling the stator magnetic field upon rotation of said rotor (6) is installed.
9. Electric motor according to Claim 8, characterized in that said program comprise a first subroutine (S,) for sampling the magnetic field detected by said first sensor means (24) in predetermined angular positions of the rotor over a maximum angular range of less than one complete revolution.
10. Electric motor according to Claim 9, characterized in that said microprocessor unit (22) comprises a memory medium (34) designed to store the sampling values detected.
11. Electric motor according to Claim 10, characterized in that the program installed in said microprocessor comprises a second subroutine (S2) for processing the data sampled and stored in said memory medium (34) and for driving said power unit (23).
12. Electric motor according to Claim 11, characterized in that said second subroutine (S2) provides the function of sequentially modulating the supply current of said at least one excitation coil (4,5) so as to establish the direction of rotation upon startup, the torque and the speed of the rotor (6) during the transient and operating phases.
13. Electric motor according to Claim 11, characterized in that said microprocessor (33) is of the programmable type, means being provided for connection to a digital interface (35) and/or to remote sensor means (36) for controlling longdistance functions and messages.
Description:
ELECTRONICALLY CONTROLLED PERMANENT-MAGNET ELECTRIC MOTOR Technical field The present invention generally relates to the technical filed of electrical machines and in particular concerns an electric motor with electronically controlled permanent magnets for industrial applications.

Background art The known electric motors of the type indicated above generally comprise a stator with one or more pairs of pole shoes and with at least one excitation coil and a permanent-magnet rotor located inside the stator. The excitation coil is usually supplied by an alternating current of predetermined frequency, for example the mains frequency.

These known motors have the particular feature that they operate at a well- defined speed determined by the frequency of the supply current and, for this reason, must be regarded as special synchronous motors.

The drawbacks of magnetic motors are therefore similar to those of synchronous motors, i. e. a very low starting torque and therefore difficulty in spontaneous start-up, uncertainty as regards the initial direction of rotation, lack of flexibility with regard to regulation, constant operating condition, risk of stoppage due to loss of synchronism in the event of even minor variations in the resistive load, no possibility of regulation of the currents and therefore the risk of demagnetisation of the stator in the event of stoppage, the vibrations caused by an imbalance in the rotor polarities, overheating of the stator due to the effect of parasitic currents, and very low power factors.

In order to overcome some of these drawbacks, in the past the rotor has been provided with a suitable squirrel cage, basically forming a"mixed"structure which results in a greater complexity of the control systems.

Other measures for overcoming the abovementioned drawbacks consist in designing the electrical part with large dimensions, deforming the pole shoes so as to cause an imbalance in the air gap thereof and therefore the electrical reluctance, or introducing cam means in order to favour stoppage of the rotor in predetermined positions.

In certain cases, however, these measures give rise to correlated effects such as, for example, wear or breakage of mechanical parts, vibrations and noisiness, risk of breakage due to fatigue, reduction in performance or magnetic imbalance due to variation in the reluctance.

In order to overcome this lack of flexibility it is possible to use suitable electronic apparatus for controlling the supply current, of the inverter type, which also allow an improvement in the efficiency and specific power.

However, these apparatus are somewhat complex and costly, are external and cannot be easily incorporated into the motor and are therefore difficult to apply especially in the case of relatively low power outputs.

In order to overcome the problem of a low torque and specific power, special magnetic alloys containing rare earths, such as samarium-cobalt and neodymium-iron-boron alloys, have been developed. However, the somewhat high cost of these materials constitutes a serious obstacle to the diffusion of these motors in all the industrial sectors where they could be applied.

To summarise, all the measures considered have managed to overcome only partially the abovementioned drawbacks and, for technical and economic

reasons, have not been able to achieve full development of permanent-magnet motors in the various applications where they could be used.

Italian patent application No. PD92A000107 discloses a permanent-magnet motor which comprises at least one static switch arranged in series between at least one stator winding and the mains frequency supply line and an electronic circuit which processes three parameters, in particular the stator current, the position and the polarity of the permanent magnet of the rotor and the polarity of the alternating voltage source. The circuit drives the static switch so that the latter conducts whenever the flux generated by the stator winding produces on the rotor a torque coinciding with the direction of rotation of the said rotor.

Even though this motor is characterised by a fairly efficient and reliable start- up, it does not allow regulation of its operating characteristics and there is no possibility of programming the control circuit according to the desired operating conditions.

Another drawback consists in the fact that the solution proposed is applicable solely to a stator of the"open"type and involves geometrical alterations to the pole shoes and the air gap which result in a considerable reduction in the efficiency and imbalances in the magnetic fluxes.

Disclosure of the invention The object of the present invention is to overcome the abovementioned drawbacks by providing a permanent-magnet motor which has characteristics of high efficiency, reliability and flexibility of use at a low cost.

Another particular object is to provide a permanent-magnet motor which is

substantially independent of the frequency of the electric mains or other electrical power supply sources.

Another particular object is that of providing a permanent-magnet motor which has a good starting torque and easy start-up in a given direction, avoiding losses in efficiency or oversizing of component parts.

A further particular object is that of providing a permanent-magnet motor which has a geometrical configuration of the stator independent of particular predetermined geometries.

Yet another object is that of designing a permanent-magnet motor which has an overall efficiency and specific power greater than the motors according to the state of the art.

A further particular object is that of providing a permanent-magnet motor which can be easily controlled in any operating condition and which allows energy savings in terms of electric power required.

Another object is that of designing a permanent-magnet motor which allows easy control of the uniformity of rotation and limitation in the presence of vibrations.

A further object is that of providing a permanent-magnet motor which allows control and regulation of the stator currents, while keeping the power factor as close as possible to its maximum value and keeping as small as possible the parasitic currents which negatively influence the dimensions and the efficiency of the motor in addition to creating overheating and the risks of seizing.

Yet another object is that of designing a permanent-magnet motor which offers a high degree of precision in control of its operating characteristics so as to allow use thereof in the most complex applications in the industrial field.

A further object, finally, is that of integrating the hardware and software components so that they are extremely flexible and adaptable to different variations in performance and to subsequent developments in use thereof.

These objects along with others which will be understood more clearly below are achieved by an electronically controlled permanent-magnet motor which, in accordance with Claim 1, comprises a stator with at least one excitation coil for the creation of a main magnetic field with a predetermined axis, a permanent-magnet rotor mounted rotatably inside said stator with a certain air gap, an electronic circuit for regulating and controlling the electrical supply of said coil, characterized in that the stator comprises at least one magnetic element located in the vicinity of the air gap so as to generate a secondary magnetic field which is angular offset with respect to the main magnetic field.

Owing to this arrangement, when the coil is not excited, the rotor will be aligned with the secondary field generated by the magnetic element so as to be angular offset with respect to the axis of the main magnetic field.

Thus, when the coil is energised, the main magnetic field generated by the latter will exert on the rotor a torque ensuring alignment in a predetermined direction, so as to ensure start-up of the rotor in a predefined direction.

Moreover, the air gap between the stator and the rotor has a substantially constant width and minimum value compatible with rotation of the rotor in any condition.

Owing to the uniformity of the air gap, it will be possible to limit the lack of regularity in the operating condition and therefore the vibrations and noisiness of the motor during operation. The minimum value for the width of the air gap will also allow the magnetic losses of the motor to be kept to a minimum, achieving the maximum increase in specific power.

The motor comprises first sensor means for detecting the magnetic field, position and angular speed of the rotor and second sensor means for detecting the overcurrents induced during rotation of the rotor.

The electronic circuit comprises a microprocessor unit which is designed to process the electric signals supplied by the first and second sensor means so as to drive a power unit intended to supply power to the excitation coil.

The microprocessor unit comprises a microprocessor in which a program for sampling the stator magnetic field during rotation of said rotor is installed.

The program may comprise a routine for sampling the magnetic field detected by the first sensor means in predetermined angular positions of the rotor over a maximum angular variation of less than 180°.

Brief description of the drawings Further features and advantages of the invention will be more clearly understood from the detailed description of some preferred, but not exclusive embodiments of a permanent-magnet electric motor according to the invention, illustrated by way of a non-limiting example with the aid of the accompanying drawings in which: Fig. 1 shows a perspective schematic view of a first embodiment of a permanent-magnet motor according to the invention;

Fig. 2 shows a schematic front view of the motor according to Fig. 1; Fig. 3 shows a schematic front view of a second embodiment of a permanent-magnet motor according to the invention; Fig. 4 shows a schematic front view of a third embodiment of a permanent-magnet motor according to the invention; Fig. 5 shows a schematic front view of a fourth embodiment of a permanent-magnet motor according to the invention; Fig. 6 shows a cross-sectional view of the motor according to Fig. 5; Fig. 7 shows a functional diagram of a control circuit for a permanent- magnet motor according to one of the preceding figures; Fig. 8 shows a block diagram of the software for controlling a permanent-magnet motor according to the invention, installed in the control circuit according to Fig. 7.

Detailed description of the preferred embodiment (s) With reference to Figs. 1 and 2, the permanent-magnet electric motor according to the invention, which is generally indicated with the reference numeral 1, essentially comprises a stator structure or stator 2 comprising an ordinary pack of laminations 3 in the form of a horse-shoe, provided with one or more windings or excitation coils 4,5 inside which a permanent-magnet rotor 6 is housed.

The stator 3 has at least one pair of pole shoes 7,8 which are substantially symmetrical with respect to a plane indicated by A and containing the axis of rotation H of the rotor 6 and by means of which the main magnetic field generated by the windings 4,5 and indicated schematically by the vector F1 is closed. The lines of the main magnetic field F1 which pass through the rotor 6, extend in a direction substantially parallel to the plane indicated by the line C, intersecting radially the axis of rotation H of the rotor.

The rotor 6, which has an essentially cylindrical shape, may be made of a homogeneous material, for example ferrite or ferritic ciay, which may be obtained by means of various techniques, such as sintering, extrusion or pressing. The rotor is then permanently magnetised so as to generate a magnetic field with a dual-lobe polar progression of the type known per se, with magnetic poles N-S oriented perpendicularly with respect to a diametral plane indicated by the line K.

Alternatively, the rotor may be constructed in a non-integral form, as shown in Figs. 3,4, i. e. with a central core 9 which is made of preferably non-magnetic material and to which there is stably fixed, for example by means of tenons or gluing, at least one insert or segment 10 consisting of material with a high magnetic retentivity and with the direction of magnetism N-S oriented radially.

Advantageously two magnetic inserts or segments 10,11 may be provided, as shown in Figs. 5,6.

In all cases, the core 9 may have a central spindle or end hubs 1 2 mounted on supports 13,14 which may be fixed to a connecting plate 15 integral with the stator 2.

Thus, the rotor 6 and the pole shoes 7,8 of the stator 2 will have a clearance or an air gap T with a width W which will allow the rotor to rotate freely and which will be crossed by the flux lines of the main magnetic field upon excitation of the coils 4,5.

The terminals 16,17 of the windings 4,5 are connected to a power supply circuit which is denoted in its entirety by the reference number 18 and which has the function of driving the voltage and the supply current supplied by the mains or by an external source.

The power supply circuit 18 may be advantageously installed on a board 19 which supports a series of components which will be described in greater detail below.

According to the invention, the stator 2 comprises at least one magnetic element 20 located in the vicinity of the air gap T with a polarity N-S directed so as to generate a secondary magnetic field indicated schematically by the vector F2. This secondary magnetic field is angular rotated with respect to the main magnetic field F1 through a predetermined angle a which may range between 0° and 180°.

In the absence of the main field F1, i. e. in an initial or stopped condition of the motor, the rotor 6 will be"forced", i. e. will tend to align its segments 10,11 with the magnetic element 20 and with its secondary magnetic field F2.

When the coils 4,5 are energised by the circuit 18, the primary magnetic field F1 which has a strength considerably greater than that of the secondary field F2 will impart to the rotor 6 a torque of predetermined magnitude and direction, with consequent spontaneous and controlled start-up of the motor.

According to a further particular aspect of the invention, the air gap T has a substantially constant width W so as to avoid reluctance vibrations and hence the vibrations and noisiness which are typical of the permanent-magnet motors of the past. This feature will help make operation of the motor more continuous and uniform, independently of the characteristics of its control circuit.

Moreover, the width W of the air gap T has a minimum value which may be equal, for example, to a few hundredths of a millimetre in the case of a rotor in air, even with rotation of the rotor in any operating condition.

Owing to this very small air gap, it will be possible to reduce to a minimum the magnetic losses and maximise the stator magnetic flux, so as to increase the efficiency and the specific power of the motor which are proportional to this flux.

The embodiment shown in Fig. 3 differs from the preceding embodiments solely in that it has, instead of a single magnetic element, a pair of these elements 20 which are identical and fixed to the stator in diametrically opposite positions with respect to the axis of rotation H of the rotor 6, so that the secondary magnetic field induced by them has an inclination a with respect to the direction of the primary field F1.

The embodiment shown in Figs. 4 and 5 differs from the preceding embodiment solely with regard to the form and structure of the lamination pack 3 of the stator 3 which in this case is in the form of a closed ring. In this case also, at least one magnetic element 20 is envisaged and the air gap T has a constant width equal to a minimum value compatible with the condition of free rotation of the rotor 6.

It can be easily understood from this further example that the particular characteristics of the invention, with regard to the magnetic element 20 and the form of the air gap T, are applicable to any form of the stator and enable the same results and the same advantages to be achieved.

A further special feature of the invention relates to the particular configuration of the electronic control circuit 18 and its control software.

With reference to Fig. 7, the electronic circuit 18 comprises an input stage 21, a microprocessor-type calculating unit 22 and a driving or power unit 23 which will be described in detail below.

Moreover, the control circuit comprises a first sensor 24 facing the rotor 6, for detecting the magnetic field variations generated by rotation thereof, so as to determine its position and angular speed at any moment.

Preferably, the sensor 24 is of the linear type so as to provide a sinusoidal type signal. Thus, the sensor 24 will not merely detect the passing movement of a magnetic dipole but will also determine in an instantaneous manner the position and angular speed of the rotor.

By way of a non-limiting example, the sensor 24 may be of the Hall-effect type, with output terminals 25,26 connected to the calculating unit 22.

A second sensor 27, which is arranged in series along the power supply terminals 16,17 of the coils 4,5 for detecting overcurrents induced during rotation of the rotor and provided with output terminals 28,29 connected to the calculating unit 22, is also envisaged.

The input stage 21 may be powered by a source of alternating electric current, for example the electric mains at 220 V ac and 10 A or a low voltage independent source (48 V ac, 24 A) or also a direct-current source supplied by a 12 V or 24 V dc battery.

In the case illustrated, an alternating current power supply with two phase terminals 30,31 and an earth terminal 32 is envisaged.

The input unit 21 may have an input filter, a protection stage to ensure compliance with the EMI regulations and a rectifier stage for supplying the high-voltage motor and the low-voltage calculating unit 22.

The microprocessor unit 22 comprises a programmable microprocessor 33

which processes the signals sent from the first sensor 24 and the second sensor 27 and generates a suitable sequence of output signals which have the function of driving the power unit 23 intended to supply the stator windings 4,5 of the motor 1.

Software for modulating the power supply of the stator windings is installed in the microprocessor so as to control all the operating characteristics of the motor, such as spontaneous start-up in the desired direction, regulation of the torque in order to compensate for variations in external load, protection against any overcurrents, or compliance with the user's requirements, such as limitation in the maximum number of rotations or the maximum resistive load.

In particular, the control software, a block diagram of which is shown in Fig.

8, comprises among other things a subroutine Si for sampling the magnetic field detected by the sensor 24 over an angular range of less than one complete revolution by means of predetermined angular increments and a second subroutine S2 for calculating the modulation of the current supplying the stator windings based on the PWM (Pulse Width Modulation) method able to determine the direction of rotation at start-up, the torque and the speed of the motor during the transient and operating phases as well as other desired operating parameters.

The data detected by the sensor 24 and sampled by the processor 33 are stored on a suitable memory medium 34 inserted into the microprocessor unit 22.

Moreover, the programmable microprocessor 33 comprises means for connection to a digital interface 35 for modifying the control program after initial programming thereof.

Optionally, the microprocessor unit 22 may be connected to additional remote sensors 36 which provide data for operation management, or remote systems for sending alarm signals or long-distance messages in the case where the motor has to operate in such conditions.

From that described above, it is obvious that the permanent-magnet motor according to the invention achieves all the predefined objects and in particular allows control of all the operating variables of the motor such as, for example, the position and angular speed of the rotor, the driving torque independent of the variations in the resistive torque applied, the power output, the efficiency, rotor demagnetisation and the temperature of the various parts of the motor.

Moreover, the programmable microprocessor control system ensures a high degree of operating flexibility which allows the diffusion of these motors in all the application sectors. Finally, the extremely simple and economical structure of the control hardware ensures a notable compactness and therefore allows direct installation thereof onto the said motor, even in the case of relatively low power outputs, avoiding the use of complex and bulky external regulating devices.

The instant application is based upon and claims priority of patent application No. SM-A-199900003, filed on 17.12.1999 in San Marino, the disclosure of which is hereby expressly incorporated herein by reference thereto.




 
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