Conveyors: power-driven – Conveyor or accessory therefor specialized to convey people – With means to control the operation of the section
Reexamination Certificate
2002-05-15
2003-09-30
Bidwell, James R. (Department: 3651)
Conveyors: power-driven
Conveyor or accessory therefor specialized to convey people
With means to control the operation of the section
Reexamination Certificate
active
06626279
ABSTRACT:
TECHNICAL FIELD
The invention concerns a device and a method of controlling the drive of a conveyor installation in the form of an escalator or a moving sidewalk, which can be switched between the load and the idle operation mode. This conveyor installation comprises a line voltage connection which supplies an essentially constant line frequency, an electric drive motor, particularly in the form of an induction or a synchronous motor, and a conveyance demand signal generator that signals a demand to switch operation modes.
BACKGROUND ART
A typical conveyor installation in the form of an escalator or a moving sidewalk for transporting passengers comprises a number of closely adjacent tread plates in the form of an endless band which are moved in the desired direction by the drive motor.
To reduce the power consumption and the wear of such conveyor installations, they now move only when conveyance is needed, otherwise they are brought to a standstill. To that end a conveyance demand signal generator is provided, for example in the form of a tread plate, a photosensitive relay or a manually activated switch, whereby the existence of a conveyance demand can be determined. If there is a conveyance demand, for example because a passenger has stepped on the tread plate, the conveyor installation goes into the conveyance mode for a predetermined period of time, and is then switched off if no further conveyance demand has been determined within a predetermined period of time.
To avoid peak loads during frequent on and off switching of the conveyor installation, it is known from WO 98/18711 not to switch the drive motor abruptly on and off, but to allow its RPM to increase or decrease linearly when switching. Such conveyor installations predominantly use induction motors. Since the RPM of an induction motor depends on the frequency of the supplied alternating voltage, which in the case of the direct supply from an alternating voltage network with constant line frequency means a constant RPM of the induction motor, a controllable frequency converter is used whereby the supplied line frequency can be converted in a controllable manner into an output frequency which differs from the line frequency.
The cost of a frequency converter which also supplies the drive motor of an escalator or a moving sidewalk in the load operation mode is high, since it increases enormously with the output power that a frequency converter must be able to produce.
To lower the acquisition and operation costs, WO 98/18711 provides that the conveyor installation only moves at full conveyance speed in the load operation mode, and that in the stand-by or idle mode when no conveyance is required, it only operates at a reduced idle operation speed, and the frequency converter only supplies the drive motor during idle mode and switch-over processes, while it is directly supplied by the line voltage source in the load operation mode. This creates the possibility of designing the maximum output of the frequency converter to be much lower, which leads to considerable cost savings as compared to a frequency converter whose maximum output is adapted to the conveyor installation's load operation. If no further conveyance demand is signalled after a conveyance order has been carried out, the conveyor installation in WO 98/18711 first changes into the idle mode, and only goes into standstill if no new conveyance demand is signalled during a predetermined period of time since the change-over to the idle mode.
The cited measures have achieved a considerable reduction in load peaks and abrupt speed changes of the conveyor installation. However, high transition currents can always occur when changing between the line supply and the frequency converter supply of the drive motor, namely due to the drive motor's own characteristic voltage which can overload the frequency converter and cause jerky movements of the conveyor installation. The present invention will overcome such events.
DISCLOSURE OF INVENTION
This is achieved with a method according to the invention as claimed in claim 1, and a device according to the invention as claimed in claim 11, where further developments of the method or the device are indicated in the dependent claims.
SUMMARY OF INVENTION
The method of the invention as well, the drive motor in the load operation mode is supplied with a line voltage, and in the idle mode with an output voltage of a frequency converter. To achieve the target of the invention, the line voltage and the frequency converter's output voltage are compared with respect to frequency and phase position, and the frequency converter is set to an output frequency which has a predetermined spacing from the line frequency. If a conveyance signal generator signals a demand for a switch-over of the conveyor installation from load to idle mode or vice versa, at the point in time after the demand to change the mode was signalled, when the output frequency of the converter has the same spacing with respect to the line frequency and has also reached a predetermined phase spacing between the output frequency of the converter and the line frequency, a signal is produced which triggers the switch-over of the drive motor between the frequency converter supply and the line supply.
The switching devices used to switch between the line supply and the frequency converter supply, usually contactors, are not delay-free on the one hand, and on the other require a zero current time between the turn-off of one contactor and the activation of the other in order to prevent a short circuit in the line through the frequency converter. There is a certain inherent reaction delay between the production of a switch-over signal and the previously conducting contactor turning off, and finally the activation of the other contactor, which depends on the special components of the special conveyor installation.
A smooth transition between the frequency converter supply and the line supply and vice versa can therefore not be achieved by monitoring the line voltage and the frequency converter's output voltage for a match in the frequency and phase position, and by producing a switch-over signal at the time when such a match is determined. By the time the inherent reaction delay actually produces a switch-over, a frequency and phase deviation would already have occurred and no smooth transition could take place between the line supply and the frequency converter supply.
The invention therefore produces a switch-over process “in advance”, i.e. it plans the reaction delay and the frequency and phase changes that occur during the reaction delay between the line frequency and the output frequency of the converter. To that end it determines in a special conveyor installation the inherent reaction delay, the frequency change in the motor voltage which occurs during the reaction delay, and the change in the phase difference between the line frequency and the output frequency of the converter which occurs during the reaction delay, and produces the switch-over control signal at a time in advance, when a predetermined frequency spacing as well as a predetermined phase spacing exist between the line voltage and the frequency converter's output voltage. The frequency and phase match needed for a smooth switch-over is then available at the end of the reaction delay.
The sign of the predetermined frequency spacing depends on the switch-over direction. Due to inherent friction losses in the conveyor installation, the motor RPM decreases during the zero current time when the drive motor is not supplied by either the line or the frequency converter. Since we start with a constant line frequency, which the motor RPM must match at the end of a switch-over process, the predetermined frequency spacing of the converter's output voltage is above the line frequency when the frequency converter supply is switched over, and it is under the line frequency when the line supply is switched over to the frequency converter supply.
The method of the invention can be carried out
Henkel Reinhard
Markus Richard
Spannhake Stefan
Stripling Ralph
Bidwell James R.
Otis Elevator Company
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