Small power supply devices have incorporated more and more new technologies. These include switching power supplies, hard switching, soft switching, parametric voltage regulation, linear feedback voltage regulation, magnetic amplifier technology, digitally controlled voltage regulation, PWM, SPWM, electromagnetic compatibility, and so on.
Practical needs directly drive the continuous development and progress of power supply technology. In order to automatically detect and display current, and to have automatic protection functions and more advanced intelligent control when harmful conditions such as overcurrent and overvoltage occur, power supply technology with sensing detection, sensing sampling and sensing protection is gradually becoming a trend. As a result, sensors for detecting current or voltage have emerged and have begun to be favored by a large number of power supply designers in my country.
Current sensors are key components in frequency converter control, ensuring the performance of the frequency converter. Currently, many frequency converter failures on the market are caused by damage or improper use of current sensors.
The following describes the application of current sensors in frequency converters. Generally, current sensors are paired with corresponding power supplies of ±12V or ±15V. The conditioning circuit constitutes a current sampling module, which converts the three-phase stator current into basic binary code, providing real-time current information and completing the current loop adjustment. This system uses three JCE modules; some frequency converter manufacturers use two. The third module uses an algorithm to detect the currents of phases a, b, and c respectively. The current information is converted into voltage information through sampling resistors, and then converted into digital signals by the AD conversion module in the frequency converter's main control chip. After digital filtering and calibration, current feedback information is provided for current loop adjustment. The hardware schematic diagram of the detection circuit is shown below:
The JCE current sensor converts current information into voltage information. Furthermore, it converts the signal detected by the LEM into a voltage range suitable for the control chip. First, the voltage signal is transformed into symmetrical positive and negative voltages, and then a corresponding offset is provided to convert it into a voltage range suitable for the control chip. The JCE samples the currents of phases a, b, and c, and after passing through the detection module, stores them in the corresponding registers for use by the main control system.
Generally, a three-wire system is used, where the sum of the three-phase stator currents of the motor is equal to zero. The LEM element samples the currents of phases a, b, and c, and the conversion result after passing through the detection module serves as the basis for detecting the three-phase current imbalance rate. In addition, after the inverter cuts off the output, the LEM element can be used to detect the flow of the currents of phases a, b, and c to determine whether the power module has been reliably shut down, thus eliminating potential safety hazards.
In applications, the JCE sensor demonstrates numerous advantages:
Strong overload capacity: When the primary current is overloaded, it can automatically protect itself, and the sensor will not be damaged even if the overload current is twenty times the rated value;
The capacitance between the primary and secondary sides is very weak, and in many applications, the effects of various modal voltages can be ignored.
High sensitivity: It can distinguish weak signals at high frequencies, such as AC components of a few milliamperes at a DC current of several hundred amperes;
High reliability: failure rate λ=0.43×10-6/h; strong resistance to external magnetic fields: experimental results show that the error caused by magnetic field interference generated by a magnetic field twice the working current at a distance of 5 to 250px from the sensor is ≤0.5%.
It can measure current of any waveform, such as DC, AC, pulse waveforms, and even transient peak values;
The primary circuit and the secondary circuit are completely and highly insulated, with an insulation withstand voltage of 3-6kV, which can reach 6-50kV for special requirements;
High measurement accuracy: Accuracy is better than 10% within the operating temperature range; this accuracy is suitable for measuring any waveform.
Good linearity: better than 0.5%;
Excellent dynamic performance: response time less than 1μs, tracking speed di/dt greater than 50A/μs;
Operating frequency bandwidth: accuracy reaches 1% in the range of 0 DC to 100 kHz, and improves to 0.5% in the range of 0 to 5 kHz;
Practice has proven that the current detection circuit consisting of the JCE current sensor and conditioning circuit can fully meet the current detection requirements of the frequency converter in closed-loop control.
