CH658953A5 - CIRCUIT ARRANGEMENT FOR MONITORING THE HEATING OF ELECTRICALLY OPERATED MOTORS. - Google Patents

Description

The invention relates to a circuit arrangement for monitoring the heating of electrically operated motors in accordance with the preamble of patent claim 1.

For the monitoring of electric motors, electronic protective devices are required which record the temperature of the current-carrying proteges and which put the object to be protected out of operation when the permissible operating temperature is exceeded. In the majority of applications, the rotor is subjected to particularly high thermal loads during motor starts. It is generally known, in a first approximation, to electrically simulate and evaluate the current of the stator, which contains the rotor current, squarely in an electronic timer in a running machine. In contrast to simple "thermal images" with indirectly heated bimetal systems, motor protection relays with an electrically simulated thermal time constant offer the possibility of fulfilling the requirement that at least one motor start-up can also take place from a warm operating condition (AEG-TFK - Mitteilungen, 1968, H6, page 372 and EP-OS 00 19 885).

From DE-OS 28 44 706 a circuit is known in which the time step in the motor protection relay is acted upon by a function transmitter above the nominal current and by another function transmitter below the nominal current, so that the heating of the rotor in nominal operation is hereby simulated. The disadvantage here is that the very large cooling time constant is not fully taken into account when the machine is stopped. With the use of RC elements, very large cooling time constants can only be achieved with difficulty, especially since they fluctuate greatly from motor to motor.

The object of the invention is to design a circuit arrangement of the type mentioned at the outset in such a way that it detects the differently long cooling times of both the running and the stationary machines.

The stated object is achieved according to the invention with the features specified in the characterizing part of patent claim 1. Advantageous embodiments of the invention are characterized in claims 2 and 3. The invention is explained in more detail with reference to an embodiment shown in the drawing.

Show it

1 shows the block diagram of a motor protection relay,

2 shows the voltage-time diagram of the electronically simulated temperature profile of the protective relay according to FIG. 1 for a motor,

Fig. 3, the protection relay of FIG. 1 in a modified embodiment and

Fig. 4 shows a circuit design for the time control unit.

In Fig. 1, 1 denotes three lines of a three-phase system for a motor M, the currents of which are supplied to an electronic evaluation circuit via current transformers 2. A pulse generator 3 converts the currents into pulses. This pulse generator sends out pulses above the nominal current, the pulse spacing of which decreases quadratically with increasing current. These pulses charge a capacitor 7 as a storage element via a resistor 5 and a diode 6. If the voltage of the memory element reaches the trigger limit of the threshold switch 8, this triggers the relay 9, which excites the trigger coil of a circuit breaker 10, which interrupts the circuit of the protege M. After the current has been switched off, the capacitor 7 is discharged via the resistor 11. In this respect, the circuit design of a motor protection relay is known. As described below, this structure is expanded and improved by the invention.

The output currents of the current transformers 2 feed two threshold switches of the timing control unit 14. The threshold switch 12 checks whether the current is above or below C times the nominal current, where C = 1.2 ... 2.5 times the nominal current as a criterion for starting the motor. If this value is exceeded, the threshold switch 12 responds. The threshold switch 13 determines whether the current is zero, i.e. whether the engine M is switched off. The threshold switch 13 can be replaced by an auxiliary contact which is coupled to the circuit breaker 10 in the off position.

A time evaluation stage 28 counts the motor starts per unit of time. During the counting process, stage 28 evaluates the cooling time of the running rotor and the cooling time of the stationary motor. 2 shows the curve of the charging voltage U of the capacitor 7 over time t as a graph. If the motor M is switched on at the time T, the time evaluation stage 28 in the first stage A superimposes a small voltage Uva on the capacitor 7 via the diode 15 at the time TA. The voltage Uva simulates the heating of the rotor during rated operation after the first start.

As can be seen in FIG. 2, after a further engine start in its second stage B at time TB, time evaluation stage 28 increases the precharge voltage to voltage Uvb-After another third engine start, stage 28 blocks at stage TC in its third stage C via the auxiliary relay 16 a further switching on of the circuit breaker 10. At the same time, an increased voltage Uve is superimposed on the capacitor 7. The stages B and C of the time evaluation stage and the auxiliary relay 16 are reset with the cooling time of the running rotor, the stage A with the cooling time of the stationary motor M. This fulfills the frequently asked requirement that three starts from cold and two starts allow the engine to be at operating temperature.

The motor protection relay shown in FIG. 3 essentially corresponds in its structure and mode of operation to the embodiment according to FIG. 1. However, instead of precharging via a diode, the base point of the discharge resistor 11 is switched over to a voltage divider with the divider resistors RI, R2 and R3. This has the effect that, when the time evaluation stage 28 is switched back to a lower stage, the discharge of the capacitor 7 takes place slowly. The dashed part of the curve in FIG. 2 shows the profile of the discharge voltage U for the version with discharge resistor 11.

4 shows a possible embodiment for the time control unit 14.

After each motor start, the flip-flop 12 turns on

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

3rd

658 953

Given a signal to the set input a of a shift register 17. The outputs AI ... A3 of the shift register act on the MOS channel switches 18, which supply a partial voltage via the diode 15 to the memory 7 for precharging in accordance with the switching stage.

When the motor is switched on, the flip-flop 13 controls the timing stage 20 via the bistable flip-flop 19. The cooling time of the rotor is set at this time stage while the machine is running. After the set time has elapsed, the reset input b of the shift register 17 is applied via the & stage 21 and the OR operation 22. The resetting of the time stage 20 can only take place due to the & stage 21 and the OR operation 23 if the shift register is in the second or third stage A2, A3.

Each time the time stage 20 responds, the reset input R of the bistable flip-flop 19 is applied via the OR gate 24, so that the time stage 20 is not restarted until either a signal from the flip-flop 13 at the static input S of the bistable flip-flop 19 is present, that is, the rotor is rotating, or when the motor is switched on, so that the rocker member 13 responds again. The bistable flip-flop is reset via the

OR operation 24 also from the inverting output of the flip-flop 13, so that the time sequence for resetting the time stage 20 is interrupted when the engine is switched off.

When the engine is switched off, the time stage 26 is controlled by a further inverting output of the flip-flop 13 via the bistable flip-flop 25. This time stage is set to the cooling time of the stationary engine. After the set time has elapsed, the shift register is reset to the starting position via the OR link 22.

Each time the time stage 26 responds, the reset input R of the bistable flip-flop 25 is applied via the OR gate 27, so that the time stage 26 is not restarted until either of the flip-flops 13 at the static input S of the bistable flip-flop 25 is still on Signal 15 is present, that is to say the motor is switched off, or if the motor is switched off, so that the flip-flop 13 falls back again.

The bistable flip-flop 25 is reset via the OR link 27 also from the output of the flip-flop 20 13, so that the time sequence for resetting the time stage 26 is interrupted when the motor is switched on.

The function of this circuit can also be implemented with a microcomputer.

v

3 sheets of drawings

Claims (3)

658 953 1.Circuit arrangement for monitoring the heating of electrically operated motors, which acts on a memory element as a function of the square of the motor currents of the motors to be monitored, the memory content of which serves as a measure of the temperature of the motor, with a switching stage which, after reaching a predetermined memory content, the circuit of the Motor interrupts, characterized by a time control unit (14) which detects the number of times the motor (M) is switched on and off per unit of time and, depending on this, changes the content of the memory element (7) in such a way that its memory content represents a measure of the operating temperature of the motor . 2. Circuit arrangement according to claim 1, characterized in that the timing control unit has a first flip-flop (12) which generates a signal when the starting current of the motor (M) exceeds a value above the C times the nominal current, 1.2 s C < 2.5, and a second flip-flop (13), which generates a signal when the motor (M) is at a standstill. 2nd PATENT CLAIMS 3. Circuit arrangement according to claim 1, characterized in that the content of the memory element (7) is changed in stages, the size of each stage being a measure of the instantaneous change in the heating state of the motor (M).