Frequency Converter Knowledge: Basics

Basic knowledge of inverter

The frequency converter is a device that converts the industrial frequency power supply (50Hz or 60Hz) into AC power supply of various frequencies to achieve variable speed operation of the motor. The control circuit completes the control of the main circuit, the rectifier circuit converts AC power into DC power, the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit converts DC power into AC power. For frequency converters that require a lot of calculations, such as vector control frequency converters, a CPU for torque calculation and some corresponding circuits are sometimes required. Variable frequency speed regulation achieves the purpose of speed regulation by changing the frequency of the power supply to the motor stator winding.

Frequency conversion technology was born to meet the needs of stepless speed regulation of AC motors. After the 1960s, power electronic devices have gone through the development process of SCR ( thyristor ), GTO (gate turn-off thyristor), BJT (bipolar power transistor ), MOSFET (metal oxide field effect transistor), SIT (static inductance transistor), SITH (static induction thyristor), MGT (MOS control transistor), MCT (MOS control thyristor), IGBT (insulated gate bipolar transistor), HVIGBT (high voltage insulated gate bipolar thyristor), and the update of devices has promoted the continuous development of power electronic conversion technology. Since the 1970s, the research on pulse width modulation variable voltage and frequency ( PWM -VVVF) speed regulation has attracted great attention. In the 1980s, the PWM mode optimization problem, as the core of frequency conversion technology, attracted people's strong interest, and many optimization modes were obtained, among which the saddle wave PWM mode had the best effect. Since the second half of the 1980s, VVVF inverters from developed countries such as the United States, Japan, Germany, and the United Kingdom have been put on the market and have been widely used.

There are many ways to classify inverters. According to the main circuit working mode, they can be divided into voltage type inverters and current type inverters; according to the switching mode, they can be divided into PAM control inverters, PWM control inverters and high carrier frequency PWM control inverters; according to the working principle, they can be divided into V/f control inverters, slip frequency control inverters and vector control inverters; according to the purpose, they can be divided into general inverters, high-performance special inverters, high-frequency inverters, single-phase inverters and three-phase inverters.

VVVF: variable voltage, variable frequency CVCF: constant voltage, constant frequency. The voltage and frequency of the AC power supply used in various countries, whether for home or factory, are 400V/50Hz or 200V/60Hz (50Hz), etc. Usually, the device that converts the AC power with fixed voltage and frequency into the AC power with variable voltage or frequency is called "inverter". In order to generate variable voltage and frequency, the device must first convert the AC power of the power supply into direct current (DC).

The frequency converter used for motor control can change both voltage and frequency.

Working principle of frequency converter

We know that the synchronous speed expression of AC motor is:

n＝60 f（1－s）/p （1）

In the formula

n——the speed of asynchronous motor;

f——the frequency of the asynchronous motor;

s——motor slip rate;

p——The number of motor pole pairs.

From formula (1), we can see that the speed n is proportional to the frequency f. The speed of the motor can be changed by changing the frequency f. When the frequency f varies within the range of 0 to 50 Hz, the motor speed adjustment range is very wide. The frequency converter achieves speed regulation by changing the power supply frequency of the motor. It is an ideal high-efficiency and high-performance speed regulation method.

Inverter block diagram

Figure 1

Inverter control mode

The low voltage general frequency conversion output voltage is 380~650V, the output power is 0.75~400kW, the operating frequency is 0~400Hz, and its main circuit adopts 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 are simple control circuit structure, low cost, good mechanical properties, and can meet the smooth speed regulation requirements of general transmission. It has been widely used in various fields of the industry. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, which reduces the maximum output torque. In addition, its mechanical characteristics are not as hard as those of DC motors, and its dynamic torque capacity and static speed regulation performance are not satisfactory. In addition, the system performance is not high, the control curve will change with the load, the torque response is slow, the motor torque utilization rate is not high, and the performance is reduced and the stability is poor due to the existence of stator resistance and inverter dead zone effect at low speeds. Therefore, people have studied vector control variable frequency speed regulation.

2. Voltage space vector (SVPWM) control method

It is based on the overall generation effect of the three-phase waveform, with the purpose of approaching the ideal circular rotating magnetic field trajectory of the motor air gap, generating a three-phase modulated waveform at one time, and controlling it in the way of an inscribed polygon approaching a circle. After practical use, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; the feedback estimation of the flux amplitude can eliminate the influence of the stator resistance at low speed; the output voltage and current are closed-loop to improve the dynamic accuracy and stability. However, there are many control circuit links, and no torque adjustment is introduced, so the system performance has not been fundamentally improved.

3. Vector control (VC) method

The method of vector control variable frequency speed regulation is to convert the stator current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system into the AC current Ia1Ib1 in the two-phase stationary coordinate system through three-phase-two-phase transformation, and then convert it into the DC current Im1 and It1 in the synchronous rotating coordinate system through the directional rotation transformation according to the rotor magnetic field (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque). Then imitate the control method of the DC motor to obtain the control quantity of the DC motor, and realize the control of the asynchronous motor through the corresponding coordinate inverse transformation. Its essence is to convert the AC motor into a DC motor and control the speed and magnetic field components independently. By controlling the rotor flux, the stator current is decomposed to obtain the torque and magnetic field components, and the coordinate transformation is used to realize orthogonal or decoupling control. The introduction of the vector control method is of epoch-making significance. However, in practical applications, since the rotor flux is difficult to observe accurately, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is relatively complex, making it difficult for the actual control effect to achieve the ideal analysis result.

4. Direct Torque Control (DTC)

In 1985, Professor DePenbrock of Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology has largely solved the shortcomings of the above-mentioned vector control, and has developed rapidly with its novel control ideas, concise and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC transmission for electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the magnetic flux and torque of the motor. It does not need to equate the AC motor to a DC motor, thus eliminating many complex calculations in the vector rotation transformation; it does not need to imitate the control of the DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.

5. Matrix cross-cross control method

VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all types of AC-DC-AC frequency conversion. Their common disadvantages are low input power factor, large harmonic current, large energy storage capacitors required for DC circuits, and regenerative energy cannot be fed back to the power grid , that is, four-quadrant operation is not possible. For this reason, matrix AC-AC frequency conversion came into being. Since matrix AC-AC frequency conversion eliminates the intermediate DC link, it eliminates the large and expensive electrolytic capacitors. It can achieve a power factor of 1, a sinusoidal input current, and can operate in four quadrants, and the system has a high power density. Although this technology is not yet mature, it still attracts many scholars to conduct in-depth research. Its essence is not to indirectly control current, magnetic flux, etc., but to directly use torque as the controlled quantity. The specific method is:

Control the stator flux and introduce the stator flux observer to achieve speed sensorless mode;

Automatic identification (ID) relies on accurate motor mathematical models to automatically identify motor parameters;

Calculate the actual values ​​corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. to calculate the actual torque, stator flux, and rotor speed for real-time control;

Realize Band-Band control to generate PWM signal according to the Band-Band control of flux and torque to control the switching state of the inverter.

Matrix AC-AC frequency conversion has fast torque response (<2ms), very high speed accuracy (±2%, without PG feedback), and high torque accuracy (<+3%); it also has high starting torque and high torque accuracy, especially at low speed (including 0 speed), and can output 150%~200% torque.

Problems encountered in the use of frequency converters and fault prevention

Incorrect use or unreasonable setting environment may easily cause the inverter to malfunction or fail, or fail to meet the expected operating effect. In order to prevent accidents, it is particularly important to carefully analyze the cause of the failure in advance.

External electromagnetic induction interference

If there are interference sources around the inverter, they will invade the inverter through radiation or power lines, causing the control circuit to malfunction, resulting in abnormal operation or shutdown, and even damage the inverter in severe cases. It is important to improve the inverter's own anti-interference ability, but due to the limitation of device cost, it is more reasonable and necessary to take noise suppression measures externally to eliminate interference sources. The following measures are specific methods to implement the "three no" principle for noise interference: All control coils of relays and contactors around the inverter need to be equipped with absorption devices to prevent impact voltage, such as RC absorbers; try to shorten the wiring distance of the control circuit and separate it from the main line; specify the use of shielded wire loops, which must be carried out in accordance with regulations. If the line is long, a reasonable relay method should be used; the inverter grounding terminal should be carried out in accordance with regulations and cannot be mixed with welding and power grounding; a noise filter should be installed at the input end of the inverter to avoid interference introduced by the power supply line.

Installation Environment

The inverter is an electronic device, and its specification sheet contains detailed requirements for the installation and use environment. In special cases, if these requirements cannot be met, appropriate suppression measures must be taken as much as possible: vibration is the main cause of mechanical damage to electronic devices. For occasions with large vibration impact, vibration-proof measures such as rubber should be used; moisture, corrosive gases and dust will cause electronic devices to rust, poor contact, reduced insulation and form short circuits. As a preventive measure, the control panel should be treated with anti-corrosion and dustproof treatment, and a closed structure should be used; temperature is an important factor affecting the life and reliability of electronic devices, especially semiconductor devices. Air conditioning should be installed or direct sunlight should be avoided according to the environmental conditions required by the device.

In addition to the above three points, it is also necessary to regularly check the air filter and cooling fan of the inverter. For special high-cold occasions, in order to prevent the microprocessor from not working properly due to low temperature, necessary measures such as setting up space heaters should be taken.

Power supply abnormality

Power supply abnormalities can be manifested in various forms, but they can be roughly divided into the following three types, namely phase loss, low voltage, power outage, and sometimes a combination of these. The main causes of these abnormalities are mostly caused by wind, snow, and lightning strikes on the transmission lines, and sometimes by ground short circuits and phase-to-phase short circuits in the same power supply system. Lightning strikes vary greatly by region and season. In addition to voltage fluctuations, some power grids or self-generating units also experience frequency fluctuations, and these phenomena sometimes occur repeatedly in a short period of time. In order to ensure the normal operation of the equipment, corresponding requirements are also put forward for the inverter power supply.

If there are direct start motors and induction cookers nearby, in order to prevent the voltage drop caused by these devices when they are put into use, they should be separated from the inverter power supply system to reduce mutual influence; for occasions that require continued operation after a momentary power outage, in addition to selecting a suitable inverter, the speed reduction ratio of the load motor should also be considered in advance. The inverter and the external control circuit adopt the instantaneous stop compensation method. When the voltage is restored, the overcurrent during acceleration is prevented by speed tracking and speed measurement of the motor detection; for equipment that must be operated, the inverter should be equipped with an automatic switching non-stop power supply device.

Although inverters with diode input and single-phase control power supply can continue to work in the phase loss state, the current of some devices in the rectifier is too large and the pulse current of the capacitor is too large. If it runs for a long time, it will have an adverse effect on the life and reliability of the inverter and should be inspected and handled as soon as possible.

Lightning strike, induced lightning

The impulse voltage caused by lightning strike or induced lightning strike can sometimes damage the inverter. In addition, when the primary side of the power system is equipped with a vacuum circuit breaker , the opening and closing of the short circuit breaker can also generate a high impulse voltage. When the vacuum circuit breaker on the primary side of the transformer is disconnected, a very high voltage impulse peak is formed on the secondary side through coupling.

In order to prevent overvoltage damage caused by impact voltage, it is usually necessary to add varistor and other absorption devices at the input end of the inverter to ensure that the input voltage is not higher than the maximum voltage allowed during the inverter main circuit. When using a vacuum circuit breaker, it is necessary to use an impact-forming additional RC surge absorber as much as possible. If there is a vacuum circuit breaker on the primary side of the transformer, the inverter should be disconnected before the vacuum circuit breaker is activated in the control sequence.

In the past, transistor inverters had the following main disadvantages: easy to trip, difficult to restart, and low overload capacity. Due to the rapid development of IGBT and CPU, the inverter has added a complete self-diagnosis and fault prevention function, which greatly improves the reliability of the inverter.

If the "full-range automatic torque compensation function" in the vector control inverter is used, the fault causes such as "insufficient starting torque" and "output reduction caused by environmental conditions change" will be well overcome. This function uses the high- speed calculation of the microcomputer inside the inverter to calculate the torque required at the current moment, and quickly corrects and compensates the output voltage to offset the change in the inverter output torque caused by changes in external conditions.

In addition, as the software development of the inverter is more complete, various fault prevention measures can be set in advance inside the inverter, and the inverter can continue to run after the fault is resolved. For example: restarting the motor during free parking; automatically resetting internal faults and maintaining continuous operation; automatically adjusting the operating curve when the load torque is too large to avoid trips; and being able to detect abnormal torque in the mechanical system.

The impact of frequency converter on peripheral equipment and fault prevention

The installation and use of the inverter will also have an impact on other equipment, and sometimes even cause other equipment failures. Therefore, it is very necessary to analyze and explore these influencing factors and study what measures should be taken.

Power supply high harmonics

Since almost all current frequency converters use PWM control, this pulse modulation form generates high-order harmonic currents on the power supply side when the frequency converter is running, causing voltage waveform distortion and serious impact on the power supply system. The following treatment measures are usually adopted: use a dedicated transformer to power the frequency converter and separate it from other power supply systems; install filter reactors or multiple rectifier bridge circuits on the input side of the frequency converter to reduce high-order harmonic components. For occasions with phase-leading capacitors, high-order harmonic currents will increase the capacitor current and cause serious heating. Therefore, a reactor must be connected in series in front of the capacitor to reduce the harmonic components. The inductance of the reactor should be reasonably analyzed and calculated to avoid the formation of LC oscillation.

Motor temperature is too high and operating range

When the existing motor is modified for variable speed, the cooling capacity of the self-cooling motor decreases when running at low speed, causing the motor to overheat. In addition, because the high-order harmonics contained in the inverter output waveform will inevitably increase the iron loss and copper loss of the motor, after confirming the load state and operating range of the motor, take the following corresponding measures: force cooling and ventilation of the motor or improve the motor specification level; replace the motor dedicated to variable speed; limit the operating range and avoid the low-speed area.

Vibration, 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. Noise is usually divided into frequency converter noise and motor noise. Different treatment measures should be taken for different installation sites: During the commissioning process of the frequency converter, the pulse torque component should be minimized while ensuring the control accuracy; the mechanical resonance point should be confirmed during the commissioning, and the frequency shielding function of the frequency converter should be used to exclude these resonance points from the operating range; since the frequency converter noise is mainly generated by the cooling fan motor reactor, low-noise devices should be selected; AC reactors should be reasonably set between the motor and the frequency converter to reduce the high-order harmonics caused by the PWM modulation method.

High-frequency switching generates spike voltages that are detrimental to motor insulation

The output voltage of the inverter contains high-frequency spike voltage. These high-order harmonic impact voltages will reduce the insulation strength of the motor windings, especially for PWM control inverters. The following measures should be taken: try to shorten the wiring distance between the inverter and the motor; use a blocking diode surge voltage absorption device to process the inverter output voltage; for PWM inverters, try to add a filter on the motor input side.

Forecast of the development direction of inverter technology

The frequency converter is the power converter in the motion control system. Today's motion control system covers a variety of technical fields, and the general development trend is: AC drive , high frequency power converter, digitalization, intelligence and networking of control . Therefore, as an important power conversion component of the system, the frequency converter has developed rapidly by providing controllable high-performance variable voltage and frequency AC power supply.

With the application of new power electronic devices and high-performance microprocessors and the development of control technology, the performance-price ratio of frequency converters is getting higher and higher, and the size is getting smaller and smaller. Manufacturers are still making new efforts to continuously improve reliability and realize the further miniaturization, lightness, high performance, multi-function and pollution-free of frequency converters. The performance of frequency converters depends on the influence of the harmonics of the output AC voltage on the motor, the harmonic pollution of the power grid and the input power factor, and the energy loss of the frequency converter itself. Here, we only take the AC-DC-AC frequency converter, which is widely used, as an example to explain its development trend:

The main circuit power switch components are self-shutdown, modular, integrated and intelligent; the switching frequency is continuously increased and the switching loss is further reduced.

The topology of the main circuit of the inverter. The grid-side converter of the inverter often uses a 6-pulse converter for low-voltage and small-capacity devices, and a multiplexed 12-pulse converter for medium-voltage and large-capacity devices. The load-side converter often uses a two-level bridge inverter for low-voltage and small-capacity devices, and a multi-level inverter for medium-voltage and large-capacity devices. For the rotation of four-quadrant operation, in order to realize the inverter regenerative energy feedback to the grid and save energy, the grid-side converter should be a reversible converter. At the same time, dual PWM converters with bidirectional power flow have appeared. Proper control of the grid-side converter can make the input current close to a sine wave and reduce pollution to the grid.

The control methods of pulse width modulation transformer frequency converter can adopt sinusoidal pulse width modulation control, PWM control to eliminate harmonics of specified order, current tracking control, and voltage space vector control (flux tracking control).

The progress of AC motor variable frequency control methods is mainly reflected in the development from scalar control to high dynamic performance vector control and direct torque control and the development of speed sensorless vector control and direct torque control systems .

The progress of microprocessors has made digital control the development direction of modern controllers . Motion control systems are fast systems, especially high-performance control of AC motors requires storage of multiple data and fast real-time processing of large amounts of information. In recent years, major foreign companies have launched DSP ( digital signal processor )-based cores, equipped with peripheral functional circuits required for motor control, integrated in a single chip called DSP single-chip motor controllers, which have greatly reduced prices, reduced size, compact structure, convenient use, and improved reliability. Compared with ordinary single-chip microcomputers , DSP has a 10-15 times higher digital computing capability, which can ensure that the system has better control performance. Digital control simplifies hardware, and flexible control algorithms make control very flexible, can realize complex control laws, and make the application of modern control theory in motion control systems a reality. It is easy to connect with the upper system for data transmission, facilitate fault diagnosis, strengthen protection and monitoring functions, and make the system intelligent (such as some inverters have self-adjustment functions).