SU1403199A1 - Device for overload protection of a.c. motor - Google Patents

Description

(21) 3950501 / 24-07

(22) 07/11/85

(46) - 06/15/88. Bul No. 22

(71) Belarusian Institute of Agricultural Mechanization

(72) C. V. Turin and A. A. Geleisha (53) 621.316.925 (088.8)

(56) USSR Copyright Certificate No. 993378, cl. H 02 H 7/08, 1983. USSR Copyright Certificate

№ 1319147, 05/28/85.

(54) DEVICE FOR PROTECTION OF AC MOTOR PROTECTION AGAINST OVERLOAD

(57) The invention relates to the field of electrical engineering, namely to devices for protecting electric motors from emergency operation. The purpose of the invention is to improve the accuracy of the protection device by improving the matching of the optimal time of the current characteristics of the device and the motor in a wide range.

V С

capacities. The device contains a current transformer 1, the secondary winding of which is bounded by a resistor 2, an AC amplifier 3 with an adjustable resistor 4, a rectifier 5, a comparator 6, an amplifier 7 equipped with a light indicator 8, a functional converter 9, a voltage-frequency converter 10, a matching element voltage levels And, the first element 2I-NOT 12, the first inverter 13, the first reversible counter 14, the second element 2I-NOT 15, the third element 2I-NOT 16, the reset pulse generator 17, the second inverter 18, transistor switch 19, the element 12 LEE 20, second and third reversible

one

The invention relates to electrical engineering, in particular, to devices for protecting electric motors from emergency operation.

The purpose of the invention is to increase the accuracy of the protection device by improving the matching of the optimal time of the current characteristics of the device and the motor in a wide power range.

FIG. 1 shows a circuit diagram of a protection device with three four-bit counters of the time delay correction block; in fig. 2 - voltage converter characteristic - frequency.

The device contains a current transformer 1 with primary winding leads for connecting a motor into the phase, the secondary winding of which is ground by a resistor 2, the output of which is connected to an AC amplifier 3, the coefficient is. which is regulated by a resistor 4. The output of the AC amplifier 3 is connected via rectifier 5 to the input of the threshold element in the form of a comparator 6, with an amplifier 7 equipped with a light indicator 8. The output of rectifier 5 is connected through a functional converter 9 that approximates the optimum time current characteristic of the electric motor , the input of the voltage converter 10 is the frequency, the output of which is connected through the voltage level matching element 11 to the second input of the first element 2I-NOT 12, the first input of which is connected nen yield of the first inverter 13 having an input coupled to an output amplifier 7 and the output of the first NOR element 2I 12 is connected to the counting input of combining the first down counter 14, the count input of which is connected to subtractor

counters 21, 22, output amplifier 23 with executive relay 24, reset button 25, unit for setting zero counters 26. When the motor is continuously overloaded, comparator 6 switches and functional converter 9 comes on and voltage-frequency converter 10 generates a frequency proportional to the input signal , through the element 2И-НЕ 12 the frequency pulses arrive at the counting input of the addition of the counter 14. The counters 14, 21, 22 are filled, the output of the counter 22 is a logical zero signal and the output amplifier 23 is open Switches the motor off. 2 Il.

with the output of the second element 2I-NOT 15, the second input of which is connected to the output of the third element 2I-NOT 16, the first input of which is connected to the input of the first inverter 13. The second input of the third element 2I-

NOT 16 is connected to the input of the generator 17 of the reset pulses, and the first input of the second element 2I-NOT 15 is connected to the output of the second inverter 18, the input of which through transistor switch 19 is connected to the output of the element 12 OR 20, the inputs of which are connected to the outputs of reversible counters 14, 21 and 22, which are a time delay correction block. The output of the reversible counter 22 is connected to

the input of the output amplifier 23 executive relay 24, the power of which is turned on via the reset button 25. Block 26 installation of zero meters is connected to the inputs of the installation of zero meters 14, 21 and 22.

The security device works as follows

0 way.

Suppose that the protected motor runs at a current that exceeds the rated current. In this case, the comparator 6 tuned to the current (1.05-1.1) is switched, the indicator 8 is lit, indicating an overload of the electric motor, and a functional converter 9 is put into operation. A voltage, voltage and a voltage converter 10 — the frequency begins to generate a frequency,

0 proportional to the input signal. This frequency passes through element 11 and is fed to the second input of the first element 2I- NOT. At the same time, the first input of the first element 2I-NO 12 enters the “1” and the frequency of the converter 10 passes to the counting input of the addition of the first reversible counter 14. The counters are filled. After some time, the output “Transfer with the addition of P1 of the last counter 22 results in a signal“ O and the output amplifier 23 turns on the relay 24. The electric motor is disconnected from the network. Relay

24 it remains on until the maintenance staff returns the device to its original position by pressing the button.

25 “Reset. After the electric motor is disconnected from the mains, the generator 17 of the reset pulses comes into operation. Its signal passes through the elements 2I-NOT 16 and 15 to the counting input of the subtraction of the reversible counter 14. The pulses recorded in the counters are subtracted with the frequency of the generator 17. As the frequency of this generator is regulated, it is possible depending on the constant cooling of the motor and its mode Work more fully simulate the cooling of the electric motor.

Suppose that the overload lasted for some time during which the protection did not activate. In this case, when the motor current drops below the nominal value, the comparator 6 switches and the input 1 of the third element 2I-NO 16 enters "1, the third element 2I-NO 16 opens for the passage of pulses from the generator 17 of the reset pulses. If by this time some number is recorded in the counters 14-22, then at the output of element 12 or 20 there is "1, the key 19 opens and the element 2I-NOT 15 enters" 1. The pulses from the generator 17 pass through the element 2I-NOT 15 and arrive at the counting input of the subtraction of the counter 14. When reading the last pulse from the counter 14, the output of the element 12 OR 20 appears, the voltage O, the key 19 is closed and the element 15 is closed. Pulse reading is stopped.

The optimum protection characteristic for asynchronous motors in the wide power range has the same values. By using an amplifier 3 with an adjustable gain at any rated motor current, the output voltage of the rectifier 5 can be obtained at the same level.

Since the functional converter 9 approximates the optimal protective characteristic and has the same signal level at the input for any nominal motor currents, the device therefore protects the electric motors in a wide power range.

In order to match the optimum protective characteristic of the electric motor and the capabilities of the time delay correction block on the counters, the voltage converter frequency f is required - the frequency corresponds to the ratio f - t where N is the counter discharge;

Wed-1

p - NUMBER of counters; required response time at optimum protection

characteristic at the i-th current ratio. Since the frequency of the transducer 10 is proportional to the input voltage, then to obtain the relationship shown in phig. 2, it is required that the functional

Converter 9 provided a similar dependence of output9 (Uexg), where L Bux.sf, and UBxgssKi, where i is the ratio of the load current

15

(f). Jh

Thus, supplying the protection device with an AC amplifier with an adjustable gain, and a time delay correction block with a functional converter at the input, approximating the optimal time, the current protective characteristic of the electric motors, and the voltage-frequency converter with an output voltage matching element allows

improve the accuracy of matching protection and motor performance in a wide power range. Performing a threshold element with an indication improves the tuning accuracy, and hence

coordination of the specified characteristics.

Equipping the device with a separate generator of pulse pulses with adjustable frequency and logic elements allows more accurately simulating the cooling of electric motors of different power

various modes of operation.

The voltage-frequency converter frequency received in the device with the design of the meters and the response time of the protection also improves the accuracy of the protection device.

The economic effect of the use of the device is associated with an increase in the reliability of the electric motors with a more precise matching of the time characteristics of the electric motors and protection.

Claims (2)

Invention Formula . Device for protection of an AC electric motor against overload, containing a current transformer with primary windings for connecting an electric motor to the phase, the secondary winding of which is bounded by a resistor, a rectifier, to the output of which a threshold element is connected, and a time delay correction unit having -reverse N-spread meters, three elements 2I-NOT, a diode element OR, two inverters. a transistor switch, a zero metering unit, an output amplifier with an output relay, the input of which is connected to the output of the time delay correction block, while the output of the threshold element is connected via the first inverter to the input of the first element 2I-NOT, the output of which is connected to the counting input of the first reversible input the counter, while the output of the transistor switch is connected to the output of the diode element OR, whose inputs are connected to the bits of the meters, and the output of said key is connected via a second inverter to the input of the second elec ment 2I-NOT, the output of which is connected to the counting input of the subtraction of the first reversible counter, and the second input of the second element 2I-NOT connected to the output of the third element 2I-NOT, one of the inputs of which is connected to the input of the first inverter, improving the accuracy of work by improving the matching of the optimal time of the current characteristics of the device and the electric motor in a wide power range; it is additionally equipped with an AC amplifier with adjustable gain, an impulse generator cos reset with adjustable frequency, the functional converter an- proksimiruyush.im optimum time Toko the characteristic of the motor, a voltage-frequency converter with a voltage matching element at the output, and the threshold element is designed as a comparator with an output amplifier equipped with a light indicator, while the AC amplifier is connected between the secondary winding of the current transformer and the input of the rectifier, the output which through the functional converter is connected to the input of the voltage-frequency converter, the output of which through the voltage level matching element is connected to the second input 2I house NOR first element, and an output reset pulse generator coupled to a second input of the third NOR element 2I. 2. A device according to claim 1, characterized in that the frequency of the voltage-frequency converter is related to the pulse counters and the protection response time by N ifi ttp-t N - counters; n is the number of counters; icp.1 is the response time of the protection according to the optimal characteristic at the i-th current ratio, sec.