1 Introduction
Variable frequency drives (VFDs) are mainly used for speed regulation of AC motors. Besides their excellent speed control performance, VFDs also offer significant energy savings, making them ideal speed control devices for enterprise technological upgrades and product modernization. However, the output characteristics of the VFD itself and the coupling effect of cable distributed capacitance limit its output distance.
2. Cause Analysis
The length of the cable from the inverter output to the motor is affected by many factors, the main reasons for which are as follows:
(1) Distributed capacitance. Distributed capacitance refers to a distributed parameter formed by non-capacitive structures. It generally refers to the capacitance formed between lines or between upper and lower layers of a printed circuit board or other circuit forms. The problem of limited output distance of frequency converters is closely related to the distributed capacitance of cables. It is not only capacitors that have capacitance; in fact, capacitance exists between any two insulated conductors. For example, between wires and between a wire and the ground, they are separated by insulation layers and air, so capacitance exists in both cases. Figure 1 shows the equivalent distributed capacitance structure of 4-core and 7-core cables.
Normally, this capacitance value is very small (generally around 15-30 nF/100m), and its actual impact is negligible when the cable length is short. However, if the cable is very long or the transmission signal frequency is high, the effect of distributed capacitance must be considered. In long-distance cable laying systems, the cable capacitance becomes more noticeable, affecting the control circuit and even the control function. This is especially true for control circuits of frequency converters controlling ordinary low-voltage motors, where faults often manifest as overcurrent and start/stop failures, posing significant safety hazards to production and maintenance. The resonance of distributed capacitance and inductance on the output line generates surge voltage, which is superimposed on the output voltage. The higher the switching frequency of the transistor or IGBT and the longer the cable, the higher the surge voltage, sometimes reaching twice the DC voltage. In this situation, overvoltage and overcurrent protection can be easily triggered, potentially even burning out the module.
Distributed capacitance is a distributed parameter whose value varies not only from cable manufacturers, but also from cable laying methods, operating conditions and external environmental factors. This needs to be considered comprehensively during the design process.
(2) Inverter output problem
Currently, almost all frequency converters use PWM (pulse width modulation) technology. However, because the power switching devices in the frequency converter operate in a switching state, the high-speed switching action of these devices causes voltage and current to jump rapidly. This results in a large number of harmonic components in the voltage and current waveforms. Higher-order harmonics can increase the output current of the frequency converter, causing the motor windings to heat up, generating vibration and noise, accelerating insulation aging, and potentially damaging the motor. Simultaneously, harmonics of various frequencies can emit radio interference into space, potentially causing malfunctions in other equipment. Therefore, it is desirable to place the frequency converter near the controlled motor. However, due to space limitations in the production site, a certain distance is often required between the frequency converter and the motor.
(3) Power of the frequency converter
The power of the frequency converter directly determines the length of the cable from the frequency converter output to the controlled motor. The higher the power of the frequency converter (without the output reactor connected), the longer the corresponding output cable will be.
The above three aspects directly affect the cable length from the inverter output to the motor. Based on the analysis of the above reasons, the following will further study the improvement methods.
3 Improvement Plans
3.1 Adjust the carrier frequency to reduce harmonic interference.
The carrier frequency of a frequency converter is the frequency that determines the number of times the power switching devices (such as IGBTs) of the inverter are turned on and off.
It mainly affects the following aspects:
(1) The effect of carrier frequency on the inverter itself
The power loss of the IGBT power module is related to the carrier frequency. The higher the carrier frequency, the greater the inverter's losses, the lower the output power, and the more heat the power module generates. If the ambient temperature is high, the dead zone of the two inverter transistors in the inverter bridge during the alternating conduction process will become smaller, which can lead to a short circuit in the bridge arm and damage the inverter in severe cases.
(2) The effect of carrier frequency on the waveform of the secondary current output by the frequency converter
The higher the carrier frequency, the larger the duty cycle of the voltage wave and the smaller the higher harmonic components of the current. In other words, the higher the carrier frequency, the smoother the current waveform. This results in less harmonics and less interference, and vice versa. The higher the carrier frequency, the smaller the current that the inverter can output. The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitor (because xc=1/2πfc), and the greater the leakage current caused by high-frequency pulses.
(3) The effect of carrier frequency on motor
When the carrier frequency is too low, the effective torque of the motor decreases, losses increase, and the temperature rises. At the same time, the rate of change of the output voltage dv/dt increases, which has a significant impact on the motor insulation. When the carrier frequency is too high, the motor vibration decreases, the operating noise decreases, and the motor heat generation also decreases. However, the frequency of the harmonic current increases, the skin effect of the motor stator becomes more severe, the motor loss increases, and the output power decreases.
(4) The impact of carrier frequency on other equipment
The higher the carrier frequency, the more severe the interference of high-frequency voltage on electronic equipment through electrostatic induction, electromagnetic induction, and electromagnetic radiation.
In practical applications, all the above factors should be considered to select the appropriate carrier frequency for the frequency converter. Generally, the larger the motor power, the lower the load factor should be.
3.2 Add a common-mode choke to the output terminal
A common-mode choke, also called a common-mode inductor, consists of coils wound symmetrically in opposite directions with the same number of turns on a closed magnetic ring. Essentially, a common-mode inductor is a bidirectional filter: on the one hand, it filters out common-mode electromagnetic interference on signal lines; on the other hand, it suppresses its own electromagnetic interference, preventing it from affecting the normal operation of other electronic devices in the same electromagnetic environment.
Common-mode chokes can transmit differential-mode signals, including DC and very low-frequency differential-mode signals, while exhibiting high impedance to high-frequency common-mode noise. Therefore, they can be used to suppress common-mode current interference.
3.3 Add input and output reactors
The following options can be added to the input side of the frequency converter:
(1) Input line reactor: The input reactor can suppress harmonic current, improve the power factor and weaken the impact of surge voltage and current in the input circuit on the frequency converter, and weaken the influence of power supply voltage imbalance. Under normal circumstances, an input line reactor must be added.
(2) Input EMC radio interference filter. The purpose of the EMC filter is to reduce and suppress electromagnetic interference generated by the frequency converter.
The following options can be added to the output side of the frequency converter:
(1) Output reactor: When the cable length from the inverter to the motor exceeds the product specification, an output reactor should be added to compensate for the charging and discharging effect of the coupling capacitor during the operation of the long motor cable, thus preventing overcurrent in the inverter. There are two types of output reactors: one is an iron-core reactor, used when the inverter's carrier frequency is less than 3kHz; the other is a ferrite reactor, used when the inverter's carrier frequency is less than 6kHz.
(2) Output dv/dt reactor. The output dv/dt reactor is used to limit the rate of rise of the inverter output voltage, reduce the output harmonic components, prevent overvoltage protection cables, reduce motor noise, and ensure the normal insulation of the motor.
(3) Sine wave filter: With the continuous research on the output distance of frequency converters, various manufacturers have launched output filters for frequency converters. The sine wave filter is used at the output end of the frequency converter. It can improve the output waveform of the frequency converter, making the output voltage and current of the frequency converter approximately sinusoidal, reducing the motor harmonic domain coefficient and motor insulation pressure. Compared with the output reactor and dv/dt filter, the sine wave filter has a capacitor filter circuit at the end, making the output waveform of the frequency converter close to a sine wave.
The following describes this novel filtering technology. Compared to traditional filter layout techniques, the new sinusoidal filter can simultaneously achieve three functions: converting phase-to-phase voltage into a sinusoidal signal, suppressing common-mode current, and converting conductor-to-ground voltage into a sinusoidal voltage. Figure 2 shows a schematic diagram of the installation location of the sinusoidal output filter.
The sine filter mainly consists of the following components: high-frequency output reactor, RC circuit, common-mode reactor, etc., as shown in Figure 3.
The output of a frequency converter is a pulse sequence with equal amplitude but unequal width. Since the voltage waveform output by the frequency converter is not a sine wave, it contains a large number of harmonic components, as shown in Figure 4, which is the output voltage waveform of the frequency converter. After using a sine filter, the waveform will approximate a sine wave, as shown in Figure 5.
As can be seen from Figures 4 and 5, after adding a sine filter to the output of the frequency converter, the output is close to a sine wave, which can significantly improve the harmonic content of the output and reduce eddy current losses. After testing, the longest cable length from the output of the frequency converter to the motor after adding a sine filter can reach 1000m.
Compared to traditional filter technologies, inverter output filter technology can simultaneously convert phase-to-phase voltage into a sinusoidal signal, suppress common-mode current, and convert conductor-to-ground voltage into a sinusoidal voltage waveform. It offers various advantages, such as significantly extending motor lifespan by suppressing harmful voltage peaks in the motor windings and reducing bearing current to a negligible level.
4. Conclusion
The output distance of frequency converters has always been a difficult problem to solve completely. This article analyzes the possible reasons that may cause the output distance to be limited by frequency converters and cables, and proposes some practical solutions, which are of reference value for practical engineering applications.
