METHOD OF CONTROLLING A THREE-PHASE INDUCTION MOTOR

Abstract: A method of controlling a three-phase induction motor wherein an ac input is converted into a direct current, and a three-phase induction motor (d) is vector-controlled by the output of an inverter (c) which is served with the above con­ verted direct current. A voltage (Vdc) is detected at a dc link portion converted into a direct current, and a magnetic flux instruction () is changed depending upon the detected dc voltage, so that the motor will produce a constant output irrespective of fluctuations of the power source voltage.

Description: This invention relates to a three-phase induction motor control method. Recent control of three-phase induction motors is in many cases performed through either of the following methods: (1) a method of controlling the frequency and voltage of the motor power supply by an inverter, and (2) 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. 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, irrespective of whether the inverter method or vector control method is employed. This is true for example in the case of EP-A-0214301, which is relevant under Art.54(3)EPC and in which a flux or excitation command is produced in dependence upon a value of flux excitation frequency. Countermeasures devised in order to prevent such a fluctuation in output include varying pulse width in dependence upon the AC input voltage in the inverter method and varying the amount of maximum slip in dependence upon the AC input voltage in the vector control method. Nevertheless, satisfactory results are not obtained in terms of holding the output of the induction motor constant. The present invention has been devised to solve the aforementioned problem of the prior art and its object is to provide a three-phase induction motor control method in which output is controlled so as to be substantially constant, even if there is a fluctuation in power supply voltage, by sensing a DC voltage, which is converted from an AC input, as the voltage of a DC link section, and varying a flux command to deal with this voltage. According to the present invention there is provided a three-phase induction motor control method according to the attached claim. Thus, in accordance with the present invention, the output of the induction motor is controlled so as to be substantially constant in an ordinary vector control-type control circuit, even when there is a variation in the AC input voltage. Preferably, the output of the induction motor can be held substantially constant even if the input voltage fluctuates by ±10% of the rated input voltage. Brief Description of the Drawings Fig. 1 is a block diagram of an embodiment for performing the present invention, Fig. 2 is an equivalent circuit, Fig. 3 is a vector diagram, and Figs. 4 through 9 are views for explaining characteristics. One example of the present invention will now be described in detail with reference to the drawings. Fig. 1 is a control block diagram in which the method of the present invention is applied to vector control of a three-phase induction motor pulse-width controlled by a PWM circuit. Before describing the arrangement shown in Fig. 1, let us refer to Figs. 2 through 6 to describe the technique on which the present invention is based. Fig. 2 is an equivalent circuit of one phase of a three-phase induction motor, Fig. 3 is a vector diagram, and Figs. 4 and 5 are views for explaining characteristics. Vector control of a three-phase induction motor is performed in accordance with an operating sequence of the following kind: (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. 4. (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. 2, and values of magnetization current I 0M , excitation resistance R₀ and excitation reactance L₀ are found from the vector diagram of Fig. 3. (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. More specifically, 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 Similarly, secondary current I₂ is obtained as follows by taking the vector sum between a component I ˙ 2M along the I axis and a component I ˙₂ω along the E₁ axis: I ₂ = I 2M + I ₂ω The flux Φ is obtained as Φ = L₀· I 0M from Therefore, the Φ-axis component I ˙ 0M of excitation current I ˙₀ and the El-axis component I ˙ 0i are obtained from the characteristic diagram of Fig. 5 and from I 0M = Φ/L₀ I0i = Kω₀Φ (K is a constant) (9) The E₁-axis component I ˙ 2w of secondary current I ˙₂ is obtained from I 2w = (T c /Φ) (10) The slip frequency ωs is determined from the characteristic diagram of Fig. 6. (11) The Φ-axis component I ˙ 2M of the secondary current I ˙₂ is obtained from I 2M = I 2w x ω s x (L₂/R₂) (12) The component of primary current I ˙₁ in the Φ direction is obtained from I₁(Φ) = I 0M + I 2M (13) The component of primary current I ˙₁ in the E₁ direction is obtained from I₁(E₁) = I 0i + I 2w (14) The excitation frequency ω₀ is obtained from ω₀ = ω m + ω s Returning to the block diagram of Fig. 1, let us describe circuitry for controlling a motor d. A rectifier circuit a is connected to an AC power supply to obtain a DC voltage applied to an inverter c. A DC voltage sensing circuit b, the details of which are described below, is connected to this DC link circuit. The output voltage of the inverter c is pulse-width controlled by a PWM circuit/current control circuit m before being impressed upon a three-phase induction motor d. The velocity command ω c and the induction motor velocity ω m , which is obtained, via a F/V converter o, from a voltage signal detected by a tachogenerator (PG) e, 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 proportional integrator (PI) f and a clamping circuit g to form an actual torque command T m . Thereafter, the primary current I₁ and excitation frequency ω₀ are outputted on the basis of the above-described operating sequence (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 circuit/current control circuit m. The input current signals to the motor d are fed back to the circuit m by CT1, CT2 to be compared with the current commands Iu, Iv, iw outputted by the 2-to-3 phase converter circuit ℓ, whereby the circuit m delivers a commanded current I in each phase to the inverter c. Described next will be the operation of the DC voltage sensing circuit b, which is a characterizing feature of the present invention. If a triangular wave is formed by a triangular wave generating circuit, not shown, within the PWM circuit and the triangular wave is applied to the circuit m, then the output voltage of the PWM circuit/current control circuit m will differ depending upon the degree to which the triangular wave is utilized, considering the relationship between a voltage command and the actual terminal voltage of the motor. Let B represent the amplitude of the triangular wave applied to the PWM circuit/current control circuit m, and let A represent the amplitude of the PWM command signal, as shown in Fig. 7(a). If the amplitude ratio A/B is less than 1, then the output voltage V u from a point u produced by turning on one transistor of the inverter c, as shown in Fig. 8, is as follows: V u = (T ON1 /T) x √2V ac {(1+sinϑ)/2} where T represents the period of the triangular wave. The line voltage across u-v applied to the motor d is V u-v = (T ON1 ) x √2V ac x {√3 cos (0-60 o )/2} Expressing this as an effective value gives us where Vac: effective value Vdc: set value (DC voltage) More specifically, the effective value V u-v is determined by the period T of the triangular wave, the ON period T ON1 of one transistor of the inverter and the DC voltage Vdc. It will be understood that if the PWM command signal A is increased in magnitude by enlarging the flux command Φ in a case where the DC voltage Vdc drops, then the motor terminal voltage will increase and a constant output will be obtained. Next, let us consider control by a base signal of the transistor constituting the inverter c for a case where the amplitude ratio A/B is greater than 1. As shown in Fig. 7(b), there is a method in which the base signal of Tr in the inverter is turned off whenever the amplitude ratio (A/B) exceeds 1. However, this method involves certain problems, namely the fact that the output voltage will not rise, switching loss is great, etc. Specifically, since there are a large number of switching operations, as at t₁, t₂, t₃,..., in a time period α over which the amplitude ratio (A/B) is greater than 1, switching loss is great. In addition, the output voltage in this time period takes on the average value of 1 ○ - 5 ○, as a result of which an increase is suppressed. Fig. 9 illustrates the manner in which the line voltage V u-v varies with respect to the amplitude ratio A/B. In the present invention, the aforementioned problem of the prior art is solved by turning all the transistors Tr on, as shown in Figs. 7(c), (d), in a range where the amplitude A of the PWM command signal exceeds the amplitude B of the triangular wave applied to the circuit m. 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 d 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 sensed voltage V dc of the DC voltage sensing circuit b to a data map i indicating the relationship between excitation frequency ω₀ and flux Φ, and correcting the flux Φ in dependence upon the DC voltage V dc . It should be noted that the DC voltage V dc sensed by the DC voltage sensing circuit b is also applied to an induced voltage compensating circuit n. Here the induced voltage E₁ is expressed by by the process of step (8), (9), as described above. Though this is expressed as a function of the magnetization current I ˙ 0M , the latter may be expressed I 0M = I ₀ - I 0i where I ˙₀ is the excitation current and I ˙ 0i the core loss current. Accordingly, the induced voltage is influenced by the core loss current I 0i due to these relationships. Therefore, the influence of the core loss current I 0i is compensated for in the induced voltage compensating circuit so that the current I in each phase outputted by the PWM circuit/current control circuit m is obtained correctly. In accordance with the three-phase induction motor control method of the present invention, output can be controlled so as to be substantially constant even if there is a fluctuation in the voltage applied to the three-phase induction motor. It is possible for the three-phase induction motor control method of the present invention to have all processing executed by software using a high-speed processor.

Claims: A three-phase induction motor control method for vector control of a three-phase induction motor (d) supplied by an inverter (c) which is itself supplied from a DC link section with direct current obtained by rectification of an AC input voltage from an AC power supply, the inverter (c) being controlled by a PWM circuit (m) operating in dependence upon a PWM command signal, having an amplitude (A), which cooperates with a triangular wave, the PWM circuit (m) receiving motor current commands (I U ,I V ,I W ) produced by circuitry including a data map (i) outputting a value of flux command (Φ) in dependence upon an input value of excitation frequency (ω₀), the data map (i) having a characteristic defining a region of excitation frequency (ω₀) wherein the value of flux command (Φ) decreases for increasing values of excitation frequency (ω₀), and wherein the voltage of the DC link section, which voltage is proportional to said AC input voltage, is sensed and when it is low the flux command (Φ) outputted by the data map (i) is raised, by reducing said region, together with the amplitude (A) of the PWM command signal with respect to said triangular wave thereby to raise motor terminal voltage, and when the voltage of the DC link section is sensed to be high, the flux command (Φ) outputted by the data map (i) is reduced, by increasing said region, together with the amplitude (A) of the PWM command signal with respect to said triangular wave, thereby to lower the motor terminal voltage.

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