1. Change the installation location of the current transformer
The main reasons for shifting the sampling of inverter current from the input side to the output side are:
(1) Although the output current of the inverter also contains a large number of high-order harmonics, the output current waveform is close to a sine wave because the inverter uses sine wave SPWM modulation. The effective value is 1.2 to 1.5 times the average value. When the rectifier system instrument is used for display, its error can be compensated in an appropriate way.
(2) Since the current waveform on the input side of the inverter power supply is an intermittent pulse with double peaks at the peak of the input voltage waveform, and the voltage waveform on the output side is a rectangular pulse with equal height and width varying according to a sine wave, the current waveforms on the input and output sides are formed under the same voltage (maximum value). The current on the input side and the output side should be basically the same. Measuring the inverter current on the output side will not cause a large error. Moreover, measuring the current on the output side is more in line with reality from the perspective of the motor.
2. Selection of ammeter
With technological advancements, more and more measuring tools are available to display the effective value of current, but they are all relatively expensive. Inverter manuals recommend using electromagnetic ammeters, which utilize the magnetic field generated by the current signal to attract or repel a fixed and a movable iron piece, causing the measuring mechanism to deflect and indicate the current value. The deflection angle of the measuring mechanism is approximately proportional to the square of the measured current, and it can basically reflect the effective value of current containing high-order harmonics. However, this type of ammeter has relatively low accuracy, with larger errors at lower currents; because it uses a rotating magnetic field and its own magnetic field is relatively weak, it is easily affected by external magnetic fields, sometimes resulting in larger errors. After moving the current transformer to the output side, since the current waveform tends to be sinusoidal, the difference between the effective value and the average value is not too large. In situations where the current display requirements are not too high, a 1T1 moving iron ammeter or a rectifier system instrument (but compensation is required) can be used. We still used the original 42L6-A20/5 rectifier system ammeter to display the inverter current.
3. Methods to address the inherent errors of current transformers
By increasing the primary current, a certain number of turns are added to the primary side of the current transformer, and the primary current of the current transformer is adjusted to about 100A, so that the magnetization force and leakage flux of the current transformer itself are relatively small.
Therefore, we made the following modifications: A 200/5 current transformer was added to the inverter output side. The primary winding of the 200/5 current transformer has 13 turns, resulting in an actual transformation ratio of 15.385/5. On the secondary side, it measures 30% more current than a 20/5 current transformer, which can be used to compensate for errors between the effective and average values and various losses in the secondary circuit. The actual current transformer is calculated based on a 20/5 transformer. The purpose of this is that when the current value on the primary side of the current transformer is increased, the actual current transformer ratio remains unchanged, consistent with the original design (only the original and actual transformation ratios need to be labeled on the replaced current transformer for future verification). Based on a normal operating current of 10A for the washing pump motor circuit, the current on the primary side of the current transformer can reach approximately 130A after 13 turns of winding. This results in a relatively small magnetization force of the current transformer itself and leakage flux caused by higher harmonics. Furthermore, the various losses caused by higher harmonics, such as hysteresis and eddy currents, will not increase significantly due to the demagnetizing effect of the secondary circuit, and will remain within a relatively small range.
4. Methods to resolve current errors in the field control box
The cable from the frequency converter cabinet to the field control box is approximately 30 meters long. To minimize transmission errors caused by the cable, we took advantage of the relatively large size of the field control box and the small cross-section of the main circuit cable for the washing pump. We installed a current transformer inside the field control box, threaded the main circuit cable through it, and obtained the current signal from one phase of the main circuit. A 42L6 ammeter is then used to display the signal directly on the control box.
5. Methods to resolve current errors in the variable frequency speed control motor in the main control room
Because the inverter room is close to the motor load, the distance between the inverter cabinet and the main control room is relatively increased. In order to reduce the transmission error caused by the cable, a 200/5 current transformer is added to the output side of the inverter. On the basis of 13 turns of primary winding, a BS4I type current transmitter is added to the inverter cabinet to convert the 0-5A current signal containing a large number of high-order harmonics in the current transformer circuit into a 4-20mA DC signal, which is transmitted to the computer cabinet in the main control room through the original cable. An RZG-2100 4-20mA/4-20mA signal isolator is used on the computer cabinet to isolate the field signal from the main control room signal and ensure the safety of the computer system.
