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.
Function of frequency converter
The direct function of frequency converters:
1. By changing the voltage and frequency of the motor, the speed of the motor can be infinitely adjusted.
2. Soft start energy saving and power factor compensation energy saving.
Indirect effects of frequency converters:
1. Energy saving (electricity saving). Traditional speed control methods for equipment such as fans and pumps adjust the air and water supply by regulating the opening of inlet or outlet baffles or valves. This method has high input power, and a large amount of energy is consumed in the flow throttling process of the baffles and valves. When using variable frequency speed control, if the flow requirement decreases, the requirement can be met by reducing the speed of the pump or fan, thus reducing power consumption.
2. Improve the automation level of production equipment. Currently, there are many brands of frequency converters available.
Inverter wiring diagram
Inverter working principle
The main circuit is the power conversion section that provides voltage and frequency adjustable power to the asynchronous motor. The main circuit of a frequency converter can be broadly divided into two categories: voltage-source converters, which convert DC voltage to AC, and current-source converters, which convert DC current to AC, and current-source converters, which use inductors for DC circuit filtering. It consists of three parts: a rectifier that converts the mains frequency power to DC power; a smoothing circuit that absorbs voltage ripples generated in the converter and inverter; and an inverter that converts DC power to AC power.
Inverter structure and working process diagram
1. Rectifier
Recently, diode converters have been widely used to convert mains frequency power into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can operate regeneratively due to its reversible power direction.
2. Smoothing circuit
The DC voltage after rectification contains a pulsating voltage at six times the frequency of the power supply. Furthermore, the pulsating current generated by the inverter also causes DC voltage fluctuations. To suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). For small device capacities, if the power supply and main circuit components have sufficient capacity, the inductor can be omitted, and a simple smoothing circuit can be used.
3. Inverter
Unlike rectifiers, inverters convert DC power into AC power at the required frequency. A three-phase AC output is obtained by switching six switching devices on and off at predetermined times. The switching time and voltage waveforms are shown as an example using a voltage-source PWM inverter.
control circuit
The inverter control circuit is a loop that provides control signals to the main circuit that supplies power (voltage and frequency are adjustable) to the asynchronous motor. It consists of a frequency and voltage "operation circuit", a main circuit "voltage and current detection circuit", a motor "speed detection circuit", a "drive circuit" that amplifies the control signals from the operation circuit, and a "protection circuit" for the inverter and the motor.
(1) Operational circuit: It compares and calculates the external speed, torque and other commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) Voltage and current detection circuit: isolated from the main circuit potential to detect voltage, current, etc.
(3) Drive circuit: The circuit that drives the main circuit devices. It is isolated from the control circuit to turn the main circuit devices on and off.
(4) Speed detection circuit: The speed signal is sent to the operation circuit by the speed detector (tg, plg, etc.) installed on the shaft of the asynchronous motor. According to the instructions and operations, the motor can run at the commanded speed.
(5) Protection circuit: detects the voltage and current of the main circuit. When an overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor, the inverter stops working or suppresses the voltage and current values.
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
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:
1) Control the stator flux linkage by introducing a stator flux linkage observer to achieve a sensorless speed mode;
2) Automatic identification (ID) relies on a precise mathematical model of the motor to automatically identify motor parameters;
3) Calculate the actual values corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
4) Implement Band-Band control: Generate PWM signals based on flux linkage and torque to control the inverter switching state.
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.
