JPS6043729B2 - Temperature detection device for 3-phase induction motor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temperature detection device used to protect the rotor of a three-phase induction motor from temperature increases.

Heat generation in the rotor of a three-phase induction motor is determined by the product of the rotor resistance RDo and the square of the positive-sequence current 1, and the product of the negative-sequence resistance RAc and the negative-sequence current 12, which take into account the skin effect of the rotor. Heat is generated by the sum of the squares, but RAc is generally larger than RDc,
If we take the rotor's heat resistance, which is said to be about 3 to 6 times higher, it becomes 1,"・RDc+122・RAc=,
If RAC/RDC=α=3~6, then 1, ゜+α10”
=, and if the rated current of the induction motor is equal to the rated current of CA, then if we take the secondary rated current of CT, then from the case where there is no negative phase current (12■0), we can find ■25,
It can be expressed as 1,2+α122■25.

In other words, a curve of 112 ■ α 122 ■ 25 (in Figure 1, α ■
4 curve is shown. ) If the following current flows, it is necessary to stop the motor (three-phase induction motor) after a certain period of time to prevent burnout. FIG. 2 shows an overcurrent detection section of a conventional motor protection relay device, and its operation will be explained. In FIG. 2, 1 indicates a three-phase induction motor, and 2 indicates an electromagnetic switch or a power switch for starting or stopping the motor. 3 indicates a current transformer CT.

When electric power is supplied to the three-phase induction motor 1 by the electromagnetic switch 2, the CT 13 and the positive-phase and negative-phase filter 4 derive a positive-sequence current 1 and a negative-phase current 12 from the three-phase current, and then Convert to DC voltage with wave rectifiers 5 and 6, set values -V, B
-Vβ is compared by a comparator 9, and if either input is larger than the set value, the driver circuit 10 operates the thermal element detection relay 12. In this way, the I overcurrent region is detected by dividing the current into a positive-sequence current and a negative-sequence current, and setting a set value for each. An example of the detection area is shown in FIG. FIG. 3 shows that the overcurrent detection relay operates when the positive-sequence current exceeds the value or when the negative-sequence current exceeds 2.5 A. When the thermal element detection relay 12 operates in this way, the 2-square circuits 11-1, 11-
The comparator 13 calculates V,2+αV22 based on the output V, ``and ri.'' of 2, and outputs a trip signal to the shear breaker (or switch) 2 when the calculated value reaches a certain value. In the conventional method, the detection characteristic is as shown in FIG. 3, but as mentioned above, it is desirable for the motor protection device to have a detection characteristic that matches the curve of 112+α122=K as shown in FIG.

This invention was made in order to eliminate the above-mentioned problems, and in order to have a detection characteristic close to an elliptic curve such as 112+α122=25 shown in FIG.
It attempts to approximate with book straight lines Yl, y2, y3,
A current exceeding the minimum value of the three straight lines is detected and a thermal element detection relay is activated.

The heat generation of the rotor of a three-phase induction motor is 112+α122, which is calculated from the positive sequence current 11 and the negative sequence current 12 as described above.
is proportional to.

An embodiment of the present invention will be described below. An embodiment of the present invention is shown in FIG. 5, and the three-phase induction motor 1, electromagnetic switch or breaker 2, and current transformer 3 are used in the same manner as in FIG. The positive-sequence current of the positive-phase and negative-phase filters 4 is converted into a DC voltage by a full-wave rectifier 5, and the positive-sequence current is set as a positive-sequence voltage V1.
Similarly, the reverse phase current is converted into a DC voltage by the full wave rectifier 6, and is set as the reverse phase voltage V2.

FIG. 4 shows a curve obtained by passing the curves 11 and 12 of the curve 112+α122=2\however α=4 in FIG. As an example, this filter is 1V when 3-phase positive phase is included.
This is an example of a voltage of . In order to obtain the straight line y1 connecting points A and B in FIG.
If V−+−>11t1, the positive saturation voltage changes to negative RlR.
Focusing on the inversion to the saturation voltage of 2, Rl, R2, and R3 are set so that the above relationship is satisfied at points A and B.

Then, in the area above the straight line y1, the Arithmetic amplifier 9-1
operates, and driver circuit 1 is connected through diode D1.
0 is activated, the thermal element detection relay 12 is activated, and a signal is given to the delay circuit 13. Similarly, point B in Figure 4
and the resistance R4 in Fig. 5 so that a straight line Y2 connecting point C is formed.
R5 and R6 are set, and in the region above the straight line Y2, the operational amplifier 9-2 connected to the diode D2 operates and the thermal element detection relay 12 operates. Also, resistors R7 and R8 are connected so as to obtain a straight line Y3 connecting point C and point D in Fig. 4.
, R9 are set, the operational amplifier 9-3 connected to the diode D3 operates in the region beyond the straight line Y3, and then the thermal element detection relay 12 operates. In addition, even if the set voltage -■8 is constant, by changing the values of the resistors 7R1, R2 to R9, the set voltage -■8 is constant and the straight lines Yl, y2,
The area of y3 can be variably set. In this way, it is possible to simulate a square curve using a plurality of straight lines. In the above embodiment, a contact/point relay is used as the overcurrent detection relay 12, but the same effect can be obtained even if a semiconductor such as a transistor is used.

Further, in the above embodiment, the square curve was simulated using three straight lines, but the same effect can be obtained even if the square curve is simulated using three or more straight lines. As described above, according to the present invention, since the temperature rise due to overcurrent of the three-phase motor is simulated by a plurality of straight lines, it is possible to accurately detect the actual temperature rise.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the positive-sequence current, negative-sequence current, and rotor temperature rise, Figure 2 is a circuit diagram showing a conventional temperature detection circuit for a three-phase motor, and Figure 3 is a diagram of a conventional temperature detection circuit. FIG. 4 is a characteristic diagram showing an example of the operating range of the temperature detection device for a three-phase motor according to the present invention, and FIG. 5 is an example of the temperature detection device for a three-phase motor according to the present invention. FIG. In the figure, 1 is a three-phase electric motor, 2 is a breaker, 3 is a current transformer, 4 is a filter, 9-1, 9-2, 9-3 are operational amplifiers, 12 is a detection relay, Rl, R2, R3, R4
, R5, R6, R7, R8, and R9 are resistors, 11 is a set voltage, 11 is a positive sequence current, and 12 is a negative sequence voltage.

Claims (1)

[Claims] 1 A filter that derives a positive-sequence current and a negative-sequence current from each phase current of a three-phase induction motor, and a current that converts each of the currents into voltages and outputs them as a positive-sequence voltage V_1 and a negative-sequence voltage V_2. /voltage converting means, inputting the positive-sequence voltage, the negative-sequence voltage, and the set voltage to generate V_1^2+α
V_2^2 = a plurality of operational amplifiers that obtain detection characteristics that approximate a certain elliptic curve; and a detection element that operates upon receiving the output of one of the operational amplifiers that outputs an output when the detection characteristic exceeds the elliptic approximate curve. Temperature detection device for 3-phase induction motor equipped with