GB2120877A

GB2120877A - Thermal imaging of electric motors

Info

Publication number: GB2120877A
Application number: GB08313988A
Authority: GB
Inventors: Hubert Rollecke
Original assignee: Hartmann and Braun AG
Current assignee: ABB Training Center GmbH and Co KG
Priority date: 1982-05-21
Filing date: 1983-05-20
Publication date: 1983-12-07
Also published as: SE8302829D0; CH658953A5; GB8313988D0; DE3219199C2; GB2120877B; SE451783B; SE8302829L; DE3219199A1

Abstract

A motor protective relay 10 with electrically imaged thermal time constants 7,11 allows at least one motor run-up even in the warm state of the motor, and takes account of the different cooling periods when running and stationary. Circuit 2,3,5,6 supplies store capacitor 7 with a signal which depends upon the square of the motor current to be monitored, and a time control unit (14) determines the number of times the motor (M) switches on and off in unit time and in dependence thereon alters the content of the store (7) in such a way that this represents a measure of the operating temperature for the motor. <IMAGE>

Description

SPECIFICATION A circuit arrangement for monitoring the heating of electric motors The invention concerns a circuit arrangement for monitoring the heating of electric motors.

For monitoring electric motors, electronic protective devices are required which determine the temperature of the protected member receiving current, and which, when the permitted operating temperature is exceeded, takes the member out of operation.

When running up a motor, in most cases the rotor is subject to particularly high thermal stress. It is generally known, for running machines, as a first approximation to electrically generate the square of the current in the stator, which includes the rotor current, in an electronic time member and to evaluate it. By contrast with simple "thermal imaging" with indirectly heated bimetal systems, motor protective relays with electrically formed thermal time constants provide the possibility of meeting the requirement that even from the warmed state at least one motor run-up can be achieved (AEG-TFK Disclosures, 1968, H6, page 372, and EP-OS 00 19 885).

A circuit is known from DE-OS 28 44 706, in which the time stage in the motor protective relay is supplied above the rated current from one function generator and below the rated current from another function generator, so that in this case the heating of the rotor in rated operation is simulated. The disadvantage of this is that the very long cooling time for stationary machines is not fully taken account of. Using RC members, the large cooling time constants can only be achieved with difficulty, the more so since this varies greatly from motor to motor.

It is the object of the invention to provide a circuit arrangement which can take account of the different cooling times both of running and of stationary machines.

Accordingly, the invention provides a circuit arrangement for monitoring the heating of electric motors, which puts in a store a value which serves as a measure of the temperature of the motor, having a switching stage which, when the store content reaches a pre-determined value interrupts the current circuit of the motor, including a time control device which determines the number of times per unit time the motor switches on and off and in dependence thereon alters the content of the store so that its content forms a measure of the operating temperature of the motor. The invention will be more clearly expiained with reference to exemplary embodiments shown in the drawing.

Figure 1 shows the block circuit diagram of a motor protective relay; Figure 2 shows the voltage/time diagram electronically formed for the temperature curve of the protective relay according to Figure 1; Figure 3 shows the protective relay similar to Figure 1 in an alternative form; and Figure 4 shows a circuit example for the time control device.

In Figure 1, three lines 1 of a three phase system of motor M are shown, their currents being supplied via current converter 2 to an electronic evaluation circuit. A pulse generator 3 forms the currents into pulses. Above rated current, this pulse generator provides pulses of which the pulse separation shortens as the square of the increasing current.

These pulses charge up a capacitor 7 used as a store via a resistor 5 and a diode 6. When the voltage of the store reaches the set value of a threshold circuit 8, this energises a relay 9, which in turn energises the release coil of a power switch 10 which interrupts the current circuit of the protected member M. After switching off the current, the capacitor 7 is dis charged via a resistor 11. So far, this circuit construction of a protective relay is known. This construction is, as will now be described, extended and improved by the invention.

The output current of the current converter 2 supplies two level switches of a time control mem ber 14. A threshold switch 12 checks whether the current lies above or below C times rated current, the value for a motor run-up being C = 1.2 ... 2.5. If this value is exceeded, the threshold circuit 12 responds.

A threshold circuit 13 determines whether the current is zero, that is whether the motor M is switched off. The threshold circuit 13 may be replaced by an auxiliary contact which is coupled with the power switch 10 in the off position.

Atime evaluation stage 28 counts the number of motor starts per unit of time. During the counting process, the stage 28 evaluates the cooling time of the running rotor and the cooling time of the stationary motor. In Figure 2, the curve is shown of the charge voltage U of the capacitor 7 over time t. If the motor M is switched on at the time T, the time evaluation stage 28 in the first stage A at the time TA supplies a small voltage UVA via the diode 15 to the capacitor 7. The voltage UVA simulates the warming of the rotor at rated operation after the first start.

As can be seen from Figure 2, the time evaluation stage 28 after a further motor start increases the bias voltage to the voltage Uve in its second stage B at the time TB. After a third motor start, the stage 28 at time TC in its third stage C prevents by means of an auxiliary relay 16 a further switch on of the power switch 10. Simultaneously, the capacitor 7 receives an increased voltage Uvc. Resetting of the stages B and C of the time evaluation stage and of the auxiliary relay 16 occurs according to the cooling time of the running rotor, while the resetting of stage A occurs according to the cooling time of the stationary motor M. The circuit thus permits three starts from cold, and two run-ups from warm, for the motor.

The version of a protective relay shown in Figure 3 corresponds in construction and method of operation substantially with the version shown in Figure 1.

However, in place of biassing via a diode, here the base of the discharge resistor 11 is switched between the resistors R1, R2 and R3 of a voltage divider. This ensures that when switching back the time evaluation stage 28 to a lower stage, discharge of the capacitor 11 occurs slowly. The dotted part of the curve in Figure 2 shows the curve of the discharge voltage U for the version with the discharge resistor 11 connected as shown.

Figure 4 shows a possible embodiment for the time control member 14. After each motor start, the threshold switching member 12 passes a signal to the set input a of a shift register 17. The output A1...A3 of the shift register are connected to an MOS-channel switch 18, which, corresponding to the switching stage, provides a partial voltage via the diode 15 for pre-biassing the store 7.

When the motor is switched on, a timer stage 20 is controlled by the switching member 13 via a bistable switching member 19. The cooling time of the rotor for running machines is set at this timer stage.

After expiry of the set time, the reset input b of the shift register 17 receives a signal via the AND-gate 21 and the OR-gate 22. Resetting from the timer stage 20 can however be achieved through the AND-gate 21 and the OR-gate 23 only if the shift register is in the second or third position A2, A3.

Every time the timer stage 20 responds, the reset input R of the bi-stable member 19 receives a signal from the OR-gate 24, so that renewed operation of the timer stage 20 only occurs if either there is still a signal applied by the switching member 13 at the static input S of the bi-stable member 19, that is the rotor is turning, or if a switch-on of the motor has occurred so that the switching member 13 again responds. Resetting of the bi-stable member occurs via the OR-gate 24 also from the inverted output of the switching member 13, so that when the motor is switched off the time period for re-setting the timer stage 20 is interrupted.

When the motor is switched off, the timer stage 26 is controlled from a further inverted output of the switching member 13 via a bi-stable switching member 25. This timer stage is set to the cooling time for the stationary motor. After expiry of the set time, the shift register is re-set to its starting position via the OR-gate 22.

Each time the timer stage 26 responds, the re-set input R of the bi-stable switching member 25 receives a signal via the OR member 27, so that renewed running of the timer stage 26 only occurs if either a signal still appears at the static input S of the bi-stable switching member 25 from the switching member 13, that is the motor is switched off, or if switching off of the motor occurs so that the switching member 13 again falls back.

Resetting of the bi-stable switching member 25 is also produced via the OR-gate 27 also from the output of the switching member 13, so that when the motor is switched on, the time period for re-setting of the timer stage 26 is interrupted.

The function of this circuit can also be achieved by means of a micro-computer.

Claims

1. A circuit arrangement for monitoring the heating of electric motors, which puts in a store a value which serves as a measure of the temperature of the motor, having a switching stage which, when the store content reaches a pre-determined value interrupts the current circuit of the motor, including a time control device which determines the number of times per unit time the motor switches on and off and in dependence thereon alters the content of the store so that its content forms a measure of the operating temperature of the motor.

2. A circuit arrangement according to claim 1, wherein the time control unit has a first threshold member which produces a signal if the start-up current of the motor exceeds a value C times the rated current, with 1.2 < C < 2.5, and a second threshold member, which produces a signal when the motor is stationary.

3. A circuit arrangement according to claim 1, wherein the content of the store is varied in stages, the magnitude of each stage representing a measure of the instantaneous heating state alteration of the motor.

4. A circuit arrangement according to claim 1, wherein the content of the store is altered to a level at which the current circuit of the motor is interrupted after three successive starts have occurred.

5. A circuit arrangement for monitoring the heating of electric motors substantially as described with reference to the accompanying drawings.