SU1394321A1 - Device for protecting three-phase asynchronous motor supplied by frequency converter against operating in two phases - Google Patents

Abstract

The invention relates to electrical engineering. The purpose of the invention is to increase the reliability by controlling the input current of the inverter and eliminate the false triggering of the protection at the moment the motor starts. The device consists of a frequency converter 1, which includes a controlled rectifier 2, connected via a power filter 3 and a current sensor 4 to a voltage inverter 5. An asynchronous motor 6 is connected to the output of the inverter 5, for example fe / n. The inverter 5 control input is connected to the first output of the control unit 7, the second output of which is connected to the control input of the rectifier 2. The first input of the control unit 7 is energized set the rotational speed of the asynchronous engine 6. The current sensor 4 output is connected to the input of the module 8 module allocation, the output of which through serially connected filter 9, the threshold element 10 and logic element 11 is connected to the output terminal 14. When the break one of the phases of the asynchronous motor 6 with pulse sequence of the current sensor 4 varies substantially. During the non-current interval, the voltage of the threshold element reaches the threshold value, and at the output, a positive polarity pulse is formed. This pulse through the element I passes to the control unit, as a result of which the currents of the asynchronous motor 6 are reduced to zero. % il. r c & (L with with 4 00 to

Description

The invention relates to electrical engineering and can be used for the protection of three-phase asynchronous motors powered from a voltage converter with an intermediate poston current, if one of the phases of the wiggle breaks.

The purpose of the invention is to increase the alarm by controlling the input current of the inverter and eliminate the false triggering of the protection at the time of the start of the electric motor.

1 shows a block diagram of the device; Fig. 2 shows graphs of 15 stresses on corresponding elements.

The device consists of a frequency converter 1, which includes a controlled rectifier 2, connected 20 through a power filter 3 and a current sensor 4 to a voltage inverter 5. An asynchronous motor 6 is connected to the voltage of the inverter 5. The control input of the voltage inverter 5 is connected to the first output of the control unit 7, the second output of which is connected to the control input of the rectifier 2. The first voltage of the control unit 7 is supplied with a voltage , frequency 30 of rotation of the asynchronous motor 6. The output of current sensor 4 is connected through series-connected block 8 module and filter 9 to the input of the threshold element 10. The output of the last, c connected to the first input of the logic element And 11, the second input to torogo through the delay unit 12 and an input terminal 13 connected to the third vkody unit 7 control. The output Q of the logic element And 11 is connected With the output terminal 14.

The device works as follows.

Information about the current flowing in the phases of the stator winding of the induction motor 6 is removed from the current sensor 4 connected to the input power circuit of the voltage inverter 5. In the simplest case, a resistor made of chrome wire can be used as a current sensor 4. In the normal operation of the asynchronous motor 6, the voltage on the current sensor 4 is a sequence of pulses with a frequency six times the frequency of the output voltage of the frequency converter 1. Depending on the power factor asynh50

55

d

0 5 0 c Q

with

0

five

An external motor and its mode of operation (motor or generator modes) can be bipolar, unipolar, without a constant component, or unipolar with a constant component.

In the initial state, when frequency converter 1 is turned off, the output voltages of current sensor 4, module allocation unit 8 and filter 9 are zero. At the output of the threshold element. 10 and, respectively, the first input of the logic element 11 and there is a signal of logic 1, which indicates the absence of current in the phases of the asynchronous motor 6. However, the further passage of this signal is blocked by the presence of the signal of the logic 0 coming to the second input of the logic element 11 from the output 12 delays. At the output terminal 14 there is a logical signal O ..

When the frequency converter 1 is turned on (time t), the input terminal 13 receives a logical 1 signal, and a voltage appears at the output of the converter, the frequency of which is determined by the amplitude of the ultrasonic voltage. In the time interval tf - t in an asynchronous motor, an electromagnetic transient is established to establish phase currents. During this time, the flow and the envelope of the pulses of the input current of the voltage inverter 5 increase exponentially with the time constant of the slow aperiodic component that creates the main flow when the stationary asynchronous motor is turned on.

The voltage supplied from the current sensor 4 is converted by the module allocation unit 8 into a positive polarity voltage. The output voltage 9 smoothly increases and at time tj reaches the value of the reference voltage of the threshold element 10. The latter switches and a logical signal O is set at its output, which indicates the absence of a phase loss of the asynchronous motor 6.

At time tj, the output of the delay block 12 is set to a logical 1 signal and the lock from the AND 11 gate is released. The protection device is ready-to-work.

3139

On the time interval t, - t, the normal operation of an asynchronous motor 6 takes place (there is no phase failure). In this mode, as already noted, the current pulses on the sensor A can be double-ended or unipolar with or without a constant component, depending on the power factor of the induction motor and the mode of its operation.

When one of the phases of the asynchronous motor 6 is disconnected (time t.), The type of pulse train from current sensor 4 changes significantly compared to normal operation. The pulse frequency decreases by 3 times, and their amplitude increases by 1.5 times or more. In the sequence of pulses, twice during the period Tcg of the output voltage of the frequency converter 1, current-free intervals occur, which can be used to identify the phase failure mode in an induction motor. To do this, it is sufficient to choose the time of the filter 9 so that the voltage at its output during the current-free interval, decreasing in amplitude, will reach the value of the reference voltage of the threshold element 10,

In this case, a positive polarity pulse is generated at the output of the threshold element 10, which passes through the logic element 11 to the output protection terminal 14 and then to the control unit 7 of the frequency converter 1. The algorithm of the frequency converter 1 must be built so that with the arrival of the first impulse control unit 7 from the output terminal 14, the formation of control impulses to the rectifier 2 ceases, resulting in the output voltage of the frequency converter 1 and, accordingly, the asynchronous currents This motor 6 is reduced to zero. Simultaneously with the interlock of the controlled rectifier 2, it is possible to block the formation of control pulses on the voltage inverter 5.

By the end of the current-free interval, the output {voltage of the filter 9 should decrease to

the reference voltage of the threshold element 10. Since the duration of the current-free interval during a phase failure is always 1/6 of the period

one

the output voltage of the frequency converter 1, then for reliable operation of the device, the time constant Cf of the filter 9 must be selected for the mode of operation of the frequency converter with the maximum frequency of the output voltage, for which the current interval is minimal. The condition must be met.

5 0 5 o

Q

five

0

five

T (f fMC, ")

 v. or -y, 4 iHiff i

H C,

 where T- (f fjinouc) is the period corresponding to the maximum frequency of the output voltage of the frequency converter. During its operation, the induction motor current can vary widely from idle current to nominal, and in transient conditions, the load current may briefly exceed the rated current by a factor of 3-5 and more. In addition, in practice, quite often from one frequency converter it is necessary to power a group of asynchronous motors, which also causes a wide range of variation of the load current of the converter, especially when the number of motors connected to it changes. A change in the current in a wide range leads to a corresponding change in the output voltages of the module 8 extraction module and filter 9. In this case, at low frequencies the output voltage of the frequency converter 1 and low load currents can occur, the protective device can operate in the absence of an asynchronous phase loss engine

In order to extend the allowable range of variation of the induction motor current (the load current of the frequency converter), the gain of the modulus allocation unit 8 must be chosen such that the output voltage reaches the saturation voltage already at a motor current of 0.1-0.2 from nominal. In this case, the amplitude of the output voltage of the filter 9 remains uncorrected and equal to the maximum value when the current is changed 5-10 times lower than the nominal value.

Thus, by choosing the appropriate gain values

block 8 of module allocation and constant time of filter 9 can provide reliable protection of induction motor 6 against phase failure within specified limits of variation of frequency and amplitude

current.

i

The invention makes it possible to increase the reliability of protection while simplifying the design of the device. The use of the invention in a frequency converter used to power a group of high-speed asynchronous motors for precision machining machines will provide a significant economic cost.

Claims (1)

Invention Formula 20 A device for protection of a three-phase asynchronous motor, powered by a frequency converter, from operation on two phases, made in the form of a series-connected controllable 25 rectifier, a silt filter and voltage inverter, containing a current ddtchik with terminals for inclusion in a controlled circuit, the output of which is connected through the module allocation unit to the filter, the logical element And, the input and output terminals, characterized in that, in order to increase reliability by controlling the input current of the inverter and prevent false triggering of the protection at the time the motor starts up, it is additionally equipped with a threshold element and a delay unit, while the terminals of the current sensor are designed to be connected to a power filter and a voltage inverter , the first input of the logic element AND is connected via a threshold element to the output of the filter, the second input is through a delay unit to the input terminal connected to the output of the frequency converter control unit, and the output of the logic element AND is connected to the output terminal connected with the input of the converter control unit frequencies.