Abstract: This paper introduces the classification of inverter-specific output reactors, and then elaborates on the functions and selection methods of inverter-specific output reactors in smoothing filtering and compensating for long-line distributed capacitance.
Keywords: Category function selection
1. Classification of output reactors for frequency converters
The inverter-specific output reactor is installed between the inverter's output line and the motor. Based on the different core materials used, inverter-specific output reactors are divided into the following two categories:
1) Dedicated output reactor for iron-core frequency converters. As shown in Figure 1, a dedicated output reactor for iron-core frequency converters should be selected when the carrier frequency of the frequency converter is less than 3kHz.
2) Ferrite-type frequency converter dedicated output reactor. As shown in Figure 2, a ferrite-type output reactor should be selected when the carrier frequency of the frequency converter is less than 6 kHz.
The core of the dry-type output reactor used in the variable frequency speed control system is made of high-quality, low-loss cold-rolled silicon steel sheet, and the air gap is made of epoxy laminated glass cloth plate to ensure that the air gap of the variable frequency drive-specific output reactor does not change during operation. The coil of the variable frequency drive-specific output reactor is wound with H-grade enameled flat copper wire, which is tightly and uniformly arranged, without an outer insulation layer, and has good heat dissipation performance.
The fasteners for the core section of the inverter-specific output reactor are made of non-magnetic materials to reduce eddy current heating during operation. All exposed components are treated with anti-corrosion coating, and the leads are made of tin-plated copper tubing. Compared to general-purpose iron-core dry-type output reactors, the inverter-specific output reactor has advantages such as smaller size, lighter weight, and more aesthetically pleasing appearance.
2. Function of the output reactor for frequency converters
The output of the frequency converter is a voltage wave modulated by PWM, as shown in Figure 3. Since the inductive nature of the motor windings enables the current to be continuous, the current is basically sinusoidal. Pulse Width Modulation (PWM) has steep rising and falling edges of the voltage, that is, the du/dt is very large, which causes the output leads to emit a large amount of electromagnetic interference. It also generates a large pulse current between the leads and ground, between the motor winding turns, and between the motor windings and ground.
To reduce the interference of inverter output du/dt to the outside world, reduce output waveform distortion and leakage current to meet environmental protection standards, reduce voltage surges to the motor windings causing insulation damage, reduce motor temperature rise and noise, reduce eddy current losses, prevent damage to the inverter output power transistors due to du/dt and excessive pulse surge currents, and reduce damage to the inverter caused by load short circuits, a reactor should be added between the inverter output terminal and the motor.
Adding a reactor to the output side of the inverter can passivate the steepness of the inverter's output voltage (switching frequency), reduce disturbances and impacts on the power components in the inverter, and effectively suppress inrush current at the moment of load closing, protecting the inverter device and other components in the circuit from overcurrent impact.
Because the output voltage of a frequency converter contains many high-order harmonics, the transmission line between the frequency converter and the motor should not be too long. If the wire is too long, the distributed capacitance will increase. Under the influence of harmonic voltage, the high-order harmonic current will also be excessive. Using a frequency converter-specific output reactor can compensate for the effect of long line distributed capacitance, extending the effective transmission distance from the frequency converter to the motor (the maximum permissible motor cable length mainly depends on the switching frequency and output voltage of the drive device).
Theoretically, the allowable length of motor cable varies depending on the power rating of the frequency converter, and also differs between frequency converters from different manufacturers. Therefore, to determine the minimum distance between the frequency converter and the motor cable that requires the installation of an output reactor, please refer to the user manual provided by the respective frequency converter manufacturer. Table 1 shows data provided by Siemens.
Table 1 Output Filter Reactor and Allowable Wire Length
3. Output Reactor Selection
When selecting the output reactor for a variable frequency speed control system, factors such as rated AC current, voltage drop, inductance, inductance corresponding to the rated current, and cable length should be considered.
1) Selection of rated AC current. The rated AC current is selected based on the heating aspect of the reactor's long-term operating current. At the same time, sufficient high-order harmonic components should be considered, that is, the actual current flowing through the output reactor is the current of the motor load driven by the frequency converter.
2) Voltage drop. Voltage drop refers to the actual voltage drop across the reactor coil at 50Hz and corresponding to the actual rated current. Typically, a voltage drop of around 4V to 8V is selected.
3) Inductance Selection. The rated inductance of the reactor is also an important parameter! An inappropriate inductance selection will directly affect the voltage drop under rated current, potentially causing a malfunction. The inductance depends on the cross-sectional area of the reactor core, the number of turns in the coil, and the adjustment of the air gap. The selection of the output reactor inductance is based on the cable length within the rated frequency range. Then, based on the actual rated current of the motor, the required core cross-sectional area and conductor cross-sectional area are selected to determine the actual voltage drop. Table 2 shows the inductance and cable length corresponding to the rated current.
Table 2 shows the inductance and cable length corresponding to the rated current.
An ideal reactor should maintain a constant inductance at or below the rated AC current, gradually decreasing as the current increases. When the rated current exceeds twice the rated current, the inductance decreases to 0.6 times the rated inductance. When the rated current exceeds 2.5 times the rated current, the inductance decreases to 0.5 times the rated inductance. When the rated current exceeds 4 times the rated current, the inductance decreases to 0.35 times the rated inductance.
In practical applications, as long as the load is inductive, a reactor with 1% impedance or even lower is feasible. This is because the PWM modulation frequency is much higher than the fundamental frequency, and the inductance of the output reactor should ideally be such that the voltage across the reactor when the fundamental current flows through it does not exceed the rated voltage. Therefore, the inductance of the output AC reactor can be calculated using the following formula:
For example: Selection of AC reactor on the output side of a 380V, 90kW, 50Hz, 170A frequency converter:
mH
Choose a reactor with an inductance of around 0.041mH and an operating current of 170A.
The position of the wires wound on the magnetic core of the output-side AC reactor is related to the degree of interference energy emitted by the inductor. The cross-sectional structure of the AC reactor on the output side of the frequency converter is shown in Figure 4. The winding head 1 is in the inner layer and the tail 2 is in the outer layer. Therefore, it is better to connect 1 to the output of the frequency converter and 2 to the load motor. In this way, the strong interference at the output of the frequency converter is shielded by the outer layer, reducing the interference emitted outward.
If the suppression frequency of the output-side AC reactor is in a high frequency range, a ferrite core should be used to reduce losses, but this results in a larger size. If a transformer is installed between the frequency converter and the load, the leakage reactance of the transformer input winding and the transformer losses greatly weaken the modulation wave, effectively acting as an output-side reactor; therefore, the output-side AC reactor can be omitted.
