1：Adjust the installation position of the current transformer (CT)

We moved the VFD current sampling from the VFD input side to the output side mainly for these reasons:

- Even though the VFD output current also has plenty of high-order harmonics, the drive uses sine-wave SPWM modulation, so the output current waveform is close to a sine wave. Its RMS value is 1.2 to 1.5 times the average value. When shown on a rectifier-type meter, we can compensate for the error with proper adjustments.

- The VFD input current waveform is an intermittent pulse with double sharp peaks at the peak of the input voltage. The output voltage waveform is rectangular pulses with equal height and width that changes sinusoidally. Since the input and output current waveforms are formed under the same peak voltage, the input and output currents are basically the same. Measuring current on the output side won’t cause big errors, and it’s also more representative of the actual current seen by the motor.

2：Selecting the Right Ammeter

As technology advances, more and more instruments can measure true RMS current, but they tend to be quite expensive.

VFD manuals recommend using a moving‑iron (electromagnetic) ammeter. This type of meter uses the magnetic field produced by the current to attract or repel its fixed and moving iron vanes, which deflects the meter movement to show the current value. The deflection angle is roughly proportional to the square of the measured current, so it can reasonably indicate the RMS value of current that includes high‑order harmonics.

However, this type of ammeter has relatively low accuracy, with large errors at low current levels. Since it operates using a weak internal magnetic field, it is easily affected by external magnetic fields, which can sometimes make the error even bigger.

After moving the current transformer (CT) to the output side, the current waveform becomes closer to a sine wave, so the difference between RMS and average current is small. For field applications where current display requirements are not strict, we can use either a 1T1 moving‑iron ammeter or a rectifier‑type meter (with proper error compensation). For our application, we still use the original 42L6‑A 20/5 rectifier‑type ammeter to display the VFD current.

3：Methods to Compensate for the Inherent Errors of Current Transformers (CTs)

By adopting the method of increasing the primary current, a specified number of turns are added on the primary side of the current transformer (CT) to adjust the primary current of the CT to approximately 100A, so that the inherent magnetizing force and leakage flux of the CT itself can be kept at a relatively low level.

For this purpose, we have made the following modifications: a 200/5 current transformer is installed on the inverter output side, with 13 turns wound on its primary side, resulting in an actual transformation ratio of 15.385/5. The secondary side of this CT measures 30% more current than a standard 20/5 current transformer, which can be used to compensate for the error between the RMS (effective) value and the average value, as well as various losses in the secondary circuit. For metering and calculation purposes, the CT is still processed in accordance with the 20/5 transformation ratio.

The purpose of this approach is to ensure that the nominal transformation ratio of the CT remains unchanged and fully consistent with the original design when the current value on the primary side of the CT is increased. It is only required to affix a nameplate on the replaced CT to mark both the original and actual transformation ratios for future verification.

In this way, based on the normal loop current of 10A for the washing liquid pump motor, the primary side current of the CT can reach approximately 130A after 13 turns are wound. This enables the inherent magnetizing force of the CT and the leakage flux caused by high-order harmonics to be kept at a relatively low level. Meanwhile, various losses such as hysteresis and eddy current losses induced by high-order harmonics will not increase significantly due to the demagnetization effect of the secondary circuit, and will remain within a relatively small range.

4：Solution to Current Measurement Error of On-site Control Box

The control cable running from the VFD cabinet to the field control box is approximately 30 meters long. To minimize the transmission error caused by the cable, we leveraged two favorable conditions: the relatively large interior space of the field control box and the small cross‑section of the main circuit cable for the washing liquid pump.

We installed an additional current transformer inside the field control box, routed the main circuit cable through the box, and picked up the current signal from one phase of the main circuit. The current value is displayed directly on the control box using a Type 42L6 ammeter.

5：Methods for Solving Current Error of Variable Frequency Speed-Regulating Motor in Main Control Room

Since the frequency converter room is close to the motor load, the distance between the frequency converter cabinet and the main control room is relatively increased.

To reduce the transmission error caused by the cable, a 200/5 current transformer is installed at the output side of the frequency converter.

On the basis of 13 turns on the primary side, a BS4I current transmitter is additionally installed in the frequency converter cabinet, converting the 0–5 A current signal with high harmonics from the current transformer circuit into a 4–20 mA DC signal.

This signal is transmitted through the original cable to the computer cabinet in the main control room.

An RZG-2100 4–20 mA/4–20 mA signal isolator is adopted in the computer cabinet to isolate the signals between the field and the main control room, ensuring the safety of the computer system.