Change the installation position of the current transformer

The main reasons for shifting the current sampling from the input side to the output side are as follows:

(1) although there are a lot of high-order harmonics in the output current of the converter, the frequency converter adopts sinusoidal SPWM modulation, the output current waveform is close to sine wave, and the effective value is 1.2-1.5 times of the average value. 

(2) because the current waveform at the input side of the inverter power supply is a discontinuous pulse with double peaks at the peak of the input voltage waveform, and the voltage waveform at the output side is a rectangular pulse of equal height and varying width according to the sinusoidal waveform, the input and output current waveforms are formed under the same voltage (maximum) , the input and output current should be basically the same, the output side of the inverter current measurement will not cause large error, and in the output side of the current measurement, from the electrical, mechanical point of view is more realistic.

Select the type of ammeter

With the development of technology, there are more and more tools which can reflect the current RMS, but they are expensive. An electromagnetic ammeter is recommended in the specification of a frequency converter. It uses the magnetic field generated by an electric current signal to make the fixed and movable iron sheets attract or repel each other and drive the measuring mechanism to deflect to indicate the current value, the deflection angle of the measuring mechanism is approximately proportional to the square of the measured current, and can basically reflect the effective value of the current containing high-order harmonics. But this type of ammeter accuracy is relatively low, in the current is small, the error is larger; because it uses magnetic field rotation and its own magnetic field is weak, vulnerable to external magnetic field influence, sometimes the error will be larger. After the position of the current transformer is moved to the output side, the difference between the effective value and the average value is not too large because the current waveform tends to be sinusoidal, in the field of current display is not too high requirements can be used 1T1 dynamic iron ammeter or rectifier instrument (but need to be compensated) , we still use the original 42L6-A 20/5 rectifier ammeter to display the variable frequency current.

The method of solving the inherent error of current transformer

Using the method of increasing the primary current, increasing a certain number of turns on the primary side of the current transformer, the primary current of the current transformer is adjusted to about 100A, the magnetization force and leakage flux of the current transformer itself are reduced to a relatively small degree. For this reason, we have made the following changes: adding 200/5 current transformer at the output side of the converter, winding 13 turns at the primary side of the 200/5 current transformer, the actual conversion ratio is 15.385/5, the measurement current of the secondary side is 30% more than that of the 20/5 current transformer, which can be used to compensate the error between the effective value and the average value and various losses in the secondary circuit. The actual current transformer is calculated as 20/5. The purpose of this is that when the current value is increased at the primary side of the current transformer, the actual current transformer ratio does not change, in accordance with the original design (only after the replacement of the current transformer listed with the original conversion ratio and actual conversion ratio, for future verification) . In this way, the current of the circuit of the motor of the washing pump can reach about 130 a at the primary side of the current transformer after 13 turns of winding, so that the magnetizing force and the high-order harmonic of the current transformer itself can be achieved, the magnetic flux leakage caused by the wave is relatively small, and the hysteresis, eddy current and other losses caused by the high-order harmonics are not obviously increased due to the demagnetization of the secondary circuit, keep within a relatively small range.

The method of solving the current error on the field control box

There are about 30m point cables from the converter cabinet to the field control box. In order to reduce the transmission error caused by the cables, we take advantage of the relatively large field control box and the relatively small section of the cable in the main circuit of the washing pump, a current transformer is installed in the field control box, and the main circuit cable is penetrated into the control box, and the current signal is obtained from one phase of the main circuit. The 42L6 Ammeter is used to display directly on the control box.

The method of solving the current error of the inverter-fed motor in the main control room

Because the converter chamber is close to the motor load, the distance between the converter cabinet and the main control room is increased. In order to reduce the transmission error caused by the cable, a current transformer of 200/5 is added at the output side of the converter, on the basis of winding 13 turns at one side, a BS4I current transmitter is added to the converter cabinet, the current signal of current transformer circuit 0-5a, which contains a lot of high-order harmonics, is transformed into 4-20mA DC signal, which is transmitted to the computer cabinet of main control room by the original cable, rZG-21004-20mA/4-20mA signal isolator is used in computer cabinet to isolate the signals from the main control room and ensure the security of computer system.

Source: internet (for study only, intrusion and deletion)