ELEVATOR MACHINE BRAKING

An elevator machine assembly includes a motor and an elevator drive configured to control power supply to the motor. The elevator drive includes at least a plurality of converter switches and an energy storage device that is situated to be charged in response to the motor generating a back emf as the motor rotates in response to a torque applied to the motor when the elevator drive is not providing power to the motor. A short-circuiting module that is selectively coupled with the energy storage device through the converter switches selectively discharges the energy storage device to limit the back emf of the motor and a corresponding speed at which the motor rotates.

BACKGROUND

Elevator systems are in widespread use for carrying passengers between various levels in buildings, for example. Some elevator systems are traction-based in which roping that suspends the elevator car and a counterweight facilitates movement of the elevator car when needed. The roping moves in response to movement of a traction sheave, which is driven by a machine motor or prevented from rotating by a machine brake. In most situations, the machine brake is applied or dropped to stop the elevator car. Alternative or additional ways of braking movement of the elevator car at the machine are needed to comply with some elevator codes, for example.

SUMMARY

An illustrative example embodiment of an elevator machine assembly includes a motor and an elevator drive configured to control power supply to the motor. The elevator drive includes at least a plurality of converter switches and an energy storage device that is situated to be charged in response to the motor generating a back emf as the motor rotates in response to a torque applied to the motor when the elevator drive is not providing power to the motor. A short-circuiting module that is selectively coupled with the energy storage device through the converter switches selectively discharges the energy storage device to limit the back emf of the motor and a corresponding speed at which the motor rotates.

In addition to one or more of the features described above, or as an alternative, the short-circuiting module comprises a plurality of resistors and a switch that selectively couples the resistors and the converter switches.

In addition to one or more of the features described above, or as an alternative, the switch of the short-circuiting module couples the resistors to the converter switches in response to the elevator drive disconnecting the motor from power.

In addition to one or more of the features described above, or as an alternative, the switch of the short-circuiting module comprises a relay switch that closes in response to the elevator drive disconnecting the motor from power.

In addition to one or more of the features described above, or as an alternative, the elevator drive comprises a brake control that determines a voltage of the energy storage device and controls the converter switches to selectively couple the short-circuiting module and the energy storage device to maintain the voltage of the energy storage device within a preselected range.

In addition to one or more of the features described above, or as an alternative, the converter switches each comprise an IGBT or a MOSFET and the brake control turns on the IGBTs in response to the voltage exceeding a preselected threshold.

In addition to one or more of the features described above, or as an alternative, the brake control turns the IGBTs or MOSFETs on and off repeatedly to allow the voltage of the energy storage device to repeatedly increase and decrease within the preselected range.

In addition to one or more of the features described above, or as an alternative, the energy storage device comprises a capacitor.

In addition to one or more of the features described above, or as an alternative, the short-circuiting module comprises a plurality of resistors and a switch that selectively couples the resistors to the converter switches, the converter switches each comprise an IGBT or a MOSFET, and the energy storage device comprises a capacitor.

An illustrative example embodiment includes a method of using an elevator machine to control movement of an associated elevator car. The elevator machine includes a motor configured to selectively move the elevator car and an elevator drive configured to control power supply to the motor, the elevator drive having an energy storage device and converter switches. The method includes determining that a voltage of the energy storage device exceeds a preselected threshold. The voltage of the energy storage device is based on a back emf of the motor rotating when the elevator drive is not supplying power to the motor, using the converter switches to couple the energy storage device and a short-circuiting module based on the voltage of the energy storage device exceeding the preselected threshold, and discharging the energy storage device using the short-circuiting module to maintain the voltage of the energy storage device and the back emf of the motor within respective preselected ranges.

In addition to one or more of the features described above, or as an alternative, the method includes using the converter switches to uncouple the energy storage device from the short-circuiting module once the voltage of the energy storage device drops to a selected value.

In addition to one or more of the features described above, or as an alternative, the method includes allowing the back emf of the motor to recharge the energy storage device and repeating each of determining that the voltage of the energy storage device exceeds the preselected threshold, using the converter switches to couple the energy storage device and the short-circuiting module, and discharging the energy storage device.

In addition to one or more of the features described above, or as an alternative, the converter switches comprise IGBTs or MOSFETs and using the converter switches to couple the energy storage device and a short-circuiting module comprises selectively turning on the IGBTs or MOSFETs.

In addition to one or more of the features described above, or as an alternative, the short-circuiting module comprises a plurality of resistors and a switch that selectively couples the resistors and the converter switches in response to the elevator drive disconnecting the motor from power.

In addition to one or more of the features described above, or as an alternative, the switch of the short-circuiting module comprises a relay switch that closes in response to the elevator drive disconnecting the motor from power.

In addition to one or more of the features described above, or as an alternative, the method includes repeatedly turning the converter switches on and off to allow the voltage of the energy storage device to repeatedly increase and decrease within the preselected range.

In addition to one or more of the features described above, or as an alternative, the energy storage device comprises a capacitor.

In addition to one or more of the features described above, or as an alternative, the short-circuiting module comprises a plurality of resistors and a switch that selectively couples the resistors and the converter switches, the converter switches each comprise an IGBT or a MOSFET, and the energy storage device comprises a capacitor.

The various features and advantages of an example embodiment will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

DETAILED DESCRIPTION

FIG.1schematically illustrates selected portions of an elevator system20. An elevator car22is supported by a roping arrangement or suspension assembly24that includes a plurality of load-bearing suspension members26. The elevator car22is coupled to a counterweight28by the suspension members26. A machine30includes a traction sheave32for controlling movement of the elevator car. As the suspension members26move in response to rotation of the traction sheave32, the elevator car22and counterweight28move vertically. A machine brake (not illustrated) selectively applied a braking force to prevent the traction sheave32from rotating and, thereby, prevents the elevator car22from moving.

FIG.2schematically illustrates selected portions of an example elevator drive40that provides machine braking that can be used even when the machine brake is unable to apply a sufficient braking force because the machine brake is malfunctioning or the load on the elevator car is too large for the machine brake to resist rotation of the traction sheave32.

The example elevator drive40receives power from a power source, such as a utility grid, through a filter42when switch contactors44are turned on or conducting. The drive40includes known converter switches46and inverter switches48that are controlled to provide power to the motor50of the elevator machine30when needed for moving the elevator car22. In the illustrated example embodiment, the converter switches46are IGBTs or MOSFETs and the inverter switches48are IGBTs or MOSFETs. The switch contactors44turn off or disconnect the power from the grid when the elevator car stops. The machine brake drops and applies a braking force to prevent the traction sheave32from rotating while the elevator car should remain stationary.

If the traction sheave32were to rotate under those conditions, the motor50generates back emf in a known manner that charges an energy storage device52, which in this example embodiment comprises a capacitor. The back emf increases as the motor50rotates faster. The voltage across the capacitor52indicates the magnitude of the back emf. A controller54controls the converter switches46or turns them on to selectively discharge the capacitor52through a short-circuiting module56. The controller54selectively controls the converter switches46based on the voltage across the capacitor to keep the back emf within a preselected range. The controller54maintains the voltage on the capacitor52and, therefore, the back emf below a predetermined threshold to limit the speed at which the motor50may rotate. Controlling the back emf of the motor50in this way effectively brakes rotation of the motor50and limits a speed at which the roping or suspension assembly24and the elevator car22may move. In some embodiments, the motor50is allowed to rotate at a maximum speed corresponding to movement of the elevator car22at 0.3 m/second.

In the illustrated example embodiment, the short-circuiting module56includes a switch58and resistors60. The size or resistance of the resistors60is based on factors such as the duty load of the elevator car22and characteristics of the motor50. The controller54monitors current flow through the resistors60in the illustrated example embodiment to verify that the short-circuiting module56is working as intended.

The illustrated example embodiment includes three resistors60, one for each phase of the motor50. Another embodiment includes two resistors connected across two of the phases instead of all three phases. Another embodiment includes a single resistor across the DC link capacitor with one switch.

The switch58is on or closed when the capacitor52should be discharged through the resistors60. In some embodiments, the switch58turns on whenever the switch contactors44disconnect the drive40from the utility power grid or the elevator car is idle. The switch58comprises a relay switch in some embodiments.

The controller54is schematically shown in broken lines because it may be embodied in a physical component or be a function performed by the elevator drive40. In some embodiments, the controller54comprises software or firmware added to the elevator drive40. In other embodiments, at least one dedicated hardware component performs the function of the illustrated controller54.

FIG.3is a flowchart diagram70summarizing an example method of controlling movement of the elevator car using elevator machine braking as described above. At72, the elevator car22is idle and the machine brake is applied or dropped. The switch58closes or turns on at74. The controller54determines whether the voltage across the capacitor52exceeds a preselected threshold at76. When that voltage exceeds the threshold, at78, the controller54turns on the converter IGBTs46to discharge the capacitor52. The controller54monitors the voltage across the capacitor52at80and determines whether that voltage reaches a preselected base magnitude at82. The brake control turns off the converter IGBTs46and allows the back emf of the motor50to recharge the capacitor52at84and continues to monitor the voltage across the capacitor at76.

The method summarized inFIG.3repeatedly allows the back emf of the motor50to charge the capacitor52and then discharges the capacitor52to keep the back emf of the motor50within a desired range to maintain a speed of movement of the elevator car below a desired threshold. Controlling the back emf of the motor50in this way provides elevator machine braking even if the machine brake is unable to prevent the traction sheave32from rotating.

Some features or functions mentioned above as being included in an embodiment may be combined with at least one feature or function described as part of another embodiment. The features and functions described above may be combined in other ways to realize other embodiments.