Over the past decade, advancements in large-scale integrated circuits and computer control technologies, coupled with the practical implementation of modern control theories, particularly the application of vector control methods, have significantly enhanced AC variable frequency drive technology. This technology now offers a broad speed range, high stability in speed accuracy, rapid dynamic responses, and excellent technical performance across all four quadrants for reversible operations, matching the speed regulation characteristics of DC motor drives.
In the realm of AC speed control, frequency conversion has seen steady improvements in both performance and reliability, with prices gradually decreasing. Its energy-saving benefits are particularly notable, and implementing frequency conversion has become increasingly convenient. As a result, inverters are widely adopted in various applications requiring speed control, thanks to their ease of operation, compact design, and superior control capabilities.
1. General-purpose inverter development:
Industrial frequency converters are typically categorized into general-purpose and specialized types. General-purpose inverters cater to general industrial drives, while specialized inverters target specific applications. These general-purpose inverters offer diverse functionalities, making them suitable for various scenarios. By the mid-1980s, general-purpose inverters evolved in two directions: those focused on energy savings and those aimed at automation. Automation-focused inverters are commonly used in transportation equipment, machine tools, and elevators. Most small to medium-sized general-purpose inverters employ V/f ratio control, known as VVVF control. The majority of inverters available today are V/f controlled and widely utilized. Future trends in these inverters include reducing noise, minimizing harmonic impacts on power supplies, enhancing low-speed torque, and achieving more compact designs. To achieve smaller sizes, intelligent power modules are increasingly being employed. These modules integrate the inverter's three-phase main circuit, IGBT driver circuits, partial detection circuits, and protection circuits into a single module. Users only need to design the control circuit and power supply, along with selecting an appropriate filter capacitor, to create a complete inverter, which can reduce the overall size by more than 30% compared to conventional inverters.
To meet the demands for higher performance and more compact inverters, newly developed general-purpose inverters adopt sensorless vector control. Traditional vector control systems typically require a motor-speed sensor, which limits flexibility. The new sensorless vector control system features a parameter self-tuning function, automatically measuring motor parameters upon startup and adjusting the system's control parameters, such as PI regulator settings, ensuring optimal performance across varying conditions. This approach not only improves general-purpose inverter performance but also simplifies vector control, eliminates the need for speed sensors, and enhances system reliability.
2. Rational Selection of Frequency Converters:
Frequency converters are now extensively applied in industries such as metallurgy, textiles, mining, industrial control, and home appliances. Their primary purposes include speed regulation based on work requirements, energy savings, and enabling soft starts and braking for motors. When selecting a frequency converter, several critical considerations must be taken into account:
2.1 Inverter Capacity Calculation:
Determining the appropriate size of an inverter is crucial when selecting one. Proper capacity selection ensures energy savings, cost-effectiveness, and operational safety. Based on existing data and experiences, a simpler method involves: (1) Calculating the inverter current based on the motor's rated current (Im) and considering its overload coefficient (K). (2) When one inverter controls multiple motors, consider the scenario where some motors are already running while others are starting. (3) If the motor's actual load is less than its rated load, calculations can be performed using the theoretical formula, discounting the active current based on actual power plus the reactive current.
However, it’s generally unwise to excessively reduce the inverter's capacity. This is because the motor's starting current is independent of the load size and should not exceed the inverter's overcurrent capacity. Large motors have smaller reactances, leading to larger current ripples, which can easily trigger inverter overcurrent issues. Additionally, the motor's maximum torque is less than its maximum torque at power frequency, necessitating increased inverter and motor capacities in the starting and low-speed zones. For instance, a 715kW motor might be used under a 215kW load, requiring a 317kW inverter for current calculations, with a possible option of a 515kW inverter considering current pulsations.
2.2 V/f Type Selection:
The inverter selection process primarily involves matching one inverter with a motor for optimal performance. V/f type selection includes the highest frequency, basic frequency, torque type, and other parameters. The highest frequency represents the maximum frequency the inverter-motor system can operate at. Since the inverter's maximum frequency may be high, when the motor's maximum frequency is lower than the inverter's highest frequency, it should be set according to the motor's requirements and its load. The basic frequency serves as the boundary between constant power and constant torque control of the motor, and should be set according to the motor's rated voltage. The torque type refers to whether the load is a constant power or constant torque load. V/f control enables constant torque (i.e., constant current) speed regulation at low frequencies. However, due to poor cooling at low speeds, the temperature rise may be 2-3 times higher than at rated conditions. Thus, for constant torque loads, the motor and frequency converter's capacity should be appropriately increased. General-purpose inverters come equipped with multiple V/f curves for user selection, and users should choose the appropriate curve based on the load's nature.
2.3 Consider Other Factors:
Apart from the aforementioned essential considerations, when selecting inverters from different brands, certain parameters' influence on usage must also be considered. For instance: (1) Ambient temperature: Different brands specify the working environmental temperature, such as Hitachi’s J100 series having a range of 10°-40°C. A 1°C temperature drop reduces the rated current by 2.5%. (2) Harmonics: Harmonic currents generated by inverters reduce the motor's power factor and efficiency, increasing current by 10% under the same load conditions, effectively reducing the motor's effective power. Derating should be applied. (3) Noise: Inverter harmonic currents increase motor noise. If necessary, reactors can limit this. When the harmonic frequency matches the system's natural frequency, severe resonance may occur, destabilizing the system. Thus, frequency converters should bypass these resonance points during frequency adjustments, referred to as frequency cross-jump functionality.
3. Rational Use of the Inverter:
Manufacturers of inverters have carefully considered user needs, designing numerous adjustable settings and protection functions. However, to maximize these functions and use inverters effectively, attention must be paid to the following aspects:
(1) Reasonable Adjustment of Starting Torque:
Adjusting the starting torque enhances the low-speed performance of the inverter during startup, ensuring the motor's torque output meets production load requirements. In asynchronous motor variable frequency speed control systems, torque control is complex. At low frequencies, the influence of leakage reactance cannot be ignored due to resistance. Keeping V/f constant reduces magnetic flux, thereby decreasing motor output torque. To address this, voltage compensation is applied at low frequencies to increase torque. However, leakage impedance affects both frequency and motor current magnitude, making precise compensation challenging. Typically, users manually compensate. Recently, inverters with automatic compensation have been developed, though they require complex calculations, hardware, and software. Inverters usually provide two options: automatic setting and automatic torque boost. Excessive voltage compensation can saturate the motor core, increasing excitation current and overloading the motor.
(2) Reasonable Setting of Acceleration/Deceleration Time:
The motor's motion equation is Jdω/dt = Te - Tz, where Te is the electromagnetic torque and Tz is the load torque. During acceleration or deceleration, the rate depends on the acceleration torque (Te - Tz), and the inverter sets the frequency change rate during braking. If the motor's inertia (J) or load torque (Tz) changes, using the preset frequency change rate may lead to insufficient acceleration torque, causing the motor to stall, i.e., inconsistency between motor speed and inverter output frequency, resulting in overcurrent or overvoltage. Therefore, the acceleration and deceleration times should be properly set based on motor inertia and load, ensuring coordination between the inverter's frequency change rate and the motor's speed change rate. A reasonable setting can be verified by initially selecting an acceleration time based on experience and setting the deceleration time. If overcurrent occurs during startup, extend the acceleration time; if overcurrent occurs during braking, increase the deceleration time. Conversely, overly long times negatively impact production efficiency, especially in frequent braking scenarios.
(3) Reasonable Setting of Frequency Jump Points:
When a V/f controlled inverter drives an asynchronous motor, the motor's current and speed may oscillate in certain frequency bands. Severe oscillations can prevent the system from functioning, even triggering overcurrent protection during acceleration, preventing normal motor startup. This issue becomes more severe under light loads or when the torque inertia is small. Many inverters are equipped with frequency jump functions, allowing users to set jump points and widths on the V/f curve based on the oscillation frequency points. During acceleration, these frequency segments are automatically skipped to ensure stable system operation. Two values can be set: the jump frequency and jump width. Common inverters provide three set points for users to adjust the jump frequency and width based on debugging oscillation points.
(4) Reasonable Selection of Braking Resistor:
When the motor brakes, the kinetic energy stored in the motor is fed back to the DC side via the PWM inverter. Once the filter capacitor voltage exceeds the set value, the energy is dissipated by the braking resistor. Small-capacity inverters have built-in braking resistors, whereas large-capacity inverters allow users to select braking resistors based on load nature, size, duty cycle, etc. The braking resistor's resistance value determines the braking current magnitude, and its power is calibrated under a short-time working system. When selecting, pay attention to the braking energy requirements under various operating conditions, check the most severe conditions, and determine the braking resistor accordingly.
(5) Reasonable Setting of Inverter Protection:
Improper settings, load changes, external operating conditions, or component damage in the inverter can lead to malfunctions. Upon detecting faults, the inverter must provide fast and reliable protection. General-purpose inverters offer various protections, including overcurrent, overvoltage, reverse connection, external trips, instantaneous power failures, EEPROM errors, CPU errors, and power module failures. Correctly setting protection parameters and matching them with motor and inverter settings are essential. When an external fault occurs, the inverter should handle it coordinately, stopping operation. Some faults result from unreasonable parameter settings, requiring resetting. For example, an unreasonable acceleration time setting may cause overcurrent during acceleration, which can be mitigated by extending the acceleration time. Some faults are transient, like short-term overvoltage caused by poor power quality. When power returns to normal, the inverter can resume normal operation.
With the continuous progress of power electronics technology and the emergence of new devices, various high-voltage integrated circuits and intelligent integrated circuits are continually innovating, enhancing the performance and functionality of general-purpose inverters. Maximizing their performance according to user requirements remains a challenging task.
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