1. Main circuit of frequency converter
Question 1: Under normal circumstances, how is the internal main circuit of a frequency converter constructed?
1) Basic Structure Low-voltage medium and small capacity frequency converters all adopt the "AC-origin DC-origin AC" conversion method. Their basic circuit consists of two main parts: rectification and inversion, as shown in Figure 1.
Question 2: What is the role of the resistor connected in parallel with the filter capacitor in the circuit?
To date, the withstand voltage of electrolytic capacitors can only reach 500V. However, after full-wave rectification, the peak DC voltage of a three-phase 380V power supply is 537V, and the average is 513V. Therefore, the filter capacitor can only be composed of two (or two sets of) electrolytic capacitors connected in series. To increase the capacitance and improve the filtering effect, several electrolytic capacitors are first connected in parallel to form a group in the inverter, and then two sets of capacitors (CF1 and CF2) are connected in series, as shown in Figure 2.
Since the capacitance of each capacitor cannot be exactly the same, especially for electrolytic capacitors, whose capacitance varies considerably, the difference in capacitance between the two sets of capacitors connected in parallel becomes quite significant. When connected in series, the voltage distribution across the two capacitor banks will be uneven. This will lead to inconsistent lifespans for the two sets of capacitors.
The solution to voltage imbalance is to connect equal-resistance resistors RC1 and RC2 in parallel across the two capacitor banks, as shown in Figure 2. The principle is as follows.
Because the resistance value of resistors can be made relatively accurately, the effect of voltage equalization is guaranteed.
Question 3: Why is a parallel circuit of resistors and switching devices connected between the rectifier bridge and the capacitor?
In terms of the basic processes of rectification and filtering, low voltage and high voltage are the same.
The key issue is that before the power is switched on, the capacitor has no charge and its voltage is 0V, and the voltage across the capacitor cannot change abruptly. This means that at the instant the switch is switched on, the rectifier bridge (between P and N) is essentially short-circuited. Therefore, two problems arise when the power is switched on:
The first problem is that there is a large inrush current, as shown by the curve in Figure 3, which may damage the rectifier tube.
The second problem is that the voltage at the input line drops to 0V instantaneously, as shown by the curve in Figure 3.
These two characteristics are exactly the same in both high-voltage and low-voltage rectifier circuits. However, low-voltage rectifier circuits require a transformer to step down the voltage. The transformer winding is a large inductor, which acts as a barrier, limiting the inrush current during switching on, as shown by curve λ in Figure 3. In the rectifier circuit of a frequency converter, there is no such barrier, so the inrush current is much more severe, as shown by curve λ in Figure 3.
Regarding the voltage waveform on the input side, in a low-voltage rectifier circuit, the secondary voltage of the transformer will instantly drop to 0V, as shown by curve φ in Figure 3(a). However, this instantaneous voltage drop is buffered on the primary side of the transformer, as shown by curve φ in Figure 3(a), so it does not interfere with other devices in the same network. In contrast, the inverter rectifier circuit lacks the buffering effect of a transformer; its input voltage is the grid voltage. Therefore, at the instant of closing the circuit, the grid voltage drops to 0V, as shown by curve φ in Figure 3(b), which will affect the normal operation of other devices in the same network, commonly referred to as interference.
Therefore, a current-limiting resistor RL needs to be connected between the rectifier bridge and the filter capacitor. On the one hand, this reduces the inrush current when the capacitor is energized, as shown by curve RL in Figure (c). On the other hand, the instantaneous voltage drop is also reduced across the current-limiting resistor, thus resolving the voltage waveform issue on the secondary side. Once the voltage across the capacitor rises to a certain level, the current-limiting resistor is short-circuited. This is the reason for connecting the current-limiting resistor in parallel with the switching device.
Question 4: Why isn't the power indicator for the DC circuit mounted on the panel?
The power indicator showing that the frequency converter is powered on is displayed on the screen.
The power indicator for the DC circuit is shown in Figure 4. Its function is not to indicate whether the frequency converter is powered on, but to indicate whether there is power on the filter capacitor.
When the inverter is powered off, the discharge process of the filter capacitors will be very slow because the inverter bridge has stopped working. Therefore, when maintenance personnel open the inverter cover, there is often still a high DC voltage on the filter capacitors, which may pose a threat to the personal safety of the maintenance personnel.
Therefore, the function of the DC circuit power indicator is to warn maintenance personnel that the filter capacitor has not been fully discharged and that they should not touch the live parts.
Question 5: Why are there anti-parallel diodes connected next to each inverter tube?
Next to each inverter transistor in the inverter bridge, a diode is connected in anti-parallel, as shown by VD7 and VD12 in Figure 1. Its main function is to provide a circuit for the inductive feedback energy of the stator winding.
The stator equivalent circuit of an asynchronous motor is a resistive-inductive circuit, as shown in Figure 5. The change in current (curve 0) will lag behind the change in voltage (curve 0).
In segment 0t1: the current flows in the opposite direction to the voltage u, and the self-induced electromotive force (i.e., back electromotive force) of the winding overcomes the power supply voltage to do work (the magnetic field does work). At this time, the current will flow through the anti-parallel diode into the DC circuit to charge the filter capacitor;
During the t1 to t2 segment: the current and voltage u are in the same direction, indicating that the power supply voltage is doing work by overcoming the self-induced electromotive force of the winding (power supply doing work). At this time, the current is the discharge from the filter capacitor flowing to the motor through the inverter tube.
Without anti-parallel diodes, the inverter tube can only conduct in one direction, and the magnetic field of the winding cannot exchange energy with the power supply, resulting in distortion of the motor's current waveform.
Question 6: What are the wiring terminals of the main circuit of the frequency converter?
The arrangement of the main circuit terminals is roughly as shown in Figure 6.
The explanation is as follows:
1) Connect the input terminals of the R, S, and T frequency converters to the power supply;
2) Connect the output terminals of the U, V, and W frequency converters to the motor;
3) The original and secondary terminals of the DC circuit after P and N filtering;
4) The output terminals of the P1 rectifier bridge are shorted by a copper strip at the factory. When it is necessary to connect the DC reactor DL, remove the copper strip and connect DL between P1 and P.
5) PE grounding terminal.
Figure 6(b) shows the case where a DC reactor, braking unit, and braking resistor are connected.
2. Selection of external main circuit and components for frequency converters 7. How to select the capacity of air circuit breaker?
1) Factors to Consider: Because air circuit breakers have overcurrent protection, to prevent malfunctions caused by the inverter being powered on, the following factors must be considered when selecting an air circuit breaker (as shown in Figure 7):
(1) At the instant the power is turned on, the charging current of the capacitor by the frequency converter can be up to (2 to 3) times the rated current (in the case of a current-limiting resistor).
(2) The input current of the frequency converter is a pulse current with a lot of high-order harmonic components. When the fundamental current reaches the rated value, the effective value of the actual current is greater than the rated current.
(3) The frequency converter itself has a certain overload capacity, usually 150℃, 1min.
2) Selection Method: To avoid malfunction, the air circuit breaker should be selected...
Question 8: Is it necessary to add a contactor in front of the frequency converter?
Generally speaking, an "input contactor" should be connected between the air circuit breaker and the frequency converter. Its main function is shown in Figure 8.
1) Convenient control: The inverter can be easily controlled to turn on and off via a push-button switch;
2) The inverter power supply can be automatically cut off in the event of a fault. This includes two aspects:
(1) When the inverter itself malfunctions and the alarm output terminal is activated (the B and C terminals in the figure are disconnected), the power supply of the inverter can be quickly cut off.
(2) When there are other fault signals in the control system (such as the AL contact being open as shown in the figure), the power supply of the frequency converter can also be quickly cut off.
Question 9: Should an output contactor be connected between the frequency converter and the motor?
1) One frequency converter controls one motor and no switching is required. When a frequency converter controls only one motor and does not require switching from the mains power supply, an output contactor should not be connected between the frequency converter and the motor. The main reason is that if an output contactor is connected, the motor may be started directly when the frequency converter's output frequency is high, resulting in a large starting current and causing the frequency converter to trip.
2) Situations where an output contactor must be connected: There are two main situations where an output contactor must be connected:
(1) When one frequency converter is connected to multiple motors, each motor must have a contactor for individual control, as shown in Figure 9(a);
(2) When switching between frequency converter and power frequency is required, the connection between the motor and the frequency converter must be disconnected when the motor is connected to the power frequency power supply. Therefore, a contactor between the motor and the frequency converter is necessary, as shown in Figure 9(b).
Question 10: Is a thermal relay required between the frequency converter and the motor?
Similar to output contactors, when a frequency converter controls only one motor and does not require switching between inverter and mains frequency, a thermal relay is unnecessary because the frequency converter itself has thermal protection. However, when a frequency converter connects to multiple motors, since each motor's capacity is much smaller than the frequency converter's, the frequency converter cannot provide thermal protection for each motor. Therefore, each motor must be protected by its own dedicated thermal relay. When the motor needs to switch between inverter and mains frequency control, a thermal relay is also necessary because the frequency converter cannot provide thermal protection for the motor when operating at mains frequency.
Question 11: Why are thermal relays prone to malfunction in the output circuit of frequency converters?
Although the output current of the frequency converter is very close to a sine wave, it still contains higher harmonic components similar to the carrier frequency. Therefore, with the same motor output power, the effective value of the current in each phase is greater than the phase current during power frequency operation. This is why thermal relays are prone to malfunction when the motor is running at rated conditions. Solutions include:
1) Increase the operating current of the thermal relay. Generally speaking, the operating current of the thermal relay should be increased by about 10%.
2) Connect a bypass capacitor in parallel next to the heating element of the thermal relay to prevent high-order harmonic current from flowing through the heating element of the thermal relay, as shown in Figure 10.
Question 12: Why do the output lines of a frequency converter sometimes need to be thickened?
Because the output voltage of a frequency converter changes along with its output frequency, a very low output frequency results in a very low output voltage. Therefore, the proportion of voltage drop across the line increases, reducing the actual voltage received by the motor, and in severe cases, preventing normal operation. The line voltage drop between the motor and the frequency converter is generally defined as...
Therefore, when the distance between the motor and the frequency converter is large and the operating frequency is low, the effect of line voltage drop must be considered, as shown in Figure 11. If necessary, the output lines of the frequency converter should be appropriately thickened.
Question 13: What measures should be taken when the distance between the motor and the frequency converter is far?
Since the output voltage of the frequency converter is a high-frequency pulse voltage, when the distance between the motor and the frequency converter is far, the distributed capacitance between the lines and the leakage inductance of the motor may be close to the resonance point, which may cause the input voltage of the motor to be too high, thus making the slot insulation of the motor easy to be damaged or causing vibration during operation.
The solution is to connect an output reactor to the output side of the frequency converter.
If the motor capacity is small and the distance between it and the frequency converter is not far, then the three output wires of the frequency converter can be wound together in the same direction on the high-frequency magnetic core, as shown in Figure 12.
