A device for controlling a synchronous electric motor with a permanent magnet rotor

Abstract: The device comprises a stator (2) including a winding (5) disposed around a stack (3) of laminations and with which is associated a permanent magnet rotor (4). Controlled conductor devices (12) are connected to the stator winding (5) and to an AC source (13, 14). One or more position sensors (10) determine the application to the control inputs of the said controlled conductor devices (12) of electrical control signals having to states which vary as a function of the angular position of the rotor (4). The arrangement is such that the current is caused to flow in the stator winding (5) in a direction which depends only on the polarity of the voltage of the source and the instantaneous position of the rotor (4).

Description: The present invention relates to a device for controlling the rotation of an electric motor comprising a stator including a winding disposed around a stack of laminations having a permanent magnet rotor associated therewith, of the kind defined in the preamble of Claim 1. A control device of this kind for use in connection with low speed brushless motors, such as motors used for the control of washing machines, is known from WO-86 05336-A. According to this document the stator winding of the motor is divided into two or more portions, which are independently coupled to an a.c. voltage source through respective bidirectional controlled semiconductor devices (TRIACs), which are driven by a common piloting circuit on the basis of control signals provided by Hall-sensors used to detect the angular position of the rotor. This prior control device allows to drive the motor at a speed which is independent of the frequency of the supply voltage. Control circuits for starting a synchronous electric motor having a permanent magnet rotor are disclosed in EP-A-0 574 823. These circuits generally include one or more bidirectionally-conducting semiconductor devices (TRIACs) controlled as a function of three parameters, i.e. the current flowing through the motor winding, the angular position of the rotor and the polarity of the supply voltage. DE-A-44 10 005 discloses circuit arrangements with Hall-sensors detecting the angular position of a permanent magnet rotor of a d.c. motor. US-A-4 678 974 discloses other control circuits for driving d.c. or a.c. motors having a permanent magnet rotor. The position detection of the machine's rotor is achieved with a phototransistor embodiment which consists of a single phototransistor arrangement which is suitable to detect the movement of a rotating opaque segment of a defined angular extent. A problem of single-phase synchronous motors is that the initial direction of rotation of the rotor is in general not pre-determined, that is to the say the motor is, per se, not of a unidirectional starting type. A further problematic aspect of the synchronous motor is represented by the fact that its speed of rotation under normal running conditions is strictly tied to the frequency of the supply voltage. The object of the present invention is to provide a device for controlling the rotation of a synchronous electric motor with a permanent magnet rotor which will have a simple structure, which can be made economically, and which has a high operating reliability. This and other objects are achieved according to the invention with a device the main characteristics of which are defined in the attached Claim 1. Further characteristics and advantages of the invention will become apparent from the following detailed description, given purely by way of non-limitative example, with reference to the attached drawings, in which: Figure 1 shows a synchronous electric motor with a permanent magnet rotor; Figure 2 is a block-schematic diagram of a control device for a synchronous electric motor of the type illustrated in Figure 1; Figure 3 is a detailed electric diagram of a first control circuit for a synchronous motor ; Figures 4a and 4b are timing diagrams of signals and other quantities in the device of Figure 3, respectively in the starting phase and in the normal running condition; Figures 5 and 6 are electric diagrams of two variant embodiments of the a control device for a synchronous motor; Figure 7 is a diagram which shows a variant embodiment of a power supply circuit included in a control device for a synchronous motor; Figure 8 is an electrical diagram of a further variant embodiment of a control device for a synchronous motor; Figure 8a is a timing diagram of signals present in operation of the device of Figure 8; Figure 9 is a partial perspective view which shows one way of forming position sensor devices associated with the rotor of a synchronous motor; Figure 10 is a diagram of a device according to the invention; and Figure 10a illustrates the signals provided by the position sensors of Figure 9. In Figure 1 there is illustrated a single-phase synchronous electric motor generally indicated with the reference numeral 1. The stator 2 of this motor 1 comprises a stack 3 of substantially U-shape laminations between the ends of which is rotatably mounted a permanent magnet rotor 4. The stator 2 does not, however, necessarily have to have a U-shape form. The stator 2 further includes a winding generally indicated 5 which in the exemplary embodiment illustrated is formed by two half-windings 5a and 5b connected together (in series or in parallel) wound on the two parallel arms of the lamination stack 3. The distal ends of the two arms of the lamination stack 3 are shaped in such a way as to define, with respect to the rotor 4, an asymmetric air gap. In particular, this air gap comprises two interstices 6a and 6b of increasing width in the clockwise sense as seen in Figure 1. In the embodiment illustrated the rotor 4 has two counter posed magnetic poles indicated N and S. Due to the effect of the shape of the air gap 6a, 6b the rotor 4 at rest is disposed with the magnetisation axis M R inclined with respect to the axis M S of the pole pieces of the stator, forming an angle  with respect thereto. At rest the rotor 4 can in particular be disposed in the position illustrated in Figure 1, or in a position at 180° with respect to that illustrated. In Figure 2 there is shown, in block diagram form, a device for controlling the rotation of a synchronous electric motor of the type described above with reference to Figure 1. In general this device comprises one or more position sensors 10 associated with the rotor 4 to provide, to a piloting circuit 11, electrical control signals of two states, which vary in dependence on the angular position of this rotor. The output of the piloting circuit 11 is connected to one or more electronic controlled conduction devices 12, operable to allow current to flow selectively in the two senses in the stator winding 5 of the motor, and which are connected in series between two terminals 13 and 14 intended to be connected to an AC supply source such as the mains network. The device schematically illustrated in Figure 2 is 'such that a current is caused to flow in the stator winding 5 of the motor in a direction which depends only on the polarity of the voltage applied between the terminals 13 and 14 and the instantaneous position of the rotor 4. A first embodiment of this device according to Figure 2 is illustrated in Figure 3. In this embodiment the rotor 4 of the motor is associated with a Hall-effect sensor 10. The conduction of current in the stator winding 5 is controlled by means of a Triac T, connected in series with this winding between the supply terminals 13 and 14. The piloting circuit 11 of Figure 3 comprises a transistor Q1 which in the embodiment illustrated is of pnp type and the base of which is connected to the output of the sensor 10. The collector of Q1 is connected to the terminal 13 by means of a resistor 15. The base and the emitter of Q1 are connected to the output of a DC voltage supply circuit 16 by means of respective resistors 17 and 18. The supply circuit 16 has an input terminal 19 intended to receive an alternating voltage, preferably a reduced voltage with respect to that applied between the terminals 13 and 14. This supply circuit comprises for example a resistor 20 followed by a rectifier diode 21, a stabiliser (Zener) diode 22 and a levelling capacitor 23. The piloting circuit 11 further includes two diodes 24 and 25 having their anodes and cathodes connected respectively to the gate of the Triac T by means of a PTC resistor 26. The cathode of the diode 24 is connected to the emitter of the transistor Q1. The anode of the diode 25 is connected to the output of the sensor 10 via a resistor 27. A resistor 28 is connected between the gate of the Triac T and ground. The device of Figure 3 acts in a manner which will now be described by making reference also to the graphs of Figure 4a and 4b in which V M represents the supply voltage applied between the terminals 13 and 14, H indicates the signal provided at the output from the sensor 10, I indicates the current flowing in the stator winding 5 of the motor, and CEMF indicates the back-electromotive force developed in the motor. The graphs of Figure 4a show the variations of V M , H, I and CEMF in the starting phase, that is to say in the starting transient of rotation of the rotor, whilst the graphs of Figure 4b show the variation of the same quantities in the normal running conditions. When the output signal from the Hall-effect sensor 10 is at high level ("1"), the transistor Q1 is switched off and a current flows in towards the gate of the Triac T. Therefore, as long as the output of the sensor 10 is at high level, the Triac T allows only the positive half-waves of the supply current to flow through the stator winding 5, as is shown by the lines representing the current I illustrated in Figures 4a and 4b. The Triac T operates in the first quadrant of the voltage-current plane. When, upon rotation of the rotor 4, the output of the sensor 10 passes to low level ("0"), the transistor Q1 passes into conduction. A current flows out from the gate of the Triac T, which therefore operates in the third quadrant, allowing only the negative half-waves of the current to flow in the stator winding 5 of the motor. If the transition from high level to low level of the signal H provided by the sensor 10 occurs in correspondence with a positive half-wave of the supply voltage V M , as is shown at the instant t 1 in Figure 4a, the current in the stator winding 5 does not immediately invert, but rather does this upon subsequent passage through 0 of the voltage V M , as is shown at instant t 2 in the said figures. Still with reference to Figure 4a, up to the instant t 0 the back-electromotive force CEMF and the current I are both positive so that a positive driving torque develops in the motor. Between the instants t 0 and t 2 the current I in the motor is still positive, but the back-electromotive force CEMF is negative so that in this short interval a negative torque develops. After the instant t 2 , the current I and the back-electromotive force CEMF are both negative and a positive torque again develops. Similar situations can arise when the signal H at the output of the sensor 10 passes from low level to high level. The possibility that a negative torque is generated for a short time interval represents, however, an inconvenience of practically negligible magnitude. Moreover, the signal H does not always pass from high to low level (and vice versa) in coincidence with a positive (or negative) half-wave of the supply voltage. In Figure 5 there is shown a further variant of a control device, in which in place of a Triac there are utilised two controlled rectifiers SCR1 and SCR2 disposed in antiparallel with one another'in series with the stator winding 5. The piloting circuit 11 comprises a photodiode PD the anode of which is connected to the output of the sensor 10, in this case also constituted for example by a Hall-effect sensor, and the cathode of which is connected to the gate of one of the two SCRs and coupled optically to a phototransistor PT the collector of which is connected to the gate of the other SCR. With this arrangement the two SCRs are piloted in counterphase by the output of the sensor 10, one by means of the photodiode PD and the other by means of the phototransistor PT which acts as an inverter. In the variant of Figure 6 the bidirectional controlled conductor means 12 comprise a controlled rectifier diode SCR1 in parallel with which a Triac T and a diode D are disposed in series in such a way as to allow the passage of current in the opposite sense from that of the current in the controlled diode SCR1. In general the Triac T and the diode D are substantially equivalent to a controlled rectifier diode (SCR) in antiparallel with the controlled diode SCR1. The piloting circuit 11 shown by way of example in Figure 6 comprises a transistor Q2 of npn type the base of which is connected to the output of the (Hall-effect) sensor 10 via a resistor 34, the emitter of which is connected to ground, and the collector of which is connected to the gate of the Triac T and to the cathode of the diode D via a resistor 35. The output from the sensor 10 is connected to the gate of SCR1 via a resistor 36. The anode of a diode 37 is connected to the gate of SCR1 and its cathode is connected to ground via a thermal protection resistor 38 of NTC type. A resistor 39 is connected between the cathode of the diode 37 and the collector of the transistor Q2. A DC supply circuit 16 provides for the supply of the sensor 10 and biasing of the base of the transistor Q2 via a resistor 40. The operation of the device illustrated in Figure 6 is similar to that of the device illustrated in Figure 5: the controlled diode SCR1 is piloted directly by the output of the sensor 10 via the resistor 36, whilst the gate of the Triac T is piloted by the signal from the sensor 10 inverted by the transistor Q2. In the various embodiments described above, as in some of those which will be described hereinafter, the generation of the DC supply voltage necessary for the sensor 10 associated with the rotor, and possibly for the biasing of other components, can conveniently be achieved by adopting the arrangement which will now be described with reference to Figure 7. In this Figure a rectifier/stabiliser circuit is generally indicated 16, the input 19 to which is connected to an intermediate tap 5c of the stator winding 5. This arrangement makes it possible particularly to contain the resistance value of the input resistor 20 of the supply circuit 16. The intermediate tap 5c is positioned in such a way as to allow a fraction of the supply voltage applied across the terminals 13 and 14 to be taken off. In Figure 8 is illustrated a further variant of a control device for a synchronous motor. In this embodiment the stator winding 5 is divided into two half-winding 5a and 5b with a central tap 5c connected to the supply terminal 14. In series with the half-windings 5a and 5b are connected respective Triacs T1 and T2. These Triacs have respective upper terminals connected to the supply terminal 13. The sensor 10 associated with the rotor 4 of the motor may also be for example a Hall-effect sensor the output of which is coupled to the gates of T1 and T2 via two circuit branches 11a and 11b of the piloting circuit 11, each comprising the same circuitry A in advance of which, in the branch 11a only, is positioned a logic inverter stage 41. The circuitry A in each of the branches of the piloting circuit 11 may have for example the structure of the circuit generally indicated 11 in Figure 3. The two half-windings 5a and 5b of the rotor winding are wound in opposite senses. The device of Figure 8 operates in a manner which will now be 'described with reference to the signals the timing diagram of which are shown in Figure 8a. In Figure 8a V M indicates the alternating supply voltage applied in operation between the terminals 13 and 14, H indicates an output signal from the sensor 10 associated with the rotor 4 of the motor. I2 and I1 indicate the currents flowing in the Triacs T1 and T2 (see also Figure 8). When the output from the sensor 10 is at high level the Triac T2 causes a current I2 constituted by the positive half-wave of the supply current, to flow in the half-winding 5b. At the same time the Triac T1 causes a current I1, corresponding to the negative half-wave of the supply current, to flow through the half-winding 5a. Conversely, when the signal H at the output from the sensor 10 is at low level, the current I2 flowing in the half-winding 5b is constituted by the negative half-wave of the supply current whilst the current I1 flowing in the half- winding 5a is constituted by the positive half-wave of the same supply current. Since the half-windings 5a and 5b are wound in opposite senses, the magnetic fluxes produced by the currents circulating in them are in concordance and sum, substantially as if these half-windings were wound in the same sense and had currents flowing in the same direction. With the above-illustrated arrangement both the positive half-waves and the negative half-waves of the supply current are successfully exploited for the purposes of generating torque. The various variant embodiments described up to now comprise a single position sensor associated with the rotor 5 of the synchronous motor and are affected by the (modest) disadvantage represented by the fact that the generation of negative torque can be generated for a brief time interval when the passage of the sensor output from low (high) level takes place in correspondence with a positive (negative) half-wave of the supply voltage. This disadvantage can be eliminated with the variant embodiments which will now be described with reference to Figure 9 and thereafter, in which in general two sensors are utilised for detecting the position of the rotor 4 of the motor. With reference to Figure 9, in a first embodiment the rotor 4 of the motor is associated with a member 42 rotatable fixedly with it. This member has for example the form of disc and has an opaque peripheral arcuate sector 43 extending over. a predetermined angular extent. The reference numerals 44 and 45 indicate two photodiode-photo transistor pairs disposed in respective positions fixed along the circular path of the opaque sector 43. This sector can pass between the photodiode and the phototransistor of each of these pairs during its rotating motion together with the rotor 4. When the rotor 4 of the motor rotates, the signals correspondingly provided by the phototransistors of the devices 44 and 45 exhibit a behaviour of the type illustrated in Figure 10a. If the devices 44 and 45 are disposed at 180° with respect to one another the signals P1 and P2 provided by respective phototransistor are phase-displaced by 180°. Each of these signals is at high level for a time period corresponding to the angular extent of the opaque sector 43. Figure 10 shows a first embodiment of a motor control device according to the invention utilising the rotor position detection system shown in Figure 9. In the device according to Figure 10, the controlled bidirectional conduction means 12 comprise two controlled rectifier diodes SCR1 and SCR2 disposed in antiparallel in series with the stator winding 5. The gates of SCR1 and SCR2 are piloted by the phototransistors of the photo-couplers 44 and 45 respectively. The opaque sector 43 associated with the photo-couplers 44 and 45 of the device of Figure 10 has an angular extent selected as a function of the desired running speed for the electric motor. This angular extent lies in particular between a minimum value corresponding to the condition that the torque generated should always be positive and a maximum value corresponding to the condition that the average torque generated should be positive. In the device of Figure 10 the photodiodes of the photo- couplers 44 and 45 are connected in antiparallel with one another, in series with a resistor 48, and in parallel with the controlled diodes SCR1 and SCR2. The resistor 48 (which, however, is not essential) allows the current in the said photodiodes to be conveniently limited in the case of mechanical jamming of the rotor 4 of the motor. The collectors of the phototransistors of the photo-couplers 44 and 45 can conveniently be connected to the gates of the corresponding controlled diodes SCR1 and SCR2 by means of resistors 46 and 47 so that when each of these phototransistors becomes conductive its collector-emitter voltage does not exceed the gate-cathode voltage of the associated controlled diode. The collector of the phototransistor of the photo coupler 45 is connected to the emitter of the phototransistor of the photo coupler 44 through a resistor 149. Similarly, a resistor 148 interconnects the collector of the phototransistor of the photo coupler 44 to the emitter of the photo coupler 45. Two resistors 150 and 151 are connected essentially in parallel to the collector-emitter path of the two photo- couplers 44 and 45. The resistor 150 and 151, the presence of which is not strictly necessary, are convenient to ensure the extinction of the SCR in all likely operating temperature ranges. The control device according to the invention can be used for starting a synchronous motor with high inertia for the purpose of bringing it to the synchronism speed. It further allows, via a suitable dimensioning of the stator winding, a synchronous motor to be operated at a different speed from the synchronism speed corresponding to the frequency of the alternating supply voltage. Naturally, the principle of the invention remaining the same, the embodiments and the details of construction can be widely varied with respect to what has been described and illustrated purely by way of non-limitative example, without by this departing from the ambit of the invention as defined in the attached claims.

Claims: A device for controlling the rotation of a synchronous electric motor (1) comprising a stator (2) including a winding (5) disposed around a stack (3) of laminations and having an associated permanent magnet rotor (4); the device comprising: controlled conductor means (12) connected to the stator winding (5) and to an AC source (13,14), position sensor means (10) operable to determine the application to the control inputs of the said controlled conductor means (12) of electrical control signals of two states which vary as a function of the angular position of the rotor (4); the arrangement being such that the current is caused to flow in the said stator winding (5) in a direction which depends only on the polarity of the voltage of the said source and the instantaneous position of the rotor (4), characterised in that for controlling the rotation of a synchronous motor (1) the controlled conductor means (12) comprise two controlled rectifiers (SCR1, SCR2) disposed in antiparallel with' one another and in series with the stator winding (5) between the terminals (13, 14) intended to be connected to the AC source; the control inputs of the said rectifiers being coupled to the position sensor means (10) via a piloting circuit (11) operable to apply to them a logic control signal corresponding to the signal of the position sensor means (10) and the inverse of this signal respectively; the position sensor means comprising two photodiode-phototransistor pairs (44, 45) disposed in respective fixed positions along the path of an interceptor member (42, 43) rotatable with the rotor (4) and having a predetermined angular extent, said piloting circuit (11) comprising the phototransistors of said photodiode-phototransistor pairs (44, 45), said phototransistors having their respective outputs coupled to the control inputs of said controlled rectifiers (SCR1, SCR2). A device according to Claim 1, in which the phototransistors of the said photodiode-phototransistor pairs (44, 45) are of npn type and are connected in antiparallel with one another between the terminals (13, 14) intended to be connected to said AC voltage source. A device according to Claim 2, in which the photodiodes of said photodiode-phototransistor pairs (44, 45) are connected in antiparallel with one another between said terminals (13, 14) intended to be connected to the AC voltage source. A device according to Claim 3, in which the said photodiodes are connected in series with current-limiting means (48). A device according to Claim 1 or 2, in which the collectors of said phototransistors are connected to the gates of said controlled rectifiers (SCR1, SCR2) through respective transistors (46, 47). A device according to Claim 5, in which respective resistors (150, 151) are connected essentially in parallel to the collector-emitter path of said phototransistors.

Inventors: ACQUAVIVA SEBASTIANO, ALVARO NICOLINO, ACQUAVIVA, SEBASTIANO, ALVARO, NICOLINO
Applicants: PLASET SPA, PLASET S.P.A.
Application year: 1997
Main classification: H02P6
IPC codes: H02P6, H02K29