Abstract : This paper introduces the structure and function of the dedicated input line reactor for frequency converters , and elaborates on the design calculation of the dedicated input line reactor for frequency converters, the function and inductance value selection of the dedicated DC reactor for frequency converters, and the measurement of the inductance of AC and DC reactors.
Keywords: Functional calculation and measurement
1. Inlet reactor for frequency converter
A reactor, also called an inductor, is an electrical device that generates a magnetic field within a certain space when current flows through it. Therefore, all current-carrying conductors possess inductance in a general sense. However, the inductance of a long, straight current-carrying conductor is relatively small, and the magnetic field it produces is not strong. Therefore, practical reactors are made by winding wire into a solenoid, called air-core reactors. Sometimes, to give this solenoid a larger inductance, an iron core is inserted inside, called an iron-core reactor. Reactance is divided into inductive reactance and capacitive reactance. A more scientific classification is that inductors and capacitors are collectively called reactors. However, because inductors were developed first and were called reactors, the term "capacitor" now refers to a capacitive reactance, while "reactor" specifically refers to an inductor.
Common inverter-specific input reactors are generally dry-type with iron cores. The iron core of these reactors is made of high-quality, low-loss cold-rolled silicon steel sheets, as shown in Figure 1. The air gap of these reactors typically uses epoxy-laminated glass cloth as a spacer to ensure that the air gap remains unchanged during operation. The coils are wound with H-grade enameled flat copper wire, arranged tightly and evenly, without an outer insulation layer, providing excellent aesthetics and good heat dissipation.
The fasteners of the core column of the inverter-specific input reactor are made of non-magnetic materials to reduce eddy current heating during operation. All exposed parts of the inverter-specific input reactor are treated with anti-corrosion measures, and the lead-out terminals are made of tin-plated copper tube terminals. Compared with similar domestic products, the inverter-specific input reactor has the advantages of small size, light weight, and beautiful appearance, and can be compared with well-known foreign brands.
After the coil and core of the inverter-specific input reactor are assembled into one unit, they undergo a process of pre-baking → vacuum impregnation → hot-baking curing. H-grade impregnation varnish is used to firmly bond the coil and core of the reactor together, which not only greatly reduces the noise during operation, but also has an extremely high heat resistance rating, ensuring that the reactor can operate safely and quietly even at high temperatures. The fasteners of the core column of the input reactor are made of non-magnetic materials to reduce eddy current heating during operation. All exposed parts are treated with anti-corrosion measures, and the lead-out terminals are made of tin-plated copper tube terminals.
A dedicated input reactor for frequency converters is an electrical device that relies on the inductive reactance of a coil to impede changes in current, suppressing high-order harmonics generated by the frequency converter. It is typically connected in series between the frequency converter's input terminals and the power supply, hence its name. The dedicated input reactor for frequency converters has the following functions:
1) Limit the voltage drop on the grid side during converter commutation;
2) Suppress high-frequency harmonics generated during the rectification process of the frequency converter and decouple parallel converter groups;
3) Limit voltage jumps in the power grid;
4) Reduce the current surge generated during power grid system operation;
5) Improve the power factor of the frequency converter system;
Speed control systems composed of frequency converters are frequently subjected to surge currents and surge voltages during operation, which can severely damage the performance and lifespan of the frequency converter and speed controller. Therefore, an input line reactor must be added between the power supply and the frequency converter to suppress surge voltages and surge currents, effectively protect the frequency converter, extend its lifespan, improve its power factor, reduce motor noise, and reduce eddy current losses.
Because frequency converters use frequency conversion to regulate speed, they often generate high-order harmonics and waveform distortion during speed regulation, which can affect the normal operation of the equipment. Therefore, an input line reactor must be installed at the input end to improve the power factor of the frequency converter, suppress harmonic current, filter out harmonic voltage and harmonic current, and improve the power grid quality.
2. Design calculation of dedicated input reactor for frequency converter
Once the rated voltage drop ΔUL of the reactor is selected and the rated operating current In is calculated, the inductive reactance XL of the reactor can be calculated. The inductive reactance XL of the reactor is calculated using the following formula:
XL=ΔUL/In
With the above data, the reactor structure can be designed. The relationship between the reactor core cross-sectional area S and the reactor voltage drop ΔUL is shown in the following formula:
In the formula: ΔUL is in V; f is the power supply frequency (Hz); B is the magnetic flux density (T); N is the number of turns of the reactor coil; Ks is the core lamination coefficient, Ks=0.93.
The relationship between the reactor core window area A, the current In, and the number of coil turns N is shown in the following formula:
A = InN/(jKA)
In the formula: j is the current density, which can be selected from 2 to 2.5 A/mm2 depending on the capacity; KA is the window fill factor, which is approximately 0.4 to 0.5.
The product of the core cross-sectional area and the window area is shown in the following formula:
SA = PK / (4.44fBjKsKA × 10－4)
As can be seen from the above formula, based on the reactor capacity PK (=ΔULIn) value, an appropriate iron core is selected so that the product energy of the cross-sectional area SA conforms to the following formula.
Assuming B=0.6T, j=200A/cm², Ks=0.93, KA=0.45, and A=1.5S, the relationship between the reactor core cross-section and capacity is as follows:
Cross-sectional area of reactor core:
Once the core cross-sectional area is determined, the number of coil turns can be calculated using the following formula:
To ensure good linearity of the input reactor, an appropriate air gap should be present in the core. The air gap size can be initially selected within the range of 2 to 5 mm, and the inductance can be changed by adjusting the air gap based on the measured inductance value.
3. DC reactor for frequency converter and its design calculation
The DC reactor for frequency converters, also called a smoothing reactor, is connected in series in the DC bus (terminals P1 and P+). Its main function is to reduce the high-order harmonic components of the input current and improve the power factor of the input power supply (to 0.95). This reactor can be used simultaneously with an AC reactor; however, a DC reactor should only be considered when the frequency converter power is greater than 30kW.
A DC reactor is connected before the filter capacitor. It suppresses the amplitude of the rectified inrush current entering the capacitor, improves the power factor, and reduces AC ripple on the bus. The higher the power of the frequency converter, the more important it is to use a DC reactor. Without a DC reactor, the capacitor filter of the frequency converter will cause severe distortion of the current waveform and the grid voltage waveform, and it will also be very detrimental to the lifespan of the frequency converter's rectifier bridge and filter capacitor.
DC reactors are used to improve the input current waveform distortion caused by capacitor filtering (currently, capacitor filtering is the main filtering method in voltage-type frequency converters), improve the power factor, and reduce and prevent rectifier bridge damage and capacitor overheating caused by inrush current. DC reactors are needed when the resistance of the power transformer and transmission line is low, or when the power grid experiences frequent transients. DC reactors can make the inverter stage more stable and limit short-circuit current.
The inductance value of the DC reactor is generally selected to be 2 to 3 times the 3% impedance inductance of the AC reactor input to the frequency converter, and at least 1.7 times.
LCD = (2~3)LAC
Example: Calculation of DC reactor for a three-phase 380V 90kW frequency converter:
LCD=(2～3)LLA1=(2～3)×0.123=0.246～0.369mH
Select a reactor with an operating current of 170A and an inductance of 0.2mH.
4. Measurement of reactor inductance
(1) Determination of LDC inductance of DC reactor
The inductance of a core reactor is highly dependent on its operating conditions and exhibits a non-linear behavior. Therefore, measurements should be taken under actual operating conditions as much as possible. Figure 2 shows the circuit for measuring a DC reactor. A DC current Id and an AC current Ij are applied to the reactor. The AC and DC circuits are separated by a capacitor C = 200μF. The AC voltage Uj and AC current Ij across LDC are measured. The inductance value L can be approximately calculated using the following formula:
XL=Uj/Ij=ωL
L=XL/ω
(2) Measurement of the inductance of the AC reactor LAC
The inductance of an AC reactor with an iron core should not be measured using a bridge circuit, because the power supply frequency of a bridge circuit for measuring inductance is generally 1000Hz. Therefore, a bridge circuit for measuring inductance can only be used to measure air-core reactors.
For AC reactors made of stacked silicon steel sheets, the inductance can be measured using the AC voltage and ammeter method of the power frequency power supply, as shown in Figure 3. The current through the reactor can be slightly less than the rated value. For accuracy, the internal resistance rL of the reactor coil can be measured using a bridge circuit. The inductance value per phase can be calculated using the following formula:
In the formula: U is the reading of the AC voltmeter (V); I is the reading of the AC ammeter (A); rL is the resistance of each phase coil of the reactor (Ω).
Since the internal resistance rL of the reactor coil is very small, it can often be ignored in engineering calculations.
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