SU1310943A1 - Device for overload protection of three-phase induction electric motor - Google Patents

Abstract

The invention relates to the field of electrical engineering, and in particular to the protection of electrical installations from abnormal conditions. The purpose of the invention is to increase the reliability of the protection device. The constant component of the output signal of the current sensor 33, is proportional to the load current of the motor, more (L with

Description

is integrated into the chain 9 and the functional converter 10 is converted into a signal proportional to the power loss in the electric motor 1. This signal enters the input of the thermal analog 30. If the motor overheats reaches the maximum permissible value, then the signal at the output of the thermal analog 30 and the inverting input of the comparator 29 modulo the signal from the sensor 32 permissible overheating at the non-inverting input of the same comparator. As a result, a positive signal appears at the output of the comparator 29, which triggers the actuator 34 to operate. With asymmetric power, the amplitude of the variable component increases and, with a single phase break, reaches 60% of

t

The invention relates to electrical engineering, specifically to devices for protecting electrical installations from abnormal modes of operation.

The purpose of the invention is to increase the reliability of the security device.

The drawing is a schematic diagram of the device. I.

Asynchronous Security Device

th three-phase electric motor 1 contains a current-loaded transformers 2, 3 and 4; resistors 5, 6 and 7 and connected to a three-phase power supply bridge 8, the output of which through the integrating chain 9 is connected to the inputs of the functional converter 10 and the voltage divider 11 and connected through the series differentiating 12 and integrating 13 chains connected to the input of the comparator 14, the output which through diode 15 is connected to the input of adder 16, in the negative feedback circuit of which the capacitor 17 is connected in series and the amplifier 18 in the positive feedback circuit of which divider 19 is connected audio, high-impedance resistor 20 is shunted output circuit

a constant component of the output signal of the current sensor 33. As a result, the signal at the non-inverting input of the comparator 14 exceeds the signal at its inverting input; a positive potential appears at the output of the comparator 14, enabling the diodes 15 and 27. As a result, the signal from the output of the functional converter 10 is summed at the input of the thermal analogue 30 with the output signal of the comparator 14. The transmission coefficient of the amplifier 18 becomes equal to one, and the time constant of the thermal analogue 30 takes the minimum value. The output signal of the thermal analogue 30 increases with a maximum speed, which leads to the rollover of the comparator 29- and the actuation of the actuator. 1 il.

2

key 21 and through an additional resistor 22 output circuit of the key 23. The control inputs of the keys 21 and 23 through the current-limiting resistors 24 and 25 and the diodes 26 and 27 are connected to the outputs of the comparators 28 and 14. Inverting inputs of the comparators 28 and 29 are connected to the output of the adder 16, which is the output and thermal analogue of the electric motor 30.

The non-inverting inputs of the comparators 28 and 29 are connected to the outputs of the setting units 31 and 32 of the nominal and permissible overheating respectively Transformers of 2-4 phase currents loaded with resistors 5-7 and three-phase switching bridge 8 make up the sensor 33 of the load current of motor 1. To the output of the comparator

29 connected executive body 34. I

The device works as follows.

The constant component of the output signal of current sensor 33 is proportional to the load current of the electric motor, is represented by the integrating chain 9, and the functional converter 10 is converted into a signal proportional to the power loss in

electric motor 1. This signal is fed to the input of the thermal analog 30, which implements the following transfer function:

W (P)

TO

Tr + 1

(one)

where T is a constant heating time

electric motor j K is the coefficient of proportionality between the losses of the electric motor and the established overheating of the latter. The transfer coefficient of the thermal analogue 30 is equal to the transfer coefficient of the adder 16. The time constant is determined by the formula

R

t,

wasp

-nineteen

where R is the resistance in the feedback circuit of the adder 16;

wasp

C; t is the capacitance of the capacitor 17; 25 oi - the transfer ratio of the divider 19.

Insofar as ))

.1, then

and c.

when it is impossible

Because the values of constant and large R values are obtained, the required T. When the signal at the output of the thermal analog 30, proportional to the motor overheating, exceeds the value corresponding to the overheating at nominal load, specified by the nominal overheating unit 31, the comparator 28 tilts and through diode 26 and current-limiting a resistor 25 includes a switch 23, which shunts the high-resistor 20 of the divider 19 with an additional resistor 22. As a result, the transfer coefficient of the thermal analog remains unchanged and is constant in T is reduced to

R,

-

(3)

about

9

oi ,, about

the transfer ratio of the splitter 19 shunted ext 1 1 resistor

22 1

nineteen

When the overheating of the motor reaches the maximum permissible value, the signal at the output of the thermal analog and the inverting input of the comparator 29 exceeds the modulus of the signal.

with fO

15

20

25

thirty

35

40

45

50

55

The output from the setpoint adjuster 32 permissible overheating to the non-inverting input of the same comparator, as a result, the latter overturns and a positive signal appears at its output, indicating an unacceptable overheating of the motor, i.e. about the abnormal mode of its operation.

With a three-phase symmetric powering of the motor, the amplitude of the variable component of the current sensor output signal is approximately 6% of its constant component. As a result, the signal at the output of the integrating chain 9 significantly exceeds the signal at the output of the integrating chain 13, which together with the differentiating chain 12 forms a signal proportional to the variable component of the output signal of the current sensor 33. As a result, a negative potential is established at the output of comparator 1A, which blocks the diodes 15 and 27. With asymmetric power, the amplitude of the alternating increases and when one phase breaks, it reaches 60% of the current output signal 33. As a result, the signal at the non-inverting input of the comparator 14 exceeds the signal at its inverting input, the comparator 14 overturns and a positive potential appears at its output, unlocking the diodes 15 and 27. As a result, the signal from the output of the functional converter 10 is summed up at the input of the thermal analog 30 with the output the signal of the comparator 14 opens the key 21, the output circuit of which shunts the resistor 20 of the divider 19, as a result of which the transfer coefficient of the amplifier 18 becomes ft 1. At the same time, the time of the thermal analog is 30 n It takes the minimum value (j (.- C the output signal of the thermal analog 30 increases

With a maximum speed, which quickly leads to the tilting of the comparator 29 and the appearance of a positive signal at its output, indicating an abnormal mode of operation of the electric motor 1, the actuator 34 trips the electric motor by this signal. If necessary, the outputs of the comparators 14 and 29 can be connected to the inputs of the coincidence circuit, the appearance of the

telny signal at the output of which will indicate a phase failure of the motor.

The transfer ratio of divider 11 determines the acceptable ratio between the constant and variable components of the current sensor output signal, i.e. permissible degree of asymmetry of the phase currents of an electric motor

The proposed device provides protection against the most common abnormal modes of operation of an electric motor: overload, phase failure, asymmetry of the supply voltage.

Claims (1)

Formula of arr et e and A device for protection of a three-phase asynchronous electric motor against overload, containing a current sensor consisting of transformers of phase currents of an electric motor and a three-phase rectifying bridge, a series-connected functional converter, a thermal analog of an electric motor, including an adder with a capacitor in the negative feedback circuit, the first comparator, the second input of which is connected to the unit of permissible overheating of the electric motor, the executive body and the key, the output of which is connected to the output house thermal analogue, characterized in that, in order to increase reliability of the protection device is provided with a setter nominal overheating zleka-rodvigatel second and third comparator and the second key, and two integrating Editor M.Petrova Order 1899/51 Compiled by O. Muratov Tehred L. Oliynyk Proofreader S. Shekmar Circulation 619 Subscription VNIIPI USSR State Committee for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab. 4/5, Production and printing company, Uzhgorod, Projecto st., 4. 9436 one differentiating circuit, three diodes and a voltage divider, and the thermal analog is equipped with a non-inverting amplifier, included 5 in series with the capacitor and the enclosed positive feedback circuit through the second voltage divider, the high resistance resistor of which is connected through an additional resistor to the output circuit of the second switch and the output circuit of the first switch, the control input of which is connected through the series-connected newly introduced first current-limiting  5 the resistor and the first diode are connected to the output of the second comparator, which is connected via the second diode to the second input of the adder, the output of which is connected to the inverting inputs of the first and third comparators, the non-inverting input of the second comparator is connected to the non-inverting input of the third comparator rex in series with; the integrating and differentiating chains are connected to the output of the current sensor, which through the second integrating chain is connected to the input 30 of the functional converter, which is connected to the inverting input of the second comparator through the first voltage divider, the output of the third comparator is connected through a third diode and the newly introduced second current-limiting resistor is connected to the control input of the second key, and the output of the first comparator is connected to the actuator. 0 25 40