GB2107538A - Device for driving an elevator using an a-c motor - Google Patents

Abstract

The device rectifies a commercial a-c power source 1 into a direct current through a converter 4, inverts the direct current into a-c electric power of a variable frequency through an inverter 7, and drives the a-c electric motor 8 using the a-c electric power. Switching contacts 19, 20 connect said converter 4 to said inverter 7 in a first polarity when said electric motor is in a driving mode of operation, and switch the connection between said converter and said inverter to the opposite polarity when said electric motor is in regenerative braking. A detector 6 senses overvoltage across a capacitor 5 during regenerative braking in order to effect changeover of the connection between the converter and inverter. <IMAGE>

Description

SPECIFICATION Device for driving an elevator using an a-c motor The present invention relates to the improvement of a device for driving an elevator using an a-c electric motor.

Prior systems for driving elevators use an induction motor which drives the cage of the elevator, wherein an alternating-current of variable voltage and variable frequency obtained through an inverter is supplied to the induction motor. With this system, electric power is regenerated when the cage is in regenerative braking mode, such as when the cage descends carrying a heavy load, when the cage ascends carrying a light load, or when the cage is decelerated. Therefore, a device is necessary to send the regerated power to the systems' power source. In Fig.

1, a three-phase a-c power source 1, in connected to a converter 2 consisting of voltage inverters, which convert an a-c electric power of a predetermined frequency into an a-c electric power of a variable frequency. An inverter 3 for regenerating the electric power, consists of thyristors 3a to 3f connected to the a-c power source. A converter 4 which consists of thyristors 4a to 4f, is connected to the a-c power source 1 and to the d-c side of the inverter 3. A smoothing capacitor 5 is connected between the output terminals on the d-c side of the converter 4, and a voltage detector 6 consisting of resistors is connected in parallel therewith. An inverter 7 of the pulse-width modulation type, converts a d-c voltage to an a-c electric power of any voltage and any frequency.Inverter 7 consists of transistors 7A to 7F and diodes 7a to 7f and is connected to the d-c side of the converter 4. A three-phase induction motor 8 is connected to the a-c side of the inverter 7. A drive sheave 9 is driven by the electric motor 8, and a main rope 10 wound on the sheave 9, has a cage 11 and a balancing weight 1 2 connected to the respective ends of the main rope 10. An overvoltage detector 1 3 is connected to the voltage detector 6. A phase control device 14 drives an igniting circuit 1 5 which controls the phases of the thyrisors 3a to 3f of the inverter 3 in response to the output of the overvoltage detector 13, or drives an igniting circuit 1 6 which controls the phase of the thyristors 4a to 4f of the converter 4.A distributor 1 7 controls the turn on or off of the transistors 7A to 7F of the inverter 7.

That is, when the electric motor 8 is in driving mode, the overvoltage detector 1 3 produces no output, the phase control device 14 drives the igniting circuit 1 6 whereby the phases of the thyristors 4a to 4f of the converter 4 are controlled so that it functions as a rectifier. The a-c electric power of the a-c power sources 1 is rectified through the converter 4 and is supplied to the inverter 7.

Therefore, the electric motor 8 is powered by an a-c electric source having a variable voltage and a variable frequency; i.e., the electric motor 8 is driven, and the cage 11 moves accordingly.

The induction motor 8 functions as a generator when it is rotated at a speed greater than its synchronous speed as in the case of braking mode of operation. In this case, the direction of d-c current from the inverter 7 is reversed compared with the case of the driving mode of operation, thereby causing the voltage across the terminals of the smoothing capacitor 5 to rise which may damage the semiconductor elements therein. Therefore, when the voltage at the terminals of the smoothing capacitor 5 rises above a predetermined value, the overvoltage detector 1 3 produces an output to drive the igniting circuit 1 5. Thus, the phases of the thyristors 3a to 3f of the inverter 3 are controlled, and the regenerated electric power is sent back to the power source 1 via the inverter 3.Consequently, the voltage at the terminals of the smoothing capacitor 5 remains constant.

However, when the electric motor 8 which has a large capacity and which regenerates large amount of electric power, is to be driven, the inverter 3 for regenerating the electric power must be provided. Therefore, the converter 2 becomes expensive.

When an induction motor which has a small capacity and which regenerates small amounts of electric power is to be used, the regenerated electric power is consumed through a resistor (not shown). In this case, the inverter 3 is not required for regenerating the electric power. However, the consumption of electric power-inevitably increases since the regenerated electric power is not returned to the power source 1.

The present invention is to improve the above-mentioned drawbacks, and has for its object to provide a novel device for driving an elevator using an a-c motor, in which the converter and the inverter for regenerating the electric power are commonly used, so that the regenerated electric power can be sent back to the power source without the need of particularly providing an inverter for regenerating the electric power.

Another object of the present invention is to provide a cheaply constructed device for driving an elevator using an a-c motor.

Figure 1 is a diagram illustrating the setup of a conventional device for driving an elevator using an a-c motor; Figure 2 is a diagram illustrating the setup of a device for driving an elevator using an a-c motor according to an embodiment of the present invention; and Figure 3 is a diagram illustrating another embodiment of the present invention.

In the drawings the same reference numerals denote the same or corresponding portions.

Referring to Fig. 2, contacts 19a, 1 9b of an electromagnetic contactor are inserted in the lines connecting the d-c output terminals of the converter 4 to the d-c input terminals of the inverter 7, contacts 20a, 20b of an electromagnetic contactor are inserted between the d-c output terminals of the converter 4 and the d-c input terminals of the inverter 7 in the opposite polarities, and a switching device 21 is connected to the overvoltage detector 13, and controls the switching operation of said electromagnetic contactors. The remaining units are the same as those of Fig. 1. The embodiment of the present invention, however, does not require inverter 3 for regenerating the electric power.

The operation of the above-mentioned embodiment will be described therebelow.

When the electric motor 8 is in driving mode, contacts 19a, 1 9b of the electromagnetic contactor are closed, and contacts 20a, 20b of the electromagnetic contactor are left open. The igniting circuit 1 6 controls the phases of the thyristors 4a to 4f so that the converter 4 will perform its rectifying operation. Therefore, the electric motor 8 is powered with an a-c electric source having a variable voltage and a variable frequency.

if the overvoltage detector 1 3 produces an output while the electric motor 8 is in regenerative braking mode, the switching device 21 operates so that the contacts 20a, 20b are closed, and the contacts 19a, 1 9b are opened. At the same time, the igniting circuit 1 6 controls the phases of the thyristors 4a to 4f such that the converter 4 operates as an inverter. Accordingly, the electric power regenerated by the electric motor 8 is returned to the power source 1 through the converter 4, without the need of specifically providing an inverter for regenerating the electric power.

A small electric current flows through the converter 4 when the contacts 19a, 1 9b and contacts 20a, 20b are switched. Accordingly, the contacts 19a, 19b, 20a, 20b require small current breaking capacities, and may be cheaply constructed therein.

Fig. 3 illustrates another embodiment according to the present invention, in which a plurality of elevators are operated in parallel.

In Fig. 3, inverters 71 to 75 are connected in parallel to the d-c side of the converter 4, and are respectively connected to three-phase induction motors 81 to 85, which respectively operate sheaves 91 to 95, main ropes 101 to 105, cages 111 to 11 5 and balancing weights 121 to 125. The remaining portions are the same as those of Fig. 2. When the elevators are to be operated in parallel, the required number of voltage-type inverter 71 to 75 is the same as the number of the elevators. However, only one converter 4 is required. Further, part of whole of the electric power consumed by the elevators in driven mode can be supplied with the regenerated electric power supplied by the elevators in regenerative mode making it possible to reduce the comsumption of electric power as a whole. Moreover, when the electric power is regenerated, the contact points 19a, 19b, 20a, 20b can be switched so that the electric power is returned back to the power supply 1, in order to further reduce the consumption of electric power.

According to the present invention as mentioned above, when the a-c electric motor of the elevator is in driving mode, the converter is connected to the inverter in the voltage-type inverter equipment for driving the electric motor. When the electric motor is in regenerative braking mode, the connection therebetween is switched to the opposite polarities, so that the converter works as an inverter to regenerate the power. Therefore, the regenerated electric power can be sent back to the power source without the need of specifically providing an inverter for regenerating the power, and the device can be cheaply constructed.

Claims (5)

1. In a device for driving an elevator using an a-c motor, by rectifying a commercial a-c power source into a direct current through a converter, converting the direct current into an a-c electric power of a variable frequency, and driving the a-c electric motor using the thus converted a-c electric power, the improvement wherein: switching contacts are provided which connect said converter to said inverter in a first polarity when said electric motor is in the driven mode of operation, and which switch the connection between said converter and said inverter to the opposite polarities when said electric motor is in regenerative braking mode of operation.

2. A device for driving an elevator using an a-c motor according to claim 1, wherein a plurality of inverters are connected in parallel.

3. A device for driving an elevator using an a-c motor according to claim 1, wherein said a-c motor is a three-phase induction motor.

4. A device for driving an elevator using an a-c motor, in which a commercial a-c power source is rectified into a direct current through a converter, the direct current is converted by an inverter into a-c electric power having a variable frequency, and an a-c electric motor is driven by the thus converted a-c electric power to drive a cage, said device utilized for driving an elevator comprising:: a smoothing capacitor which is connected across the d-c output terminals of said converter; a voltage detector which is connected in parallel with said smoothing capacitor and which detects whole or part of said d-c output voltage; an overvoltage detector which is connected to said voltage detector and which produces an output when said d-c output voltage becomes excessively high; contacts of a first electromagnetic contactor, connected between the d-c output terminals of said converter and the d-c input terminals of said inverter in a first polarity; contacts of a second electromagnetic contactor, connected between the d-c output terminals of said converter and the d-c input terminals of said inverter in the opposite polarity; and a switching device which is connected to said overvoltage detector and which controls the switching operations of said first and second electromagnetic contactors.

5. A device for driving an elevator using an a-c motor according to claim 4, wherein said switching device connects the converter to the inverter in said first polarity when the electric motor is in a driving mode of operation, and connects the converter to the inverter in said opposite polarity when the electric motor is in the regenerative braking mode of operation.