CONTROL DEVICE FOR SYNCHRONOUS MOTOR

Abstract: An armature current-instruction (Is) in response to the velocity deviation of a synchronous motor (101) and the phase (ϑ of an induced voltage, which phase is measured with respect to the position of the field pole, are multiplied by each other in circuits (107), (108) and then current-­ instructions with the same phase as said phase (ϑ are produced through a 2 - 3 phase-converter circuit (109). The armature windings of the synchronous motor are then fed a current corresponding to the current instructions so that the synchronous motor is driven and controlled equivalently to a direct current motor.

Description: DESCRIPTION METHOD OF AND APPARATUS FOR CONTROLLING A SYNCHRONOUS MOTOR TECHNICAL FIELD The present invention relates to a method of and an apparatus for controlling a synchronous motor, and more particularly to a method of and apparatus for controlling a synchronous motor in which a rotating magnetic field is produced. BACKGROUND ART Synchronous motors with a rotating magnetic field include an armature as a stator and field poles as a rotor. By applying a three-phase AC voltage to the stator winding (armature winding), a rotating magnetic field is generated which attracts the field poles to cause the latter to rotate at the same speed as that of travel of the rotating magnetic field. As shown in Figs. l(a) and l(b), the mechanism of producing a torque in a DC shunt-wound machine is that an armature current Ia is switched by a commutator so as to flow in a direction normal to a main magnetic flux ° at all times. The torque T generated is expressed by the following equation, the torque T being proportional to the armature current Ia if the main magnetic flux ° is constant. T = k fIa (1) Designated in Fig. 1 at FM is field poles, AM an armature, and AW armature windings. If the foregoing relationship is applied to a synchronous motor with a revolving magnetic field as shown in Fig. 2, then the main magnetic flux ° corresponds to a vector fs of a main magnetic flux produced by field poles PM, and the armature current Ia to a vector Is of a current flowing through armature windings SW. A torque T generated by the synchronous motor is given by: T = k' s Is cos Y (2) where Y is the phase difference between an armature current Is and an induced voltage Eo in an equivalent circuit of the synchronous motor as shown in Fig. 3. Designated in Fig. 3 at Ya is the resistance of the armature windings SW, and Xs the synchronous reactance taking an armature reaction and an armature leakage flux into consideration. When the induced voltage Eo and the armature current Is are in phase with each other, that is, the main magnetic flux s and the armature current Is are controlled so as to be perpendicular to each other, the torque T as expressed by the equation (2) can be given as follows: T = k' fs Is (3) The synchronous motor can therefore be driven in a manner equivalent to generation of a torque by a DC motor. DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a method of and an apparatus for controlling a synchronous motor, by which the speed of rotation of the synchronous motor can easily be controlled, and instantaneous values of a stator current (armature current) can be controlled to produce a torque equivalent to that which is generated by a DC motor, for thereby effecting fine, highly responsive motor control. The above object can be achieved according to the present invention by detecting the phase of an induced voltage Eo from a main magnetic flux, that is, the positions of field poles on the basis of the fact that the induced voltage Eo is 90 degrees out of phase with the main magnetic flux, producing a current command having the same phase as that of the induced voltage, and applying the current command to armature windings of a synchronous motor for thereby controlling the latter to be driven in a manner equivalent to that for driving a DC motor. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a set of diagrams explanatory of the principles of torque generation in a DC shunt-wound machine; Fig. 2 is a diagram illustrative of a synchronous motor having a rotating magnetic field; Fig. 3 is a circuit diagram of an equivalent circuit of the synchronous motor; Fig. 4 is a block diagram of a circuit according to the present invention; Fig. 5 is a diagram illustrative of a resolver; Fig. 6 is a set of diagrams showing the waveforms of resolver outputs; Fig. 7 is a circuit diagram of a two-phase-to-threephase converter; Fig. 8 is a circuit diagram of a pulsewidth-modulation circuit; and Fig. 9 is a set of diagrams illustrating operation of the pulse-width-modulation circuit. BEST MODE FOR CARRYING OUT THE INVENTION For a fuller understanding of the present invention, an embodiment of the invention will now be described in detail with reference to the drawings. Fig. 4 is a block diagram of a circuit for carrying out a method of controlling a synchronous motor according to the present invention. Indicated at 101 is a synchronous motor with a rotating magnetic field, and 102 a resolver coupled to a shaft of the synchronous motor for detecting positions of field poles of the synchronous motor. As shown in Fig. 5, the resolver has a rotor 102a, a rotor winding 102b, two stator windings 102c, 102d disposed 900 out of phase with each other, and a carrier generator circuit 102e for generating a carrier of sin wt. When the rotor 102 is positioned at an angle e, the stator windings 102c, 102d produce voltages which are expressed respectively as follows: ea = sin 6. sin wt (4) eb = cos rosin wt (5) Therefore, the resolver 102 produce a sine voltage ea and a cosine.voltage eb dependent on the angular position e of the field poles of the synchronous motor 101. A synchronous rectifier circuit 103 rectifies the sine voltage ea and the cosine voltage eb in synchronism to generate sin 6 and cos e (Fig. 6). A tachometer generator 104 produces an actual-speed voltage TSA proportional to the speed of rotation of the synchronous motor 101. Designated at 105 is an arithmetic circuit for determining the difference ER (hereinafter referred to as a "speed error") between a speed command voltage VCMD commanded by a speed command circuit (not shown) and the actual-speed voltage TSA, 106 an error amplifier for amplifying the speed error ER to produce an amplitude Is of an armature current, 107, 108 multipliers for multiplying the output from the error amplifier with the outputs cos e, sine from the synchronous rectifier circuit 103 to produce two-phase current commands Ila (= Is sin e), Ilb (= Is cos 6), and 109 a two-phase-to-three-phase converter for converting a two-phase signal into a three-phase signal. As illustrated in Fig. 7, the two-phaseto-three-phase converter 109 has two operational amplifiers OAl, OA2, resistors R1 through R4 of 10 K n, a resistor R5 of 5.78 Kfl, and a resistor R6 of 5 Kn. With the resistors R1 - R6 having such resistances and wired as shown, the following outputs are available from terminals Tu, Tv, Tw: EMI6.1 &commat; The outputs Iu, Iv, 1w are 2p/3 out of phase with each other, and constitute three-phase current commands in phase with the induced voltage Eo. Indicated at 110U, llOV, 110W are arithmetic circuits in the respective phases for determining the differences v between the command currents Iu, Iv, 1w and actual phase currents Iau, Iav, law, 111 an arithmetic circuit for adding e Iav and law to produce the phase current Iau of a phase U, 112V, 112W current transformers for detecting the phase currents Iav, law of phases V, W, respectively, 113U, 113V, 113W current amplifiers in respective phases for amplifying current differences in the phases, 114 a pulse-width modulation circuit, 115 an inverter controllable by an output signal from the pulse-width-modulation circuit, 116 a three-phase AC power supply, and a known rectifier circuit 117 for rectifying three-phase alternating currents into a direct current, the rectifier circuit being composed of a group of diodes 117a and a capacitor 117b. As illustrated in Fig. 8, the pulse-width-modulation circuit 114 comprises a sawtooth generator STSG for generating a sawtooth signal STS, comparators COMu, COMv, COMw, NOT gates NOT1 - NOT3, and drivers DV1 - DV6. The inverter 115 has six power transistors Q1 - Q6 and six diodes D1 - D6. The comparators COMu, COMv, COMw in the pulse- width-modulation circuit 114 compare the amplitudes of three-phase AC signals iu, iv, iw with the amplitude of the sawtooth signal STS, and delivers an output of "1" when the signals iu, iv, iw are larger than the signal STS, and delivers an output of 0" when the signals iu, iv, iw are smaller than the signal STS. For example, the comparator COMu for the signal iu produces a current command iuc as shown in Fig. 9, which is subjected to pulse width modulation. Therefore, three-phase current commands iuc, ivc, iwc are generated dependent on the amplitudes of the signals iu, iv, iw by way of pulse width modulation. The current commands iuc, ivc, iwc are converted by the NOT gates NOT1 - NOT3 and the driver circuits DV1 DV6 into drive signals SQ1 - SQ6, which serve to turn on and off the power transistors Q1 - Q6 in the inverter 115. Operation of the embodiment shown in Fig. 4 will now be described with reference to an application in which a speed command is increased when the synchronous motor 101 rotates at a certain speed. To rotate the synchronous motor at a desired speed Vc, an addition terminal of the arithmetic circuit 105 is supplied with a speed command voltage VCMD having a preset analog value. The synchronous motor rotates at an actual speed Va (( Vc) at this time, and the tachometer generator 104 produces as an output an actual-speed voltage TSA proportional to the actual speed Va, the actual-speed voltage TSA being applied to a subtraction terminal of the arithmetic circuit. The arithmetic circuit determines a speed error ER which is the difference between the command speed vc and the actual speed Va, and supplies the speed error ER to the error amplifier 106, which then effects an arithmetic operation of proportional integration as expressed by the following equation: EMI8.1 The result Is in the equation (7) corresponds to the amplitude of the armature current. Accordingly, when the load varies or the speed command changes, the speed error ER (= Vc - Va) becomes greater, and the armature current amplitude Is also becomes greater. The increased armature current amplitude results in a greater torque produced, which brings the actual speed of rotation of the motor into conformity with the command speed. The two-phase sine and cosine waves sin e, cos e which are indicative of positions (at the angle e) of the field poles of the synchronous motor are produced by the resolver 102 and the synchronous rectifier circuit 103. The multiplier circuits 107, 108 carry out the following arithmetic operations: Ila = Is sin e, Ilb = Is cos e, and produce two-phase current commands Ila, Ilb. Then, the two-phase-to-three-phase converter circuit 109 effects the arithmetic operations as defined by the equations (6) and generate three-phase current commands lu, Iv, lw, respectively, which are in phase with the induced voltage Eo in the synchronous motor 101. Thereafter, the arithmetic circuits 11do, ll0V, 110W determine the differences between the three-phase current commands Iu, Iv, 1w and actual phase currents Iau, Iav, law. Three-phase AC signals iu, iv, iw indicative of such differences are amplified by the current amplifiers 113U, 113V, 113W and applied respectively to the comparators COMu, COMv, COMw in the pulse-width-modulation circuit 114. The comparators COMu, COMv, COMw compare the amplitudes of the three-phase AC signals iu, iv, iw with the amplitude of the sawtooth signal STS, and deliver as outputs three-phase current commands iuc, ivc, iwc produced by way of pulse width modulation. The outputs from the comparators are supplied to the NOT gates NOT1 - NOT3 and the drivers DV1 - DV6, which generate inverter drive signals SQ1 - SQ6. The inverter drive signals SQ1 - SQ6 are supplied to the bases of the power transistors Q1 - Q6, respectively, in the inverter 115 to turn on and off the power transistors Q1 - Q6 for supplying three-phase currents to the synchronous motor 101. The synchronous motor 101 will subsequently be controlled in a similar mode until the motor will eventually rotate at the command speed. According to the present invention, as described above, a synchronous motor can be controlled in a manner equivalent to that of controlling a DC motor by detecting the phase of an induced voltage Eo in the synchronous motor, generating a current command having such a phase, and applying the current command to armature windings of the synchronous motor. INDUSTRIAL APPLICABILITY The present invention effects momentary control of an armature current to generate a torque through a mechanism equivalent to that of torque generation by a DC motor, for fine, highly responsive control of the speed of rotation of a synchronous motor, which can be driven for synchronous operation at all times without step-out or operation out of synchronism even when a load torque changes. The present invention is applicable to operation of a variety of loads which should be rotated at accurate speeds.

Claims: WHAT IS CLAIMED IS: 1. A method of controlling a synchronous motor having a rotating magnetic field, comprising detecting the phase of an induced voltage in the synchronous motor, generating a current command having a phase which is the same as that of the induced voltage, and passing an armature current in phase with the induced voltage through armature windings. 2. A method according to claim 1, wherein the phase of said induced voltage is produced by detecting rotational positions of field poles. 3. A method according to claim 1, wherein the phase of said induced voltage is produced by two-phase position detecting means for generating sin e, cos 6 where 6 is indicative of the rotational positions of the field poles. 4. An apparatus for controlling a synchronous motor having a rotating magnetic field, comprising a DC voltage source, an inverter interposed between the synchronous motor and the DC voltage source for supplying three-phase AC currents to armature windings of the synchronous motor, an error amplifier for amplifying the difference between a speed command voltage and an actual-speed voltage, two-phase position detecting means for generating a sine wave sin 6 and a cosine wave cos e dependent on positions e of field poles of the synchronous motor, multiplier circuits for multiplying an output from said error amplifier with sin e , cos e, a two-phase-to- three-phase converter for converting a two-phase output from the multiplier circuit into a three-phase signal, current amplifiers for amplifying the differences between three-phase current commands which are the output from said two-phase-to-three-phase converter and phase currents in the synchronous motor, and a pulse-width-modulation circuit for modulating outputs from said current amplifiers by way of pulse width modulation, the arrangement being that transistors constituting said inverter can be turned on and off under the control of an output from said pulse-widthmodulation circuit to pass a current in phase with an induced voltage for thereby bringing an actual speed into conformity with a command speed.

Inventors: ISHIDA, HIROSHI
Applicants: FANUC LTD
Application year: 1982
Main classification: H02P5
IPC codes: H02P25, H02P21, H02P27, H02P6