GB1570435A - Circuit arrangements for dynamic braking of a synchronous three-phase motors - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO CIRCUIT ARRANGEMENTS FOR DYNAMIC BRAKING OF ASYNCHRONOUS THREE-PHASE MOTORS (71) We, LINDE AKTIEN GESELLSCHAFT, a German Company of D-6200 Wiesbaden, Abraham-Lincoln Strasse 21, German Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to circuit arrangements for dynamic braking of asynchronous three-phase motors.

It is possible to decelerate the squirrelcage rotor of an asynchronous motor particularly rapidly by short-circuiting one or more of the stator windings immediately after the supply to the stator windings is cut.

Such short-circuiting can be established by means of one or more triacs. Such rapid braking operations can be used to advantage wherever the load driven by an asynchronous motor has to be arrested rapidly and deliberately.

It is an object of the present invention to produce a circuit arrangement that permits rapid and positive actuation of the dynamic braking with the minimum possible cost in terms of circuitry.

The invention consists in a circuit arrangement for dynamic braking of an asynchronous three-phase motor connected to an alternating current supply through a contactor, wherein the input of a switching device is connected in parallel with one ot the contactor switches, wherein a triac is connected in parallel with one of the stator windings to provide said dynamic braking, wherein the output of said switching device is connected directly to one input of a first NAND element and through inverting delay means to the other input of said first NAND element, wherein the output of said first NAND element is connected to one input of a second NAND element the other input of which is connected to the output of a pulse generator, and wherein the output of said second NAND element is connected through an ignition device to the ignition contact of the triac to fire the triac to provide said braking.

With the aid of a circuit arrangement in accordance with the invention the triac can be ignited rapidly and positively directly the supply to the stator windings is cut. The function of the logic circuit elements is to positively prevent ignition of the triac in all operational states other than the dynamic braking state.

When choosing the switching device, there are the alternative possibilities of using a bridge rectifier followed by an optical coupler or a fast-acting relay. Both options offer the advantage of galvanic separation of the power stage from the switching stage.

The inverting delay means may consist of a two-input magnetic store with a first inverting time-delay element at one input and an inversion element followed by a second inverting time-delay element at the other input. A rectangular signal appearing at the output of the switching device is inverted by the delay means and its rising and falling flanks are both delayed. By using two separate time-delay elements, the delay times for the leading and trailing edge of the rectangular signal can be chosen independently of one another. These small delay times are essential for correct generation of the ignition pulses, in the particular embodiment described hereinafter.

If desired the output of the first NAND element may be connected to the input of the second NAND element through an adjustable time-delay element, which can be used to vary the rate of deceleration.

One method of performing the invention will now be described with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a circuit diagram of one embodiment of the invention, Figure 2 is a timing diagram showing the signals appearing at various points in the circuit illustrated in Figure 1, and Figure 3 is a graph showing the deceleration rates obtainable with and without the use of a circuit arrangement in accordance with the invention.

Three-phase stator windings 1, 2 and 3 are supplied with alternating current through power lines 4, 5 and 6. The stator windings can be cut off from the supply by means of a contactor 7 having three associated switches 8, 9 and 10 which are actuated together. A quick-action switch consisting of a bridge rectifier 11 and an optical-coupler 12 is connected in parallel with the switch 10. The optical coupler may consist, for example, of a light-emitting diode and a photo-sensitive transistor as indicated. A triac 13 is connected in parallel with the stator winding 3 through an adjustable dynamic braking resistor 35. As long as the motor is in operatioh and the switches 8, 9 and 10 are closed, there is no output from the bridge rectifier 11 because of the shortcircuit produced by the switch 10.Consequently a low signal is present at the output 14 of the optical-coupler 12. But when the switch 10 is opened, alternating current is supplied to the bridge rectifier 11 since one of its input terminals is connected to the phase line 6 while its other input terminal is connected to a zero voltage point 15 through the stator winding 3. As a result a high signal appears at the output 14 of the optical-coupler 12. The purpose of the logic circuit illustrated on the right-hand side of Figure 1 is to supply momentary ignition pulses to the ignition device 33 firing the triac 13 only when the high signal appears at point 14 after the opening of the switches 8, 9 and 10.

When the signal appearing at point 14 falls from high to low, the transition is delayed by an inverting time-delay element 16, which also serves to convert a high signal to a low signal and vice versa. The delayed and inverted signal is fed to one input 17 of a magnetic store 18. An inversion element 19 followed by a second inverting timedelay element 20 merely delays the low-tohigh transitions of the signal present at 14 and supplies this delayed signal to a second input 21 of the magnetic store. The output of the magnetic store depends on the signals applied to its inputs 17 and 21 and a signal appears at its output 22 as soon as the input 17 becomes positive relative to the input 21 and continues until input 21 becomes positive relative to input 17.The output 22 of the magnetic store is connected to one input of a first NAND element 23, the second input 24 of which receives the signal present at 14 unchanged. The output 25 of the NAND element 23 drops from high to low only when the input 24 receives a high signal when the motor is switched off and the input 22 receives a high signal from the magnetic store 18. The output of the magnetic store 18 changes to low as soon as the voltages at its inputs are reversed after the delay time following motor switch off produced by the time element 20. At that moment a high signal also reappears at the output 25 of the NAND element 23.

Thus the high signal normally present at point 25 is only converted into a low signal during the short time elapsing between the motor switching off and the output of the magnetic store dropping from high to low.

This momentary low signal is used to initiate the supply of ignition pulses to the triac. For this purpose the periodic rectangular pulses generated by a pulse generator 30, operating at 2.5 kHz for example, are fed to one input 29 of a second NAND element 31, while the output 25 of the first NAND element 23 is connected to the other input 28 through an adjustable inverting time-delay element 26. The rectangular pulses from the pulse generator appear at the output 32 of the NAND element 31 only when a high signal appears momonetarily at the input 2err. The ignition pulses produced in this way and only appearing at the output 32 directly after the motor is switched off are passed through ignition device 33 to the ignition contact 34 of the triac 13.

The signal sequences present at points 14, 17, 21, 22, 24, 25, 28, 29 and 32 are shown in the graph in Figure 2. This shows that the magnetic store is set soon after the motor is switched on and remains set until shortly after the motor is switched off.

The improvement in braking speed obtainable with the circuit arrangement of the embodiment described above can be seen from the graph in Figure 3. In the graph the speed n expressed as a percentage of the nominal running speed nO is plotted against time t after the commencement of braking at time 0. The curve 1 on the extreme right relates to the case of the motor running down without braking in the time tu. The middle curve 3 and the lefthand curve 2 show the speed reduction when dynamic braking is used. When plotting the left-hand curve, the resistor 27 which controls the delay introduced by the element 26 was set to its optimum value, whereas this resistor was set at a value corresponding to the longest braking time attainable with dynamic braking when plotting the middle curve.

WHAT WE CLAIM IS: 1. A circuit arrangement for dynamic braking of an asynchronous three-phase motor connected to an alternating current

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. Figure 1 is a circuit diagram of one embodiment of the invention, Figure 2 is a timing diagram showing the signals appearing at various points in the circuit illustrated in Figure 1, and Figure 3 is a graph showing the deceleration rates obtainable with and without the use of a circuit arrangement in accordance with the invention. Three-phase stator windings 1, 2 and 3 are supplied with alternating current through power lines 4, 5 and 6. The stator windings can be cut off from the supply by means of a contactor 7 having three associated switches 8, 9 and 10 which are actuated together. A quick-action switch consisting of a bridge rectifier 11 and an optical-coupler 12 is connected in parallel with the switch 10. The optical coupler may consist, for example, of a light-emitting diode and a photo-sensitive transistor as indicated. A triac 13 is connected in parallel with the stator winding 3 through an adjustable dynamic braking resistor 35. As long as the motor is in operatioh and the switches 8, 9 and 10 are closed, there is no output from the bridge rectifier 11 because of the shortcircuit produced by the switch 10.Consequently a low signal is present at the output 14 of the optical-coupler 12. But when the switch 10 is opened, alternating current is supplied to the bridge rectifier 11 since one of its input terminals is connected to the phase line 6 while its other input terminal is connected to a zero voltage point 15 through the stator winding 3. As a result a high signal appears at the output 14 of the optical-coupler 12. The purpose of the logic circuit illustrated on the right-hand side of Figure 1 is to supply momentary ignition pulses to the ignition device 33 firing the triac 13 only when the high signal appears at point 14 after the opening of the switches 8, 9 and 10. When the signal appearing at point 14 falls from high to low, the transition is delayed by an inverting time-delay element 16, which also serves to convert a high signal to a low signal and vice versa. The delayed and inverted signal is fed to one input 17 of a magnetic store 18. An inversion element 19 followed by a second inverting timedelay element 20 merely delays the low-tohigh transitions of the signal present at 14 and supplies this delayed signal to a second input 21 of the magnetic store. The output of the magnetic store depends on the signals applied to its inputs 17 and 21 and a signal appears at its output 22 as soon as the input 17 becomes positive relative to the input 21 and continues until input 21 becomes positive relative to input 17.The output 22 of the magnetic store is connected to one input of a first NAND element 23, the second input 24 of which receives the signal present at 14 unchanged. The output 25 of the NAND element 23 drops from high to low only when the input 24 receives a high signal when the motor is switched off and the input 22 receives a high signal from the magnetic store 18. The output of the magnetic store 18 changes to low as soon as the voltages at its inputs are reversed after the delay time following motor switch off produced by the time element 20. At that moment a high signal also reappears at the output 25 of the NAND element 23. Thus the high signal normally present at point 25 is only converted into a low signal during the short time elapsing between the motor switching off and the output of the magnetic store dropping from high to low. This momentary low signal is used to initiate the supply of ignition pulses to the triac. For this purpose the periodic rectangular pulses generated by a pulse generator 30, operating at 2.5 kHz for example, are fed to one input 29 of a second NAND element 31, while the output 25 of the first NAND element 23 is connected to the other input 28 through an adjustable inverting time-delay element 26. The rectangular pulses from the pulse generator appear at the output 32 of the NAND element 31 only when a high signal appears momonetarily at the input 2err. The ignition pulses produced in this way and only appearing at the output 32 directly after the motor is switched off are passed through ignition device 33 to the ignition contact 34 of the triac 13. The signal sequences present at points 14, 17, 21, 22, 24, 25, 28, 29 and 32 are shown in the graph in Figure 2. This shows that the magnetic store is set soon after the motor is switched on and remains set until shortly after the motor is switched off. The improvement in braking speed obtainable with the circuit arrangement of the embodiment described above can be seen from the graph in Figure 3. In the graph the speed n expressed as a percentage of the nominal running speed nO is plotted against time t after the commencement of braking at time 0. The curve 1 on the extreme right relates to the case of the motor running down without braking in the time tu. The middle curve 3 and the lefthand curve 2 show the speed reduction when dynamic braking is used. When plotting the left-hand curve, the resistor 27 which controls the delay introduced by the element 26 was set to its optimum value, whereas this resistor was set at a value corresponding to the longest braking time attainable with dynamic braking when plotting the middle curve. WHAT WE CLAIM IS:

1. A circuit arrangement for dynamic braking of an asynchronous three-phase motor connected to an alternating current

supply through a contactor, wherein the input of a switching device is connected in parallel with one of the contactor switches, wherein a triac is connected in parallel with one of the stator windings to provide said dynamic braking, wherein the output of said switching device is connected directly to one input of a first NAND element and through inverting delay means to the other input of said first NAND element, wherein the output of said first NAND element is connected to one input of a second NAND element the other input of which is connected to the output of a pulse generator, and wherein the output of said second NAND element is connected through an ignition device to the ignition contact of the triac to fire the triac to provide said braking.

z. A circuit arrangement as claimed in claim 1, wherein said switching device consists of a bridge rectifier followed by an optical coupler.

3. A circuit arrangement as claimed in claim 1, wherein said switching device consists of a fast-acting relay.

4. A circuit arrangement as claimed in any of the preceding claims, wherein the inverting delay means consist of a two-input magnetic store having a first inverting time-delay element connected to one input and an inversion element followed by a second inverting time-delay element connected to the other input.

5. A circuit arrangement as claimed in any of the preceding claims, wherein the output of the first NAND element is connected through an adjustable time-delay element to said one input of said second NAND element.

6. A circuit arrangement for dynamic braking of an asynchronous motor substantially as heretofore described with reference to Figure 1 of the accompanying drawings.