TEMPERATURE COMPENSATIVE REVOLUTION SPEED CONTROL FOR AN INDUCTION MOTOR

Abstract: A temperature compensative revolution speed control system employs a method for projecting a secondary resistance in an induction motor (20) based on a relationship between a primary current (I1) and a primary voltage (V1). Based on the projected secondary resistance, slip and secondary current are corrected so that the torque to be generated by the induction motor (20) can be held coincident with that ordered by a control signal.

Description: S P E C I F I C A T I O N Temperature compensative revolution speed control for an induction motor. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to control system for an induction motor. Mor specifically, the invention relates to a revolutio speed control system for an induction motor. Furthe particularly, the invention relates to a temperatur compensative revolution, speed control for an inductio motor. Description of the Background Art In motor speed control for controllin revolution speed df an induction motor, motor drivin torque can be illustrated by: T = K x I Q x I 2 where I Q is the exciting current; I-, is the secondary current; and K is a constant. In slip frequency control, the secondary current I 2 ca be illustrated by: I 2 = S x E χ /r 2 where S is slip E-. is a primary voltage r- is a secondary resistance. Therefore, in the motor speed control in the sli frequency control, the motor revolution speed can be determined by the slip S, the primary voltage E, and the secondary resistance r-. Therefore, in automatic motor speed control, these factors are taken as control parameters. As is well known, the secondary resistance is variable depending upon the temperature condition. Therefore, in order to precisely control the motor speed, it becomes necessary to detect the secondary resistance precisely. For this, there are various methods for obtaining the secondary resistance for precise motor speed control. For example, according to one method, the secondary resistance is theoritically and arithmetically derived on the basis of primary current, slide frequency, terminal voltage and so forth.. This method is effective for executing precise motor speed control as long as the induction motor is continuously driven under stable conditions. However, in cases where the load on the induction motor frequenctly changes and uniform acceleration and deceleration characteristics are required irrespective of temperature conditions, as in an elevator, this method cannot provide satisfactory response characteristics. In order to obtain a satisfactorily high response rate to variation of the secondary resistance, it is necessary to provide a thermosensor or so forth. Therefore, it is was difficult to control the motor speed with satisactorily high precision irrespective of the temperature condition. On the other hand, in practice, motor torque control may be performed by taking the slip frequency S and primary current I, (= I Q 2 + I 2 2) as control parameters and setting other parameters as a constant value K. In order to obtain satisfactory precision in torque control, it is also necessary to vary the constant K depending upon the temperature condition. SUMMARY OF THE INVENTION It is a principle object of the presen invention to provide a motor speed control system for a induction motor, which can provide satsfactorily hig precision speed control irrespective of temperatur variations. Another object of the invention is to provid a motor speed control system which can obtai temperature dependent secondary resistance data, wit satisfactorily high precision. In order to accomplish aforementioned an other objects, a revolution speed control system according to the present invention, projects th secondary resistance in an induction motor based on th relationship between the primary current and the primar voltage. Based on the projected secondary resistance slip and secondary current are corrected so that th torque to be generated by the induction motor can b held coincident with that ordered by a control signal. According to one aspect of the invention, control system for an induction motor including primary winding and a secondary winding comprises driver circuit connected to the induction motor fo supplying a driving power for the induction motor, first sensor monitoring a primary current in the drivin power to be supplied to the primary winding, a secon sensor monitoring a primary voltage in the driving powe to be supplied to the primary winding, first means fo deriving variation rate of a primary resistance in th primary winding on the basis of the primary current an the primary voltage, a second means for deriving secondary resistance of the secondary winding on th basis of the primary resistance variation rate, and third means for deriving a control signal to control th driving power on the basis of the primary voltage an the secondary resistance. In the preferred process, the first and second means are active while a slip frequency of the induction motor is zero. Further preferably, the first and second means are active while very low frequency current or direct current is applied to the induction motor. The first means and second means are active while a fixed constant current is applied to the primary winding in order to derive the primary resistance variation rate on the basis of variation rate of the primary voltage. According to another aspect of the invention, a method for deriving a secondary winding resistance in an induction motor which has a primary winding connected to a driving power source and a secondary winding, as a control parameter for controlling driving torque and/or revolution speed of the induction motor, comprising the steps of: monitoring a primary current in the driving power to be supplied to the primary winding; monitoring a primary voltage in the driving power to be supplied to the primary winding; deriving variation rate of a primary resistance in the primary winding on the basis of the primary current and the primary voltage; and deriving a secondary resistance of the secondary winding on the basis of the primary resistance variation rate. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a block diagram of the preferr embodiment of a revolution speed control system for a induction motor, according to the invention; Fig. 2 is a circuit diagram of the a inverter circuit which employs improved power transisto circuits, applicable in the revolution control syste for the induction motor; and Fig. 3 shows waveforms at various section i the inverter circuit of Fig. 2. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, the preferre embodiment of a revolution speed control system for a induction motor, according to the present invention generally comprises an inverter circuit 10 connected t an induction motor 20. The inverter circuit 10 include an inverter section 12 and a control section 14. Th control section 14 may comprise a digital processor fo processing data to produce a control signal for drivin the induction motor 20 at a controlled revolution speed The inverter circuit 10 may be a voltag source inverter or a current source inverter. On th other hand, the inverter main circuit in the inverte section 12 may be a transistor inverter, GTO inverter thyrister inverter, a MOS FET (metal oxide semiconducto field effect transistor) inverter or so forth. The induction motor 20 includes a primar winding 22 and a secondary winding 24. The primar winding 22 is connected to the inverter circuit 10 t receive alternating current as a driver signal. A voltmeter 30 is connected to a power suppl wiring connecting the inverter circuit 10 and th induction motor 20 to monitor the output voltage V, o the inverter circuit. An ammeter 32 is also connecte to the power supply wiring to monitor the output curren I, of the inverter circuit 10. The voltmeter 30 and th ammeter 32 are connected to the control section to input a primary voltage indicative signal DV, and a primary current indicative signal DI, . The control section 14 receives the primary voltage indicative signal DV, from the voltmeter 30 and the primary current indicative signal DI, from the ammeter 32. The control section 14 is active while preliminary excitation for applying a small level of DC current or extremely AC low frequency current for projecting a secondary resistance indicative data Dr 2 representative of projected secondary resistance r~ of the secondary winding 24, on the basis of the primary voltage indicative signal value and the primary current indicative signal value. Here, a primary resistance r, of the primary winding 22 can be derived on the basis of the primary voltage indicative signal value and the primary current indicative signal value according to the following equations: V l = H X r l r χ = V l/ When the substantially lower frequency current or DC current is applied to the induction motor 20, slip S in the induction motor is held at zero. While the slip S of the induction motor is held zero, primary resistance r, can be derived on the basis of the primary voltage indicative signal value and the primary current indicative signal value, as set forth above. In the case of the large capacity induction motors, such as these designed for driving an elevator cage, the temperature dependent variation of resistances r, and r„ in the primary and the secondary windings 22 and 24 are proportional to each other. Therefore, by deriving temperature dependent variation of the primary resistance r, in the primary winding 22, temperature dependent variation of the secondary resistance r ? can be projected based on the derived primary resistance. In the practical control, the primary current I, is set at a fixed value. By this, the relationship between variation of the primary voltage and secondary resistances r, and r 2 can be established according to the formula: 2t = r 2s x v /v^ where r 2 . is a projected secondary resistance; r 2 is a reference secondary resistance at a reference temperature; and V, is a reference primary voltage at a reference temperature. The control section 14 thus derives a motor speed control signal on the basis' of the projected secondary resistance value r 2 , . By deriving the motor speed control signal using the projected secondary resistance indicative value dr 2 , the relationship between the slip S and a msecondary current I„ can be corrected. In the shown embodiment, since only the addition of voltmeter 30 for monitoring the primary voltage is required, and the voltmeter can be built in the inverter circuit 10, no other additional elements are required for precise motor speed control. Furthermore, since the projected secondary resistance r -, can be derived by a simple arithmetic operation, the processing load on the control section 14 is not substantially increased. In addition', . since the projected secondary resistance r 2 can be derived while preliminary excitation is performed, the motor speed control and/or torque control for the induction motor can be performed from the instrant the induction motor is started. This is particularly advantagous in the speed control for the induction motor for driving elevator cages. Specifically, in case of the induction motor for the elevators, the passenger load is first monitored for driving the induction motor to generate a driving torque which corresponds to the passager load while a mechanical brake is applied. After the motor torque is increased to the required torque, the mechanical brake is released so that the elevator cage starts without any noticable shock. Furthermore, since the temperature condition of the secondary winding can be detected based on the variation of the secondary resistance, the shown method of detection of the secondary resistance can be used for detecting an over-load condition of the motor. In addition, in the case of induction motors for elevators, which are intermittently driven at invervals, the operation for projecting the secondary resistance can be performed within the intervals to provide satisfactory, practical and acceptably precise motor speed control. In cases where the preferred embodiment of the motor speed control system employs a transistor inverter, the main circuit of the inverter section may be constructed as illustrated in Fig. 2. In the circuit construction of the inverter main circuit of Fig. 2, the inverter main circuit is connected to a known converter circuit (not shown) to receive a direct current power supply therefrom. The inverter main circuit generally comprises a smoothing stage (not shown) and an inverter stage which is generally represented by the reference numeral 40. The inverter stage 40 includes pairs of the preferred embodiment of power transistor circuits 42, 44; 46, 48; and 50, 52. These pairs of the power transistor circuits 42, 44; 46, 48; and 50, 52 are connected to the induction motor 20. On the other hand the power transistors 42, 44; 46, 48; and 50, 52 ar connected to the control section 12 to control th switching timing for generating a single-phas alternating current to supply to the induction motor 20 in the manner set forth above. It should be noted that the power transisto circuits 46, 48; and 50, 52 are identical i construction to the power transistors 42 and 44 Therefore, detailed construction of these powe transistor circuits 46, 48; and 50, 52 are not shown i Fig. 1. The power transistor circuit 42 includes power transistor 421 connected to the switching signa generator of the control circuit at the base electrode. The collector electrode of the power transistor 42 receives the power source current Ic from the converter. In parallel to the collector-emitter circuit of th power transistor 421, a flywheel diode 422 is connected. A first clipper circuit 423 comprising a capacitor 42 and a first diode 425, is also provided in parallel t the the collector-emitter circuit of the power transistor 421. The junction between the capacitor 424 and the first diode 425, is connected to the negative terminal of the converter via a discharge resistor 426. A second diode 427 is also connected to the junction between the capacitor 424 and the first diode 425 in parallel to the discharge register 426. The second diode 425 is thus forms a second clipper circuit 428 with the capacitor 424. In the preferred construction, the second diode 427 of the second clipper circuit 428 is so designed and connected as to provide much greater inductance (L) for the second clipper circuit 428 than that of the first clipper circuit 423. Similarly to the power transistor circuit 42, the power transistor 44 includes a power transistor 441 connected to the switching signal generator of the control circuit at the base electrode. The collector electrode of the power transistor 441 is connected to the emitter electrode of the power transistor 421 to form a series circuit of the power transistors 421 and 441. The emitter electrode of the power transistor 441 is connected to negative terminal of the converter. Ther junction between the power transistors 421 and 441 is connected to the induction motor 20. A flywheel diode 442 is connected in parallel to the collector-emitter circuit of the power transistor 441. A first clipper circuit 443 comprising a capacitor 444 and a first diode 445, is also provided in parallel to the the collector-emitter circuit of the power transistor 441. The junction between the capacitor 444 and the first diode 445, is connected to the negative terminal of the converter via a discharge resistor 446. A second diode 447 is also connected to the junction between the capacitor 444 and the first diode 445 in parallel to the discharge register 446. The second diode 445 thus forms a second clipper circuit 448 with the capacitor 444. The second diode 447 of the second clipper circuit 448 is so designed and connected as to provide much greater inductance (L) for the second clipper circuit 448 than that of the first clipper circuit 443. The second clipper circuits 428 and 448 perform an equivalent function to the power source clipper circuit in the conventional inverter main circuit for absorbing surge voltages generated in the converter which functions as the DC power source, upon shutting OFF of the power supply. These second clipper circuits 428 and 448 are also cooperative with the first clipper circuits 423 and 443 to suppress voltage fluctuations caused in OFF-set transistion of respectively associated power transistors 421 and 441. Absorption of the voltage fluctuations in th first and second clipper circuits 423 and 428 of th power transistor circuit 42 will be discussed herebelow. When the power transistor 421 is turned OFF, loa current Ic at the collector electrode of the powe transistor 421 co mutates to flow through the capacito 424 as shunted current Ice. The current past th capacitor 424 is further shunted to flow through th diodes 425 and 427 as shunted currents I, and I 2 - B this, the capacitor 424 is charged. By this, th connector-emitter voltage V r of the power traisnsito 421 is raised according to the charge in capacitor 424. The smoothing capacitor 22 serves to absorbin surge energy for suppressing fluctuation of the power source voltage V DC . According to increase of the charge of th capacitor 424, the shunted currents I, and I 2 flowing through the diodes 425 and 427 decrease to zero. In the shown embodiment, the first and second clipper circuits 423 and 428 are so constructed as to maintain the shunted current I~ flowing the diode 427 even after the shunted current I, of the diode 425 becomes zero, as shown in Fig. 3. By this, voltage fluctuations in the collector-emitter circuit of the power transistor 421 caused by recovery of the diode 425 upon drop of the shunted current I, flowing therethrough to zero, can be held to a substantially small magnitude. The magnitude of voltage fluctuation at termination of the shunted current I, is suppressed enough that snubber circuit which is required in conventional circuits, becomes unnecessary. Upon termination of the shunted current I ? , recovery of the diode 427 causes voltage fluctuation due to L and C in the whole circuit of the inverter main circuit. However, since the second clipper circuit 428 is so designed as to have much greater L than that of the first clipper circuit, frequency of voltage fluctuation can be kept at low. Furthermore, backward resistance of the diode 427 aids suppression of the voltage fluctuation. . Therefore, even upon recovery of the diode 427, high frequency voltage fluctuation which tends to cause breakage of the power transistor 421 can be successfully suppressed. It should be noted that the voltage fluctuation suppressive operation performed by the power transistor circuit 44 is substantially the same as that of the power transistor circuit 42 as set forth above. Therefore, detailed discussion of the circuit operation of the power transistor circuit 44 is neglected in order to simplify the disclosure and to avoid redundance. In the circuit construction shown in Fig. 1 and disclosed hereabove, • it is further preferable to provide a capacitor C Q which has substantially smaller capacity than that of the capaqcitors 424 and 444. The capacity of these capacitors C 0 can be one tenth of the capacity of the capactor 424, 444, for example. The capacitor ' C~. may be effective for supressing large •magnitude- surge voltage which may be caused upon termination of the shunt current flowing through the diodes 424 and 427. Thus, the invention fulfills all of the objects and advantages sought therefor. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to . include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A control system for an induction motor including a primary winding and a secondary winding comprising: a driver circuit connected to said induction motor for supplying a driving power for said induction motor; a first sensor monitoring a primary current of said driving power to be supplied to said primary winding; a second sensor monitoring a primary voltage of said driving power to be supplied to said primary winding; first means for deriving the variation rate of a primary resistance in said primary winding on the basis of said primary current and said primary voltage; a second means for deriving a secondary resistance of said secondary winding on the basis of said primary resistance variation rate; and . a third means for deriving a control signal to control said driving power on the basis of said primary voltage and said secondary resistance. 2. A control system for an induction motor as set forth in claim 1, wherein said first and second means are active while a slip frequency of said induction motor is zero. 3. A control system for an induction motor as set forth in claim 1, wherein said first and second means are active while very low frequency current or direct current is applied to said induction motor. 4. A control system for an induction motor as set forth in claim 3, wherein said first means and second means are active while a fixed constant current is applied to said primary winding in order to derive sai primary resistance variation rate on the basis o variation rate of said primary voltage. 5. A control system for an induction motor as se forth in claim 1, which further comprises an inverte circuit for driving said induction motor, which inverte circuit including a power transistor circuit comprising: a power transistor connected to a power sourc and switching between a first state for establishing a collector-emitter circuit and a second state for blocking said collector emitter circuit for supplying drive power to a load connected thereto; and a clipper circuit for absorbing surge energy to be generated upon switching of said power transistor from said first state to said second state, said clipper circuit including a first " clipper circuit cσnnected in parallel to a collector-emitter circuit of said power transistor and including a capacitor and a first diode; and a second clipper circuit including a second diode connected to said capacitor in parallel to a discharge resistor. 6. A control system for an induction motor as set forth in claim 1, which further comprises an inverter which is connected to DC current source via a smoothing circuit including a smoothing capacitor, said inverter circuit comprising: a plurality of power transistor circuits connected to a load for suppling a driving alternating current to the latter, each of said power transistor circuits including a power transistor switching between a first state for establishing a collector-emitter circuit and a second state for blocking said collector emitter circuit; and a clipper circuit for absorbing surge energy generated upon switching of said power transistor from said first state to said second state, said clipper circuit including a first clipper circuit connected in parallel to a collector-emitter circuit of said power transistor and including a capacitor and a first diode, and a second clipper circuit including a second diode connected to said capacitor in parallel to a discharge resistor. 7. A method for deriving a secondary winding resistance in an induction motor which has a primary winding connected to a driving power source and a secondary winding, as a control parameter for controlling driving torque and/or revolution speed of said induction motor, comprising the steps of: monitoring a primary current in said driving power to be supplied to said primary winding; monitoring a primary voltage in said driving power to be supplied to said primary winding; deriving variation rate of a primary resistance in said primary winding on the basis of said primary current and said primary voltage; and deriving a secondary resistance of said secondary winding on the basis of said primary resistance variation rate. 8. A method as set forth in claim 7, wherein derivation of said second winding resistance is performed place while a slip frequency of said induction motor is zero. 9. A method as set forth in claim 8, wherein derivation of said second winding resistance is performed while very low frequency current or direct current is applied to said induction motor.

Inventors: IWASA MASAO, KITO YASUTAMI, IWASA, MASAO, KITO, YASUTAMI
Applicants: OTIS ELEVATOR CO, OTIS ELEVATOR COMPANY
Application year: 1988
Main classification: H02P7
IPC codes: H02P21, H02P23, H02P27