Basic knowledge of frequency converters
A frequency converter is a device that transforms mains frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable speed operation of a motor. The control circuit controls the main circuit, the rectifier circuit converts AC to DC, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts the DC back into AC. For frequency converters like vector control frequency converters that require a large amount of computation, a CPU for torque calculation and other corresponding circuits are sometimes also needed. Variable frequency speed control achieves speed regulation by changing the frequency of the power supply to the motor stator windings.
Variable frequency drive (VFD) technology arose to meet the need for stepless speed regulation of AC motors. Since the 1960s, power electronic devices have undergone a development process, evolving from SCR (Signal Rectifier), GTO (Gate Turn-Off Thyristor), BJT (Bipolar Junction Transistor), MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), SIT (Stationary Induction Transistor), SITH (Stationary Induction Thyristor), MGT (MOS-Controlled Transistor), MCT (MOS-Controlled Thyristor), IGBT (Insulated Gate Bipolar Transistor), to HVIGBT (High Voltage Insulated Gate Bipolar Transistor). These device updates have spurred the continuous development of power electronic conversion technology. Starting in the 1970s, research on pulse width modulation variable voltage variable frequency (PWM-VVVF) speed regulation attracted significant attention. In the 1980s, the optimization of PWM modes, a core aspect of VFD technology, attracted considerable interest, leading to the development of numerous optimization modes, among which the saddle-shaped wave PWM mode demonstrated the best performance. Starting in the latter half of the 1980s, VVVF frequency converters from developed countries such as the United States, Japan, Germany, and the United Kingdom were introduced to the market and widely used.
There are several ways to classify frequency converters. According to the main circuit operating mode, they can be divided into voltage-source frequency converters and current-source frequency converters. According to the switching method, they can be divided into PAM control frequency converters, PWM control frequency converters, and high-carrier-frequency PWM control frequency converters. According to the working principle, they can be divided into V/f control frequency converters, slip frequency control frequency converters, and vector control frequency converters. According to the application, they can be divided into general-purpose frequency converters, high-performance special-purpose frequency converters, high-frequency frequency converters, single-phase frequency converters, and three-phase frequency converters.
VVVF: Change voltage, change frequency; CVCF: Constant voltage, constant frequency. The AC power supplies used in various countries, whether for household or factory use, typically have voltages and frequencies of 400V/50Hz or 200V/60Hz (50Hz), etc. Generally, a device that converts AC power with a fixed voltage and frequency into AC power with a variable voltage or frequency is called a "frequency converter." To generate variable voltage and frequency, this device must first convert the AC power supply into DC power.
Inverters used for motor control can change both voltage and frequency.
Working principle of frequency converter
We know that the expression for the synchronous speed of an AC motor is:
n = 60f(1-s)/p(1)
In the formula
n — the rotational speed of the asynchronous motor;
f — the frequency of the asynchronous motor;
s — Motor slip;
p — Number of pole pairs of the electric motor.
As shown in equation (1), the rotational speed n is directly proportional to the frequency f. Changing the frequency f will change the motor's rotational speed. When the frequency f varies within the range of 0–50 Hz, the motor's speed adjustment range is very wide. A frequency converter achieves speed regulation by changing the motor's power supply frequency, making it an ideal, highly efficient, and high-performance speed control method.
Inverter principle block diagram
Figure 1
Inverter control method
The low-voltage general-purpose frequency converter has an output voltage of 380–650V, an output power of 0.75–400kW , and an operating frequency of 0–400Hz. Its main circuit adopts an AC-DC-AC circuit. Its control method has gone through the following four generations.
1. Sinusoidal Pulse Width Modulation (SPWM) control method with U/f=C
Its characteristics include a simple control circuit structure, low cost, and good mechanical stiffness, which can meet the smooth speed regulation requirements of general transmissions, and it has been widely used in various fields of industry. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, resulting in a reduction in the maximum output torque. In addition, its mechanical characteristics are ultimately not as stiff as those of a DC motor, and its dynamic torque capability and static speed regulation performance are not satisfactory. Furthermore, the system performance is not high, the control curve changes with the load, the torque response is slow, the motor torque utilization rate is low, and the performance degrades and the stability deteriorates at low speeds due to the stator resistance and inverter dead-zone effect. Therefore, vector control variable frequency speed regulation has been developed.
2. Voltage Space Vector (SVPWM) Control Method
It is based on the overall generation effect of three-phase waveforms, aiming to approximate the ideal circular rotating magnetic field trajectory of the motor air gap. It generates three-phase modulated waveforms in one step and controls them by approximating a circle using an inscribed polygon. After practical use, it has been improved by introducing frequency compensation to eliminate speed control errors; estimating the flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speeds; and closing the output voltage and current loops to improve dynamic accuracy and stability. However, the control circuit has many components and lacks torque regulation, so the system performance has not been fundamentally improved.
3. Vector Control (VC) Method
Vector control variable frequency speed regulation involves transforming the stator currents Ia, Ib, and Ic of an asynchronous motor in a three-phase coordinate system into equivalent AC currents Ia1 and Ib1 in a two-phase stationary coordinate system through a three-phase to two-phase transformation. Then, through a rotor magnetic field-oriented rotational transformation, these are equivalent to DC currents Im1 and It1 in a synchronous rotating coordinate system (Im1 is equivalent to the excitation current of a DC motor; It1 is equivalent to the armature current proportional to torque). The control method of a DC motor is then used to obtain the control quantities for the DC motor, and through a corresponding inverse coordinate transformation, the asynchronous motor is controlled. Essentially, it equates an AC motor to a DC motor, independently controlling the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current to obtain the torque and magnetic field components, orthogonal or decoupled control is achieved through coordinate transformation. The proposal of the vector control method is epoch-making. However, in practical applications, due to the difficulty in accurately observing the rotor flux linkage, the significant influence of motor parameters on system characteristics, and the complexity of the vector rotational transformation used in the equivalent DC motor control process, the actual control effect is difficult to achieve the ideal analytical results.
4. Direct Torque Control (DTC) Method
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed direct torque control (DTC) frequency conversion technology. This technology largely solved the shortcomings of the aforementioned vector control and has rapidly developed due to its novel control concept, simple and clear system structure, and excellent dynamic and static performance. Currently, this technology has been successfully applied to high-power AC drives for electric locomotive traction. DTC directly analyzes the mathematical model of the AC motor in the stator coordinate system, controlling the motor's flux linkage and torque. It does not require equating the AC motor to a DC motor, thus eliminating many complex calculations in vector rotation transformation; it does not require mimicking the control of a DC motor, nor does it require simplifying the mathematical model of the AC motor for decoupling.
5. Matrix-based cross-sectional control method
VVVF frequency converters, vector control frequency converters, and direct torque control frequency converters are all types of AC-DC-AC frequency converters. Their common drawbacks are low input power factor, high harmonic current, the need for large energy storage capacitors in the DC circuit, and the inability to feed regenerated energy back to the grid, meaning they cannot operate in four quadrants. To address these issues, matrix AC-AC frequency converters were developed. Because matrix AC-AC frequency converters eliminate the intermediate DC link, they also eliminate the need for large and expensive electrolytic capacitors. They can achieve a power factor of 1, sinusoidal input current, and four-quadrant operation, resulting in a high system power density. Although this technology is not yet mature, it continues to attract in-depth research from many scholars. Its essence is not to indirectly control quantities such as current and flux linkage, but rather to directly use torque as the controlled variable. The specific method is:
The stator flux linkage is controlled by introducing a stator flux linkage observer to achieve a sensorless speed operation.
Automatic identification (ID) relies on a precise mathematical model of the motor to automatically identify motor parameters;
The actual values are calculated to correspond to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and the actual torque, stator flux linkage, and rotor speed are calculated for real-time control.
Band-Band control is implemented by generating PWM signals based on flux linkage and torque to control the switching state of the inverter.
Matrix AC-AC converters have fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), and high torque accuracy (<+3%). They also have high starting torque and high torque accuracy, especially at low speeds (including 0 speed), where they can output 150% to 200% torque.
Problems and Fault Prevention in the Use of Frequency Converters
Incorrect usage or unsuitable setup can easily cause inverter malfunctions and failures, or prevent it from achieving the expected operating results. Therefore, it is crucial to carefully analyze the causes of malfunctions beforehand to prevent such problems.
External electromagnetic interference
If interference sources exist around the frequency converter, they will penetrate the converter's interior through radiation or power lines, causing control circuit malfunctions, resulting in abnormal operation or shutdown, and in severe cases, even damaging the frequency converter. While improving the frequency converter's own anti-interference capability is important, due to cost constraints, taking external noise suppression measures to eliminate interference sources is more reasonable and necessary. The following measures are specific methods for implementing the "three no's" principle for noise interference: All relays and contactors around the frequency converter must be equipped with surge voltage absorption devices, such as RC absorbers; the wiring distance of the control circuit should be shortened as much as possible and separated from the main line; the use of shielded wire circuits must be carried out according to regulations, and if the line is long, a reasonable relay method should be used; the frequency converter's grounding terminal should be grounded according to regulations and should not be mixed with welding or power grounding; a noise filter should be installed at the frequency converter input terminal to prevent interference from being introduced through the power supply line.
Installation Environment
Inverters are electronic devices, and their specifications detail the requirements for their installation and operating environment. In special circumstances where these requirements cannot be met, appropriate mitigation measures must be taken as much as possible: Vibration is a major cause of mechanical damage to electronic devices; in environments with significant vibration and impact, vibration damping measures such as rubber should be used; Moisture, corrosive gases, and dust can cause electronic devices to rust, have poor contact, and experience reduced insulation, leading to short circuits. As a preventative measure, the control board should be treated with anti-corrosion and dustproof measures and should adopt a sealed structure; Temperature is a crucial factor affecting the lifespan and reliability of electronic devices, especially semiconductor devices. Air conditioning should be installed according to the environmental conditions required by the device, or direct sunlight should be avoided.
In addition to the three points mentioned above, regular inspection of the inverter's air filter and cooling fan is also essential. For extremely cold environments, to prevent the microprocessor from malfunctioning due to low temperatures, necessary measures such as installing a space heater should be taken.
Power supply error
Power supply anomalies manifest in various forms, but can be broadly categorized into three types: phase loss, low voltage, and power outages, sometimes in combination. The main causes of these anomalies are mostly due to wind, snow, and lightning strikes on transmission lines, and sometimes short circuits to ground or between phases within the same power supply system. Lightning strikes vary significantly depending on location and season. In addition to voltage fluctuations, some power grids or self-generating units may also experience frequency fluctuations, and these phenomena sometimes recur within a short period. To ensure the normal operation of equipment, corresponding requirements are placed on the power supply for frequency converters.
If there are direct-start motors or induction cookers nearby, their power supply systems should be separated from the inverter's to prevent voltage drops when these devices are switched on, minimizing mutual interference. For applications requiring continued operation after a momentary power outage, in addition to selecting an inverter with a suitable price, the speed reduction ratio of the load motor should be considered in advance. The inverter and external control circuit should employ momentary stop compensation; once the voltage recovers, speed tracking and tachometer detection prevent overcurrent during acceleration. For equipment requiring continuous operation, an automatic switching uninterruptible power supply (UPS) device should be installed on the inverter.
Inverters with diode inputs and single-phase control power supplies can continue to operate even in the event of a phase loss. However, excessive current in some components of the rectifier and excessive pulse current in the capacitors can adversely affect the lifespan and reliability of the inverter if operated for a long period of time. They should be checked and dealt with as soon as possible.
Lightning strikes and induced lightning
Lightning strikes or induced lightning strikes can sometimes damage frequency converters due to their surge voltage. Furthermore, when the primary side of the power system has a vacuum circuit breaker, the opening and closing of the circuit breaker can also generate high surge voltages. When the vacuum circuit breaker on the primary side of the transformer opens, a very high voltage spike is generated on the secondary side through coupling.
To prevent overvoltage damage caused by impulse voltage, it is usually necessary to add absorption devices such as varistors at the input of the frequency converter to ensure that the input voltage does not exceed the maximum allowable voltage during the main circuit of the frequency converter. When using a vacuum circuit breaker, an additional RC surge absorber should be used as much as possible to form an impulse. If there is a vacuum circuit breaker on the primary side of the transformer, the frequency converter should be disconnected before the vacuum circuit breaker operates in the control sequence.
Traditional transistor frequency converters had the following drawbacks: they were prone to tripping, difficult to restart, and had low overload capacity. Due to the rapid development of IGBTs and CPUs, frequency converters now incorporate comprehensive self-diagnostic and fault prevention functions, significantly improving their reliability.
Using the "full-range automatic torque compensation function" in a vector control frequency converter can effectively overcome faults such as "insufficient starting torque" and "output reduction caused by changes in environmental conditions." This function utilizes the high-speed computation of the frequency converter's internal microcomputer to calculate the required torque at the current moment and quickly correct and compensate the output voltage to offset changes in the frequency converter's output torque caused by changes in external conditions.
Furthermore, due to the more sophisticated software development of frequency converters, various fault prevention measures can be pre-set within the frequency converter, and the converter can continue to operate even after the fault is resolved. For example, it can restart the motor during the free stop process; automatically reset internal faults and maintain continuous operation; automatically adjust the operating curve to avoid trips when the load torque is too high; and detect abnormal torque of the mechanical system.
Impact of frequency converters on peripheral equipment and fault prevention
The installation and use of frequency converters will also affect other equipment, sometimes even causing malfunctions. Therefore, it is essential to analyze and discuss these influencing factors and study what measures should be taken.
Power supply high harmonics
Since most current frequency converters use PWM control, this pulse modulation generates high-order harmonic currents on the power supply side during operation, causing voltage waveform distortion and severely impacting the power system. Common solutions include: using a dedicated transformer to power the frequency converter, separating it from other power supply systems; adding filter reactors or multiple rectifier bridge circuits to the input side of the frequency converter to reduce high-order harmonic components; and, in cases with leading capacitors, where high-order harmonic currents increase capacitor current and cause severe heating, connecting a reactor in series before the capacitor to reduce harmonic components. Furthermore, the inductance of the reactor should be carefully analyzed and calculated to avoid LC oscillations.
Overheating of motor and operating range
When retrofitting existing motors with variable frequency speed control, self-cooled motors may overheat due to reduced cooling capacity at low speeds. Furthermore, the high-order harmonics in the inverter's output waveform inevitably increase the motor's iron and copper losses. Therefore, after confirming the motor's load condition and operating range, the following measures should be taken: provide strong cooling ventilation or upgrade the motor's specifications; replace with a dedicated variable frequency motor; limit the operating range and avoid low-speed zones.
Vibration and noise
Vibration is usually caused by the pulsating torque of the motor and the resonance of the mechanical system, especially when the pulsating torque coincides with the mechanical resonance voltage. Noise is generally divided into inverter noise and motor noise, and different measures should be taken for different installation sites: During the commissioning of the inverter, the pulsating torque component should be minimized as much as possible while ensuring control accuracy; the mechanical resonance point should be identified during commissioning, and the frequency shielding function of the inverter should be used to exclude these resonance points from the operating range; since the inverter noise is mainly generated by the cooling fan and reactor, low-noise devices should be selected; an AC reactor should be reasonably installed between the motor and the inverter to reduce high-order harmonics caused by PWM modulation.
High-frequency switching generates voltage spikes that are detrimental to motor insulation.
The output voltage of a frequency converter contains high-frequency spike voltages. These high-order harmonic surge voltages will reduce the insulation strength of the motor windings, especially for PWM-controlled frequency converters. The following measures should be taken: minimize the wiring distance between the frequency converter and the motor; use surge voltage absorption devices with blocking diodes to process the output voltage of the frequency converter; and add a filter to the motor input side for PWM-type frequency converters whenever possible.
Forecast of the development trend of frequency converter technology
Variable frequency drives (VFDs) are power converters in motion control systems. Modern motion control systems encompass multiple technical fields, and the general development trend is towards AC drives, higher frequency power converters, and digital, intelligent, and networked control. Therefore, VFDs, as crucial power conversion components in systems, have experienced rapid development in providing controllable, high-performance variable voltage and frequency AC power.
With the application of new power electronic devices and high-performance microprocessors, as well as the development of control technology, the performance-price ratio of frequency converters is becoming increasingly higher, and their size is becoming smaller. Manufacturers continue to strive to improve reliability and further miniaturize, lighten, increase performance, enhance multifunctionality, and reduce pollution in frequency converters. The performance of a frequency converter depends on three factors: first, the impact of harmonics in its output AC voltage on the motor; second, its harmonic pollution to the power grid and input power factor; and third, its energy loss. This section will use the widely used AC-DC-AC frequency converter as an example to illustrate its development trend:
The main circuit power switching elements are self-turn-off, modular, integrated, and intelligent; the switching frequency is continuously increasing, and the switching losses are further reduced.
Regarding the topology of the inverter's main circuit: For low-voltage, small-capacity devices, the grid-side inverter typically uses a 6-pulse inverter, while for medium-voltage, large-capacity devices, a multi-stage inverter with 12 or more pulses is used. For load-side inverters, low-voltage, small-capacity devices typically use a two-level bridge inverter, while for medium-voltage, large-capacity devices, a multi-level inverter is used. For four-quadrant operation, to achieve energy feedback from the inverter to the grid and save energy, the grid-side inverter should be a reversible inverter. Simultaneously, dual-PWM inverters with bidirectional power flow have emerged. Appropriate control of the grid-side inverter can make the input current approach a sine wave, reducing pollution to the grid.
The control methods for pulse width modulation (PWM) transformer frequency converters can include sinusoidal pulse width modulation control, PWM control to eliminate harmonics of a specified number, current tracking control, and voltage space vector control (magnetic flux tracking control).
The progress in AC motor frequency conversion control methods is mainly reflected in the development from scalar control to vector control and direct torque control with high dynamic performance, and in the development of sensorless vector control and direct torque control systems.
Advances in microprocessors have made digital control the future direction of modern controllers. Motion control systems are fast systems, especially high-performance control of AC motors, which requires storing various types of data and processing large amounts of information quickly in real time. In recent years, major international companies have launched DSP (Digital Signal Processor)-based cores, coupled with peripheral functional circuits required for motor control, integrated into a single chip, known as DSP single-chip motor controllers. These controllers offer significantly reduced prices, smaller size, more compact structures, ease of use, and improved reliability. Compared to ordinary microcontrollers, DSPs have 10-15 times greater digital processing capabilities, ensuring superior system control performance. Digital control simplifies hardware, and flexible control algorithms provide great flexibility, enabling the implementation of complex control laws. This makes the application of modern control theory in motion control systems a reality, facilitates data transmission with upper-level systems, and enhances fault diagnosis, protection, and monitoring functions, making the system intelligent (e.g., some frequency converters have self-adjustment functions).
