A METHOD OF, AND APPARATUS FOR, OPERATING AN INDUCTION MOTOR

Abstract: A carrier wave (B) of constant amplitude and frequency, generated by a generator (5), is modulated in multipliers (6 and 6′) by respective sinusoidal signals (e a and e b ). The resulting modulated signals are applied to stators (L a , L b ) of induction motor (1) via power amplifiers (7,7′) to drive the motor. The sinusoidal signals (e a , e b ) are both of one frequency, which is dependent upon the difference between a commanded speed (P s ) and actual motor speed (P n ). The motor (1) can be driven a very low speed and is suitable for use in a machine tool for driving a workpiece table.

Description: A METHOD OF, AND APPARATUS FOR, OPERATING AN INDUCTION MOTOR This invention relates to a method of, and apparatus for, operating an induction motor. When the table of a machine tool carries a workpiece to a prescribed position, there are instances where this movement must be effected at a high speed. It is also required that the table be moved slowly at times, as when the workpiece is subjected to a slow machining operation. Hence the motor which is connected to the feed shaft that moves the table ordinarily has to be capable of having its speed controlled over a very wide range, for example from 400 r.p.m. to an extremely slow speed on theorder of 0.1 r.p.m. DC motors are well suited for connecting to such feed shafts insofar as they are capable of offering such a wide range of speed control and are therefore employed to drive feed shafts in machine tools. It has become possible to ?vary the speed of an induction motor over a broad range through the utilization of variable frequency control systems, and indùction motors, which are easy to inspect and maintain, are now finding use in such applications as spindle drive motors for machine tools. However, it is not possible for an induction motor to run stably at as low a speed as a few r.p.m. or less through the use of a control system that merely reduces the operating frequency. The reason for this is as follows. The magnitude of a rotating magnetic flux which is linked to the secondary conductor of an induction motor is proportional to a voltage derived by subtracting a voltage drop, which is due to primary resistance and primary leakage reactance, from a terminal voltage applied to the primary winding of the motor. This terminal voltage becomes small at extremely low frequencies but, since voltage drop in the above-mentioned resistance and leakage reactance remains substantially constant irrespective of frequency, there is an increase in the disparity between the magnitude of the rotating magnetic flux and the terminal voltage as frequency assumes very small values. Difficulties are encountered in controlling and accurately detecting the rotating magnetic flux and the secondary current which it induces in the secondary conductor. In other words, smooth and stable speed control cannot be achieved where there are fluctuations in load torque. Further, while it might be possible to conceive of an arrangement in which a prescribed torque could be produced by sending a large current into the primary side of the induction motor when rotating the motor at a low speed of only a few r.p.m., the magnetic circuit of the induction motor would saturate and this would prevent the production of the desired torque. These and other problems raise doubts about the expediency of using a control system which merely reduces operating frequency to provide effective low speed control. In view of the foregoing it has not been possible to employ an induction motor in the driving of a machine tool table when the motor may be required to turn through one revolution over a period of several minutes. Accordingly, replacement of all of the conventional motors used in machine tools with readily maintained and inspected squirrel-cage induction motors has not been possible, and the partial use of DC motors has been unavoidable. According to one aspect of the present invention there is provided a method of operating a N-phase induction motor, characterised by modulating a carrier wave of constant frequency and amplitude with N sinusoidal signals of N different phases, all of one frequency dependent upon a commanded motor speed, to produce N modulated signals, and applying the respective modulated signals to respective stators of the motor, to energise those stators. According to another aspect of the present invention there is provided apparatus for operating an N-phase induction motor, characterised by means for generating a carrier wave of constant frequency and amplitude; means for generating N sinusoidal signals of N different phases, all of one frequency dependent upon a commanded motor speed, and means operable to modulate the carrier wave with the N sinusoidal signals to produce N modulated signals, for application respectively to respective stators of the motor, to energise those stators. Embodiments of this invention can provide for variable speed operating systems that are capable of rotating induction motors at a very low speed. Further, embodiments of this invention can provide for variable speed operating systems that are capable of rotating induction motors stably in regions of very low speeds. Moreover, embodiments of this invention can provide for variable speed operating systems that are capable of rotating induction motors stably while producing prescribed torques in regions of very low speeds. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a block diagram of af'two-phase induction motor drive apparatus embodying the present invention; Fig. 2 is a waveform diagram illustrating signals associated with respective parts of the two-phase induction motor drive apparatus shown in Fig. 1; Figs. 3A and 3B are respective vector diagrams of magnetomotive force for assistance in explanation of the operation of a two phase induction motor under control of apparatus embodying this invention; and Figs. 4A to 4F are vector diagrams of magnetomotive force for assistance in explanation of the operation of a two phase induction motor under control of apparatus embodying this invention. Illustrated in Fig. 1 is a block diagram of a two-phase induction motor driven by drive apparatus embodying the present invention. The arrangement of Fig. 1 includes a two-phase induction motor 1 having stator windings LaJ Lb; an error register 2 comprising means such as a reversible counter for computing the difference between two input pulse trains P5 and Pns where the pulse train P5 serves as a speed command signal, and is supplied from outside the circuitry shown in the Figure, whilst the pulse train Pn is indicative of actual motor speed and is the output of a pulse generator 8 coupled directly to the induction motor 1; a digital-to-analog (D-A) converter 3 for converting error pulses from error register 2 into an analog signal, and an arithmetic unit 4 for producing two-phase sinusoidal waves ea, eb that are displaced from each other by 90 degrees, the frequency and peak value of the sinusoidal waves ea, eb varying in proportion to the voltage level of the analog signal obtained from D-A converter 3. In the present embodiment the arithmetic unit 4 produces two-phase output signals in order to operate the two-phase induction motor 1, but the unit can be adapted to produce N-phase signals in a situation where an N-phase induction motor is to be driven. The arrangement of Fig. 1 further includes a carrier wave generator 5 for enerating a sinusoidal signal B having a constant amplitude as well as a constant frequency, such as 50Hz, which is high in comparison with the frequency of the two-phase sinusoidal waves ea, eb; multipliers 6, 6' for multiplying tÏgether the two-phase sinusoidal waves ea, eb obtained from arithmetic unit 4, and carrier waves ec, ec' of identical phase obtained from carrier wave generator 5, thereby to produce output signals C, D which are the carrier waves ec, ec' as modulated by the sinusoidal waves ea, eb; and power amplifiers 7, 7' which amplify the outputs C, D of respective multipliers 6, 6' and then apply the amplified signals to stator windings La, Lb as voltages Era web In terms of operation, the error register 2 receives at one of its input terminals the speed command pulse train P5 of a prescribed frequency which is determined in accordance with a desired motor speed. The error register receives at its other input terminal the pulse train Pn indicative of actual motor speed and subtracts the number of pulses in this train from those in the pulse train Ps to provide an error signal E which is indicative of the difference. The D-A converter 4 converts the digital error signal E into an analog error voltage signal A the level of which corresponds to the error signal E. The form of signal A can be understood from Fig. 2. The arithmetic unit 4 accepts the error voltage signal A and generates a pair of sinusoidal signals ea, eb, as shown in Fig. 2, which differ in phase by 90 degrees and have a frequency and amplitude that depend upon the size of the error voltage. Meanwhile the carrier wave generator 5 produces the carrier wave B whose frequency and amplitude are constant, as depicted in Fig. 2, the carrier wave being applied to multipliers 6, 6' as respective carrier waves ec, ec of identical phase. The multipliers 6, 6' multiply the carrier waves ec, ec' and arithmetic unit outputs ea, eb thereby to modulate the carrier waves ec and ec' by the sinusoidal signals ea, respectively and thus produce the modulated outputs C, eb D shown in Fig. 2. These outputs C, D are then amplified by the power amplifiers 7, 7' into signals Ba Eb that are supplied to respective stator windings Las % of twophase induction motor 1. The induction motor responds in such a manner that an induced voltage appears in a secondary conductor owing to an alternation in flux attributed to the carrier wave frequency, the induced voltage giving rise to a secondary current thais large enough to generate a torque. In addition, the field established by the stator winding currents revolves as the sinusoidal outputs ea, eb of arithmetic unit 4 vary in phase, thereby producing a rotating field. What is of special note here is that the frequency of the sinusoidal signals ea, eb, namely the synchronous speed of the motor, must not be allowed to exceed a value which is half that of the carrier wave frequency. Moreover, a change in the speed of induction motor 1 is effected by changing the frequency of the speed command pulse train P5. A prescribed speed and torque are established when the modulated, very low frequency outputs C, D are impressed upon theinduction motor 1. The principle which accounts for this may be explained as follows. Two-phase induction motor 1 cannot operate at very low speeds, for the reasons set forth above, when the stator windings La, Lb receive solely the signals ea, eb having a frequency and amplitude which depend upon the error voltage at the output of error register 2. Furthermore, assume now-that only the constant frequency and constant amplitude carrier wave output of carrier wave generator 5 is applied to the respective induction motor stator windings Lass Lb after having been split into the two portions ec, ecl of identical phase. This will give rise to a magnetomotive force fa resulting from E and a magnetomotive force b resulting from Eb, re a b sulting from Eb, and the magnetomotive forces will be of the same magnitude (that is, a = b) because Ea and Eb are identical in magnitude as well as phase. Accordingly, as shown in Fig. 3A, the resultant magnetomotive force Qm between ia and merely alternates at a slope of 45 degrees, that is, at an angle of 45 degrees with respect to bothk and ifs Such a condition cannot produce a rotating field. Thus, merely applying the carrier wave to the stator windings LaX Lb will not cause the induction motor to rotate. However, if the voltages Ea, Eb applied to the stator windings LaX Lb can be made to undergo a slight progressive or step-by-step variation, a corresponding variation will occur-in the magnitudes of the magnetomotive forces #a, #b, and the a' b resultant magnetomotive force #m will rotate slowly, as depicted in Fig. 3B which illustrates sucessive' step-by-step values # a1 to # a3, and #b1 to # b3 for example, of magnetomotive force produced by successive values of EaX Eb applied to the stator windings to generate a rotating resultant magnetomotiveforce, #m1,#m2,#m3 for example. More specifically, a rotating magnetic field is generated by applying a component that yields a turning effort, attributed to the outputs ea, eb of arithmetic unit 4, to the carrier wave component which is incapable of producing such turning effort, while the carrier wave component results in an induced voltages large enough to cause a secondary current to fiow in the secondary conductor. It is this feature that allows the rotor to rotate smoothly even at very low speeds. The operation of the two-phase induction motor drive circuit can be more fully understood from Figs. 4A to 4F which give an analysis of such operation with the passage of time and in terms of the rotating field. The various times t1, t2, t3 ... t5, t6 shown in Fig. 4 correspond to the pcints in time similarly labelled along the time axis of the respective signal waveforms in Fig. 2. In other words Fig. 4A to 4F illustrate the conditions under which the rotating field is produced. Thus Fig. 4A shows that only the modulated output voltage Ea is present at time t=t1, with output voltage Eb being 90 degrees out of phase and having a magnitude of zero, so that the magnetomotive force #a, which is attributed a' entirely to Ea, is the total magnetomotive force, i.e., Fig. Fig. 4B shows an instance where Ea:Eb = #3 : 1 at time t=t2, so that both the output voltages Ea , Eb now contribute to the resultant magnetomotive force in between the magnetomotive forces #a, #b, with # b being attributed to Eb. The vector of the resultant magnetonotive force #m has thus rotated through 30 degrees between times t1 and t2. Fig. 4C illustrates an instance where Ea:Eb = 1 : a at time t=t3, with the vector of the resultant magnetomotive forcem having rotated through an additional 30 degrees. Fig. 4D shows that only the output voltage Eb is present at time t=t4, with output voltage E a being 90 degrees out of phase and having a magnitude of zero, so that the magnetomotive forced is the total magnetomotive force, i.e., = b m Beret has rotated by an additional 30 degrees for a m total of 90 degrees in comparison with its orientation at time t=t1. Fig. 4E shows the condition at time t"t5 and Fig. 4F at time t=t6. It can be seen that the vector of the output voltage E a has reversed direction as compared with t=t1 and t=t2, and that this is accompanied by a reversal in the direction of the magnetomotive force vector i . The resultant a magnetomotive force vector can thus be understood to continue rotating successively by 30 degree steps without altering direction. Thus the magnetomotive forces attributed to the respective output voltages Ea, Eb also rotate smoothly owing to the mutual changes in magnitudes and directions of the Ea and Eb vectors. The two-phase induction motor can therefore run stably at a very low speed while generating a prescribed torque. While the embodiment of the present invention which has been described and illustrated concerns a simple two-phase induction motor in order to facilitate description, it is in no way limited to such an arrangement but can obviously be applied to induction motors that operate on three or more phases. Inrsuch a case the arithmetic unit 4 would be required to generate N-phase output signals of a phase angle 2f/N, and the number of multipliers and power amplifiers would have to agree with the number of phases N. It should also be understood that the feedback system in the circuitry of Fig. 1 may employ analog signals instead of the digital signals. It has thus been shown that the present invention makes it possible to operate a durable, maintenance-free squirrel-cage induction motor smoothly and stably while producing a prescribed torque, as in the case of a DC motor, even at a very low speed of almost several r.p.m. or less. The economic advantages of this are self-evident. The application of this operating system enables a squirrel-cage induction motor to be employed as a feed shaft drive motor in machine tools. Thus, embodiments of the present invention provide a system for operating an induction motor at variable speeds, which is particularly effective when an induction motor is to rotate at a very low speed of at most several r.p.m. or less. The system produces polyphase sinusoidal signals of a frequency which corresponds to the difference between a command speed and the actual speed of the induction motor, the number of phases of the sinusoidal signals being equal to the number of phases at which the induction motor operates, and also produces a sinusoidal carrier wave having a constant frequency in the range from 50 Hz to 400 Hz for example and a constant amplitude, and-polyphase induction motor drive signals are formed by modulating the carrier wave by the polyphase sinusoidal signals. The induction motor drive signals are amplified up to a desired value for application to the induction motor to energise the stator for each respective input phase so that the rotor of the induction motor can be caused to rotate at a very low speed.

Claims: CLAIMS 1. A method of operating a N-phase induction motor, characterised by modulating a carrier wave of constant frequency and amplitude with sinusoidal signals of N different phases, all of one frequency dependent upon a commanded motor speed, to produce N modulated signals and applying the respective modulated signals to respective stators of the motor, to energise those stators. 2. A method as claimed in claim 1, wherein N is equal to two and the two sinusoidal signals differ in phase by it/2. 3. A method as claimed in claim 1, wherein N is greater than or equal to three and the sinusoidal signals differ in phase, from one to the next nearest in phase, by 2f/N. 4. Apparatus for operating an N-phase induction motor, characterised by means for generating a carrier wave of constant frequency and amplitude, means for generating N sinusoidal signals of N different phases, ail of one frequency dependent upon a commanded motor speed, and means operable to modulate the carrier wave with the N sinusoidal signals to produce N modulated signals, for application respectively to respective stators of the motor, to energise those stators. 5. Apparatus as claimed in claim 4, wherein N is equal to two, and the means for generating the two sinusoidal signals generate signals differing in phase ffl2. - 6. Apparatus as claimed in claim 4, wherein N is greater than or equal to three, and the means for generatsignals ing tne N slnusoleaiIgenerate signals tittering in pnase, from one to the next nearest in phase, by 2n/N. 7. Apparatus as claimed in claim 4, 5 or 6, in combination with an N-phase induction motor operated thereby. 8. Apparatus as claimed in claim 7, wherein the N-phase induction motor is a motor of a machine tool for driving a workpiece table. 9. A machine tool including apparatus as claimed in claim 7 or 8.

Inventors: ISHIDA, HIROSHI, KAWADA, SHIGEKI
Applicants: FANUC LTD
Application year: 1980
Main classification: H02P5
IPC codes: H02P27, H02P29, H02P23