METHOD AND ELECTRONIC CIRCUIT FOR STARTING A RELUCTANCE MOTOR AND RELUCTANCE MOTOR PROVIDED WITH SUCH AN ELECTRONIC CIRCUIT

Abstract: A reluctance motor comprises a stator (S) provided with coils (W1, W2), and a rotor (R). The rotor R has pairs of positions of stable equilibrium and pairs of positions of unstable equilibrium. The reluctance motor is started by first bringing the rotor into a first position of stable equilibrium (ϑ=-ϑms) by means of a first energising current and subsequently rotating it towards a second (ϑ=0) of stable equilibrium by means of a second energising current.

Description: The invention relates to a method of starting a reluctance motor having a rotor, a stator and an energising coil by means of a suitable energising current applied to the energising coil, the rotor and/or the stator being constructed in such a way that the rotor has pairs of positions of stable equilibrium, each constituted by a first and a second position of stable equilibrium and corresponding pairs of positions of unstable equilibrium, each constituted by a first and a second position of unstable equilibrium. The invention also relates to an electronic circuit and a reluctance motor. EP 0,105,851 (herewith incorporated by reference) describes a method of starting a single-phase reluctance motor by applying current pulses to the stator windings. The amplitude and duration of the pulses are selected in such a way that periodically a part of the stator is magnetically saturated and the rotor oscillates about a position of equilibrium with increasing amplitude until it is set into rotation. A disadvantage of this known method is that starting of the motor proceeds comparatively slowly, inter alia because current pulses with a repetition rate of 1 Hz are utilized. Further, EP 0,108,732 (herewith incorporated by reference) also discloses a method of starting a reluctance motor. This known method is based on the use of a position indicator to indicate when the rotor of the reluctance motor to be started is in areas of positive torque. The position indicator is connected to a pulse generator via a logic circuit to activate the pulse generator on start. The position indicator is an indispensable element for carrying out the known method. It is an object of the invention to improve the known methods of starting a reluctance motor and to this end the invention provides a method of the type defined in the opening paragraph, which is characterized by the steps of applying, independently of the initial position of the rotor, a programmed first energising current which is controlled in such a way that during a first pre-determined interval it increases to a second value for which at least a part of the rotor and/or the stator is saturated via a first value for which no saturation occurs in the rotor and/or the stator, during a second predetermined time interval it is sustained in at least the second value for maintaining saturation in the rotor and/or the stator and during a further predetermined time interval it is reduced towards a third value for which no saturation occurs in the rotor and/or the stator, in order to bring the rotor to or near one of the two positions of stable equilibrium of one of said pairs and to stabilise the rotor in this position of stable equilibrium, and subsequently applying an impulse-like second energising current of a value greater than the second value of the first energising current in order to rotate the rotor towards the nearest other of said two positions of stable equilibrium. This enables the rotor to be brought into a desired initial position by means of a single impulse-like signal and subsequently to be started in the desired direction of rotation. Therefore, the starting process of the motor can be very short. The inventive detectorless method renders the reluctance motor better suited for use in appliances which have to be started frequently and which require a short stating time, such as vacuum cleaners and fans. It is to be noted that the motor always rotates in the same direction after starting. The method in accordance with the invention may be further characterized in that increasing the first energising current to the second value via the first value and subsequently reducing it to the third value is effected gradually in time. In accordance with the invention the rotor can be brought into the desired initial position in a very controlled manner if the first value of the first energising current is maintained for some time. The invention also provides an electronic circuit for supplying a suitable first energising current during starting in accordance with the inventive method, which circuit is provided with means for switching on the current through the energising coil and is characterized in that it comprises means for determining time intervals, which means supply signals to and up/down counter, to reset the counter at the beginning of a first time interval, cause the counter to count up for the first time interval, hold the counter at a first count for a second time interval and at a second count for a fourth time interval, cause the counter to count down for a third time interval, and set the counter to a preset count upon termination of a fourth time interval, which counter supplies a binary output signal, means for generating a periodically incremented binary signal, and means for comparing the value of the binary output signal of the counter with the value of the periodically incremented signal, the means for switching on said current when the value of the binary output signal of the counter is larger than the value current when the value of the binary output signal of the counter is larger than the value of the periodically incremented signal and switching off said current when the value of the binary output signal of the counter is smaller than the value of the periodically incremented signal. The invention further provides a reluctance motor comprising a stator, a rotor, an energizing coil and an electronic circuit for supplying an energising current to the coil and provided with means for switching on the current through the energising coil, which motor is provided with a magnetically saturable area so that the rotor has pairs of first and second positions of stable equilibrium and corresponding pairs of first and second positions of unstable equilibrium, which motor is characterized in that the electronic circuit comprises means for determining first, second, third, and fourth time intervals and having an output at which signals are supplied, an up/down counter, responsive to the signals supplied by the means for determining the time intervals, which counter is reset at the beginning of the first time interval, counts up during the first time interval, holds a first count during the second time interval, counts down during the third time interval, holds a second count during the fourth time interval, and is set to a preset count upon termination of the fourth time interval, the counter supplying a binary output signal, means for generating a periodically incremented binary signal, and means for comparing the value of the binary output signal of the counter with the periodically incremented signal, the means for switching on the energizing current switching on said current, when the binary output signal of the counter is larger than the value of the periodically incremented signal, and switching off the energizing current, when the binary output signal of the counter is smaller that the periodically incremented signal, so that the rotor is first brought to or near one of the first and second positions of one of the pairs of positions of stable equilibrium and subsequently the rotor rotates toward the other of the first and second positions of the one of the pairs of stable equilibrium. The invention will now be described in more detail, by way of example, with reference to the drawings, in which: Figure 1 shows diagrammatically a reluctance motor; Figure 2 shows the torque as a function of the angular rotation of the rotor of the reluctance motor, which torque is exerted on the rotor for two different energising currents; Figures 3a and 3b show the energising currents as a function of time during starting of the motor in accordance with preferred embodiments of the inventive method; and Figure 4 shows the basic diagram of a circuit arrangement by means of which the inventive method can be carried out. The methods in accordance with the invention will be described for a single-phase reluctance motor comprising two stator-teeth, as shown in Figure 1. However, the invention is equally applicable, for example, to two-phase reluctance motors and reluctance motors comprising a multi-pole rotor and/or stator. The invention can also be used with a reluctance motor comprising a rotor of a cross-section in accordance with EP 0,108,732 or a reluctance motor having a stator teeth cross-section in accordance with EP 0,105,851. The rotor R of the motor shown in Figure 1 comprises a lamination assembly mounted on a shaft. The stator S is also laminated. Series-connected energising coils W1 and W2 are arranged around the stator teeth. The direction of the currents in these coils are indicated schematically. A current I is applied to the coils W1 and W2. The current I through the coils W1 and W2 can be controlled by means of a single switch (not shown). When the coils are energised with a sufficiently large current in the rotor position indicated in solid lines a torque T is developed in the indicated direction, causing the rotor to rotate about an axis of rotation A. When the rotor passes through the vertical position (line ϑ=0) the torque will become negative and act in the other direction. Therefore, the said switch is suitably opened before the vertical position is reached. The switch is closed again when the rotor approaches the horizontal position etc. In practical embodiments the switch will be an electronic switch controlled by an electronic circuit or a suitably programmed microprocessor. The stator teeth comprise projections A1 and A2 to define, as will become apparent hereinafter, a position of stable equilibrium for the rotor from which the motor can be started. The rotor radius Rr has a constant value, whilst the radius of the stator Rs (ϑs) is variable, as can be seen in Figure 1. The value of the stator radius is a minimum near the projections A1 and A2 of the stator teeth S1 and S2 and a maximum at the sides A3 and A4 of the stator teeth. As a result of the projections A1 and A2 and the varying stator radius a well-defined position of stable equilibrium of the rotor is reached when the projections A1 and A2 are not magnetically saturated. This is the position shown in the drawings, the angular position ϑ of the rotor then being -ϑms. A major problem with the present type of motor is starting from an arbitrary position, such as the two positions of unstable equilibrium of the motor as indicated in broken lines. In accordance with the invention this problem is solved by the method described hereinafter. Assuming that a current I=Im flows through the coils W1 and W2 of the motor shown in Figure 1, Im having such a value that the areas A1 and A2 are not yet saturated. For this value of the current I the preferred position of the rotor is now given by the angle ϑ=-ϑms + k.π, where k=0,1,2,... This is the position of stable equilibrium of the rotor for the value Im (the position shown). When the rotor is brought out of this position of stable equilibrium by a small angular rotation (for example < π / 8 rad) and is subsequently released the rotor will return to the area around this position of stable equilibrium if I=Im. The position of unstable equilibrium is given by the angle ϑ=ϑm1 + k.π, where k=0,1,2,... When for the value Im of the energising current the rotor is brought to the unstable position related to this current outside this angle the rotor will rotate towards the position of stable equilibrium. If the energising current I is now increased to a value Ih, for which the areas A1 and A2 are saturated, the position of stable equilibrium of the rotor is given by the angle ϑ=0+kπ, where k=0,1,2,... The corresponding position of unstable equilibrium is given by the angle ϑ=π/2+kπ, where k=0,1,2,... The torques for the two values of the current Im and Ih as a function of the angular rotation ϑ of the rotor vary as shown in Figure 2, the torques being indicated by T(ϑ)Im and T(ϑ)Ih respectively. It is assumed that in the stationary condition the load of the motor exerts a maximum torque on the motor given by Tf and -Tf. When in the stationary condition the rotor position is situated outside the range indicated by Lm, the rotor will move towards the range Gm or G′m (the desired starting position) in the case of a current Im. However, if in the stationary condition the rotor position is situated within the range Lm, the rotor remains stationary because within this range the current Im cannot produce a torque larger than Tf. To set the rotor in motion the current is subsequently increased to Ih, causing the rotor to move to a position in or near the range Gh (or G'h). Subsequently, the current is reduced to Im, causing the rotor to assume a position within said range Gm (or G'm). This is the desired starting position. The motor can always be started from this position. From this position a current pulse (Is ≧ Ih) will always cause the rotor to rotate in the same direction, in the present example clockwise, so that the starting problem is solved. Figures 3a and 3b show how the current I may vary as a function of time during starting of the motor for two versions of the inventive method. The current I must be reduced when the rotor approaches the vertical position (0 ≈ k.π), where k=0, 1,2, ... as is indicated in Figures 3a and 3b. Figure 3a shows how the current preferably varies when the motor is switched on. In a first time interval between the instant t=0 and the instant t=t1 a first energising current increases to a value Ih via a first value Im, the portions A1 and A2 of the stator being magnetically saturated. This value Ih of the current is sustained in a second time interval t1-t2 and, subsequently, in a third time interval t2-t3 it is reduced to a value which may be equal to Im. During a time interval t3-t4 this value of the current is maintained again. At the end of this fourth time interval the rotor will assume the position shown in solid lines in Figure 1 and the motor can always be started by applying an impulse-like second energising current to the stator winding, which current has a value Is larger than the value Ih. This current is preferably terminated before the rotor is in line with the stator teeth. Subsequently, the rotation of the motor can be sustained in a manner known per se . Figure 3b shows how the current I varies during starting as an alternative to the situation of Figure 3a. Now the first energising current is maintained at a value Im for some time before it is increased to a value Ih and subsequently reduced to a value Im'. Since the current is maintained at the value Im for some time, the rotor is allowed to stabilise at the assumed position of equilibrium, so as to prevent overshooting. The values Im and Im' may differ but may also be equal. This applies also to the values Im and Im' indicated in Figure 4 to be described hereinafter. The methods described can be carried out by means of an electronic circuit which can supply a first energising current in accordance with Figures 3a or 3b, and which can also produce the other current impulses required in operation. For this purpose a programmed microprocessor may be employed which is capable of supplying the signals necessary for generating the energising currents in accordance with, for example, Figures 3a and 3b. Figure 4 shows an electronic circuit by means of which the methods described can, in principle, be carried out. In principle, the circuit shown in Figure 4 is capable of supplying the first energising current shown in Figure 3a. Therefore, Figure 4 is described in relation to Figure 3a. In Figure 4 CL1 is a clock generating a clock signal which is applied to a counter C and to a count input CC of an up/down counter UDC. The output signal of the counter C, which may be for example a 8-bit signal, is applied to comparators C1, C2, C3, C4 and C5. The output signals of the comparators C1 to C5 are applied to the RESET input R, the count-up input U, the HOLD input H, the count-down input D and the SET input S respectively for setting the up/down counter UDC to a specific count. The output signal of the comparator C5 is also applied to a circuit N which in normal operation supplies the control signal for the energising current in the motor. A binary output signal I1 of the up/down counter UDC is applied to a comparator C6, to which also a binary output signal I2 of a clocked counter CL2 is applied. The binary signals I1 and I2 may be, for example, 8-bit signals. The clocking frequency of the clocked counter CL2 must be high relative to the frequency of the clock CL1. When the value of the binary signal I1 is larger than the value of the binary signal I2, the SET input of a flip-flop FF is energised and the flip-flop supplies an output signal, causing the switch for energising the coils to be closed and energising current to be applied to the coils. When the binary value of the signal I1 is smaller than the value of the signal I2 the signal on the SET input S' of the flip-flop FF becomes logic 0 and the RESET input R′ of the flip-flop FF becomes logic 1, causing the switch to be opened, so that no current is applied to the energising coils of the motor. During normal operation the circuit N supplies the control signal for the switch of the motor under control of a suitable signal T, which is generated for example by photo-electric means to produce pulses having a specific amplitude at the appropriate instants. At the instant t=0 the comparator C1 supplies a RESET signal to the input R of the counter UDC, so that in fact it functions as a starting circuit. In the interval 0-t1 (see Figure 3a) the comparator C2 supplies a signal to the input U of the counter UDC, so that the counter counts up. In the intervals t1-t2 and t3-t4 the comparator C3 supplies a signal to the input H of the counter UDC to hold the counter at the count reached at the instants t1 and t3 respectively. In the interval t2-t3, the comparator C4 supplies a signal which causes the counter UDC to count down, which signal is applied to the input D. In the time interval after the instant t4 the comparator C5 supplies a signal to the SET input S of the counter UDC for setting the counter to a preset count. This signal is also applied to the circuit N. If the clock frequency of the clocked counter CL2 is sufficiently high relative to the frequency of the clock CL1, for example a factor of 10 higher, the current through the energising coils is switched in accordance with the pulse-width modulation principle. In the interval 0-t1 the current through the energising coils is continually switched on and off, the on-interval increasing relative to the off-interval until the instant t1. As a result of the presence of a freewheel circuit the current through the motor coils will oscillate about the linearly increasing value in accordance with Figure 3a until the instant t1. In the interval t1-t2 the current through the coil will then oscillate about the constant value Ih, in the interval t2-t3 the current through the coils will then oscillate about the gradually decreasing value of the current, and in the interal t3-t4 the on-interval of the current through the motor coils will be constant relative to the intervals in which the current is switched off (this is also the case for the interval t1-t2), so that in the interval t3-t4 the current will oscillate about the value Im. Variants of the circuit described are possible, for example, it is possible to apply the output signal of the up/down counter UDC to a digital-to-analog converter and to control the current through the energising coils by means of the analog signal thus obtained.

Claims: A method of starting a reluctance motor having a rotor (R), a stator (S) and an energising coil (W1, W2) by means of a suitable energising current applied to the energising coil, the rotor and/or the stator being constructed in such a way that the rotor has pairs of positions of stable equilibrium, each constituted by a first and a second position of stable equilibrium and corresponding pairs of positions of unstable equilibrium, each constituted by a first and a second position of unstable equilibrium, characterized by the steps of applying, independently of the initial position of the rotor, a programmed first energising current which is controlled in such a way that during a first predetermined interval it increases to a second value (Ih) for which at least a part (A1, A2) of the rotor and/or the stator is saturated via a first value (Im) for which no saturation occurs in the rotor and/or the stator, during a second predetermined time interval it is sustained in at least the second value for maintaining saturation in the rotor and/or the stator and during a further predetermined time interval it is reduced towards a third value (Im; Im') for which no saturation occurs in the rotor and/or the stator, in order to bring the rotor to or near one of the two positions of stable equilibrium of one of said pairs and to stabilise the rotor in this position of stable equilibrium, and subsequently applying an impulse-like second energising current of a value (Is) greater than the second value of the first energising current in order to rotate the rotor towards the other of said two positions of stable equilibrium. A method as claimed in Claim 1, characterized in that increasing the first energising current to the second value (Ih) via the first value (Im) and subsequently reducing it to the third value (Im; Im') is effected gradually in time. A method as claimed in Claim 1 or 2, characterized in that the first value (Im) of the first energising current is maintained for some time. An electronic circuit for supplying the first energising current in accordance with a method as claimed in Claim 1, 2 or 3, provided with means (FF) for switching on the current through the energising coil, characterized in that it comprises means (CL1, C, C1-C5) for determining time intervals (0-t1, t1-t2, t2-t3, t3-t4 and t>t4), which means supply signals to an up/down counter (UDC) to reset (R) the counter at the beginning (t=0) of a first time interval, cause the counter to count up (U) for the first time interval (0=t1), hold (H) the counter at a first count for a second time interval (t1-t2) and at a second count for a fourth time interval (t3-t4), cause the counter to count down (D) for a third time interval (t2-t3), and set the counter to a preset count (S) upon termination of a fourth time interval (t3-t4), which counter supplies a binary output signal (I1), means (CL2) for generating a periodically incremented binary signal (I2), and means (C6) for comparing the value of the binary output signal (I1) of the counter (UDC) with the value of the periodically incremented signal (I2), the means (FF) for switching on the current switching on said current when the value of the binary output signal of the counter is larger than the value of the periodically incremented signal (I1>I2) and switching off said current when the value of the binary output signal (I1) of the counter is smaller than the value of the periodically incremented signal (I1<I2). A reluctance motor comprising a stator (S), a rotor (R), an energizing coil (W1, W2) and an electronic circuit for supplying an energising current to the coil and provided with means (FF) for switching on the current through the energising coil, which motor is provided with a magnetically saturable area (A1; A2) so that the rotor has pairs of first and second positions of stable equilibrium and corresponding pairs of first and second positions of unstable equilibrium, characterized in that the electronic circuit comprises means (CL1, C, C1-5) for determining first (0-t1), second (t1-t2), third (t2-t3), and fourth (t3-t4) time intervals and having an output at which signals are supplied, an up/down counter (UDC), responsive to the signals supplied by the means for determining the time intervals, which counter is reset (R) at the beginning of the first time interval, counts up during the first time interval, holds (H) a first count during the second time interval, counts down during the third time interval, holds a second count during the fourth time interval, and is set to a preset count (S) upon termination of the fourth time interval (t3-t4), the counter supplying a binary output signal (I1), means (CL2) for generating a periodically incremented binary signal (I2), and means (C6) for comparing the value of the binary output signal (I1) of the counter with the periodically incremented signal (I2), the means (FF) for switching on the energizing current switching on said current, when the binary output signal of the counter is larger than the value of the periodically incremented signal, and switching off the energizing current, when the binary output signal of the counter is smaller that the periodically incremented signal, so that the rotor is first brought to or near one of the first and second positions of one of the pairs of positions of stable equilibrium and subsequently the rotor rotates toward the other of the first and second positions of the one of the pairs of stable equilibrium.

Inventors: COMPTER, JOHAN CORNELIS, HEIJMANS, PETER MICHEL SILVESTER MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H02P7
IPC codes: H02P6, H02K19, H02P1