VECTOR CONTROL APPARATUS FOR INDUCTION MOTOR

Abstract: A vector control apparatus for an induction motor con­ trols rotation of the induction motor by the respective phase current command values calculated on the basis of a torque command and a secondary flux command and includes a memory means (22) for storing a function representing a pre­ determined relation between a rotation speed, a torque and a second flux of the induction motor and for outputting a sec­ ondary flux corresponding to given rotation speed and torque data, and a means (1, 19) for supplying at least torque data to the memory means (22). The function is set such that a loss is reduced in a high-load operation of the induction motor and a variation in rotation speed of the induction motor is reduced in low-load operation.

Description: The present invention relates to a vector control apparatus for an induction motor and, more particularly, to a vector control apparatus for an induction motor wherein rotational smoothness of the motor at a low rotation speed with a small torque can be greatly improved and a loss of the motor with a large torque can be greatly reduced. Conventional induction motors have been used as constant speed motors using a power source of a predetermined frequency in a variety of applications due to rigidness and low cost. Along with the recent development of electronic devices, microcomputers, and software, a power source having a wide variable frequency range can be obtained to drive an induction motor. The field of applications of the induction motors is changing from constant speed motors to servo motors. The variable frequency power source is operated according to vector control. Basic parameters in vector control are a torque current i 1q , an exciting current i₀ for generating a secondary flux Φ₂, and a slip speed ω s and are defined as follows: i 1q = (L₂₂/M) (T/Φ₂) i₀ = {Φ₂ + (L₂₂/R₂)·(dΦ₂/dt)} /M ω s = TR₂/Φ₂² = (R₂/L₂₂)·(i 1q /i₀) for the steady state, i.e., dΦ₂/dt = 0 where L₂₂ is the inductance of the secondary winding, M is the mutual inductance between the primary and secondary windings, T is the torque, Φ₂ is the flux generated by the primary winding and crossing with the secondary winding, and R₂ is the resistance of the secondary winding. Relational equations (1), (2) and (3) are referred to as vector relational equations. The secondary flux Φ₂ is a predetermined value in vector control. The torque T is a command value supplied to the vector controller for dΦ₂/dt = 0 as follows: T = (M²/R₂)ω s i₀² = (M²/L₂₂)i₀i 1q That is, a so-called inverter is controlled by the slip speed ω s , the exciting current i₀, and the torque current i 1q to supply power to the induction motor so as to allow the motor to operate at desired ratings. Fig. 1 is a block diagram of a conventional vector control system shown in "New Drive Electronics", Naohiko Kamiyama, P, 205. In other words, Fig. 1 shows a basic arrangement of conventional "slip frequency type vector control". Referring to Fig. 1, reference numeral 1 denotes a speed control amplifier for generating a torque T; 2, a divider; 3, a constant setter for outputting the torque current i 1q ; 4, a vector analyzer; 5, a multiplier; 6, a converter; 7, a current amplifier; 8, a power converter; 9, an induction motor; 11, a speed detector; 12, a differentiator; 13, 14, 15, and 16, constant setters for generating the exciting current i₀; 17, a divider for generating the slip speed ω s ; 18, a vector oscillator; and 20, an adder. With the above arrangement, the torque can be controlled in accordance with an instantaneous current. Please refer to PP. 205 - 206 in the above reference for the operation of the circuit shown in Fig. 1. In the case of the slip frequency type vector control circuit shown in Fig. 1, an expected value Φ E of the secondary flux Φ₂ is generally constant within the entire rotation speed range and the entire torque range (this is called as constant torque characteristics, so that an output from the induction motor is increased in proportion to the motor rotation speed), as shown in Fig. 2. If a constant output of the induction motor in a high-speed range is required, the secondary flux Φ₂ is kept constant at a rotation speed below a predetermined rotation speed ω r11 , as shown in Fig. 3. However, at a rotation speed exceeding the predetermined rotation speed ω r11 , the secondary flux Φ₂ is in inverse proportion to the rotation speed ω r (i.e., constant output characteristics). In this case, the secondary flux Φ₂ is a function of the rotation speed ω r . However, if the induction motor is used as a servo motor, the following problems are presented. The servo motor must satisfy the following requirements: (1) smooth rotation, i.e., a small variation in rotation speed is required in mainly a low-speed range in order to achieve high-precision control, for example, in table feed finish machining in a machine tool; and (2) a heat loss must be minimized and a torque must be maximized in a high-output operation in, e.g., table feed coarse machining in the machine tool. In EP-A1-75023 a method of controlling an AC motor and device thereof is described. In this method, the exciting current I₀ of an AC motor is kept constant, and the primary current I₁ is changed in response to the velocity difference Δn so as to control the AC motor. A memory device (18) previously stores the torque-to-slip frequency ω s as characteristics, based on the secondary leakage reactance, and characteristics of the slip frequency ω s -to-primary current vectors I₁, ₁. According to the torque command T obtained from the velocity difference Δn, with the slip frequency ω s and the primary current vectors I₁, 1, are read out and then 3-phase current, IU, IV, IW are delivered. This method and device suppresses torque irregularities and improves response. In conventional slip frequency type vector control, the requirements of the servo motor cannot be sufficiently satisfied. More specifically, the loss of the output in a high torque operation cannot be reduced and smooth rotation at a low speed with a small load cannot be achieved. It is an object of the present invention to provide a vector control apparatus for an induction motor wherein a loss can be reduced in an operation with a high torque, and smooth rotation at a small load can be achieved. A vector control apparatus according to the present invention comprises: means for outputting a torque command; means for outputting a secondary flux command; means for calculating a current command value of each phase of an induction motor on the basis of the torque command and the secondary flux command; and means for supplying a current represented by the current command value to the induction motor; the secondary flux command output means comprising memory means for storing a function representing a predetermined relationship between a rotation speed, a torque, and a secondary flux of the induction motor, and for outputting the secondary flux corresponding to rotation speed data and torque data, and means for supplying at least the data corresponding to one of the torque command T and the actual speed ω r to the memory means, and said predetermined relation between ω r T, and ₂ being determined such that when T is large, the secondary magnetic flux ₂ is increased within a range where the ratio of the motor loss L to T (T/L) can reach a maximum value, in such a way as to minimize a slip speed ω s , and when T is small and the actual speed is less than a predetermined value, the secondary magnetic flux ₂ is reduced in such a way as to reduce ripple of said actual speed ω s , and to increase said slip speed ω s , within an allowable range of the loss L. The invention will be described in more detail, by way of example, and with reference to the accompanying drawing in which: Fig. 1 is a block diagram of a conventional vector control apparatus for an induction motor; Figs. 2 and 3 are graphs for explaining the operation of the vector control apparatus shown in Fig, 1; Fig. 4 is a graph for explaining the principle of operation of the present invention; Fig. 5 is a block diagram of an induction motor control apparatus according to an embodiment of the present invention; Fig. 6 is a graph showing the contents of a memory shown in Fig. 5; Figs, 7, 8, and 9 are graphs showing different contents of the memory shown in Fig. 5; Fig. 10 is a block diagram of a vector control apparatus according to another embodiment of the present invention; and Fig. 11 is a block diagram of a vector control apparatus according to still another embodiment of the present invention. The principle of the present invention will be described so as to best understand the present invention prior to a description of a preferred embodiment. In general, when a load such as a machine tool is driven, a large torque at a load-side portion on which a torque (force) acts causes a stress when a large torque acts on the load. In this case, a high output must be generated. A loss caused by heat dissipation of the induction motor must be minimized even if smooth rotation thereof is sacrificed. Losses of the induction motor are classified into a copper loss, a core loss, and a mechanical loss. Of these losses, the copper loss is the major loss. The copper loss is increased in proportion to the square the primary and secondary currents. The core loss is regarded to increase in proportion to the 1.6 to second power of the flux density (which is proportional to the exciting current if a relatively small hysteresis loss is neglected) and to the square (second power) of a primary frequency ω₁ of power applied to the induction motor. For the sake of simplicity, the core loss is assumed to increase in proportion to the square of the flux density or the exciting current. The mechanical loss is generated by other causes and will not be described herein. In order to reduce the loss, reduction of the losses with respect to the exciting current i₀ and the torque current i 1q is very important. The main components of the loss L can be represented as follows: In equation (5), the first and second terms correspond to the copper loss, and the third term corresponds to the core loss. L Equations (1) and (3) yield i 1q ² = (L₂₂²/M²)·(T²/Φ₂²) = (T/M²)-(L₂₂²/R₂)·ω s A substitution of this equation into equation (6) yields the following equation relating the loss L and the torque T: Equation (7) can be rewritten as follows: That is, (T/L is equal to the right-hand side if ω s = ( R₁′/R₁ ·R₂)/ L₂₂ ² + R₂M²/R₁ , that is, T/L = maximum) A relationship between a speed variation Δω r and a harmonic component ΔT of the torque T is calculated. The speed variation Δω r is calculated by the equation of motion of the rotor of the induction motor as follows: Equation of Motion: Jd(ω r + Δω r )/dt + Dr(ω r + Δω r ) + T L = Ta + ΔT where J is the inertia, Dr is the speed viscosity coefficient, ω r is the rotation speed, T L is the torque loss, and Ta is the output torque. In the steady state, dω r /dt = 0 and Drω r + T L = Ta therefore JdΔω r /dt + DrΔω r = ΔT If a harmonic component of the first order frequency ω₁ is given as mpω₁ , where p is the number of pole pairs, then Δω r = ΔT/ (Jmpω₁)² + Dr ² ≦ ΔT/Jmpω₁ Since ω₁ = ω r + ω s , then Δω r = (ΔT/Jmp){1/(ω r + ω s )} In order to reduce Δω r at a given torque harmonic component ΔT, ω s is increased. If ω s is multiplied with k₂, equation (14) can be rewritten as follows: Δω r = (ΔT/Jmp) x {1/(ω r + k₂·ω s )} The term ΔT/Jmp in equation (15) is a contant, and the term 1/(ω r + k₂·ω s ) is a variable. In the variable, i.e., 1/(ω r + k₂·ω s ) in equation (15), if the rotation speed ω r is low, the speed variation Δω r can be reduced by increasing k. That is, if the value of the rotation speed ω r is small, the speed variation Δω r can be reduced by increasing the slip speed ω s . As is apparent from the above description, in order to increase the T/L or decrease the Δω r , the most important factor is the slip speed ω s . A relationship between the torque T, the loss L, and the slip speed ω s will be described with reference to Fig.. 4 showing a coordinate system plotting i₀ and i 1q along the abscissa and ordinate. The loss L is given by equation (6) described above: L = R₁′i₀² + (R₁ + R₂M²/L₂₂²)i 1q ² Equation (6) indicates that i₀ and i 1q for a constant loss L form a part of a ellipse as shown with, for example, symbols L₁ and L₂ in Fig. 4. At the same time, the torque T is defined by equation (4): T = (M²/L₂₂)i₀i 1q The "torque T = a constant curve" in the i₀-i 1q coordinate system is a hyperbola as shown with, for example, T₁ and T₂ in Fig. 4. An allowable heat loss of the induction motor is predetermined. When a specific torque T is given, the upper limit of the slip speed ω s as a function of the allowable loss L is determined. For example, referring to Fig. 4, upper limits of slip speeds ω s for the torques T₁ and T₂ at the allowable loss L₁ are calculated. For example, upper limit values ω s1 of the slip speeds ω s for the torques T₁ and T₂ at the allowable loss L₁ are ω sA and ω sB , respectively. If a ratio (ω sE /ω s1 ) of an expected value ω sE to a slip speed upper limit value ω s1 is given as k₀², k₀² is the maximum value. That is, if k₀² is increased, the loss exceeds the allowable heat loss. In other words, when the slip speed ω s is increased, the loss is increased. The advantage obtained by increasing the slip speed ω s is given as follows. Since the slip speed ω s is given as a function of the torque T as in equation (3), the slip speed ω s is larger than the rotation speed ω r at a low speed with a large torque T. If the k₀ is larger then 1, k₀²ω s is larger than ω r . For this reason, when ω r shown in equation (15) is small, the speed variation Δω r falls within a sufficiently narrow range. The induction motor as the servo motor has a structure for inhibiting electromagnetic saturation even if a large torque for acceleration or deceleration is required. Therefore, the induction motor is regarded to have a structure wherein the motor is not electromagnetically saturated even if an exciting or torque current is partially increased or decreased. By properly selecting the slip speed ω s = (R₂/L₂₂)(i 1q /i₀), i.e., a ratio of the exciting current i₀ and the torque current i 1q , the loss L can be minimized. In addition, the speed variation Δω r can be reduced. The present invention is based on the principle described above. Fig. 5 is a block diagram of a vector control apparatus for an induction motor according to an embodiment of the present invention. The same,reference numerals as in Fig. 1 denote the same parts in Fig. 5, and a detailed description thereof will be omitted. Referring to Fig. 5, a signal representing the rotation speed ω r from a speed sensor 11 is input to a speed control amplifier 1. This signal is A/D-converted by an A/D converter 21. The output terminal of the A/D converter 21 is connected to an input terminal 22A of a memory 22. The output terminal of the speed control amplifier 1 is connected to an intput terminal 22B of the memory 22 through an A/D converter 19. An output terminal 22C of the memory 22 is connected to a divider 2, a differentiator 12, a constant setter 15, and a divider 17 through a D/A converter 23. The parts comprising the converters 19, 21, and 23, and the memory 22 are external elements added to the conventional vector control system (Fig. 1) and are referred to as a "memory system". As illustrated in the embodiment of Fig. 10 and 11, there can be a case where either of the converters 19 and 20 is not used, but the memory means is always used. The data which are used or registered in this memory means show the relationship between inputs (torque T and speed ω r ) and outputs (secondary magnetic flux 2) is called the "magnetic flux 2 relation" as referred to in the following. The operation of the vector control apparatus having the above arrangement in Fig. 5 will be described below. The signal representing the rotation speed ω r is input to the memory 22 through the A/D converter 21. A signal representing the torque T from the speed control amplifier 1 is input to the memory 22 through the A/D converter 19. Three-dimensional plane data representing the relationship between the rotation speed ω r , the torque T, and the secondary flux Φ₂ is stored in the memory 22. Fig, 6 shows an example of data stored in the memory 22. Referring to Fig. 6, the ω r -Φ₂ plane of the rotation speed ω r and the secondary flux Φ₂ indicates only a case wherein the torques are zero and T₁. More specifically, if T = 0, then the secondary flux Φ₂ is gradually increased when the rotation speed ω r is less than ω r1 , as indicated by a curve Φ₂₁. In the range between the rotation speeds ω r1 and ω r2 , the secondary flux Φ₂ has a predetermined maximum valve. If the rotation speed exceeds ω r2 , the secondary flux Φ₂ is gradually decreased. However, if the torque T is increased and reaches T₁ , the secondary flux Φ₂ has a maximum value in the range between ω r0 and ω r2 , as indicated by a curve Φ₂₂. The torque T and the rotation speed ω r are input as address signals to the memory 22 from the input terminals 22B and 22A, respectively. The secondary fluxes Φ₂ (e.g., Φ₂₁ and Φ₂₂) are solely output from the memory 22. The data value of the secondary flux Φ₂ is determined to cause the loss L to fall within the allowable range. The secondary flux Φ₂ is used together with the torque T and the rotation speed ω r to calculate slip speed ω s , the exciting current i₀, and the torque current i 1q according to equations (1), (2), and (3) which maintain the relations in vector control. Data stored in the memory 22 is not limited to the one shown in Fig. 6. Data shown in Figs. 7, 8, and 9 may be stored in place of the data shown in Fig. 6. Arrows in fig. 7, 8, and 9 show directions of increases in torque T. The data shown in Fig, 7 is used in a case wherein both the induction motor and the power source have sufficient margins and a large torque is required even at a high speed. The data shown in Fig. 8 is used in a case wherein the induction motor has a sufficient margin but a large torque is required at intermediate and low speeds. The data shown in Fig. 9 is used in a case wherein neither the induction motor nor the power source have margins and a large torque is required at a low speed. Since the vector control parameters are represented by equations (1), (2), and (3), equation (2) yields relation Φ₂ = Φ₂(i₀). A substitution of this relation into equations (1) and (3) yields equations (1) and (3) as a function of i₀. Therefore, it is apparent that the secondary flux Φ₂ need not be used. For example, instead of determining the secondary flux Φ₂ by the torque T and the rotation speed ω r , the exciting current i₀ having one-to-one correspondence with the secondary flux Φ₂ is determined by the torque T and the rotation speed ω r . By using equations obtained by eliminating the secondary flux Φ₂ from equations (1) and (3), the torque current i 1q and the slip speed ω s are calculated to obtain the same result as described above. Thus, a technique without using the secondary flux Φ₂ can be obtained. Fig. 10 is a block diagram showing another embodiment of the present invention. The same reference numerals as in Fig. 5 denote the same parts in Fig. 10, and a detailed description thereof will be omitted. Referring to Fig. 10, data representing rotation speed ω r is input to a speed control amplifier 1. The output terminal of the speed control amplifier 1 is connected to an input terminal 22B of a memory 22 through an A/D converter 19. An output terminal 22C of the memory 22 is connected to a divider 2, a differentiator 12, a constant setter 15, and a divider 17 through a D/A converter 23. In operation, the data of rotation speed ω r is input to the speed control amplifier 1. Data of the torque T as an output from the speed control amplifier 1 is input to the memory 22 through the A/D converter 19. The data shown in Fig. 6 is prestored in the memory 22. Even if the torque T is large, the second flux Φ₂ is set to be large (Φ₂₂) at a low speed. The secondary flux Φ₂2 is changed by only the signal of the torque T. The shape of the curve (Φ₂₂) is not changed. In other words, the curve in a region below ω r1 cannot be changed as in Φ₂₁. In this case, the secondary flux Φ₂ is determined so that the loss L falls within the tolerance. Using the secondary flux Φ₂ as a parameter, the torque T and the rotation speed ω r are used to obtain a slip speed ω s , an exciting current i₀ , and a torque current i 1q , all of which maintain the relations of vector control. Fig. 11 shows still another embodiment of the present invention. The same reference numerals as in Fig. 5 denote the same parts in Fig. 11, and a detailed description thereof will be omitted. Referring to Fig. 11, data of rotation speed ω r is inputted to an input terminal 22A of a memory 22 through an A/D converter 21. An output terminal 22C of the memory 22 is connected to a divider 2, a differentiator 12, a constant setter 15, and a divider 17 through a D/A converter 23. A DIP (Dual Inline Package) switch 24 is connected to an input terminal 22B of the memory 22. Data of a desired torque T can be externally entered by properly setting the DIP switch 24. The memory 22 prestores secondary fluxes Φ₂ corresponding to values of the rotation speed ω r and the values of the torque T. For example, the data shown in Fig. 6 where the value of the torque T is predetermined is stored in the memory 22. By supplying the torque T by setting the DIP switch 24 and the rotation speed ω r as address signals, the corresponding secondary flux Φ₂ can be solely output from the memory 22. The data to be stored in the memory 22 may be ones shown in Figs. 7, 8, and 9. In the above three embodiments, the relation stored in the memory 22 is determined so as to satisfy the following two points: (1) When the absolute value |ω r | of the rotation speed ω r is small and the torque T is small, the speed variation Δω r is reduced to obtain smooth rotation. For this purpose, the loss L is increased within the tolerance, and the slip speed ω s is increased. The slip speed ω s is determined such that the torque T and the rotation speed ω r are designated to obtain the corresponding secondary flux Φ₂ and that the torque T and the secondary flux Φ₂ are used to calculate the slip speed ω s , the exciting current i₀ , and the torque current i 1q so as to maintain the vector control relations (i.e., to establish equations (1), (2), and (3)). (2) When the torque T is increased, the loss L is reduced. In this case, the speed variation Δω r is sacrificed to reduce the slip speed ω s . Therefore, the slip speed ω s is determined to minimize the loss L. Since ω s = TR₂/Φ₂², an increase in torque T causes a proportional increase in slip speed ω s in the conventional constant torque and output characteristics. However, according to the present invention, the secondary flux Φ₂ is determined by the torque T and the rotation speed ω r . As compared with the conventional case wherein the slip speed ω s is increased in proportion to the torque T, the slip speed ω s can be greatly reduced. Therefore, the loss L can be minimized at a constant torque T. As shown in Fig. 4, the conventional loss characteristic curve is an elliptical curve as indicated by symbol L₁ . In this case, the corresponding torque T1 is represented by the alternate long and short dashed line. The operating point of the induction motor is represented by symbol P . However, according to the present invention, the operating point P can be shifted to Pm by controlling the slip speed ω s thereby reducing the loss as indicated by symbol L₂. (3) A means for realizing the present invention can be implemented by adding relatively inexpensive memory elements and the like.

Claims: 1. A vector control apparatus comprising: means (1) for outputting a torque command T; means for outputting a secondary flux command ₂; means for calculating a current command value of each phase of an inductin motor on the basis of the torque command T and the secondary flux command ₂; and means for supplying a current represented by the current command value to said induction motor (9); characterized in that said secondary flux command output means comprising memory meand (22) for storing a function representing a predetermined relationship between a rotation speed ω r , a torque T, and a secondary flux ₂ of said induction motor (9), and for outputting the secondary flux ₂ corresponding to rotation speed data and torque data, and means (19;21) for supplying at least the data corresponding to one of the torque command T and the actual ω r to said memory means (22); and said predetermined relation between ω r , T, and ₂ being determined such that when T is large the secondary magnetic flux ₂ is increased, within a range where the ratio of the motor loss L to T (T/L) can reach a maximum value, in such a way as to minimize a slip speed ω s , and when T is small and the actual speed is less than a predetermined value, the secondary magnetic flux ₂ is reduced in such a way as to reduce ripple of said actual speed ω r , and to increase said slip speed ω s , within an allowable range of the loss L. 2. An apparatus according to claim 1, wherein said means for supplying at least said data corresponding to one of the torque command T and the actual speed ω r to said memory means (22) comprises: means (11) for detecting the rotation speed of said induction motor (9); first means (21) for supplying the digitized rotation speed data to said memory means (22); and second means (19) for supplying the digitized torque data to said memory means (22). 3. An apparatus according to claim 1, wherein said means for supplying said torque data to said memory means (22) comprises means (19) for supplying the digitized torque data to said memory means (22). 4. An apparatus according to claim 1, wherein said means for supplying said at least data corresponding to one of the torque command T and the actual speed ω r to said memory means (22) comprises: means (11) for detecting the rotation speed of said induction motor (9); means (21) for supplying the digitized rotation speed data to said memory means (22); and DIP switch means (24) for supplying the torque command to said memory means (22). 5. An apparatus according any of to claims 1 to 4 wherein the function is set such that a value to the secondary flux Φ₂ or an exciting current i₀ of said induction motor (9) is determined by the command torque T and the rotation speed ω r and that the excitting current i₀, the torque current i 1q , and the slip speed ω s of said inductiong motor (9) are determined so as to maintain relations of vector control on the basis of the secondary flux Φ₂ and the exciting current i₀. 6. An apparatus according to claim 5, wherein the function is set such that when the command torque T exceeds a predetermined value, the slip speed ω s is changed depending on the commanded torque T; the slip speed ω s is determined by values of a mutual inductance M, a secondary inductance L₂₂, a primary resistance R₁, and a secondary resistance R₂, all of which are circuit constants of said induction motor (9); and the slip speed ω s causes the secondary flux Φ₂ or the exciting current i₀ to increase to a predetermined value so as to minimize a loss (a sum of primary and secondary losses) of said induction motor (9) with respect to the commanded torque T, thereby greatly reducing the loss at a constant torque. 7. An apparatus according to any of claims 1 to 4, wherein the function is set such that: when the command torque T is less than a predetermined value, if an absolute value |ω r | of the rotation speed of said induction motor (9) is less than a predetermined value, a slip speed ω s is changed depending on a value of the rotation speed ω r ; if the absolute value |ω r | of the rotation speed is decreased, the secondary flux Φ₂ or the exciting current i₀ is decreased from a normal value to a predetermined value; and if the absolute value |ω r | of the rotation speed is increased toward the predetermined value, the secondary flux Φ₂ or the exciting current i₀ is returned to the normal value, thereby reducing a variation in rotation speed ω r at a low speed of said induction motor (9). 8. An apparatus according to any of claims 1 to 4, wherein the function stored in said memory is set such that: when a range of changes in slip speed ω s is set in accordance with a value of a command torque T acting on said induction motor (9) and the value of the command torque T is less than a predetermined value, the range of changes in slip speed ω s has a lower limit a slip speed ω s1 in a rated operation (i.e., a rated torque and a rated rotation speed) of said induction motor (9) and as an upper limit a slip speed ω s2 for generating a loss equal to that of said induction motor (9) in the rated operation with respect to the value of the command torque T; when the value of the command torque T exceeds the predetermined value, the range of changes in slip speed ω s has the slip speed ω s1 as an upper limit and as a lower limit a slip speed ω s3 determined by circuit constants (i.e., a mutual conductance M, a secondary inductance L₂₂, a primary resistance R₁, and a secondary resistance R₂) of said induction motor (9), the slip speed ω s3 serving to minimizing the loss; whereby when the commanded torque T is reduced, the slip speed ω s is increased and when the commanded torque T is increased, the slip speed ω s is reduced, so that when the commanded torque T is small, a variation in rotation speed ω r of said induction motor (9) is reduced and that when the command torque T is large, the loss of said induction motor (9) is reduced. 9. An apparatus according to claim 8, wherein the function is set such that an increase/decrease in secondary flux Φ₂ or exciting current i₀ is controlled so as to increase/decrease the slip speed ω s by an increase/decrease in the commanded torque T, thereby setting the slip speed ω s and a torque current i 1a so as to maintain relations of vector control according to the command torque T as well as the secondary flux Φ₂ or the exciting current i₀. 10. An apparatus according to any of claims 1 to 4, wherein the function is set such that: when a command torque T is less than a predetermined value |T E |, if a rotation speed |ω r | is reduced within a range smaller than a predetermined rotation speed |ω r |, a slip speed |ω s | of said induction motor (9) causes to reduce a secondary flux Φ₂ or an exciting current i₀ so as to prevent the slip speed |ω s | from exceeding a maximum slip speed |ω s | determined by the commanded torque T and an allowable loss L of said induction motor (9), and if the rotation speed |ω r | exceeds the predetermined rotation speed |ω r |, the secondary flux Φ₂ or the exciting current i₀ is set to be an expected value Φ 2E or i 0E ; and when the commanded torque T exceeds the predetermined value |T E |, the slip speed |ω s | of said induction motor (9) is determined to increase the expected secondary flux Φ 2E or the expected exciting current i 0E within a partial or entire range of rotation speeds |ω r | which do not exceed a minimum slip speed |ω s ′| for minimizing the loss L of said induction motor (9), whereby controlling the slip speed ω s and the torque current i 1q of said induction motor (9) by the commanded torque T, the secondary flux Φ₂ or the exciting current i₀ so as to satisfy relations of vector control. 11. An apparatus according to claim 10, wherein the function is set so that, when the command torque T is smaller than the predetermined value T E , the expected secondary flux Φ 2E or the expected exciting current i 0E linearly decreases as a function of the rotation speed ω r under a condition i.e., rotation speed |ω r | ≦ the predetermined speed |ω r |. 12. An apparatus according to claim 10, wherein the function is set so that when the command torque T exceeds the predetermined value T E , as the rotation speed ω r is decreased, the expected secondary flux Φ 2E or the expected exciting current i 0E linearly increases for a second predetermined rotation speed |ω r ′| larger than the predetermined speed |ω r | under a condition, i.e., the rotation speed |ω r | ≦ the second predetermined rotation speed |ω r ′|.

Inventors: AMAGAI, HIDEYUKI NIPPON ELECTRIC INDUSTRY CO. LTD., OKAMOTO, KIYOKAZU C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H02P5
IPC codes: H02P21