RU1794889C - Method of measuring load current of bridge inverter - Google Patents

Abstract

The invention relates to a conversion and measuring technique and may find application in AC drives with frequency-current control to control the current in the winding of a controlled electric motor. The purpose of the invention is to increase the accuracy of current measurement by eliminating measurement errors that occur when switching the load current circuit. At the beginning of the switching of the load current flow circuit, the measured current value is recorded and for the whole time the circuit is switched, a fixed value is taken as the current value in the load. 1 ill.

Description

The alleged invention relates to a conversion and measuring technique and may find application in AC drives with frequency-current control to control the current in the winding of a controlled electric motor.

A known method for measuring the load current of a bridge inverter using a current meter connected directly to the load current flow circuit.

A disadvantage of the known method is the need for galvanic isolation of the high voltage load circuit of the bridge inverter and the low voltage load circuit of the current meter. The use of magnetic amplifiers greatly complicates the current meter and introduces a delay in the current measurement channel.

The closest in technical essence to the claimed method is a method of measuring the load current of a bridge inverter, based on the use of shunts as measuring elements,

A known method of measuring the load current of a bridge inverter, the power source of which is connected by one of the terminals to a common bus, based on the use of shunts as measuring elements included in the bridge inverter in the flow circuit of the load current so that the shunts are under the potential of the common bus, and measuring the load current by the difference in the output voltages of one of the shunts.

A disadvantage of the known method is the low accuracy of measuring current while switching the load current flow circuit in a bridge inverter. The result of measuring the load current is distorted by the errors generated by the dynamics

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The aim of the invention is to improve the accuracy of current measurement by eliminating measurement errors that occur when switching the load current circuit in a bridge inverter.

This goal is achieved by the fact that according to the known method of measuring the load current of a bridge inverter, the power source of which is connected by one of the terminals to a common bus based on the use of shunts as measuring elements included in the bridge inverter in the load current flow circuit so that the shunts are under the potential of the common bus, and on the measurement of the load current by the difference in the output voltages of one of the shunts, the value of the measured current is fixed. At the moment of the beginning of switching the flow circuit load current in the inverter bridge and receive chains at the switching time recorded value of the current as the load current.

The following is the basis for solving the stated problem. When measuring the load current of the bridge inverter using shunts connected to the bridge inverter in the load current flow circuit, the measurement result is distorted by errors due to the dynamics of the switching of the load current flow circuit. Current measurement errors appear as spurious short pulses appearing on the measuring shunts.

It is possible, by smoothing the measurement result, to substantially reduce the amplitude of these pulses. However, the inertia inherent in the filters leads to a noticeable dynamic error in the measurement of the load current.

It is proposed that the arising false pulses not be smoothed, but cut out from the output signal of the current meter. The implementation of this is to fix the value of the measured current at the moment of switching the load current flow circuit and to use the fixed current value as an estimate of the load current during the time of switching the current flow circuit.

With this approach, the accuracy of the current measurement will increase, since the errors caused by the dynamics of the switching circuit of the flow of the load current are eliminated, and the assumption that the load current remains constant during the fixing time practically does not introduce an additional error in the current measurement.

It should be noted that this assumption that the load current is unchanged is based on the inductive nature of the inverter load, which is the motor control winding.

The proposed method of measuring current

the load of the bridge inverter, the power supply of which is connected by one of the terminals to the common bus, consists in using shunts as measuring elements included in the bridge inverter in the load current flow circuit so that the shunts are under the common bus potential; measuring the load current by the difference in output voltage of one of the shunts; they fix the value of the measured current at the moment of switching the circuit of the flow of the load current in the bridge inverter, and for the time of switching the circuit take a fixed current value as the load current.

The drawing shows a device for measuring the load current of a bridge inverter, in which the accuracy of current measurement is carried out by

the proposed method.

A device for measuring the load current 1 of a bridge inverter 2, the power supply of which is connected by a negative terminal to a common bus.,

Contains shunts 3 and 4 included in the shoulders of a bridge inverter 2, the second terminals of which are combined and connected to a common bus, a subtraction circuit 5 on an operational amplifier, inverting

whose input is connected to the first output of shunt 3, and the non-inverting input is connected to the first output of shunt 4.

The control inputs of the keys 6 and 9, 7 and 8 of the bridge inverter 2 are connected respectively to the outputs 10,11 of the control unit 12 of the bridge inverter.

The device also contains an analog switch with memory 13 connected by an information input to the circuit output

subtract 5, and the control input through a two-input single-shot 14 short pulses to the outputs 10 and 11 of the control unit 12.

The device operates as follows.

The current flow circuit of the load 1 in the mos-TQBOM inverter 2 is controlled by block 12. The high level of signals at outputs 10 and

11 of the control unit 12 corresponds to the closed state of the keys 6,7,8 and 9 of the bridge inverter 2.

The control unit 12 provides a symmetrical operation mode of the bridge inverter 2, which is characterized by the simultaneous activation of the keys 6 and 9 or 7 and 8. Moreover, the inclusion of one of the key pairs is accompanied by the obligatory shutdown of the other pair. The load current 1 flows through one of the shunts 3 and 4, while a positive load current 1 corresponds to the current flowing through the shunt A towards the common bus or along the shunt 3 in the opposite direction.

The output voltage of the analog memory switch 13 with memory exactly repeats the input voltage in the on state, and in the off state, it remains the same as it was at the time of the shutdown.

Let the load current 1 flow through the return diodes of the keys 7 and 8. At the moment of closing of the keys 6 and 9, the current flow circuit of the load 1 does not instantly switch, since these diodes, due to their inertia, will retain a conducting state for some time. In the bridge inverter 2 there is a short-term through current. which will distort the information about the load current, taken from the measuring shunts 3 and 4, In addition,

the signal at the output 10 of the control unit 12, closing the keys 6 and 9, will start the single-shot 14. The output pulse of the single-shot 14 will turn off the analog for a short time

switch 13. At the output of the current meter, a constant signal will be received corresponding to load track 1 before switching on the keys 6 and 9. False pulses that occur in the output voltages of shunts 3 and 4 are not

pass to the output of the current meter.

We arrive at a similar result. if the load current 1 flows through the reverse diodes of the keys 6 and 9. At the moment of closing the keys 7 and 8, the one-shot 14 will be triggered by the signal from the output 11 of the control unit 12 and will also block the passage of false information to the output of the current meter,

Thus, when measuring the load current of the bridge inverter using shunts connected to the bridge inverter in the flow circuit of the load current, fixing the value of the measured current at the moment

starting the switching of the load current flow circuit and using the fixed current value as an estimate of the load current during the switching time of the current flow circuit allows to increase

accuracy of measurement of load current.

Claims (1)

SUMMARY OF THE INVENTION A method for measuring the load current of a bridge inverter, the power supply of which is connected to a common bus by one of the terminals, which consists in using shunts as measuring elements connected to a common bus and included in the load current flow circuit, according to the voltage difference load shunts are measured on shunts, characterized in that, in order to improve the accuracy of current measurement by eliminating measurement errors that occur when switching the load current flow circuit, the measured current value is recorded at the moment of switching the load current flow circuit and for the entire time the circuit is switched, the fixed value is taken as the load current value.