Working principle of frequency converter

Working principle of frequency converter

We know that the synchronous speed expression of AC motor is:
n = 60 f (1-s) / p (1) where
n is the speed of asynchronous motor;
f is the frequency of asynchronous motor;
s is the motor slip;
p is 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 speed adjustment range of the motor is very wide. The frequency converter achieves speed adjustment by changing the power supply frequency of the motor. It is an ideal high-efficiency and high-performance speed regulation method.

Inverter control mode
The output voltage of low-voltage general-purpose inverter 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 mode has gone through the following four generations.
1U/f=C sinusoidal pulse width modulation (SPWM) control mode
Its characteristics are simple control circuit structure, low cost, good mechanical characteristics hardness, 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 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 change of load, the torque response is slow, the motor torque utilization rate is not high, and the performance is reduced and the stability is poor at low speeds due to the existence of stator resistance and inverter dead zone effect. 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 modulation waveform at a time, and controlling it in the way of inscribed polygon approximating the 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 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.
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 independently control the speed and magnetic field components. 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.
Direct Torque Control (DTC) Method
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 transmissions for electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system to control the 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. Matrix AC
-AC 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 capacitor required for DC circuit, 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 capacitor. It can achieve a power factor of 1, a sinusoidal input current and can operate in four quadrants, and the power density of the system is high. 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 and other quantities, but to directly use torque as the controlled quantity to achieve it. The specific method is:
- Control the stator flux to introduce the stator flux observer to realize the speed sensorless mode;
- Automatic identification (ID) relies on the accurate motor mathematical model to automatically identify the motor parameters;
- Calculate the actual value corresponding to the 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 signals according to the Band-Band control of flux and torque to control the inverter switching state.
Matrix AC-AC frequency conversion has fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (<+3%); At the same time, it also has high starting torque and high torque accuracy, especially at low speed (including 0 speed), it can output 150%~200% torque.