METHOD OF CONTROLLING A THREE-PHASE INDUCTION MOTOR

Abstract: A method of controlling a three-phase induction motor in which a primary current is formed as a current command by taking all of the constants of an induction motor (d) into con­ sideration to obtain a linear output torque with respect to the torque command (Tm), and a voltage is detected at a DC link portion to change the excitation flux command based upon the DC voltage (V). Therefore, the output can be maintained constant irrespective of changes in the power source voltage. Provision is further made of a power factor adjusting means (q) to adjust the phases of the voltage and current of the AC power source side during the regenerative braking operation, so that the reactive power decreases.

Description: This invention relates to a three-phase induction motor control method for performing motor drive control and regenerative control. Preferably, but not essentially, the control is based on digital values. Recent control of three-phase induction motors is in many cases performed by a vector control method of controlling the instantaneous value of motor stator current and generating a torque equivalent to that of a shunt DC machine. Using vector control of this type to perform motor drive control and regenerative control of a three-phase induction motor is disclosed in e.g. Japanese Patent Application Laid-Open No. 59-14385. If an induction motor is subjected to vector control in cases where velocity control is performed up to a region of high rotational velocity as in the spindle motor of a machine tool or the like, or in cases where control is performed to weaken excitation in accordance with a torque command at a constant rpm, the torque command, excitation current, secondary current and slip frequency become non-linear owing to such effects as the secondary leakage impedence and core loss of the motor. As a result, the output torque also becomes non-linear with respect to the torque command, so that a linear relationship between the torque command and output torque cannot be accurately maintained. Furthermore, since the output of an induction motor generally varies in proportion to the square of the voltage impressed upon the motor, the output will fluctuate when the AC input voltage undergoes a large variation. In the vector control method, a measure devised for preventing such a fluctuation in output entails varying the amount of maximum slip in dependence upon the AC input voltage. Nevertheless, satisfactory results are not obtained in terms of holding the output of the induction motor constant. A further attempt at maintaining constant output when the AC input voltage fluctuates is disclosed in EP-A-0218729, which is relevant under Article 54(3) EPC. At time of regenerative braking, moreover, power-factor fluctuates due to the capacity of a phase advancing capacitor on the input side, and there is an increase in power supply distortion and reactive power. The present invention has been devised to solve the foregoing problems and its object is to provide a three-phase induction motor control method in motor drive control and regenerative control based on the vector control method using a power amplifier comprising a converter and an inverter, in which a linear output torque can be obtained with respect to a torque command without using a flux sensor, and in which the output of the three-phase induction motor can be held constant even when there are fluctuations in power supply voltage. According to the present invention, there is provided a three-phase induction motor control method for obtaining a linear torque command and performing constant output control irrespective of a fluctuation in power supply voltage, the method being based on vector control for converting an AC input into a direct current at the time of motor drive control to drive the three-phase induction motor at a variable velocity by the output of a first inverter to which the direct current resulting from the conversion is applied, and for converting an output current of the three-phase induction motor into a direct current at the time of regenerative control to regenerate the output of a second inverter, to which the direct current resulting from the conversion is applied, in an AC power supply, the method including vector analyzing an excitation current command and secondary current command into an excitation flux direction component and a motor electromotive force direction component, respectively, on the basis of a torque command with respect to the three-phase induction motor and an excitation flux command decided to accommodate the three-phase induction motor, determining a primary current command of the motor based on a synthesis of the vector components, detecting the voltage of a direct current link portion of a circuit for converting the AC input into a direct current, and correcting the excitation flux command in dependence upon the detected voltage. A development of the invention comprises adjusting a phase between voltage and current on the side of the AC power supply at the time of regenerative control, thereby performing control in such a manner that reactive power is reduced. Brief Description of the Drawings Fig. 1 is a block diagram of an embodiment for performing the present invention, Fig. 2 is a circuit diagram, Fig.3 is an equivalent circuit, Fig. 4 is a vector diagram, and Figs. 5 - 10 are explanatory views. The method of the invention will be described in conjunction with the drawings. Fig. 1 is a block diagram of a control circuit for practicing the method of the invention, and Fig. 2 is a circuit diagram of a power amplifier section shown in Fig. 1. Before describing the control circuit of Fig. 1, let us refer to Figs. 3 through 7 to describe the technique on which the present invention is premised. Fig. 3 is an equivalent circuit of one phase of a three-phase induction motor, Fig. 4 is a vector diagram, and Figs. 5, 6 and 7 are views for explaining characteristics. Vector control of a three-phase induction motor is performed as follows: (1) A velocity command ω c is set. (2) Motor velocity ω m is sensed. (3) A torque command T c is obtained by performing a subtraction between ω c and ω m . (4) An estimated value ω' s of slip is obtained as follows: ω' s = K₀ x T c x ω m (K₀ is a constant) (5) An estimated value ω'₀ of excitation frequency is obtained as follows: ω'₀ = ω m + ω' s (6) A flux Φ corresponding to the excitation frequency ω'₀ is obtained from the characteristic diagram of Fig. 5. (7) Winding resistance measurement, a no-load test and a lock test are carried out and values of primary induction voltage E₁ and core loss current I 0i of the excitation current I₀ are found from the equivalent circuit of Fig. 3, and values of magnetization current I 0M , excitation resistance R₀ and excitation reactance L₀ are found from the vector diagram of Fig. 4. (8) A revolving coordinate system is transformed into a fixed coordinate system, with the flux Φ of the revolving field serving as a reference phase. This is done as follows: 1 ○ Excitation current I ˙₀ is obtained as follows by taking the vector sum between a component I 0M along the Φ axis and a component I ˙ 0i along the E₁ axis: I ₀ = I 0M + I 0i 2 ○ Similarly, secondary current I ˙₂ is obtained as follows by taking the vector sum between a component I ˙ 2M along the Φ axis and a component I ˙₂ω along the E₁ axis: I ₂ = I 2M + I ₂ 3 ○ The flux Φ is obtained as Φ = L₀·I 0M from Next, Φ 0M = Φ/L₀ are obtained from the characteristic diagram of Fig. 6. (9) The E₁-axis component I 2w of secondary current I₂ is obtained from I 2w = (T c /Φ) (10) The slip frequency ω s is determined from the characteristic diagram of Fig. 7. (11) The Φ-axis component I 2M of the secondary current I₂ is obtained from I 2M = I 2w x ω s x (L₂/R₂) (12) The component of primary current I₁ in the Φ direction is obtained from I₁(Φ) = I 0M + I₂ (13) The component of primary current I₁ in the E₁ direction is obtained from I₁(E₁) = I 0i + I 2w The primary current so obtained is a current command for a case where all constants of the induction motor are taken into consideration, and is for the purpose of obtaining a linear output torque with respect to a torque command. (14) The excitation frequency ω₀ is obtained from ω₀ = ω m + ω s Let us return to the block diagram of Fig. 1 to describe the same. As shown in Fig. 2, a power amplifier a, which comprises a converter b and an inverter c respectively provided on AC input and DC output sides, is connected to an AC power supply. The converter b is constituted by a full-wave rectifier bridge composed of diodes, each of which is connected in parallel with a transistor inverter. A DC voltage is obtained by the converter b of the power amplifier a and is applied to the inverter c. The output voltage of the inverter c is pulse-width controlled by a PWM/current control circuit m before being impressed upon a three-phase induction motor d. The velocity command ω c and the motor velocity ω m , which is obtained, via a F/v converter o, from a voltage signal detected by a tachogenerator PG, are inputted to a comparator p to obtain the torque command T c as a difference signal voltage. The torque command T c is corrected via a PI and a clamping circuit g to form an actual torque command T m . Thereafter, the primary current I₁ and excitation frequency ω₀ are obtained as described above in (4) - (14). These are applied to a 2-to-3 phase converter circuit ℓ. The 2-to-3 phase converter circuit ℓ converts orthogonal two-phase currents into three-phase currents to form current commands Iu, Iv, Iw in the respective three phases, these commands being applied to the PWM/current control circuit m. The input currents to the motor are fed back to the circuit m by CT1, CT2 to be compared with the output currents of the 2-to-3 phase converter circuit, whereby the circuit delivers a commanded current I in each phase to the inverter c. The operation of Fig.1 will now be described in terms of detecting voltage Vdc on the DC side of the power amplifier and correcting the flux Φ and induced voltage V. If a triangular wave is formed within the PWM circuit, then the output voltage will differ depending upon the degree to which the triangular wave is utilized, considering the relationship between a voltage command amplitude and the actual terminal voltage of the motor. Let B represent the amplitude of the triangular wave, and let A represent the amplitude of the PWM signal, as shown in Fig. 8(a). If the amplitude ratio A/B is less than 1, then the output voltage V u from a point u in Fig. 9 is as follows: where T represents the period. The line voltage across u-v applied to the motor is where Vac: effective value Vdc: set value (DC voltage) Next, let us consider control by a base signal of the transistor constituting the inverter for a case where the amplitude ratio A/B is greater than 1. Though a method exists in which the base signal of Tr in the inverter is turned off whenever the amplitude ratio (A/B) exceeds 1, as shown in Fig. 8(b), this method involves certain problems, namely the fact that the output voltage will not rise, switching loss is great, etc. Fig. 10 illustrates the manner in which the line voltage V u-v varies with respect to the amplitude ratio A/B. In the present embodiment, all of the transistors Tr turn on, as shown in Figs. 8(c), (d), in a range where the amplitude A of the PWM signal exceeds the amplitude B of the triangular wave. In other words, when the DC voltage is low, the flux command Φ above a base velocity is increased, the amplitude of the PWM circuit with respect to the triangular wave is raised, and the terminal voltage of the motor is raised to obtain a constant output. When the DC current is high, the flux command is decreased from a region below the base velocity, the amplitude of the PWM command signal with respect to the triangular wave is reduced, and the terminal voltage of the motor is lowered to obtain a constant output. As shown in the block diagram of Fig. 1, such control is performed by inputting the DC voltage V dc sensed by the inverter to a data map i indicating the relationship between excitation frequency and flux, and correcting the flux Φ in dependence upon the DC voltage V dc . It should be noted that an induced voltage compensating circuit n corrects the value of induced voltage with respect to the motor velocity signal ω m and applies the correced value to the PWM/current control circuit m. The regenerative control operation of the present invention will now be described with reference to Fig. 2. A regenerative control circuit P is provided with a voltage control circuit r and a power-factor adjusting circuit g. When regenerative control is performed, voltage on the DC side of the power amplifier and the DC voltage command value are compared. When the voltage on the DC side is larger, an offset signal is inputted to the power-factor adjusting circuit g through the voltage control circuit r. The input voltage on the AC side of the power amplifier is applied to the power-factor adjusting circuit via the PT, and the regenerative current command signal is outputted to the current control circuit in such a manner that the currents on the AC input side of the power amplifier sensed by CT3, CT4 take on the same phase as the voltage on the AC side, i.e. in such a manner that the power-factor becomes 1. The PWM circuit controls the transistor inverter on the basis of this commanded value. Thus, in motor drive control and regenerative control based on the vector control method using a power amplifier comprising a converter and an inverter, the three-phase induction motor control method of the present invention enables a linear output torque to be obtained with respect to a torque command without using a flux sensor, and enables the output of the three-phase induction motor to be held constant even when there are fluctuations in power supply voltage.

Claims: A three-phase induction motor control method for obtaining a linear torque command and performing constant output control irrespective of a fluctuation in power supply voltage, the method being based on vector control for converting an AC input into a direct current at the time of motor drive control to drive the three-phase induction motor at a variable velocity by the output of a first inverter (c) to which the direct current resulting from the conversion is applied, and for converting an output current of the three-phase induction motor into a direct current at the time of regenerative control to regenerate the output of a second inverter (b), to which the direct current resulting from the conversion is applied, in an AC power supply, the method comprising: a step of vector analyzing an excitation current command (I₂) and a secondary current command (I₀) into an excitation flux direction component and a motor electromotive force direction component, respectively, on the basis of a torque command (Tm) with respect to the three-phase induction motor and an excitation flux command (Φ) decided to accommodate the three-phase induction motor; a step of determining a primary current command (I₁) of the motor based on a synthesis of said vector components; a step of detecting the voltage (V dc ) of a direct current link portion of a circuit for converting said AC input into a direct current; and a step of correcting the excitation flux command (Φ) in dependence upon said detected voltage. A three-phase induction motor control method according to claim 1, characterized by adjusting a phase between voltage and current on the side of the AC power supply at the time of regenerative control of said three-phase induction motor, thereby performing control in such a manner that reactive power is reduced. A three-phase induction motor control method according to claim 1, characterized in that the step of correcting said excitation flux command comprises turning on switching elements constituting said inverter at all times in a range where the amplitude of a PWM signal for the first inverter exceeds the amplitude of a triangular waveform.

Inventors: FUJIOKA, YOSHIKI, KOUNO, SHINICHI, SHINOHARA, TATSUO
Applicants: FANUC LTD.
Application year: 1986
Main classification: H02P5
IPC codes: H02P21, H02P27