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.
There are several ways to classify frequency converters . According to the main circuit operating mode, they can be divided into voltage-type frequency converters and current-type 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=60 f(1-s)/p (1)
In the formula
n — the 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 changes 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 wiring diagram:
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.
Sinusoidal Pulse Width Modulation (SPWM) control method with 1U/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.
Voltage Space Vector (SVPWM) Control
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 the circuit by approximating a circle with 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.
Vector Control (VC)
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.
Direct Torque Control (DTC)
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.
Matrix-based cross-channel control
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:
—Controlling the stator flux linkage 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;
—Calculate the actual values corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and then calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
— To achieve Band-Band control, PWM signals are generated 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.
