SYSTEM FOR CONTROLLING TORQUE OF INDUCTION MOTOR

Abstract: A system for controlling torque of an induction motor, wherein an excitation instruction (I₀) and a secondary current instruction (I₂) are decomposed respectively into vector components of excitation magnetic flux and vector components in the electromotive direction (torque direction) of the motor (M) based upon a torque instruction (T M ) for the motor (M) and an excitation magnetic flux instruction (ø) determined in response to the motor (M), so that a linear output torque can be produced for these instructions. A primary current instruction (I₁) of the motor is obtained by synthesizing these vector values.

Description: Technical Field This invention relates to a system for controlling the torque of an induction motor. Background Art Vector control generally is employed for controlling the velocity of recent induction motors. When controlling the velocity of an induction motor in accordance with such vector control, a torgue 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 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 the torque command at a constant rpm. As a result, the output torque also becomes non-linear with respect to the torque command and an accurate torque command cannot be produced (see for example EP-A-75023). A conventional approach for dealing with this is to use external equipment such as a torque sensor to measure motor torque as means for accurately ascertaining the output torque of the motor, A problem that results is the high cost of equipment. Disclosure of the Invention The present invention solves the aforementioned problem of the prior art and its object is to provide an induction motor torque control system in which output torque is linearly controlled in response to a torque command without requiring special torque measurement equipment. A system for controlling the torque of an induction motor according to the present invention is characterized by comprising means for deciding a torque command for an induction motor and an excitation flux command from an excitation frequency, means for deciding an excitation flux component of excitation current from the excitation flux, means for deciding a component, in the direction of an electromotive force, of the excitation current from the excitation flux and excitation frequency, means for deciding a component, in the direction of the electromotive force, of a secondary current of the motor from the torque command and excitation flux command, means for deciding a slip frequency from the component, in the direction of the electromotive force, of the secondary current and the excitation flux command, means for deciding an excitation flux component of the secondary current from the component, in the direction of the electromotive force, of the secondary current and from the slip frequency, and means for obtaining an excitation flux component, as well as a component in the direction of the electromotive force, of the primary current of the motor from the excitation flux component and from component in the direction of the electromotive force of the excitation current and secondary current, respectively, and for limiting a primary current command for the motor upon combining these components. Accordingly, by obtaining a slip frequency command which will cause the required load current to flow, the present invention enables the torque command itself to be substituted for the output torque in order to linearly control the motor torque, this being performed without requiring special torque measurement equipment. Brief Description of the Drawings Fig. 1 is a block diagram illustrating an embodiment of the present invention, Fig. 2 is a flowchart illustrating a torque control operation, Fig. 3 is a view illustrating an equivalent circuit of an induction motor, and Figs. 4 and 5 are vector diagrams. Best Mode for Carrying Out the Invention An embodiment of the present invention will now be described in detail with reference to the drawings. First, the relationship among various characteristics of an induction motor, upon which characteristics the present invention is premised, will be described with reference to the equivalent circuit of Fig. 3 and the vector diagrams of Figs. 4 and 5. A secondary input P2 of an induction motor is found in accordance with P₂ = E₁ x I₁ x cosϑ₂ Letting  represent excitation flux and letting ω o denote the excitation frequency, an induced electromotive force E₁ is expressed by E₁ = d/dt = ω o x  The secondary current is expressed by The power factor is obtained from Substituting Eqs. (2), (3), (4) into Eq. (1) gives us Secondary core loss P₂ r is obtained from P₂r = I₂²R₂ = S x P₂ The output P m of the motor is expressed by P m = P₂ - P₂r = (1-S)P₂ Letting T m represent the motor torque and ω m the rotational speed of the motor, we have Pm = ω m x T m The torque T m of the motor can be written as follows using Eqs. (1), (2), (7) and (8): Note that ω s represents the slip frequency. Next, the -axis component (excitation flux component) and E₁-axis component (component in the direction of the electromotive force) of I o and I₂ are obtained as I oM , I oω , and I 2M , I₂ω respectively, as follows: Letting L o represent the excitation impedence, we have so that I oM = (1/L o ) x  I oω is the core loss component, and we let K represent a constant of proportion. This gives us I oω = K x ω o x  as an approximation. Further, transforming Eq. (9) gives us I₂ω = I₂ x cosϑ₂ = T m / The -axis component I 2M of the secondary current is obtained as follows: We obtain I₂ω from Eqs. (2), (3), (4), (12) as follows: I₂ω = 12 x cosϑ₂ = [(E₁ x (R₂/S)]/[(R₂/S)² + X2²] = ω o x  x (R₂/S)/[ (R₂/S)² + X₂²] = [ x (ω o x S x R₂)]/[R₂² + (SX₂)²] = ( x ω s x R₂)/[R₂² + (ω s x L₂)²] where X₂ = ω o x L₂ ω s = S x ω o Dividing both sides by  gives us I₂ω/ = (ω s x R₂)/[(R₂² + (ω s L₂)²] With the foregoing serving as a premise, we will now describe a specific method, in accordance with the invention, of obtaining the primary current and slip frequency conforming to a torque command. In general, the maximum value of excitation torque  is decided by [1] the maximum excitation current capable of flowing in the motor, or [2] the maximum voltage capable of being impressed upon the motor. In actual control of a motor, there are cases where excitation is weakened when the load is small in order to reduce excitation noise. In the following explanation, however, it will be assumed that the flux command gives the maximum value. When the torque command T m and flux command  are applied, I oM is found from Eq. (10), I o ω from Eq. (11), and I₂ω from Eq. (12). Next, the slip frequency ω s is decided in accordance with Eq. (14) using I₂ω and . However, since it is difficult to calculate ω s by a CPU or the like in a short period of time, values of ω s are found in advance with respect to values of I₂ω/, ω s is stored in memory as a data table, and the value of I₂ω/ is stored in memory as a data address. Thus, if I₂ω and ω s are found, I 2M can be obtained from Eq. (13). From the foregoing, the -axis component I₁() of the primary current I₁ can be obtained from: I₁() = I oM + I 2M and the E₁-axis component I₁(E₁) of the primary current I₁ can be obtained from: I₁(E₁) = I o ω + I₂ω The primary current thus obtained is a current command, which takes all characteristics of the induction motor into account, for obtaining a linear output torque with respect to a torque command. Further, I₁(), I₁(E₁) correspond to an excitation current component and load current component, respectively, in vector control of the induction motor. The specifics of the present invention will now be described with reference to the block diagram of Fig. 1 and flowchart of Fig. 2. 1 ○ A velocity command ω c and the induction motor rotational speed ω m , which is obtained by a tacho-generator TG coupled to the output shaft of the motor M, are applied to a comparator a, and the torque command T m is calculated by performing the arithmetic operation T m = K₁(ω c - ω m ) + K₂∫(ω c - ω m )dt In vector control, a calculation method in which a calculation is performed as indicated by the above equation using a velocity deviation and the result of the calculation is regarded as a torque command is commonly known; hence, a description of the foregoing is omitted. 2 ○ Excitation flux Φ on a flux characteristic curve is decided from the excitation frequency ω o . 3 ○ The -direction component I o of excitation current I o is obtained from the excitation flux  in the form I oM = (1/L o ) x  This is applied to an adder d₂. 4 ○ The E₁-direction component of I o is obtained from  and ω o in the form I o ω = K₃ x ω o x  5 ○ The E₁-direction component of the secondary current I₂ is obtained in the form I₂ω = (T m /) by the divider b₁. 6 ○ Using the I₂ω and excitation flux command  that have been obtained, the divider b2 divides I₂ω by . Then, a slip frequency ω s corresponding to the data address I₂ω/, which is the result of the calculation, is obtained from a data table stored in memory beforehand. 7 ○ I₂ ω and ω s are applied to a multiplier c₂, and the product is multiplied by (L₂/R₂). The -direction component of the secondary current is obtained in the form I 2M = (L₂/R₂) x I₂ω x ω s This is applied to the adder d₂. 8 ○ The -direction component of I₁ is obtained in the form I₁ () = I oM + I 2M by the adder d₂. 9 ○ The E₁-direction component of I₁ is obtained in the form I₁(E₁) = I o ω + I₂ω by an adder d₁. Two orthogonal phases of the primary current command I₁ are obtained from a current calculating circuit A and applied to a two-phase/three-phase converting circuit B. Command currents Iu, Iv, Iw in the respective phases of the induction motor M obtained from the two-phase/three-phase converting circuit B are applied to a current controller C. The actual currents of the respective phases of the induction motor are sensed by CTu, CTv, CTw and compared with the command currents, upon which predetermined control is performed by the current controller C. Industrial Applicability In accordance with the system for controlling the torque of an induction motor of the present invention, control is performed in such a manner that a linear output torque is obtained with respect to a torque command. This enables the output torque of the induction motor to be accurately judged at low cost.

Claims: 1. An induction motor torque control system for linearly controlling the output torque of an induction motor in response to a torque command, comprising: means for deciding a torque command (Tm) for the induction motor, and an excitation flux command (Φ) from an excitation frequency (ωo); means for deciding an excitation flux component (Io M ) of excitation current from the excitation flux; means for deciding a component (Io), in the direction of an electromotive force (E₁), of the excitation current from the excitation flux and excitation frequency; means for deciding a component (I 2W ), in the direction of the electromotive force (E₁), of a secondary current (I₂) of the motor from said torque command and excitation flux command; means for deciding a slip frequency (ω s ) from the component, in the direction of the electromotive force, of said secondary current and the excitation flux command; means for deciding an excitation flux component (I 2M ) of the secondary current from the component (I 2W ), in the direction of the electromotive force, of the secondary current and from the slip frequency (ω s ); and means for obtaining an excitation flux component I₁ (Φ) of the primary current of the motor and a component I₁(E₁) in the direction of the electromotive force of the primary current (I₁) of the motor from the excitation flux component and from the component in the direction of the electromotive force of said excitation current and secondary current, respectively, and for limiting a primary current command for the motor upon combining these components. 2. An induction motor torque control system according to claim 1, wherein said slip frequency deciding means decides the slip frequency (ω s ) from a data table in which the data address is a value obtained by dividing the component (I 2W ), in the direction of the electromotive force, of the secondary current by the excitation flux command (ω o ).

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