DRIVE CONTROL SYSTEM FOR ELECTRIC MOTOR

BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates generally to control system for controlling a power supply for a electric motor. More specifically, the inventio relates to a fail-safe system in a drive control syste of an electric motor, such as an induction motor. Further particularly, the invention relates to fail-safe system which detects abnormality in regenerated power absorbing circuit and thus detect abnormality in a power converter for performin fail-safe operation.

Description of the Background Art British Patent First (unexamined) Publicatio 2,167,252, published on May 21, 1986, discloses a elevator control system including an elevator hoist motor. The disclosed system also includes a drive circuit for the elevator hoist motor, which circuit has three phase alternating current rectifying converter. A regenerated energy absorbing circuit is connected across direct current (DC) terminals of the converter and comprises a regenerated energy absorbing or consuming resistor and a switch which comprises a switching transistor. The switch closes to permit the regenerated power to flow through the resistor for absorption thereof while the motor is in the operation of regenerative mode.

Generally, the regenerated power absorbing resistor tends to burn out in a short period when subjected to excess voltage exceeding its a rated voltage. Such excess voltage may be applied to the regenerated power absorbing resistor upon failure in a

circuit component, such as the converter/ for example. Breakage of the regenerated power absorbing resistor also tends to occur when the frequency of acceleration and deceleration becomes higher than a predetermined value. In such case, the regenerated power to be consumed by the regenerated power absorbing resistor becomes greater than the capacity of the resistor. As a result, the resistor tends to become over-heated or i the worst case broken. Furthermore, failure- of the switch, i.e. switching transistor, also results in breakage of the resistor.

Once the breakage of the regenerated power absorbing resistor occurs, the regenerated energy of the motor drive circuit cannot be consumed which causes the current voltage in the circuitry to rise. This results in stoppage of an inverter via an excess voltage trip circuit. This makes it impossible to accurately control the position of a cage of the elevator. In suc emergency situation, it is usual to generate an alar and stop the motor. When this occurs, the cage may stop at an intermediate position trapping any passangers who may be in the cage. SUMMARY OF THE INVENTION

Therefore, it is an object of the presen invention to provide a control system for an electric motor which can detect abnormality in the regenerated power absorbing ^" circuit and perform a fail-safe operation.

Another object of the invention is to provid a less expensive detector for detecting abnormality i the regenerated power absorbing circuit in the power supply for an electric motor, such as an inductio motor.

A further object of the invention is t provide a motor control system for controlling th operation of an elavator hoist motor, which performs -

fail-safe operation in response to failure of th regenerated power absorbing circuit, wherein th elevator cage is moved to a position where th passengers therein can get out at the floor which i closest to the position where failure is detected.

In order to accomplish the aforementioned an other objects, a control system for an electric motor according to the present invention, includes a detecto associated with a regenerated power absorbing circui for monitoring operation of the regenerated powe absorbing circuit for detecting an abnormal conditio thereof. The control circuit is responsive to detectio of abnormality in the regenerated power absorbin circuit for controlling operation of an inverter in fail-safe mode.

According to one aspect of the invention, power supply control system for an electric moto comprises a direct current power source means fo supplying direct current power, an inverter mean operative for receiving the direct current power fro the direct current source means and applying alternatin current driving power to the electric motor for drivin the latter in a controlled driving direction in a firs mode, and in a second mode, for outputting regenerate power from the electric motor to the direct curren power source means, a regenerated power absorbing means, interposed between the direct current power source mean and the inverter means, for absorbing the regenerate energy from the inverter means, an abnormality detector means, associated with the regenerated power absorbin means, for monitoring the operating condition of th latter to produce an abnormal condition indicativ signal when abnormality of the regenerated power absorbing means is detected, and a control means, associated with the inverter means for controlling operation of the latter to drive the electric motor in a

desired direction at a controlled speed, the control means being responsive to the abnormal condition indicative signal of the abnormality detector means for performing a predetermined fail-safe operation to control operation of the inverter means in fail-safe mode.

According to another aspect of the invention, a fail-safe system for a power supply system of a elevator hoisting motor which drives an elevator cage u and down with stops at one or more predetermined elevational positions for allowing passengers to enter into and exit from the elevator cage, comprises a direc current power source means for supplying a direc current power, which includes an alternating curren power source and a converter for converting alternatin current received from the alternating current power source into direct current power, an inverter means operative for receiving the direct current power fro the direct current source means and applying alternating current driving power to the electric motor for drivin the latter in a controlled driving direction, and for outputting to the direct current power source a regenerated power from the electric motor, a regenerate power absorbing means, interposed between the direc current power source means and the inverter means, for absorbing the regenerated energy from the inverter means, an abnormality detector means, associated wit the regenerated power absorbing means, for monitoring operating condition of the latter to produce an abnorma condition indicative signal when abnormality of th regenerated power absorbing means is detected, and a control means, associated with the inverter means fo controlling operation of the latter to drive th electric motor in a desired direction at a controlle speed, the control means being responsive to th abnormal condition indicative signal of the abnormalit

detector means for performing a predetermined fail-sa operation to control operation of the inverter means the fail-safe mode for driving the cage to one of t predetermined elevational positions. In the preferred construction, the regenerat power absorbing means includes a switch means associat with the inverter means and which is variable the swit position or switch state according to the operation mo of the inverter means between a first switch positio in which current flow through the regenerating powe absorbing means is blocked and a second switch position in which current flow through the regenerated powe absorbing means is permitted, so that the switch is hel at the first switch position while the inverter mean operates in the first mode and at the .second switc position while the inverter means operates in the secon mode. Also, the regenerated power absorbing mean comprises a resistor for absorbing the regenerate energy. In practice, the abnormality detector mean monitors the potential at the regenerated powe absorbing means. The abnormality detector mean monitors variation of the potential in the regenerate power absorbing circuit and detects abnormality of th latter based on a comparison frequency of variation o the potential thereof in relation to a predetermine frequency. The abnormality detector means produces th abnormal condition indicative signal when the frequenc of occurrence of variation of potential in th regenerated power absorbing means is lower than th predetermined frequency.

In the practical fail-safe operation, th control means maintains the revolution speed of th elevator hoisting motor at a speed so selected as t minimize the magnitude of energy regenerated by th motor.

Preferably, the motor driving speed in the fail-safe operation is determined on the basis of the loaded weight of the cage and traveling speed of the cage immediately before the failure is detected. According to a further aspect of the invention, a failure detector is provided in a power supply control system for an induction motor, the power supply system comprises a direct current power source means for supplying a direct currentspower, an inverter means operative for receiving the direct current power from the direct current source means and applying alternating current driving power to the electric motor for driving the latter in a controlled driving direction, and for outputting regenerated power from the electric motorto the direct current power source a regenerated power absorbing means, interposed betwee the direct current power source means and the inverter means, for absorbing the regenerated energy from the inverter means, and a control means, associated with th inverter means for controlling operation of the latter to drive the electric motor in a desired direction at a controlled speed, wherein the fail detector comprises a abnormality detector means, associated with th regenerated power absorbing means, for monitoring variation of the potential in the regenerated power absorbing circuit for detecting abnormality of the latter based on frequency of variation of the potentia in relation to a predetermined frequency. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood mor fully from the detailed description given herebelow an from the accompanying drawings of the preferre embodiment of the invention, which, however, should no be taken to limit the invention to the specifi embodiment but are for explanation and understandin only.

In the drawings:

Fig. 1 is a schematic block diagram of t preferred embodiment of a motor drive control syste according to the present invention, which control syst may perform a fail-safe operation upon failure of regenerated power absorbing circuit; and

Fig. 2 is a flowchart showing fail-monitori and fail-safe operation performed in practice by t control system of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly Fig. 1, the preferred embodiment of a motor dri control system, according to the present invention, h a three phase alternating current (AC) rectifying AC/ converter 10 which is of per se well known constructio As is well known, the AC/DC converter 10 is connected a power source so as to receive therefrom three phas alternating current R, S, T and rectifies or convert the received three phase alternating current into direc current (DC) . The AC/DC converter 10 thus has a pair o DC terminals connected to DC terminals of a DC/A converter 12. As is well known, the DC/AC converter 1 may comprise an inverter circuit and is designed t convert DC current received from the AC/DC converter 1 into three phase alternating current which is supplie to an electric motor 14, such as an induction motor.

A smoothing circuit 16 is connected across th DC terminals of the AC/DC converter 10. The smoothin circuit 16 includes a smoothing capacitor 18. regenerated power absorbing circuit 20 is also connecte across the DC terminals of the AC/DC converter 10. Th regenerated power absorbing circuit 20 comprises regenerated power absorbing resistor 22, a fuse 24 an switch 26. The regenerated power absorbing resistor 22 the fuse 24 and the switch 26 constitute a serie circuit. As will be appreciated, the fuse 24 serves fo

protecting the regenerated power- absorbing resistor 2 by blowing out in response to excess voltage which would otherwise tend to cause breakage of the resistor.

The regenerated power absorbing resistor 2 performs the per se well known function of absorbing or consuming ^' regenerated power from the DC/AC converter 1 which serves as regenerative inverter component. On th other hand, the switch 26 ^" comprises a switchin transistor which is switchable between a conductiv state and non-conductive state. The switchin transistor of the switch 26 is held non-conductive whil the motor 14 is accelerated by power supplied theret from the AC/DC converter 10 via the DC/AC converter 12. The switching transistor turns to a conductive stat during the regenerative mode operation of the mbtor 14.

A fail-detector circuit 30 is connecte between the regenerated power absorbing circuit 20 an the DC/AC converter 12. The detector circuit 30 i designed to detect abnormal state of the regenerate power absorbing circuit 20 for generating a detecto signal S,. The detector signal S-, is a variable leve analog signal having a value indicative of the curren level in the detector which, in turn, represent operating condition of the regenerated power absorbin resistor 22. The detector signal S, of the detecto circuit 30 is fed to a analog-to-digital (A/D) converte circuit 40 which generates a digital signal S corresponding to the detector signal value. The A/ converter circuit 40 feeds the digital signal S to CP 50. The CPU 50 controls operation of the DC/A converter 12 depending upon the level of the digita signal S ₂ input from the A/D converter 40.

In practice, the detector circuit 30 comprise a photo-coupler 32 connected to the regenerated powe absorbing resistor 22 via the fuse 24 in parallel to th switch 26, via a resistor 34. The photo-coupler 3

comprises a light emitting diode 36 and photo-transistor 38.

In the circuit construction set forth abov while the DC/AC converter 12 operates in regenerati state to apply the regenerated power to the regenerat power absorbing circuit 20, the switch 26 alternative switches its state between closed state to permit t regenerated current through the regenerated pow absorbing resistor 22 and open state to blo regenerated current to pass therethrough. This choppi operation of the switch 26 is taken place depending up the potential to be exerted thereon. Namely, the swit 26 closes to become conductive state when the potentia rises across a given level and opens to beco non-conductive state when the potential drops across th given level. At this condition, the potential applie to the light emitting diode 36 varies LOW level and HIG level depending upon the state of the switch 26 Therefore, the light emitting diode 36 is switched O and OFF alternatively according to switching of th switch 26 between closed and open states. Th photo-transistor 38 is held OFF when the light fron th light emitting diode 36 is not detected and turns ON i reponse to the light from the diode. Th photo-transistor outputs HIGH level detector signal S while it is held ON. As a result, the photo-transisto switches ON and OFF alternatively and therefore th level of the detector signal S-, vaires between HIGH an Low level. It will be appreciated, during choppin operation, the switch 26 alternates the state betwee open and closed states in a short period wit approximately regular intervals.

On the other hand, when the DC/AC converter 1 has no DC output or is not in regenerative state, th switch 26 becomes open. Therefore, the DC current fro

the AC/DC converter 10 passes through the regenerate power absorbing resistor 22 and flow through th detector circuit 30. Therefore, the potential applie to the light emitting diode 36 is maintained at HIG level to illuminate the latter. The photo-transistor 3 detects the light output of the light emitting diode 3 to output HIGH level detector signal S-,.

The A/D converter 40 outputs a digital signa having value "0" in response to the LOW level detecto _ signal S-, and having value "1" in response to the HIG level detector signal S, . The CPU 50 detects operatin condition of the motor 14 and the DC/AC converter 1 based on the digital signal to control operation of th DC/AC converter 12. Therefore, when the regenerated powe absorbing circuit 20 is in the normal condition, th digital signal value is held "1" as long as the DC/A converter 12 outputs no DC current or is not i regenerative state. On the other hand, the digita signal value alternates between "1" and "0" every tim the motor driving mode changes from acceleration mode t deceleration mode in which power is regenerated.

As will be appreciated from the foregoin discussion, the switch 26 will never held close position for a long period. Consequently, the digita signal value of the A/D converter 40 cannot be held "0 for a long period as long as the regenerated powe absorbing circuit 20 operates in normal state Therefore, failure of the regenerated power absorbin circuit 20 can be detected by monitoring the period o time, in which the value of the digital signal is hel at "0".

When breakage of the regenerated powe absorbing resistor 22 occurs, the current, i.e. the D current from the AC/DC converter 10 and the DC/A converter 12, does not get applied to the light emittin

diode 36. Thus, the light emitting diode 36 does n become illuminated. Therefore, signal level of t detector signal S, is constantly held LOW. As a result the digital signal value to be input to the CPU 5 remains "0". When the period in which the digita signal value is held "0" exceeds a predetermined perio which may be determined in view of possible longes chopping interval, CPU 50 makes a judgement that th regenerated power absorbing circuit 20 fails. In cases that the motor 14 must be driven i the deceleration mode after operating in the operatio for a predetermined period, failure of the regenerate power absorbing circuit 20 can be detected by checkin the input level from the A/D converter 40 Specifically, when the input level is held "0" despit acceleration mode operation of the motor 14, i indicates breakage of the regenerated power absorbin resistor 22.

The CPU 50 performs a fail-safe operation whe failure of the regenerated power absorbing circuit 20 i detected.

In cases where the elevator hoisting motor direction and magnitude of regenerated energy i variable depending upon direction of the cage motion i.e. up and down, and load condition, i.e. weight o passengers and luggage in the cage. That is, th regenerated energy can be minimized by appropriatel selecting driving direction of the motor in view of th direction of the cage motion and the load. Furthermore since the magnitude of the regeneration energy increase in proportion to driving speed of the cage, in othe words, by driving the motor at substantially low speed the energy to be regenerated becomes substantiall small. The CPU 50 performs fail-safe operation t control the DC/AC converter 12 for driving the motor 1 in a direction and speed selected to minimize the energ

to be regenerated.

Fig. 2 shows the process of a fail-saf operation performed by the CPU 50. As will b appreciated, the process shown in Fig. 2 may b triggered cyclically at given timings. The trigge timing may be for example, predetermined intervals.

Immediately after starting the process, th CPU 50 checks whether the regenerated power absorbin circuit 20 is in normal .condition or not. If th detected condition of the regenerated power absorbin circuit 20 is normal, process goes END and norma elevator control process contines.

On the other hand, when abnormality of th regenerated power absorbing circuit 20 is detected i the process of the step ST, , the cage travellin direction is checked at a step ST ₂ - In practice, th cage travelling direction may be judged based on th driving direction of the motor 14. When the cage i traveling upward, a process is performed to derive motor speed for the fail-safe operation, which moto speed will be hereafter referred to as "fail-safe driv speed" and motor drive direction which will be hereafte referred to as "fail-safe drive direction", and i determined on the basis of the motor driving directio and load in the cage deteced immediately before th fail-safe operation at step ST,. On the other hand when the cage is in downward traveling, a process i performed to derive the fail-safe drive speed an fail-safe drive direction on the basis of the moto driving direction and load on the cage detecte immediately before the fail-safe operation ST..

The fail-safe drive speed of the motor is s selected as to minimize the energy regenerated durin deceleration of the cage. Namely, the fail-safe driv speed is determined in a speed range where energy fo causing regenerated energy can be absorbed by power los

in a power train, for example. On the other hand, th fail-safe drive direction may be determined in view o the load on the cage to minimize the inertia moment t be exerted on the motor during deceleration. Based on the fail-safe drive speed as derive at the step ST, or ST., the CPU 50 outputs a contro signal S, to control operation of the DC/AC converter 1 for adjusting the motor driving speed to the fail-saf drive speed. By the fail-safe operation set forth above the elevator cage can be driven to an elevation wher the passengers in the cage can safely exit therefrom even when failure in the regenerated power absorbin circuit 20 occurs. It may be possible to generate an alar simultaneously to thesafe operation in the steps ST, an ST. for alarming abnormal condition of the power suppl circuit.

It should be noted that embodiments employin photo-coupler may be the preferred since it require smaller installation space and is inexpensive.

It will be appreciated that the photo-couple employed in the shown embodiment for detecting failur of the regenerated power absorbing circuit 20, can b replaced with detecany of variety of means which ca monitor the level of current flowing through th regenerated power absorbing circuit 20. Further it wil be easily recognized by those skilled in the art tha alternatives exist for all of the circuit components, not last the photo-coupler any other components, and should therefore be regarded as possible modifications for implementing the invention.

Therefore, although 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 fro the principle of the invention. Therefore, th invention should, be understood to include all possibl embodiments and modifications to the shown embodiment which can be embodied without departing from th principle of the invention set out in the appende claims.