GB2133233A - Braking circuit for electric motors - Google Patents

Abstract

A motor 8 with a brake resistor 9 is braked by means of a controlled rectifier 13 connected to control circuits 3, 5, of which circuit 5 receives inputs from a desired-value producer 6 and a tacho-generator 7; circuit 5 comprises converters (27, 25 and 30, Fig. 2), of which the first two receive the tacho- generator output and the third (a voltage-to-current converter) is connected to the desired-value producer, and an F.E.T. (33) connected between the output of the first, voltage- to-voltage converter (27) and the input of the second, voltage-to-current converter (25). The outputs of the second and third converters (25, 30) constitute the output of circuit Sand are fed via an optical coupling device to control circuit 3. <IMAGE>

Description

SPECIFICATION Braking circuit for electric motors The present invention relates to a braking circuit for electric motors and more particularly to a circuit for braking single-phase or direct-current series-wound motors. Such motors have a high starting torque and the ability to adapt themselves well to varying loads. They are therefore also used for producing high accelerations, for example for centrifuges. The circuit can be used, in an appropriate construction, for braking the motors in all fields of application.

In order to brake series-wound motors, the reversal of the torque compared with that of motor operation must be achieved by a control action.

According to the type of circuit used for this, regenerative braking, rheostatic braking or counter-current braking is possible, as is known. A large number of circuits have been developed for rheostatic braking (for example, DD--PS 85 803, 118487, DE-B 1 200424,2002 768, 2 135 324,2 228 735,2 541 113,2608581, 2640745,2824045).

The brake circuit according to DE-OS 2 640 745 contains a rheostatic braking with controlled separate excitation. A transistor connected into the energizing circuit is controlled depending on a voltage dropping via the brake resistor. If the correct polarity is selected, the field is only weakly excited at high speeds; as the speed decreases, the excitation becomes stronger. The braking torque is substantially the same over a certain speed range but drops sharply in the lower speed range. Consequently, the braking time is still relatively long. In order to shorten the braking time, this circuit has been extended by a counter-current braking circuit according to DE-OS 2 824 045. The latter is operated in the first braking phase, then the first-mentioned circuit in the second braking phase.The braking in the lower speed range is still unsatisfactory.

The arrangement for the dynamic braking of a direct-current motor according to DD-PS 118 487 uses the same active principle as that described in the DE-OS 2 640 745. The voltage taken off from the brake resistor is applied, via a characteristic-forming device, to a magnetic amplifier which can be influenced, is preliminarily permeated and fed back ånd the output of which is connected to the exciting winding. The characteristic-forming device consists essentially of two threshold value circuits. The circuit with the lower threshold value is taken to the feed-back control winding, the one with the higher threshold value is taken to the regenerative feedback control winding of the magnetic amplifier. The device realizes a constant torque reaching to the lower speed range. The braking in this range is too little.

The magnetic amplifier necessitates a comparatively high expense on circuit material (copper) and a large structural volume. The device is susceptible to trouble because of the numerous mechanical switches (6-tier switches).

Finally, a device for the controlled braking of an electric motor is described in the DE-OS 2 228 735. The device is specially designed for the transport of a magnetic-tape store with a defined distance-time relationship. It contains not only a brake circuit in the narrower sense but also mainly a control circuit for braking the motor. The control circuit consists of a speed indicator (tachometer), a distance-measuring device, a desired-speed store and a comparator.

The comparison of the actual speed signal and of the desired-speed signal is effected in the comparator. In the event of a deviation, a signal is produced for the control circuit of the motor. The control circuit contains a bridge circuit equipped with transistors, in the diagonal of which the motor armature is introduced. The four base electrodes are controlled by the comparator. The digital version of the control circuit contains a large number of components. The device described does not produce a constant braking torque. Only low brake powers can be realized with it. It is suitable for small motors and lowspeed drives.

The present invention seeks to shorten the braking time especially for motors of high-speed machines, particularly centrifuges. The invention also seeks to provide a brake circuit for motors, e.g. direct-current motors of centrifuges, which produces an adequate braking torque even in the lower speed range.

According to the present invention there is provided a control circuit for controlling the braking of electric motors comprising first and second converters with respective inputs arranged to be connected to the output of a tacho-generator coupled to the motor, a third converter with means for providing a desired value signal connected to the input thereof, and a transistor element with an input connected to the output of the first converter and an output connected to the input of the second converter.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a block circuit diagram of a motor circuit: Figure 2 shows a block circuit diagram of a control unit; and Figure 3 shows a graph of the speed-fieldcurrent curve of the circuit.

The circuit illustrated in Figure 1 comprises the following component circuits and important operational elements, the armature circuit 1, the energizing circuit 2, a control circuit 3, an optocoupler 4, a control unit 5 with desired-value indicator 6 and a tacho-generator 7.

The armature circuit is formed by the motor armature 8, the brake resistor 9 and a switch contact 1 0. The energizing circuit 2 contains the field winding 11, the secondary winding of a transformer 12 and a thyristor 1 3 as a rectifier.

The control connection 1 4 of the thyristor 1 3 is connected to the output 1 5 of the control circuit 3.

The operating-voltage line 1 6 of the control circuit 3 contains a switch contact 17 and a rectifier diode 18. The input 19 of the control circuit 3 is connected to the output stage 20 of the optocoupler 4, the input stage 21 of which is connected to the output 22 of the control unit 5.

The input 23 of the control unit 5 is in communication with the voltage terminals of the tacho-generator 7. It is taken, via a resistor 24, Fig. 2, to the input of a first voltage-current transformer or converter 25 and, via a further resistor 26, to the input of a voltage-voltage transformer or converter 27. The transformer 25 is connected to a rheostat 28 and the transformer 27 to a rheostat 29. The desired-value indicator 6, which is constructed in the form of a rheostat, is connected to the input of a second voltage-current transformer or converter 30 which is connected to a rheostat 31. The outputs of the transformers 25, 30 are in communication through a current divider circuit 32 constructed of resistors. The transformers 25, 30 jointly contain the input stage 21 of the opto-coupler 4 in their output circuits.

The input of a field effect transistor 33 is connected to the output of the transformer 27. At the output side, the field effect transistor 33 is connected in paraliel to the input of the transformer 25. Operational amplifiers may usually be used as transformers 25, 27, 30. The tacho-generator 7 is coupled to the motor shaft 38.

The operation of the circuit shown in Fig. 1 is initiated by manual or mechanical actuation of the switch contacts 10, 17. As a result, the armature circuit 1 is closed directly and the energizing circuit is closed indirectly through the control circuit 3 and the firing thyristor 13. The thyristor 13 allows part of the negative half-wave to pass through the field winding 11. The duration of the flow of current is controlled by the control unit 5 and the control circuit 3 depending on the speed of rotation of the motor armature 8. The control circuit 3 has the function of a conventional phasestart or phase-gating circuit and is also constructed as such. The opto-coupler 4 acts as a feedback-free transmission member between it and the control unit 5.The control unit 5 produces an almost constant output signal in the upper speed range, as a result of which the field current IF assumes the course illustrated in section 35 of the speed-field-current curve 34. The steepness of the section 35 is influenced mainly by the voltage-voltage transformer 27; it can be adjusted by the rheostat 29. On approaching the lower speed range, at about n = 1000 revolutions per minute, the output signal increases greatly. The field current IF increases greatly - section 36 of the curve 34. The increase in field-current strength is caused by the voltage-current transformer 30.

The voltage-current transformer 25 has a decisive influence on the decrease in current strength beginning at low speeds. It is illustrated as section 37 of the curve 34 in Figure 3. The extent of the decrease can be influenced by the rheostat 28.

The field-current peak in the lower speed range, represented by the sections 36, 37, counteracts a decrease in the armature current 1A in such a manner that a sufficiently strong, constant braking torque acts up to speeds of n = 150-200 revolutions per minute.

Claims (14)

1. A control circuit for controlling the braking of electric motors comprising first and second converters with respective inputs arranged to be connected to the output of a tacho-generator coupled to the motor, a third converter with means for providing a desired value signal connected to the input thereof, and a transistor element with an input connected to the output of the first converter and an output connected to the input of the second converter.

2. A control circuit according to claim 1 wherein the transistor element is a field-effect transistor.

3. A control circuit according to claim 1 or 2, wherein the first converter is a voltage-voltage converter.

4. A control circuit according to any preceding claim, wherein the second converter is a voltagecurrent converter.

5. A control circuit according to any preceding claim, wherein the third converter is a voltagecurrent converter.

6. A control circuit according to any preceding claim, wherein the outputs of the second and third converters are connected to the input of an optical coupling device.

7. A control circuit according to claim 6, wherein the output of the optical coupling device is connected to a second control circuit.

8. A control circuit for controlling the braking of electric motors substantially as herein described with reference to the accompanying drawings.

9. A braking circuit for an electric motor comprising an armature or rotor circuit having a brake resistor, an energising circuit comprising a controlled rectifier, and first and second control circuits for the rectifier, the first control circuit being connected to the second control circuit via an optical coupling device, and the second control circuit being connected to the rectifier.

10. A braking circuit according to claim 9, wherein the first control circuit is as claimed in any of claims 1 to 8.

11. A braking circuit for an electric motor substantially as herein described with reference to the accompanying drawings.

12. A direct current motor having a braking circuit as claimed in any of claims 9 to 11.

13. A single-phase motor having a braking circuit as claimed in any of claims 9 to 11.

14. A centrifuge having a motor as claimed in claim 12 or 13.