GB2091052A

GB2091052A - Circuit for controlling electric drives

Info

Publication number: GB2091052A
Application number: GB8200896A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-14
Filing date: 1982-01-13
Publication date: 1982-07-21
Also published as: JPS57129172A; SE8107251L; DK282A; CA1185320A; FR2498029A1; DE3100821A1; GB2091052B; FR2498029B1

Abstract

An analog-to-digital converter (12) is used in a circuit for controlling electric drives, using a rectifier (1), a DC controller (2) and an inverter (3). A desired output frequency value Fin is thereby converted and addresses a store (10) whose particular address represents a control value proportional to the desired output voltage of the inverter. After conversion in a digital-to-analog converter (14), the control value is fed to the controller (2) as a desired value, thus ensuring the optimum utilisation of energy. Different voltage frequency ratios may be selected by means of a selector (11) which enables different stored values to be addressed. <IMAGE>

Description

SPECIFICATION Circuit for controlling electric drives The invention relates to a circuit for controlling electric drives, including a rotary current rectifier, a d.c. controller and a rotary current inverter driven from an electronic control circuit.

When controlling the speed of rotary and alternating current drives, use is conventionally made of frequency inverters, since they enable the speed to be controlled with little loss. Distortion must be the minimum possible in the output voltage of the inverter over the whole speed range, and moreover the ratio between the output voltage of the inverter and its frequency must stand in a relationship which is specific for the particular speed control. Optimum energy utilisation is possible only if both conditions are met.

The known circuits for controlling the speed of electric drives use electronic step switches which, by special control of such switches, produce the required output voltage from the inverter. Complicated switching processes must take place in the step switches, for which can be used, for example, distributors, shift registers, counters or other equivalent quasi-dynamic step switches. A very complicated circuit is therefore required to meet the demands of this quasi-dynamic system. However, the quasi-dynamic nature in itself has a negative effect on the distortion in the output voltage produced, over the whole frequency range (speed range).

Since the distortion creates a deterioration in energy conversion because, for example, the energy converted into heat in an electric motor is directly proportional to the distortion, such voltage arrangements are not suitable for applications requiring optimum utilisation of energy, for example, heat pumps.

As already mentioned, the optimum conversion of electric energy into dynamic energy also depends on the ratio between the output voltage produced by an inverter and its frequency. In many applications, such ratio is primarily linear, and in the known cir cuits for controlling electric drives there is a possibility of slightly influencing the voltage-frequency ratio at the top and/or bottom end of the speed range. In this way an attempt is made to meet the requirements of any particular application to some extent.

In the case of certain electric drives such as, for example, heat pump drives or circulating and delivery pump drives, the voltage-frequency ratio is basically non-linear (e.g. cubic in the case of a ventilator drive), so that the known methods for adapting the voltage-frequency ratio of an inverter are completely inadequate for that of the particular application. Due to its poor utilisation of energy, a controllable drive operated in this way is completely uneconomic and therefore unsatisfactory.

It is therefore an object of the invention to provide a circuit for controlling electric drives with the optimum utilisation of energy.

To this end, according to the invention, a desired value of the electric drive, converted by an analogto-digital converter, addresses a store whose particular address contains a control value which is proportional to the output voltage ofthe inverter and which, after conversion in a digital-to-analog converter, is fed to the controller as a desired value.

The invention makes it possible to store in the store, for example a fixed value store, a number of desired values representing the voltage-frequency ratio, and to select such a ratio by a selector for the operation of an electric drive.

The d.c. controller can be constructed as a voltage controller and receive the desired voltage value of the rotary current inverter from the digital-to-analog converter.

The control circuit of the rotary current inverter can have a store which delivers control signals for the inverter and whose read-out speed is derived from the particular required drive speed (frequency) of the voltage-frequency ratio.

The control circuit in accordance with the invention will now be explained in greater detail with reference to the accompanying drawings, wherein:- Figure lisa circuit diagram, and Figure 2 is a graph.

The circuit diagram (Figure 1) partly in the form of a block diagram, consists of a rotary current (three phase) rectifier 1 which feeds a rotary current (three phase) inverter 3 through a d.c. voltage controller 2.

The rotary current inverter 3 comprises six n-p-n transistors 4, which are disposed in pairs with their collector-ernitter paíLi75 in series, each pair shunted across the output voltage ofthe d.c. controller2.

Each of the three connecting points of the en emitter of a transistor to the collector of another transistor of a pair forms an output U,V,W of the rotary current inverter 3. A driver stage 5 has six outputs Q0 to Q5 which control the base electrodes of the six transistors 4, so that rotary current with a freuency dependent on the operation of the transiors and an amplitude dependent on the output volt-jge of the d.c. controller 2 is produced at the outputs U,V,W.

The driver stage 5 is controlled & six outputs Q0to Q5 of a fixed value store 6, whose read-out speed can in turn be controlled by outputs Q0 to Q3 of a counter 7. A selector 8 enables particular addresses of the fixed value store 6 to be selected for calling up, so that particular wave shapes can be selected for the output voltage to be produced. The counter 7 is supplied with signals from a voltage-frequency converter 9 (which receives a desired input voltage Uin) and with a desired frequency input Fin, respectively.

The present invention is primarily concerned with the control circuit of the d.c. controller 2, which will now be described. For this purpose a fixed value store 10 stores a number of voltage-frequency ratios of particular electric drives, which can be selected by a selector 11. The store 10 is addressed via outputs Q0 to Q7 of an analog-to-digital converter 12 which is controlled by the output signal of a frequencyvoltage converter 13 fed with input Fin. As a result, in dependence on selection by the selector 11, digital data is available at outputs 0, to Q7 of the store 10, which outputs pass via a digital-to-analog converter 14 as a desired voltage input to the voltage controller 2 supplied with the actual voltage value.In this way the data of a number of voltage-frequency ratios can be stored in the store 10 and as a result a number of characteristics are available for the optimum acceleration and/or deceleration of the particular electric drive to be controlled. However, in addition to voltage control, it is also possible to control the load current and/or the power of the particular drive. The aforementioned signals Fin and Uin act on the circuit in similar manner two a super-ordinate control.

As can be seen from the graph (Figure 2) electric drives may have very different voltage-frequency ratios. In the graph, voltage is plotted on the ordinate, the frequency being plotted on the abscissa. A curve 20 indicates the course for a ventilator, while a curve 21 indicates the course for an electric motor, typically an electric motor used in heat pumps. The invention therefore enables the characteristics to be broken up into 2" portions 22 whose voltage values correspond to the data contained in the store 10. The result is an appret fably improved utilisation of energy in the con ersion of electric energy into dynamic energy, distortion of the output voltage produced being considerably reduced by the control circuit according to the invention.

Claims

1. A circuit for controlling electric drives, comprising a rotary current rectifier, a d.c. controller and a rotary current inverter driven from an electronic control circuit, characterised in that a desired input value for the electric drive, converted by an analogto-digital converter, addresses a store the particular addresses of which contain control values which are proportional to the output voltages of the inverter and which, after conversion in a digital-to-analog converter, is fed to the controller as desired control values.

2. A circuit according to claim 1, characterised in that a desired control value represents the voltagefrequency ratio of the electric drive.

3. A circuit according to claim 2, characterised in that a number of voltage-frequency ratios of particular electric drives, selectable by a selector, are stored in the store.

4. A circuit according to any one of claims 1 to 3, characterised in that the store is a fixed value store.

5. A circuit according to any one of claims 1 to 4, characterised in that the controller takes the form of a d.c. voltage controller, and the desired control value fed by the digital-to-analog converter to the voltage controller represents the desired value of the output voltage of the rotary current inverter.

6. A circuit according to any one of the preceding claims, characterised in that the electronic control circuit of the rotary current inverter has a store, preferably a fixed value store, which delivers control signals for the inverter and whose read-out speed is derived from the particular desired drive speed (fre quency) of the voltage-frequency ratio.

7. A circuit for controlling electric drives substantially as hereinbefore described with reference to the accompanying drawings.