DYNAMIC BRAKING SYSTEM WITH SPEED CONTROL FOR AN ELEVATOR CAB

Abstract: An elevator braking system of an elevator system having an elevator cab is presented. The braking system comprises a motor having a shaft coupled to the elevator cab. The motor generating AC power in response to movement of the cab. A first rectifier circuit having a first AC input and a first DC output, is electrically connected to the motor at the first AC input. A braking circuit having a resistive element and switching device is connected in series across the first DC output. The switching device shunts the resistive element across the DC output in response to a fault condition to dynamically brake the elevator cab. Advantageously, this slows the speed of the cab during a movement operation necessitated from the fault condition. Therefore allowing the movement of passengers to any level below or above the stop, depending on an unbalanced condition of the cab and a counterweight. In an alternative embodiment the resistive element comprises a variable resistor. The elevator braking system may further comprise a sensor to provide a sensed signal indicative of the speed of the cab, wherein value of the variable resistor is varied based on the sensed signal. This advantageously provides regulation of the speed of the cab.

Description: DYNAMIC BRAKING SYSTEM WITH SPEED CONTROL FOR AN ELEVATOR CAB BACKGROUND OF THE INVENTION Field of the Invention The present invention is generally directed to safety equipment used in elevator systems and more particularly, the present invention is directed to a braking system for an elevator cab of an elevator system. Prior Art In a typical elevator system three-phase alternating current (AC) power is supplied from a source, e.g., commercially supplied power, to an elevator drive system. The source AC power is rectified to provide direct current (DC) source power having relatively positive and relatively negative source voltage potentials. The source DC power is then inverted to provide AC motor input power to a drive motor. A drive shaft of the drive motor is mechanically linked to a rotating sheave. An elevator rope engaged with the sheave is attached at a distal end to the elevator cab, and attached to a counter weight at an opposing distal end. The drive motor rotates the sheave, which causes the elevator cab to ascend and descend. During a fault condition (malfunction) of the normal operation of the elevator system, e.g., a power failure, the elevator's safety system employs a series of mechanical brakes to stop and hold the elevator cab at any fixed position in which it happens to be when the malfunction occurs. Since the elevator cab will most likely be stopped between floors, it is desirable to enable movement of the cab to a nearby floor. Usually this is accomplished by manually releasing the brakes and allowing the cab to descend or ascend (depending upon whether the cab or counterweight happens to be heavier at that point in time) freely to an available floor. However, the speed of the cab is difficult to regulate and it is desirable to provide a system which reduces the difficulty of speed regulation. SUMMARY OF THE INVENTION In an exemplary embodiment of the invention, an elevator braking system of an elevator system having an elevator cab comprises a motor having a shaft coupled to the elevator cab, the motor generating AC power in response to movement of the cab. The AC power is employed to cause a dynamic braking action that is controllable with a resistive element. The system employs a first rectifier circuit having a first AC input and a first DC output and an inverter circuit connected to said first DC output and having an AC output. The motor is connected to said AC output. There is a second rectifier circuit having a second AC input which is connected to said AC output and a second DC output which is connected to said first DC output. The system also employs a braking circuit having a resistive element and a switching device connected in series to each other, this braking circuit being coupled to said first and second DC outputs. The switching device shunts the resistive element across the DC output in response to a fault condition to dynamically brake the elevator cab. Advantageously, this slows the speed of the cab during a movement operation necessitated from the fault condition. In an alternative embodiment the braking circuit may additionally be used in normal operation to dissipate the energy generated by the motor during dynamic braking periods. In another alternative embodiment of the invention, the resistive element comprises a variable resistor or several fixed resistors. The elevator braking system may further comprise a sensor to provide a sensed signal indicative of the speed of the cab, wherein a value of the variable resistor or the string of fixed resistors is varied based on the sensed signal. This advantageously provides automatic regulation of the speed of the cab. In yet another alternative embodiment of the invention, the elevator braking system further comprises a sensor to provide a sensed signal indicative of the speed of the cab and a display device which provides a manual readout based on the sensed signal. A manual speed control provides manual adjustment of the value of the variable resistor or the string of fixed resistors. This embodiment enables a user to manually regulate the speed of the cab via the manual speed control using the manual readout as a guide. BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings wherein like elements are numbered alike in the several figures: Figure 1 is a schematic block diagram of an elevator braking system in accordance with the present invention; and . Figure 2 is a representative graph of rotational speed of the motor as a function of mechanical power in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, an exemplary embodiment of an elevator braking system of an elevator system in accordance with the present invention is shown generally at 10. The braking system includes input conductors 12, 14 and 16 for receiving electrical power from a suitable three-phase AC power source 18, e.g., a commercial power source. The input conductors 12, 14 and 16 carry the three-phase AC source power to a three- phase power rectifier 20 which is operative to convert the AC source power to DC source power to provide a relatively positive DC voltage on positive conductor 22 and relatively negative DC voltage on negative conductor 24. The power rectifier 20 includes three parallel pairs of series connected diodes 26, 28, 30, 32, 34 and 36. Diode 28 and diode 26 are connected in series between the positive and negative conductors respectively. The anode of diode 28 is connected to the negative rail 24, the cathode of diode 28 and the anode of the diode 26 are connected therebetween, and the cathode of diode 26 is connected to the positive rail 22. The input conductor 12 is connected to the junction 27 between diodes 26 and 28. Similarly diode 30 and diode 32 are connected in series between the positive and negative conductors, 22 and 24 respectively. The input conductor 14 is connected to the junction 31 between diodes 30 and 32. Similarly diode 34 and diode 36 are connected in series between the positive and negative conductors, 22 and 24 respectively. The input conductor 16 is connected to the junction 35 between diodes 34 and 36. As will be more particularly discussed hereafter the braking circuit 38 in accordance with the present invention is connected across positive rail 22 and negative rail 24. The braking circuit 38 provides dynamic braking to motor 76 in response to a fault condition of the elevator system. The positive and negative conductors 22, 24 respectively provide DC source power from rectifier 20, to a power inverter 40 at power inverter input terminals 42 and 44 respectively. The power inverter 40 includes three parallel pairs of series connected n-p-n power transistors 46, 48, 50, 52, 54 and 56, and three associated parallel pairs of series connected diodes 64, 66, 68, 70, 72 and 74. Transistor 46 and transistor 48 are connected in series between the positive and negative conductors, 22 and 24 respectively. The emitter of transistor 48 is connected to the negative rail 24, the collector of transistor 48 and the emitter of transistor 46 are connected therebetween at junction 47, and the collector of transistor 46 is connected to the positive rail 22. Diode 64 and diode 66 are connected in series between the positive and negative conductors 22 and 24 respectively, and connected therebetween to junction 47. The anode of diode 66 is connected to the negative conductor 24, the cathode of diode 66 and the anode of diode 64 are connected to junction 47, and the cathode of diode 64 is connected to the positive conductor 22. Junction 47 is connected to motor 76 via motor conductor 58. Similarly, the pair or transistors 50 and 52, as well the pair of diodes 68 and 70 are each connected in series between the positive and negative conductors 22 and 24 respectively, and connected to junction 51 therebetween. Junction 51 is connected to motor 76 via motor conductor 60. Also similarly, the pair or transistors 54 and 56, as well the pair of diodes 72 and 74 are each connected in series between the positive and negative conductors 22 and 24 respectively, and connected to junction 55 therebetween. Junction 55 is connected to motor 76 via motor conductor 62. Each base of power transistors 46, 48, 50, 52, 54 and 56 are connected to a controller (not shown). The controller provides pulse width modulated signals to the base of each transistor to convert the DC source power at conductors 22 and 24 into three-phase AC motor input power at conductors 58, 60 and 62. Diodes 64, 66, 68, 70, 72 and 74 are connected in parallel across each power transistor 46, 48, 50, 52, 54 and 56 respectively to protect the power transistors from back electro-motive forces (EMF) when the power transistors are being switched on and off. Alternatively as will be more particularly discussed hereafter, diodes 64, 66, 68, 70, 72 and 74 function as a three-phase power rectifier to convert AC generated power from motor 76 to DC generated power at positive and negative rails 22 and 24. Although power transistors 46, 48, 50, 52, 54 and 56 are shown as n-p-n transistors, it will be obvious to one skilled in the art that other switching devices may also be used, e.g., p-n-p transistors or relays. Three-phase AC motor input power from conductors 58, 60 and 62 drive synchronous motor 76. The motor 76 in this embodiment is preferably a permanent magnet synchronous motor. Motor drive shaft 80 of motor 76 is mechanically linked to sheave 82. Elevator cable 84 is disposed on sheave 82 and is connected to elevator cab 86 and counterweight 88 at opposing distal ends. During normal operation motor 76 operates in a motor mode by converting AC motor input power to mechanical power at its rotating drive shaft 80. During a malfunction (fault condition), e.g., a power failure, elevator cab 86 will be stopped and held in any fixed position it happens to be at the time of the malfunction by a system of mechanical brake (not shown). To move passengers (movement operation) in elevator cab 86, the mechanical brakes are released and elevator cab 86 is allowed to descend or ascend, depending on the existing unbalanced condition between cab 86 and the counter weight 88, to an available level. Upon moving, the unbalanced condition supplies the force (mechanical power) to turn sheave 82 which in turn rotates drive shaft 80 to drive motor 76. Motor 76 may operate in either a generator mode, i.e., mechanical power is input and electrical power is output, or a motor mode, i.e., electrical power is input and mechanical power is output. When the mechanical brakes are released, mechanical power from the unbalanced condition is transmitted as an input into motor 76. Therefore motor 76 operates in a generator mode during a movement operation. Unlike prior art elevator systems, the generator mode of motor 76 is used to regulate the speed of movement of elevator cab 86 through braking circuit 38, therefore rendering movement of the cab in the faulted elevator system easier. Braking circuit 38 includes a variable resistor 90 connected in series with n-p-n transistor 92. The emitter of transistor 92 is connected to the negative rail 24, and the collector of transistor 92 is connected in series to positive rail 22 through resistor 90. The anode of diode 96 is connected to the negative rail 24. The cathode of diode 96 is connected to the anode of diode 94 at junction 91 between resistor 90 and transistor 92. The cathode of diode 94 is connected to positive rail 22. The base of transistor 92 is electrically connected to a drive circuit (not shown). The drive circuit provides a signal to the base of transistor 92 in response to a signal to reverse bias (open) transistor 92 during normal operation, and to forward bias (close) transistor 92 during a malfunction. Without a power supply, the braking circuit must be closed. Although transistor 92 is shown as an n-p-n transistor it will be obvious to one skilled in the art that other switching devices may be used, e.g., p- n-p transistors or relays. Although transistor 92 is shown as being automatically activated by the drive circuit when a malfunction occurs, it will be clear to one skilled in the art following exposure to this discussion that transistor 92 may be manually controlled. Protective diodes 94 and 96 are connected in parallel across resistor 90 and switch 92 respectively to prevent damage due to back EMF during switching operations. A battery back up may be used to provide control power to the system in the event of a power failure. During a cab movement operation of a faulted elevator system, mechanical power supplied by descending elevator 86 is converted to generated AC power by motor 76 operated in generator mode. The generated AC power is rectified by diodes 64, 66, 68, 70, 72 and 74 of inverter circuit 40 to produce generated DC power having relatively positive and relatively negative generated voltage potentials at rail 22 and 24 respectively. By closing transistor 92, resistor 90 is effectively shunted across stator terminals (not shown) of motor 76 allowing the current produced by the motor to pass through it and producing dynamic braking of the motor. This acts as a dynamic brake, which slows the speed of descent of the elevator cab to the next level. As will be more particularly discussed hereafter, by varying resistance 90 the speed of descent of the elevator cab 86 can be regulated, i.e., the lower the resistance the slower the speed (see Figure 2). Braking circuit 38 can be used also to provide dynamic braking to motor 76 during a movement operation. Figure 1 discloses three alternative exemplary embodiments of regulating the speed of the elevator cab 86 with braking circuit 38. Variable resistance 90 may be an active load whose value of resistance and therefore speed of movement of elevator cab 86, may be automatically controlled by a control circuit sensitive to the output of either the speed sensor 98, e.g., tachometer, or frequency sensing device 100, e.g., frequency meter. Alternatively, resistance 90 may be manually controlled by manual speed control 102, e.g., potentiometer, in response to a readout from speed sensor 98. In one of the three exemplary embodiments, speed sensor 98 is attached to motor shaft 80 to provide a pulse width modulated output indicative of the rotational speed of motor 76, and therefore elevator cab 86 speed. The output of 98 is electrically connected to the control circuit provides a signal to vary the resistance of resistor 90 to automatically regulate the speed of elevator cab 86. In another of the three exemplary embodiments, frequency sensing device 100 senses the frequency of the generated AC power output from motor 76 to provide pulse width modulated output indicative of speed. The output of frequency sensing device is connected to the control circuit of the resistance 90, which provides a signal to vary the resistance of 90. In another of the three exemplary embodiments, either speed sensor 98 or frequency-sensing device 100 may produce a digital readout indicative of elevator cab speed to an operator (not shown). The operator then controls the speed of cab 86 by varying the resistance of 90 through manual speed control 102. Manual speed control 102 may be a potentiometer connected to an adjustable wiper arm (not shown) on variable resistor 90. Although resistor 90 is shown as a variable resistor it will be clear to one skill in the art that 90 may also be a group of parallel (or series) resistors controlled by a series of switches. Alternatively, resistor 90 may be a non-variable resistor which provides a relatively constant dynamic braking effect throughout the entire cab movement operation when transistor 92 is closed. In addition to providing dynamic braking during a cab movement operation, braking circuit 38 may also provide the dynamic braking required during normal operations in periods when motor 76 works as a generator, e.g., in deceleration periods. That is as AC motor input power rotates the motor 76 to ascend and descend the cab 86, the DC source power at terminals 42 and 44 can be shunted through resistor 90. This provides dynamic braking required to slow elevator cab 86 down as it approaches a designated floor. The resistance of resistor 90 and speed of cab 86 may be controlled by a conventional pulse width modulator, a dictatorial regulation system or other control system during normal operation. Referring to Figure 2, the motor rotational speed of motor 76 as a function of mechanical power input from the unbalanced condition during a movement operation is plotted for different input from the unbalanced condition during a movement operation is plotted for different values resistor 90, i.e., Rl, R2 and R3. As shown in the plot of Rl, if the stator terminals of motor 76 are short-circuited (Rl=substantially zero), the rotational speed would be very low regardless of the mechanical power input due to the (usually) low resistance of motor windings. As shown in the plots for R2 and R3, intermediate values of resistance 90 make it possible to regulate the rotational speed of motor 76 and the associated speed of elevator cab 86. If the stator terminals of motor 76 are open circuited, i.e., resistance 90 substantially equals infinity, then the speed of motor 76 would be unregulated. Under unregulated conditions the elevator cab would free fall, limited only by frictional forces. While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

Claims: What is claimed is: 1. An elevator braking system of an elevator system having an elevator cab, the braking system comprising: a motor having a shaft coupled to the elevator cab, the motor generating AC power in response to movement of the cab; a first rectifier circuit having a first AC input and a first DC output, the first AC input electrically connected to the motor; a braking circuit having a resistive element and switching device connected in series across the first DC output, wherein the switching device shunts the resistive element across the DC output in response to a fault condition to dynamically brake the elevator cab. 2. The elevator braking system of claim 1, wherein the resistive element comprises a variable resistor. 3. The elevator braking system of claim 2 further comprising a sensor to provide a sensed signal indicative of the speed of the cab, wherein value of the variable resistor is varied based on the sensed signal. 4. The elevator braking system of claim 2 further comprising: a sensor to provide a sensed signal indicative of the speed of the cab; a display device providing a manual readout indicative of the speed of the cab based on the sensed signal; and a manual speed control providing manual adjustment of the value of the variable resistor. 5. The elevator braking system of claim 1 wherein the shaft is mechanically coupled to an elevator sheave, the sheave having an elevator cable disposed thereon, the cable connected to the cab at a distal end and connected to a counterweight at an opposing distal end. 6. The elevator braking system of claim 1 wherein the first rectifier circuit further comprises an inverter circuit having an inverter DC input connected across the first DC output, and having an inverter AC output electrically connected to the first AC input. 7. The elevator braking system of claim 6 further comprising a second rectifier circuit having a second AC input and a second DC output, the second DC output connected across the first DC output. 8. The elevator braking system of claim 1 wherein the motor comprises a three- phase motor. 9. The elevator braking system of claim 1 wherein the motor comprises a synchronous motor. 10. The elevator braking system of claim 3 wherein the sensor comprises a frequency meter or a tachometer. 11. A method of braking an elevator cab of an elevator system, the method comprising: allowing the cab to descend or ascend the elevator system in response to a fault condition; rotating a shaft of a motor coupled to the cab to generate AC power from the motor in response to movement of the cab; converting the AC power to DC power; and shunting a resistive element across the DC power in response to the fault condition to dynamically brake the cab. 12. The method of claim 11 , wherein the resistive element comprises a variable resistor. 13. The method of claim 12 further comprising: sensing a characteristic of speed of the cab to provide a sensed signal indicative of the speed of the cab; and varying the value of the variable resistor based on the sensed signal. 14. The method of claim 12 further comprising: sensing a characteristic of the speed of the cab to provide a sensed signal indicative of the speed of the cab; displaying a manual readout indicative of the speed of the cab based on the sensed signal; and adjusting manually the value of the variable resistor. 15. The method of claim 11 wherein the rotating further comprises: coupling an elevator sheave to the shaft; disposing an elevator cable on the sheave; connected the cable to the cab at a distal end; and connecting a counterweight to the cable at an opposing distal end. 16. The method of claim 11 wherein the motor comprises a three phase motor. 17. The method of claim 11 wherein the motor comprises a synchronous motor. 18. The method of claim 13 wherein the sensing further comprises sensing a frequency of the AC power or a rotational speed of the motor.

Inventors: FERNANDEZ FRANCISCO, SERVIA ARMANDO, FERNANDEZ, FRANCISCO, SERVIA, ARMANDO
Applicants: FERNANDEZ FRANCISCO, OTIS ELEVATOR CO, SERVIA ARMANDO, FERNANDEZ, FRANCISCO, OTIS ELEVATOR COMPANY, SERVIA, ARMANDO
Application year: 2000
Main classification: B66B1
IPC codes: B66B1