Do You Know the 5 Control Methods of Frequency Converters?

The frequency converter is a device that converts industrial frequency power supply (50Hz or 60Hz) into AC power supply of various frequencies to realize variable speed operation of the motor.

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

Working principle of frequency converter:
We know that the synchronous speed expression of an AC motor is:
n＝60 f(1－s)/p (1)
in the formula
n————The speed of asynchronous motor;
f——frequency of asynchronous motor;
s————motor slip;
p————The number of pole pairs of the motor.
It can be seen from equation (1) that the rotation speed n is proportional to the frequency f. As long as the frequency f is changed, the rotation speed of the motor can be changed. When the frequency f changes in the range of 0 to 50 Hz, the motor speed adjustment range is very wide. The frequency converter realizes speed regulation by changing the motor power frequency. It is an ideal high-efficiency and high-performance speed regulation method.

Frequency converter wiring diagram (frequency converter control mode)

The low-voltage universal variable frequency output voltage is 380~650V, the output power is 0.75~400kW, and the operating frequency is 0~400Hz. Its main circuit adopts AC-DC-AC circuit. Its control method has gone through the following four generations.

1U/f=C sinusoidal pulse width modulation (SPWM) control method
It is characterized by a simple control circuit structure, low cost, and good mechanical properties and hardness. It can meet the smooth speed regulation requirements of general transmission and has been widely used in various fields of industry. However, at low frequencies, due to the low output voltage of this control method, the torque is significantly affected by the voltage drop of the stator resistance, which reduces the maximum output torque.
In addition, its mechanical characteristics are not as good as those of DC motors, and its dynamic torque capability and static speed regulation performance are not satisfactory. Moreover, the system performance is not high, the control curve changes with the load, the torque response is slow, and the motor rotates slowly. The torque utilization rate is not high. At low speeds, the performance decreases and the stability deteriorates due to the existence of stator resistance and inverter dead zone effect. Therefore, people have developed vector control frequency conversion speed regulation.

Voltage space vector (SVPWM) control method
It is based on the overall generation effect of three-phase waveforms and aims to approximate the ideal circular rotating magnetic field trajectory of the motor air gap. It generates three-phase modulation waveforms at one time and controls them in a way that the inscribed polygon approaches a circle.
After practical use, improvements have been made, namely the introduction of frequency compensation, which can eliminate speed control errors; estimate the flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speed; close the loop of output voltage and current to improve dynamic accuracy and stability. However, there are many control circuit components 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 through According to the directional rotation transformation of the rotor magnetic field, it is equivalent to the DC current Im1 and It1 in the synchronous rotating coordinate system (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque), and then simulates the DC motor's The control method is to obtain the control quantity of the DC motor, and then realize the control of the asynchronous motor through the corresponding inverse coordinate transformation.
Its essence is to equate an AC motor to a DC motor, and independently control the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current to obtain the two components of torque and magnetic field, orthogonal or decoupled control is achieved through coordinate transformation. The proposal of vector control method has epoch-making significance. However, in practical applications, since the rotor flux linkage is difficult to accurately observe, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is 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 solved the above-mentioned shortcomings of vector control to a large extent, and has developed rapidly with novel control ideas, simple 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 to control the flux and torque of the motor. It does not need to equate AC motors to DC motors, thus eliminating many complex calculations in vector rotation transformation; it does not need to imitate the control of DC motors, nor does it need to simplify the mathematical model of AC motors for decoupling.

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, DC circuit requiring large energy storage capacitor, and regenerative energy cannot be fed back to the grid, that is, four-quadrant operation cannot be performed.
For this reason, matrix AC-AC frequency conversion came into being. Since the matrix AC-AC frequency conversion eliminates the intermediate DC link, large and expensive electrolytic capacitors are eliminated. It can achieve a power factor of l, a sinusoidal input current and four-quadrant operation. 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 the current, flux linkage, etc., but to realize the torque directly as the controlled quantity.

The specific method is:
The stator flux observer is introduced to control the stator flux linkage to realize the speed sensorless method;
Automatic identification (ID) relies on accurate motor mathematical models to automatically identify 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;
Implement Band-Band control to generate PWM signals according to the Band-Band control of flux linkage and torque to control the switching state of the inverter.
Matrix AC-AC frequency conversion has fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (<+3%); it also has high starting speed Torque and high torque accuracy, especially at low speed (including 0 speed), it can output 150% to 200% torque.