Small VFD (Variable-frequency Drive) current display error solution

1 Introduction

During the trial run of the boehmite project in our factory, five 5.5kW lotion pumps adopted Fuji 7.5kW VFD (Variable-frequency Drive) variable frequency speed regulation. The trial run went smoothly, but the VFD (Variable-frequency Drive) ), the output current has a relatively large error on the display of the control cabinet, on-site control box, and VIA FB2000 DCS distributed control system. When close to the rated load, there is a very small current on the display of the on-site control box and DCS distributed control system, which is below 3A, and below 5A on the control cabinet; when there is no load or the load is small, there is a small current on the display of the on-site control box and DCS distributed control system. There is no current display and the current on the control cabinet is very small. We inspected and analyzed the VFD (Variable-frequency Drive) current loop: the control cabinet and on-site control box are installed with a 42L6 rectifier system ammeter, the current transformer is a single-turn LMZJ1-0.5 20/5, and the current signal is taken from On the power side of the VFD (Variable-frequency Drive), the on-site control box and the main control room are not far from the frequency conversion cabinet. It is about 30m away. The current loop wiring is normal and the current transformer is not overloaded. What is the reason? According to analysis, the reasons for the current display error are as follows.

2 Causes of current display error

2.1VFD (Variable-frequency Drive) power side current waveform is distorted and contains large high-order harmonics

Since the VFD (Variable-frequency Drive) uses an uncontrollable rectifier bridge on the power supply side, and a large filter capacitor is installed in the intermediate DC circuit, the current waveform on the input side of the VFD (Variable-frequency Drive) power supply has double spikes at the peak of the input voltage waveform. Intermittent pulses, the current pulse width is slightly wider when the load is large, and very narrow when the load is small. This distorted current waveform contains a large number of harmonics, and the peak value of the current pulse is much larger than the average value. The waveform coefficient of this distorted current It is the square root of the percentage of the current pulse width. Since small VFDs (Variable-frequency Drives) generally do not install a power factor correction DC reactor in the intermediate DC circuit, the effective value reaches 2 to 3 times the average value, and is nonlinear. When the load is small, the effective value is relatively larger than the average value.

2.2 Incorrect selection of ammeter

The 42L6 rectifier system ammeter measures current based on the principle of current average. When measuring a 50Hz complete sine wave, it is calibrated according to a waveform factor of 1.11 times. When the 50Hz complete sine wave or the waveform error is not too large, it is calibrated , can correctly or basically reflect the effective value of the current. For the pulse current with such waveform distortion on the VFD (Variable-frequency Drive) input side, a large error will occur, and because the effective value of the current with this waveform distortion is different from The average value is nonlinear, and different current segments have different errors. It is difficult to take corresponding compensation and correction measures on the circuit.

2.3 The current transformer itself has inherent errors

We all know that the magnetizing force of the current transformer itself is equal to the vector sum of the primary magnetizing force and the secondary magnetizing force, that is: I0W0=I1W1+I2W2, which is mainly related to the magnetic permeability of the iron core. The size of the current error △I of the current transformer is related to the required magnetizing force, the size of the primary current, and the relatively large leakage flux and hysteresis loss at high frequency. The greater the magnetizing force required by the current transformer itself, the greater the current error △I; when the primary current of the current transformer is much smaller, the current error △I will be large; when the size of the current transformer is larger, the leakage at high frequency will The magnetic flux and hysteresis losses will be larger, and the current error △I will also become larger. Since the currently commonly used small ratio single-turn current transformers such as LMZ, LMZJ and other models have basically the same dimensions and magnetic permeability materials, their primary rated currents are all above 200A. When the primary current increases to near the rated current, the current mutual inductance The magnetizing force of the device itself and the leakage flux and hysteresis loss at high frequency can be relatively reduced to a certain extent, which can minimize the current error △I. However, the primary current of the lotion pump is too small, about 10A during normal operation. The magnetizing force generated by the primary current in the iron core is not enough to maintain the magnetizing force of the current transformer itself. In addition, the VFD (Variable-frequency Drive) power supply side contains High-order harmonics make the leakage flux and hysteresis loss of the current transformer relatively large, and the current reflected to the secondary side is very small. In this way, when the secondary load of the current transformer is not large, the current transformer's The transformation ratio error will be large.

2.4 The capacitive reactance of the control cable reduces the ammeter value

In order to shorten the length of the power cable between the VFD (Variable-frequency Drive) and the motor, the VFD (Variable-frequency Drive) is generally set not far from the motor. Relatively speaking, the control cable is not too long, but the VFD (Variable-frequency Drive) Frequency Drive) The current on the input side of the power supply contains a large number of high-order harmonics. The control cable itself has a certain capacitance. For high-order harmonics, the capacitive reactance of the control cable is relatively small, which is reflected in the secondary circuit of the current transformer. Some of the high-efficiency harmonics do not pass through the ammeter, but directly pass through the control cable capacitance and return to the power supply side. The actual current passing through the ammeter is reduced. This can be seen from the ammeter on the VFD (Variable-frequency Drive) cabinet and the ammeter on the on-site control box. Actual situations with large differences in indicated values can be confirmed.

3 Solutions

Based on the above analysis and combined with the actual situation on site, we put forward corresponding countermeasures:

3.1 Change the installation position of current transformer

The main reasons for moving the VFD (Variable-frequency Drive) current sampling from the VFD (Variable-frequency Drive) input side to the output side are:

(1) Although the current on the output side of the VFD (Variable-frequency Drive) also contains a large number of high-order harmonics, because the VFD (Variable-frequency Drive) uses sine wave SPWM modulation, the output current waveform is close to a sine wave, and the effective value is an average 1.2 to 1.5 times of the value. When using a rectifier system instrument to display the error, the error can be compensated in an appropriate way.