In fact, this defines the significance of frequency conversion. Not only can the frequency be changed to change the motor speed, but it can also allow the motor to work normally. This embodies the wisdom and ingenuity of humankind. I am but a fool, and can only express this sentiment!
How are pulse waves generated? Undoubtedly, they are generated by the alternating switching on and off of six IGBTs across three phases. So, how do these six IGBTs achieve this alternating switching? That's definitely accomplished by the IGBT drive circuit. And who controls the drive circuit for these six IGBTs? It's undoubtedly controlled by the main control circuit (CPU)! The CPU calculates in real-time based on the frequency signal and control method given by the user, and performs sinusoidal pulse width modulation (SPWM).
So, how does SPWM work? Sinusoidal pulse width modulation means that the pulse width and duty cycle change according to a sinusoidal pattern. The pulse width is the length of the pulse duration, which can be a positive or negative pulse. The duty cycle is the ratio of the pulse width to the period, representing the percentage of the period occupied by the pulse width within one cycle. In the diagram below, this is the ratio of tp to tc.
The key to sinusoidal pulse width modulation is determining the precise timing of the rising and falling edges.
When the frequency converter performs sinusoidal pulse width modulation, the rising and falling edges of each pulse are determined by the intersection of the sine wave and the equilateral triangular wave.
This is the waveform modulated within half a cycle. The equilateral triangular wave is generated by a triangular wave generator; it is the carrier wave of the frequency converter, and its frequency is the carrier frequency. The amplitude of the carrier wave determines the pulse height, while the carrier frequency determines the number of pulses within each half cycle.
A sine wave is a modulated wave generated by a sine wave generator. Its frequency is the given frequency of the frequency converter, and its amplitude is determined by the voltage-to-frequency ratio (V/F) and the given frequency.
As can be seen from the above, the pulse sequence of sinusoidal pulse width modulation is the result of modulating a carrier wave (triangular wave) with a modulating wave (sine wave). The CPU's job is to calculate the precise timing of the rising and falling edges of each pulse in real time.
At this point, you should feel like you've found a new path. The CPU outputs only high and low voltage levels, and these levels determine the IGBT's on/off state. The IGBT's on/off state, in turn, makes the inverter output a pulse wave that changes according to a sine wave.
The upper and lower IGBTs on the same bridge arm always conduct alternately. When the upper half-bridge is on, the lower half-bridge is off, and vice versa. The pulse sequence generated by the same phase serves as the control signal for turning the IGBTs on and off. For the upper half-bridge, a positive pulse turns it on, and a negative pulse turns it off; for the lower half-bridge, a negative pulse turns it on, and a positive pulse turns it off.
A frequency converter's circuit generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectifier section is a three-phase bridge uncontrolled rectifier, the inverter section is an IGBT three-phase bridge inverter with a PWM waveform output, and the intermediate DC link serves for filtering, DC energy storage, and reactive power buffering. A frequency converter is a control device that uses the switching action of power semiconductor devices to convert mains frequency power into electrical energy of another frequency. The frequency converters we currently use mainly employ an AC-DC-AC method (VVVF frequency conversion or vector control frequency conversion), first converting the mains frequency AC power into DC power through a rectifier, and then converting the DC power into AC power with controllable frequency and voltage to supply the motor. The frequency converter's circuit generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectifier section is a three-phase bridge uncontrolled rectifier, the inverter section is an IGBT three-phase bridge inverter with a PWM waveform output, and the intermediate DC link serves for filtering, DC energy storage, and reactive power buffering.
Inverter control schematic design and analysis:
1) First, confirm the installation environment of the frequency converter;
I. Operating Temperature. The inverter contains high-power electronic components that are highly susceptible to temperature fluctuations. While the standard operating temperature is 0–55℃, a margin of safety and reliability should be considered during use, ideally keeping it below 40℃. In the control box, the inverter should generally be installed at the top of the box, strictly adhering to the installation requirements in the product manual. It is absolutely forbidden to install heat-generating or easily heated components close to the bottom of the inverter. II. Ambient Temperature. High temperatures and significant temperature fluctuations can easily cause condensation inside the inverter, greatly reducing its insulation performance and potentially leading to short circuits. If necessary, desiccants and heaters must be added to the box. This problem is particularly pronounced in water treatment rooms where humidity is generally high, especially with large temperature variations. III. Corrosive Gases. High concentrations of corrosive gases in the operating environment can corrode component leads and printed circuit boards, accelerate the aging of plastic components, and reduce insulation performance. IV. Vibration and Shock. Mechanical vibration and shock can cause poor electrical contact when the control cabinet containing the inverter is subjected to mechanical vibration and shock. The Huai'an Thermal Power Plant encountered such a problem. In this case, besides increasing the mechanical strength of the control cabinet and keeping it away from vibration and impact sources, anti-vibration rubber pads should be used to secure vibrating components such as electromagnetic switches both inside and outside the control cabinet. The equipment should be inspected and maintained after a period of operation. V. Electromagnetic Interference. During operation, the frequency converter generates a lot of interfering electromagnetic waves due to rectification and frequency conversion. These high-frequency electromagnetic waves can interfere with nearby instruments and meters. Therefore, instruments and electronic systems inside the cabinet should be housed in metal casings to shield them from the frequency converter's interference. All components should be reliably grounded. Furthermore, shielded control cables should be used for connections between electrical components, instruments, and meters, and the shielding layer should be grounded. If electromagnetic interference is not properly handled, the entire system may malfunction, leading to control unit failure or damage.
2) Determine the cable and wiring method based on the distance between the frequency converter and the motor;
I. The distance between the inverter and the motor should be as short as possible. This reduces the cable's capacitance to ground and decreases sources of interference. II. Shielded cables should be used for control cables, and shielded cables should also be used for power cables, or the entire route from the inverter to the motor should be shielded using conduit. III. Motor cables should be run independently of other cables, with a minimum distance of 500mm. Long parallel runs of motor cables with other cables should be avoided to reduce electromagnetic interference caused by rapid changes in the inverter's output voltage. If control cables and power cables cross, they should cross at a 90-degree angle whenever possible. Analog signal lines related to the inverter should be run separately from the main circuit, even within the control cabinet. IV. Shielded twisted-pair cables are preferred for analog signal lines related to the inverter, and shielded three-core cables (with a larger cross-section than standard motor cables) should be used for power cables, or the inverter's user manual should be followed.
3) Variable frequency drive control schematic diagram;
I. Main Circuit: The reactor's function is to prevent high-order harmonics generated by the frequency converter from returning to the power grid through the power input circuit and affecting other electrical equipment. Whether a reactor is needed depends on the capacity of the frequency converter. A filter is installed at the output of the frequency converter to reduce high-order harmonics. A filter should be installed when the distance between the frequency converter and the motor is long. Although the frequency converter itself has various protection functions, its phase loss protection is not perfect. A circuit breaker in the main circuit provides overload and phase loss protection; its selection should be based on the frequency converter's capacity. The frequency converter's own overload protection can replace the thermal relay. II. Control Circuit: It has manual switching between power frequency and frequency conversion, allowing manual switching to power frequency operation in case of frequency converter failure. Since voltage cannot be applied to the output, power frequency and frequency conversion must be interlocked.
4) Grounding of the frequency converter;
Proper grounding of the frequency converter is crucial for improving system stability and noise suppression. The lower the grounding resistance of the frequency converter's grounding terminal, the better. The cross-sectional area of the grounding wire should be no less than 4mm², and its length should not exceed 5m. The frequency converter's grounding should be separate from the grounding point of the power equipment; they cannot share a common ground. One end of the signal cable's shielding layer should be connected to the frequency converter's grounding terminal, while the other end should remain floating. The frequency converter and the control cabinet should be electrically connected.
Common Fault Analysis:
1) Overcurrent fault: Overcurrent faults can be divided into acceleration, deceleration, and constant speed overcurrent. They may be caused by factors such as excessively short acceleration/deceleration times of the inverter, sudden load changes, uneven load distribution, or output short circuits. Solutions generally include extending acceleration/deceleration times, reducing sudden load changes, adding energy-consuming braking components, improving load distribution design, and checking the circuit. If the inverter still experiences an overcurrent fault after disconnecting the load, it indicates a loop in the inverter circuit, requiring replacement of the inverter. 2) Overload fault: Overload faults include inverter overload and motor overload. They may be caused by excessively short acceleration times, low grid voltage, or excessive load. Solutions generally include extending acceleration times, extending braking times, and checking the grid voltage. Excessive load may mean the selected motor and inverter cannot drive the load, or it may be due to poor mechanical lubrication. In the former case, a higher-power motor and inverter must be replaced; in the latter case, the production machinery must be repaired. 3) Undervoltage: This indicates a problem with the inverter's power input section and requires inspection before operation.
