HK1161581B - Method for operating an elevator in an emergency mode - Google Patents

Description

Method for operating an elevator in emergency mode

Background

Elevators comprising a car (car), possibly also a counterweight, a drive motor, a motor drive unit supplying power to and controlling the drive motor, and an emergency power supply (power supply) are known and widely used. In normal operation, the motor drive unit is connected to and receives power from the power grid and supplies power to the drive motor and thus controls the movement of the car according to corresponding commands received from the elevator controller. This type of elevator is disclosed in document WO 2005/040027a1 of the applicant of the present application, which is incorporated herein by reference in its entirety. Documents PCT/EP 2005/000174 and PCT/EP 2005/000175, which are also assigned to the applicant of the present application, relate to similar subject matter and are also incorporated herein in their entirety by reference. As is known from such prior art, it is possible to supply power to the motor drive unit from an emergency power supply in case of an emergency situation and to perform a rescue action (run) with power from the emergency power supply, which typically comprises a rechargeable battery, for example an action to run at a reduced speed to the next available platform. The rechargeable battery of the emergency power supply is typically kept under maximum load conditions to ensure sufficient capacity for any emergency operation. However, for a battery that can reliably drive the elevator car to the next available landing, a battery with a larger capacity is required. However, batteries are relatively expensive, so it is desirable to have as small a battery as possible.

Conventional motor drive units have power switching semiconductors, such as MOSFETs or IGBTs, which produce audible noise when operated at a switching frequency (switching frequency) within the spectrum of the audible noise. Conventional motor drive units therefore operate with a switching frequency in a range in order to avoid annoying noises in buildings and/or elevator cars.

It would therefore be beneficial to provide a method for operating an elevator in emergency mode and a corresponding elevator allowing to reduce the battery size for emergency power supply.

Disclosure of Invention

An exemplary embodiment of the present invention includes a method for operating an elevator in an emergency mode, wherein the elevator includes a car, a drive motor, a motor drive unit supplying power to and controlling the drive motor, and an emergency power supply, wherein the motor drive unit has a predetermined normal operation switching frequency, the method comprising the steps of:

(a) supplying power from an emergency power source;

(b) causing the motor drive unit to enter an emergency mode;

(c) determining actual emergency operation condition characteristics; and

(d) the switching frequency of the motor drive unit is set depending on the actual emergency operation condition characteristics.

Further exemplary embodiments of the invention comprise an elevator comprising a car, a drive motor, a motor drive unit connected to the drive motor and adapted to supply power to the drive motor and to control the drive motor, and an emergency power supply, wherein the motor drive unit has a predetermined normal-operation switching frequency, and wherein, in the case of an emergency situation, the elevator is adapted to operate in a normal-operation switching mode

(a) Receiving power from an emergency power supply;

(b) causing the motor drive unit to enter an emergency mode;

(c) determining actual emergency operation condition characteristics; and

(d) the switching frequency of the motor drive unit is set depending on the actual emergency operation condition characteristics.

Drawings

Embodiments of the invention are described in more detail below with reference to the accompanying drawings, in which:

fig. 1 is a diagrammatic illustration of components of an elevator according to a first embodiment of the invention;

fig. 2 presents a diagrammatic illustration with more details of an elevator according to a second embodiment of the invention; and

fig. 3 is a line graph showing different switching frequencies depending on the actual emergency operation conditions.

Detailed Description

Figures 1 and 2 show similar embodiments. Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Fig. 1 shows an elevator 2 comprising a hoisting rope 8, which hoisting rope 8 is driven by a drive motor 10 via a traction sheave 12. The hoisting line 8 may be a conventional rope or a coated steel strip or the like. The drive motor 10 drives the traction sheave 12 directly or through a gear. A brake disc 16 is arranged in connection with the traction sheave 12 and, in the present embodiment, is attached to the shaft 14 of the drive motor 10. The brake disc 16 is part of a brake 18.

Also attached to the shaft 14 of the drive motor 10 is an encoder wheel 20 which provides encoder or speed control information to a service panel 41 via line 22 and provides this information to the motor drive unit 26 via the service panel 41. The motor drive unit 26 provides the required power to the drive motor 10 via line 36. The motor drive unit 26 is connected to a power grid 28 for receiving power from the grid during normal operation. The motor drive unit 26 may be of any type, as will be described later with reference to fig. 2.

As an alternative to the encoder wheel 20, two encoding devices may be provided, one having a high resolution for normal mode operation and the second being connected to the service control panel 41 for emergency mode operation.

The elevator 2 also comprises an emergency power supply 42. The emergency power supply 42 includes a rechargeable battery 48 and battery loading and monitoring circuitry 52. The emergency power supply 42 may also include a booster 50 for supplying different output voltages. In order to supply an output voltage higher than the conventional voltage of the battery 48, a booster 50 may be necessary. For this embodiment, the emergency power supply provides three different output voltages, a lower voltage for voltage output 54, a higher voltage for output 56, and a medium voltage for output 58. Depending on the particular elevator, the voltage may vary. Typical voltage values, however, are 24 volts DC for the hoisting brake 18 and for supplying electrical control devices (e.g. speed controllers, etc.), 110 volts AC as is typical for elevator safety chains, and 520 volts DC for supplying the motor drive unit 26 and ultimately the drive motor 10 (a typical voltage in the intermediate circuit 98, which will be described below, is 400 volts DC). The latter voltage depends on the specific construction of the motor drive unit 26. Typically, such a motor drive unit 26 requires a minimum input voltage, even in emergency operation mode, the output voltage to the drive motor 10 will typically be much smaller.

In fig. 1, a lower voltage is supplied to the service control panel 41 via line 60 and can be distributed from the service control panel 41 to the brakes 18 via line 61 connecting the service control panel 41 to the brakes 18. Alternatively, a lower voltage is supplied to the motor drive unit 26 via line 60, wherein line 63 connects the motor drive unit with the brake 18. In the latter case, the motor drive unit 26 may control the brake 18. It is feasible to have only one of the lines 61 and 63 instead of two. The line 89 supplies a low voltage from the service control panel 41 to the motor drive unit 26 and/or supplies communication information between the service control panel 41 and the motor drive unit 26.

The motor drive unit 26 is preferably of a type that is capable of determining the movement condition of the elevator car, i.e. the position, direction of movement, speed and/or acceleration of the car, from the power information, i.e. the power withdrawn from the motor 10 if the motor 10 is operating in generator mode, and/or the power provided to the motor 10 in active drive mode. It should be noted that exemplary power information is voltage, current, frequency, etc. The motor drive unit 26 may comprise a memory for storing power information so that the relevant characteristics of the elevator 2 can be read from such a memory if the car has stopped in an emergency situation. Alternatively, it is possible to detect the corresponding characteristic while operating the elevator 2 in emergency mode. It is also possible to probe this power information in addition to information already stored from previous runs.

The motor drive unit 26 supplies varying power to the drive motor 10 in time for controlling the speed of the drive motor 10. Typically, power will be supplied in the form of pulse width modulated electrical pulses. To this end, the motor drive unit 26 comprises a control unit, e.g. a processor, which controls one or more electrical switches. These electrical switches are typically semiconductor devices, such as MOSFETs or IGBTs. Such devices have switching losses (swichting losses) that are more or less proportional to the number of switching actions per time unit. In other words, the switch may also generate noise, which is considered annoying by the people in the building of the user of the elevator. Therefore, the motor drive unit 26 typically has a predetermined switching voltage that is set according to a trade-off between power loss and generated noise. Such a switching frequency will not change for a conventional motor drive unit that is set up once by design.

The embodiment of fig. 2 is generally similar to fig. 1 and shows an elevator 2 including a car 4 and a counterweight 6. The car 4 and the counterweight 6 are suspended by hoisting ropes 8. The hoisting ropes 8 are driven by a drive motor 10 via a traction sheave 12. In addition to the embodiment of fig. 1, a Door Zone Indicator (DZI)64 is shown connected to door zone sensor 68 by line 70. In the embodiment of fig. 2, the door zone indicator 64 is connected to the individual speed controllers 24 by line 66. Alternatively or additionally, signal lines are provided that connect directly from the door zone sensor 68 to the speed controller 24. Once the elevator car 4 approaches and reaches the landing 72, the door zone sensor 68 sends a signal to the speed controller 24. Thus, the speed controller 24 can interrupt power to the brake 18 in the event of over-speed of the elevator car 4 or if the elevator car 4 has reached a landing. Similar door zone indicators and speed controls may be similarly provided in the embodiment of fig. 1.

Again, the motor drive unit 26 is connected by a line 30 to a main power supply 28 connected to the elevator 2 and receives control signals by a line 32. The elevator controller 34 is connected to a conventional hall call button (hall call button) and a cabin call button (cabin call button) (not shown), and receives a delivery request from these buttons. In addition, the elevator controller 34 is provided with actual operating situation information, from which the elevator controller 34 calculates an optimal travel sequence or the like, and the motor drive unit 26 is provided with a corresponding control signal for operating the car 4 accordingly.

The motor drive unit 26 includes a rectifier 94 and an inverter 96. The rectifier 94 and the inverter 96 are connected by means of a DC intermediate circuit 98. The rectifier 94 rectifies the AC current received via the line 30 and supplies the generated (quenching) DC voltage to the DC intermediate circuit 98.

In the preferred embodiment, the rectifier is a controlled rectifier or converter 94, which allows the recaptured power to be fed back to the grid 28, as opposed to a passive rectifier. The inverter 96 may be a VVVF inverter (VVVF: variable voltage variable frequency) that varies the voltage and frequency output used to control the drive motor 12 in accordance with the control signals of the elevator controller 34. Both the converter 94 and the inverter 96 comprise switching means, which, as already mentioned, are controlled by a respective control unit, for example a microprocessor. Each may have its own control unit, but it is also feasible to provide both of them with a single control unit. Similarly, both the inverter 96 and the converter 94 may have different switching frequencies.

The elevator 2 typically also includes a main power switch 86 located in the main power line 30. It is used to disconnect the main power supply 28 from the elevator 2 before the emergency drive mode operation is started in order to ensure well defined operating conditions, even if the main power supply will be re-established during the emergency mode. The main power switch 86 may be mechanically or electronically connected to a corresponding mechanism for initiating emergency operation.

In the embodiment of fig. 1 and 2, a mechanism for initiating emergency operation is provided. Also, the embodiment of fig. 1 includes a service control panel 41, which is triggered by a so-called brake release button ("BRB") 45. Similarly, the embodiment of fig. 2 includes an emergency brake switch 44 that, when closed, supplies emergency power to the brakes 18 via line 60 and lifts the brakes 18. Once the speed controller 24 detects that the car 4 reaches the desired landing 72 or an overspeed condition is reached, it interrupts the emergency power to the brake 18 by means of the speed control switch 62 (in particular a semiconductor device) so that the brake will fall (fall in) and stop the car. Instead of providing such a manually operated mechanism, an automated system may be provided. The motor drive unit 26 may be adapted to perform this task.

In general, in the case of an emergency situation (e.g. power outage, component failure, etc.), the elevator is opened, interrupting power from the main power supply to the elevator 2. In such a situation, an emergency condition may be detected by the automatic emergency drive controller (e.g., drive unit 26). To this end, the motor drive unit 26 (and the automatic emergency controller, respectively) may receive power from the emergency power supply 42 or may include its own power buffer device, such as a power storage capacitor or the like. It may then poll (poll) the necessary components for their availability to perform emergency operations, and begin emergency operations once such polling has been successfully performed. From here on, the automatic emergency control may be more or less the same as the manually initiated emergency operation.

The elevator 2 comprising the car 4 and the counterweight 6 has different actual emergency operating situation characteristics depending on the load situation in the elevator car 4 stopped in an emergency: (i) the car 4 and counterweight 6 may be in a balanced condition, i.e., the car 4 and counterweight 6 must be actively moved to the desired landing 72; (ii) the car 4 and counterweight 6 are slightly unbalanced, which requires actively initiating movement of the car and counterweight; (iii) the car 4 and the counterweight 6 are substantially unbalanced so that the car will continue to accelerate after the hoisting brake unless controlled accordingly.

It is clear that in conditions (i) and (ii) it is necessary to supply power from the emergency power supply 42 to the drive motor 10, whereas in condition (iii) the drive motor 10 acts as a generator and supplies power back to the motor drive unit 26. The present invention allows for an efficient supply of power to the drive motor 10 and/or processing of the power withdrawn from the drive motor 10 by adapting (fit) the switching frequency of the motor drive unit (i.e. the converter 94 and/or the inverter 96) depending on the actual emergency operation condition characteristics, such that an optimized operation may be performed. To this end, the motor drive unit 26 determines an actual emergency operating condition characteristic, for example, any of the conditions (i), (ii), and (iii) described above. In addition to distinguishing between these 3 conditions, the system may also distinguish between balanced and unbalanced conditions, or may distinguish a greater number of conditions than those in 3 above.

This determination can be made on the basis of elevator information, such as elevator power information stored during previous runs or actual information that can be obtained, for example, by lifting the brake while keeping the car and counterweight in place by means of the drive motor and the motor drive unit 26. It is also feasible to obtain the actual elevator situation from both sources of the elevator 2 in the same situation.

From this information, the motor drive unit 26 may determine an optimal setting of the switching frequency of the motor drive unit 26. Fig. 3 shows a simple but efficient scheme for setting the switching frequency. According to the unbalanced condition of the car 4 and the counterweight 6. On the horizontal axis of fig. 3, the relative balance/unbalance conditions are shown in relative percentage values, where 0% indicates a balanced condition, + 100% indicates a fully unbalanced condition in which the car is pulled axially by the weight of the counterweight 6, and-100% indicates a fully unbalanced condition in which the car 4 pulls the counterweight 6 axially along the axis. On the vertical axis, the switching frequency is given exemplarily, with a normal switching frequency of 5 kHz.

In the case of an emergency situation in a balanced or almost balanced condition, i.e. in the above conditions (i) and (ii), the switching frequency of the motor drive unit 26 is significantly reduced, i.e. in this example to 500 Hz. This has the effect of significantly reducing switching losses so that active operation of the drive motor 10 powered by the emergency power supply 42 can be performed more efficiently. In such emergency operating conditions, noise generation due to the reduced switching frequency is acceptable. In the case of a slightly more unbalanced condition (e.g., up to about 50%), the switching frequency is set about to the conventional switching frequency, i.e., it will typically not change. The drive motor 10 will be actively driven in this operating range, but generate a power which does not exceed the power which can be consumed in the elevator 2, in particular by the brake and/or the electric/electronic equipment. Only in the case of exceeding a certain unbalance situation, for example exceeding 50%, as shown in fig. 3, does the drive motor generate a certain amount of power that needs to be dissipated by other means than the conventional consumers (consumers) in the elevator 2. For this reason, the switching frequency is increased significantly, in this example to 20 kHz. By doing so, the switching losses increase accordingly, so that the motor drive unit 26 will act as a power consumer and dissipate the power that is withdrawn.

As already mentioned, the unbalance values, in particular the switching frequency values of fig. 3, are typical values considered practical by the inventors at this stage. The upper limit value of the switching frequency will be a compromise between the reduction in the lifetime of the switching devices in the motor drive unit 26 due to the increased thermal load in rescue operation and on the other hand the amount of power to be dissipated. Typically, the upper limit value of the switching frequency will be 2-5 times the normal switching frequency. In principle, increasing the switching frequency will result in an increase in the speed of the car during emergency operation, since in emergency operation the elevator 2 has only maximum power consumption capability and the drive motor unit 10 can only be operated in generator emergency mode at a speed corresponding to a power output equal to the maximum power consumption. Thus, increasing the switching frequency will result in an increased emergency operation speed and thus in a reduced rescue time for trapped passengers. On the other hand, this feature also allows the elimination or reduction of the capacity of the Dynamic Braking Resistor (DBR), which is required in the conventional non-regenerative elevator 2 to dissipate regenerative power from the drive motor 10. It is to be noted, however, that the invention is not limited to regenerative elevators, although they are a preferred embodiment. It is also possible to use the advantages of the invention in the case of non-regenerative elevators, i.e. to only reduce the switching frequency below the normal switching frequency for more efficient driving of the drive motor 16 etc.

It is preferable for the motor drive unit 26 (and emergency mode controller, respectively) to actively switch on all available electrical loads of the elevator 2 in case the withdrawn power needs to be dissipated.

Although a step of segmenting (stepwise) the switching frequency has been disclosed with reference to fig. 3, it will be noted that it is also conceivable to change the set frequency gradually. For example, in addition to reducing the rescue time for trapped passengers, it may be feasible to first significantly reduce the switching frequency, even in substantially unbalanced conditions, in order to support a rapid acceleration of the car 4 to a certain speed slightly below the emergency operation speed and to step up or gradually increase the switching frequency in order to set and maintain a desired rescue operation speed.

It has been shown that the present invention, at least in its preferred embodiment, allows for a maximum reduction in battery size, does not require additional circuit elements (e.g., dynamic braking resistors), and allows for a maximum increase in rescue speed. This allows reducing the component cost and maintenance cost of batteries that are periodically replaced during maintenance.

The exemplary embodiments of the invention described above allow to select (in particular change) the switching frequency of the motor drive unit during emergency operation. Therefore, when the car is actively driven by the drive motor during an emergency situation, it would be possible to significantly reduce the switching frequency, which would significantly reduce the losses generated by the motor drive unit, since the losses are proportional to the switching operation of the semiconductor device. Therefore, power consumption can be significantly reduced, and the capacity of the battery can be reduced accordingly. Although this may increase the noise generated by the motor drive unit, the noise is acceptable during emergency operation.

It is also possible to significantly increase the switching frequency of the motor drive unit in order to increase losses. This is particularly advantageous in the case of regenerative elevators, which recover energy under certain operating conditions and supply this energy back to the main input during normal operation. During emergency operation, it is generally not feasible to supply power back to the grid. If such a situation exists, a problem arises as to how to dissipate the power drawn from the drive motor. Since the battery of the emergency power supply is fully charged in this condition, it is not feasible to supply the recovered power to this battery. On the other hand, turning on all the electrical appliances of the elevator (e.g. luminaires, etc.) will typically not be sufficient to consume all the retrieved power. The conventional approach in the prior art is to use additional circuit elements, such as Dynamic Braking Resistors (DBRs), for dissipating this energy. However, the use of DBR circuits significantly increases manufacturing costs. Thus, exemplary embodiments of the present invention allow further cost reduction by providing a regenerative elevator without any additional circuit elements for power dissipation during emergency drive mode.

However, it may be advantageous to switch on all available electrical consumers during emergency operation, in which the withdrawn electrical power needs to be dissipated, i.e. as described above in terms of the emergency operating situation characteristics. It will also be noted that by increasing the dissipation of electrical power retrieved during such emergency operation, it is possible to increase the speed of the elevator car during a rescue operation and thus reduce the time for rescuing trapped passengers from the car.

In addition to the need to reduce or increase the switching frequency, there may be situations where no change in switching frequency is required, for example where the gravitational force acting on the car and/or counterweight is just sufficient to move the car at the conventional switching frequency setting, and no additional energy would need to be dissipated.

It may be preferable to continuously vary the switching frequency during emergency operation in order to provide optimal power to the drive motor or optimal electrical power dissipation during emergency operation. It is therefore possible to accelerate the car at the beginning of the emergency action with emergency running characteristics (in which the car will accelerate slowly during gravity) and to use a reduced switching frequency for driving the drive motor economically. After a certain time, or once the desired speed has been reached, the switching frequency of the motor drive unit can be changed abruptly or gradually, so that the car finally travels at its desired emergency speed.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (26)

1. Method for operating an elevator (2) in emergency mode, wherein the elevator (2) comprises a car (4), a drive motor (10), a motor drive unit (26) supplying power to the drive motor (10) and controlling the drive motor (10), and an emergency power supply (42), wherein the motor drive unit (10) has a predetermined normal operating switching frequency, the method comprising the steps of:

(a) supplying power from the emergency power supply (42) to cause a drive motor to actively move the car;

(b) -bringing the motor drive unit (26) into an emergency mode;

(c) determining actual emergency operation condition characteristics; and

(d) setting a switching frequency of the motor drive unit (46) in dependence on the actual emergency operation condition characteristic.

2. The method of claim 1, wherein the step of setting the switching frequency comprises varying the switching frequency of the motor drive unit (26) relative to the normal operating switching frequency.

3. The method according to claim 1 or 2, wherein the motor drive unit (26) comprises a converter (94) and an inverter (96), wherein the converter (94) is connected to an AC power source (28) for providing DC power to the inverter (96) in normal operation, and wherein the inverter (96) is connected to the drive motor (10); wherein the drive motor (10) and the motor drive unit (26) are adapted to operate in normal operation for withdrawing power when the drive motor (10) is driven by gravity acting on the car (4) and supplying the power back to the AC power source (28), wherein the method comprises the steps of: in the emergency mode, the switching frequency is increased relative to the normal operating switching frequency if gravity moves the car (4).

4. The method according to claim 1, wherein the motor drive unit (26) comprises an inverter (96) and a converter (94), and wherein the inverter (96) has a predetermined normal operating switching frequency, and wherein the switching frequency of the inverter (96) is set.

5. The method according to claim 1, wherein the motor drive unit (26) comprises an inverter (96) and a converter (94), and wherein the converter (94) has a predetermined normal operating switching frequency, and wherein the switching frequency of the converter (94) is set.

6. The method according to claim 1, characterized in that the method further comprises the step of: stopping the car (6) in response to an emergency prior to step (a).

7. The method according to claim 1, characterized in that the method further comprises the step of: -determining a parameter characteristic for the actual condition of the elevator (2), and-changing the switching frequency in dependence of such parameter.

8. Method according to claim 7, characterized in that the parameter is the load condition of the car (4) and counterweight (6).

9. Method according to claim 7, characterized in that the parameter is the speed of the car (4).

10. The method of claim 7, wherein the parameter is a current through an inverter (36).

11. The method according to any one of claims 7 to 10, characterized in that the method further comprises the step of: -determining, on the basis of said parameters, whether it is necessary to supply electrical power to the drive motor (10) in order to move the car (4), and-if it is necessary to supply electrical power to the drive motor (10) in order to move the car (4), reducing the switching frequency with respect to a normal operating switching frequency.

12. The method according to any one of claims 7 to 10, characterized in that the method further comprises the step of: -determining from said parameters whether the car (4) will move due to gravity, and-if the car (4) will move due to gravity-increasing the switching frequency relative to a normal operating switching frequency.

13. Method according to claim 12, characterized in that the switching frequency is only increased when the speed of the car (4) exceeds a certain limit value.

14. Method according to claim 12, characterized in that the switching frequency is only increased to the extent necessary to dissipate the excess electrical power regenerated by the drive motor (10).

15. Elevator (2) comprising a car (4), a drive motor (10), a motor drive unit (26) and an emergency power supply (42), which motor drive unit (26) is connected to the drive motor (10) and is adapted to supply power to the drive motor (10) and to control the drive motor (10), wherein the motor drive unit (26) has a predetermined normal operating switching frequency, and wherein, in the case of an emergency situation, the elevator (2) is adapted to

(a) Receiving power from the emergency power supply (42) to cause a drive motor to actively move a car;

(b) -bringing the motor drive unit (26) into an emergency mode;

(c) determining actual emergency operation condition characteristics; and

(d) setting a switching frequency of the motor drive unit (26) in dependence on the actual emergency operation condition characteristic.

16. Elevator according to claim 15, characterized in that the motor drive unit comprises a converter (94) and an inverter (96), wherein the converter (94) is connected to an AC power source (28) to provide DC power to the inverter (96) in normal operation, and wherein the inverter (96) is connected to the drive motor (10); wherein the drive motor (10) and the motor drive unit (26) are adapted to recover energy when the drive motor (10) is driven by gravity acting on the car (4) and to supply the energy back to the AC power source (28); and wherein in the emergency mode the drive motor unit (26) is adapted to increase the switching frequency relative to the normal operating switching frequency in case gravity moves the car (4).

17. Elevator (2) according to claim 15 or 16, characterized in that in the case of emergency mode the elevator (2) is also adapted to perform an emergency stop before power is supplied from the emergency power supply (42).

18. Elevator (2) according to claim 15 or 16, characterized in that in the case of emergency mode the elevator (2) is also adapted to obtain a parameter indicating the actual condition of the elevator (2) and to set the switching frequency depending on such parameter.

19. An elevator (2) as claimed in claim 18, characterized in that the parameter is the load condition of the car (4) and counterweight (6).

20. An elevator (2) as claimed in claim 18, characterized in that the parameter is the speed of the car (4).

21. An elevator (2) as claimed in claim 18, characterized in that the parameter is the electrical power generated by the drive motor (10).

22. Elevator (2) according to claim 16, characterized in that the inverter (96) has a predetermined normal operating switching frequency and wherein the elevator (2) is adapted to set the switching frequency of the inverter (96) in the case of emergency mode.

23. Elevator (2) according to claim 16, characterized in that the converter (94) has a predetermined normal operating switching frequency, and wherein the elevator is adapted to set the switching frequency of the converter (94) in the case of emergency mode.

24. Elevator (2) according to claim 18, characterized in that in the case of emergency mode the elevator (2) is also adapted to determining, on the basis of the parameters, whether the car (4) will move due to gravity or whether electric power will have to be supplied to the drive motor (10) in order to move the car (4), and to increasing the switching frequency relative to the normal operating switching frequency and to decreasing the switching frequency relative to the normal operating switching frequency if the car will move due to gravity, respectively, and to supplying electric power to the drive motor (10) in order to move the car.

25. Elevator (2) according to claim 24, characterized in that the elevator (2) is adapted to increase the switching frequency only when the speed of the car (4) exceeds a certain limit value.

26. Elevator (2) according to claim 24, characterized in that the elevator (2) is adapted to only increase the switching frequency to the extent necessary to dissipate the excess electric power regenerated by the drive motor (10).