The role of current sensor in inverter control

Small power supplies have been incorporating more and more new technologies. Such as switching power supply, hard switching, soft switching, parameter regulation, linear feedback regulator, magnetic amplifier technology, digital voltage regulator, PWM, SPWM, electromagnetic compatibility and so on.

The actual demand directly promotes the continuous development and advancement of power supply technology, in order to automatically detect and display current, and has automatic protection function and more advanced intelligent control when the overcurrent, overvoltage and other hazards occur, with sensing detection, sensing sampling, Sensing-protected power supply technology is becoming a trend, and sensors for detecting current or voltage have emerged and are beginning to be favored by power supply designers in China.

The current sensor is used as a key component in the inverter control to ensure the performance of the inverter. At present, some of the inverter failures on the market are caused by damage or poor use of the current sensor.

The following describes the current sensor in the frequency converter, generally supporting the current sensor, there is a corresponding power supply ± 12V, ± 15V; conditioning circuit constitutes a current sampling module, the module converts the three-phase stator current into a basic binary code, Provide timely information on the current to complete the adjustment of the current loop. The system uses three JCE modules (some of the inverters also use two, and the third one is completed by the algorithm) to detect the a, b, and c phase currents respectively. The current information is converted into voltage information by a sampling resistor, and then converted into a digital signal by an AD conversion module in the main control chip of the frequency converter. It is then digitally filtered and scaled to provide current feedback for current loop regulation. The hardware schematic of the detection circuit is as follows:

The JCE current sensor converts the current information into voltage information. In addition, the signal detected by the LEM is converted into a voltage range suitable for the control chip. First, the voltage signal is converted into a symmetric positive and negative voltage, and then the corresponding offset is converted. To the voltage range suitable for controlling the chip. JCE samples the three-phase currents of a, b and c, and stores them in the corresponding registers after the detection module for the main control system.

Generally, the three-wire system is adopted, and the three-phase current of the motor stator is equal to zero. The LEM component samples the three-phase currents of a, b, and c, and the conversion result after the detection module is used as a basis for detecting the unbalance rate of the three-phase current; After the output is cut off, the three-phase current flow of a, b, and c can be detected by the LEM component to determine whether the power module is reliably turned off, thereby eliminating potential safety hazards.

In the application, the JCE sensor reflects its many characteristics:

Strong overload capability: When the primary current is overloaded, it can be automatically protected. Even if the overload current is 20 times of the rated value, the sensor will not be damaged.

The capacitance between the primary and secondary sides is very weak, and the effects of the *mode voltage are negligible in many applications;

High sensitivity: It can distinguish weak and small signals on high components, such as distinguishing several milliamps of AC components on a few hundred amperes of direct current;

High reliability: failure rate λ=0.43×10-6/h; strong resistance to external magnetic field: the magnetic field interference caused by a double working current at the distance sensor 5~250px causes the error of its own ≤0.5% .

It can measure the current of arbitrary waveforms, such as DC, AC, pulse waveform, etc., and even detect transient peaks;

The primary side circuit and the secondary side circuit are completely highly insulated, the insulation withstand voltage is generally 3~6kV, and the special requirement can reach 6~50kV;

High measurement accuracy: better than 10% accuracy over the operating temperature range, which is suitable for any waveform measurement;

Good linearity: better than 0.5%;

Good dynamic performance: response time is less than 1μs, tracking speed di/dt is higher than 50A/μs;

Working frequency bandwidth: the accuracy can reach 1% in the range of 0 (DC) ~ 100kHZ, and the accuracy is increased to 0.5% in the range of 0 ~ 5kHZ;

Practice has proved that the current detection link consisting of JCE current sensor and conditioning circuit can fully meet the current detection requirements of the closed-loop control of the inverter.

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