Let's briefly understand these two devices first.
The motor is an inductive load, which hinders the change of current, and will produce a large change of current when it is started.
The frequency converter is a power control device that uses the on-off function of the power semiconductor device to convert the power frequency power supply to another frequency. It is mainly composed of two parts of the circuit, one is the main circuit (rectifier module, electrolytic capacitor, and inverter module), and the other is the control circuit (switching power supply board, control circuit board).
In order to reduce the starting current of the motor, especially for a motor with a higher power, the greater the power, the greater the starting current. Excessive starting current will bring a greater burden to the power supply and distribution network, and the frequency converter can solve this starting problem so that the motor starts smoothly without causing excessive starting current.
Another function of using the frequency converter is to adjust the speed of the motor. On many occasions, it is necessary to control the speed of the motor to obtain better production efficiency, and the speed regulation of the frequency converter has always been its biggest highlight. The frequency converter can change the frequency of the power supply to achieve The purpose of controlling the motor speed.
The five most common ways in which an inverter controls a motor are as follows:
The output voltage of low-voltage universal frequency conversion is 380-650V, the output power is 0.75-400kW, and the working frequency is 0-400Hz. Its main circuit adopts an 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, which can meet the smooth speed regulation requirements of general transmission, and has been widely used in various fields of industry.
However, when this control method is at low frequency, due to the low output voltage, 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 hard as DC motors after, and the dynamic torque capability and static speed regulation performance are not satisfactory, and the system performance is not high, the control curve will change with the load change, and the torque response is slow, and the motor turns The torque utilization rate is not high, and the performance decreases due to the existence of the stator resistance and the dead zone effect of the inverter at low speed, and the stability becomes poor. Therefore, people have developed vector control frequency conversion speed regulation.
Voltage space vector (SVPWM) control mode
It is based on the premise of the overall generation effect of the three-phase waveform and aims at approaching the ideal circular rotating magnetic field trajectory of the motor air gap. It generates a three-phase modulation waveform at one time and controls it in a way that an inscribed polygon approximates a circle.
After practical use, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; the magnitude of flux linkage can be estimated through feedback to eliminate the influence of stator resistance at low speed; the output voltage and current are closed-loop to improve dynamic accuracy and stability. However, there are many links in the control circuit, and no torque adjustment is introduced, so the system performance has not been fundamentally improved.
Vector control (VC) mode
The practice of vector control frequency conversion speed regulation is to convert the stator currents Ia, Ib, and Ic of the asynchronous motor in the three-phase coordinate system into an equivalent AC current Ia1Ib1 in the two-phase stationary coordinate system through three-phase-two-phase conversion, and then through According to the orientation 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 imitates the DC motor In the control method, the control quantity of the DC motor is obtained, and the control of the asynchronous motor is realized through the corresponding coordinate inverse transformation.
Its essence is that the AC motor is equivalent to a DC motor, and the two components of speed and magnetic field are independently controlled. By controlling the rotor flux linkage, and then decomposing the stator current to obtain the two components of torque and magnetic field, the coordinate transformation can realize the quadrature or decoupling control. The proposal of the vector control method has epoch-making significance. However, in practical applications, due to the difficulty of accurately observing the rotor flux linkage, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is more complicated, making it difficult for the actual control effect to achieve ideal analysis. result.
Direct torque control (DTC) mode
In 1985, Professor DePenbrock of Ruhr University in Germany proposed the direct torque control frequency conversion technology for the first time. This technology largely solves the above-mentioned shortcomings of vector control 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 the high-power AC transmission of electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the flux linkage and torque of the motor. It does not need to equate the AC motor to a DC motor, thus saving many complicated calculations in the vector rotation transformation; it does not need to simulate 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 mode
VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all AC-DC-AC frequency conversions. Their common disadvantages are low input power factor, large harmonic current, large energy storage capacitors for DC circuits, 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. Because the matrix AC-AC frequency conversion eliminates the intermediate DC link, the bulky and expensive electrolytic capacitors are omitted. It can realize the power factor is l, the input current is sinusoidal and can run in four quadrants, and the power density of the system is high. Although the technology is not yet mature, it still attracts many scholars to do 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:
Control the stator flux linkage and introduce the stator flux linkage observer to realize the speed sensorless mode;
Automatic identification (ID) relies on the precise 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. Calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
Realize Band-Band control According to the Band-Band control of flux linkage and torque, PWM signals are generated to control the switching state of the inverter.
Matrix AC-AC frequency conversion has a fast torque response (<2ms), high-speed accuracy (±2%, no PG feedback), and 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.
The wiring of the inverter to control the motor is relatively simple, similar to the wiring of the contactor. The three main power supply lines enter and then go out to the motor.
First of all, let's take a look at the terminals of the inverter. Although there are many brands and different wiring methods, most of the terminals of the inverter are not too different. It is generally divided into forward and reverse switch input, which is used to control the forward and reverse rotation of many motors. The feedback terminal is used to feedback the running status of the motor, including the running frequency, speed, fault status and so on. For speed reference control, some frequency converters use potentiometers, while others use keys directly.
It is controlled by physical wiring, and another way is to go through the communication network. Many inverters now support communication control. Through this communication line, the motor can be controlled to start and stop, forward and reverse, adjust speed, etc., and at the same time feedback information is also transmitted via communications.
The starting torque and maximum torque when driven by a frequency converter are smaller than those directly driven by a commercial frequency power supply.
When the motor is powered by a commercial frequency power supply, the shocks of starting and accelerating are very large, but when the motor is powered by a frequency converter, these shocks will be weaker. Power frequency direct starting will generate a large starting current. When using a frequency converter, the output voltage and frequency of the frequency converter are gradually added to the motor, so the starting current and impact of the motor are smaller.
Generally, the torque produced by the motor decreases as the frequency decreases (speed decreases). The reduced actual data will be explained in some inverter manuals.
By using the frequency converter controlled by a magnetic flux vector, the lack of torque at the low speed of the motor will be improved, and the motor can output sufficient torque even in the low-speed area.
When the frequency converter is adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease
The usual motor is designed and manufactured according to the voltage of 50Hz, and its rated torque is also given within this voltage range. Therefore, the speed regulation below the rated frequency is called constant torque speed regulation. (T=Te，P<=Pe)
When the output frequency of the frequency converter is greater than 50Hz, the torque generated by the motor will decrease in a linear relationship that is inversely proportional to the frequency.
When the motor runs at a frequency greater than 50Hz, the load of the motor must be considered to prevent insufficient motor output torque.
For example, the torque produced by the motor at 100Hz will be reduced to about 1/2 of the torque produced at 50Hz.
Therefore, the speed regulation above the rated frequency is called constant power speed regulation. (P=Ue*Ie)
As we all know, for a specific motor, its rated voltage and rated current are constant.
For example, the rated values of the inverter and the motor are both: 15kW/380V/30A and the motor can work above 50Hz.
When the rotating speed is 50Hz, the output voltage of the inverter is 380V and the current is 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the inverter can only be 380V/30A. Obviously, the output power remains unchanged, so we call it constant power speed regulation.
Because P=wT(w; angular velocity, T: torque), because P remains unchanged and w increases, the torque will decrease accordingly.
We can also look at it from another angle:
Motor stator voltage U=E+I*R (I is current, R is electronic resistance, E is induced potential)
It can be seen that when U and I are unchanged, E is also unchanged.
And E=k*f*X (k: constant; f: frequency; X: magnetic flux), so when f changes from 50-->60Hz, X will decrease accordingly
For the motor, T=K*I*X (K: constant; I: current; X: flux), so the torque T will decrease as the flux X decreases
At the same time, when it is less than 50Hz, since I*R is very small, when U/f=E/f is constant, the magnetic flux (X) is constant. The torque T is proportional to the current. This is why the overload (torque) capacity of the inverter is usually described by its over-current capacity, and it is called constant torque speed regulation (the rated current does not change --> the maximum torque does not change)
Conclusion: When the output frequency of the inverter increases from above 50Hz, the output torque of the motor will decrease
The heat generation and heat dissipation capability determine the output current capability of the inverter, thus affecting the output torque capability of the inverter.
Carrier frequency: Generally, the rated current of the inverter is based on the highest carrier frequency, which can guarantee continuous output at the highest ambient temperature. If the carrier frequency is reduced, the motor current will not be affected. But the heating of components will be reduced.
Ambient temperature: Just because it detects that the ambient temperature is relatively low, it will not increase the inverter protection current value.
Altitude: As altitude increases, it affects both heat dissipation and insulation performance. Generally, it can be ignored below 1000m, and it is enough to reduce the capacity by 5% every 1000m above.
In the above arrangement, we have learned why the inverter is used to control the motor, and how the inverter controls the motor. The frequency converter controls the motor, and it can be summed up in two points: first, the frequency converter controls the starting voltage and frequency of the motor; achieving a smooth start and smooth stop; second, use the frequency converter adjusts the speed of the motor, and adjust the speed of the motor by changing the frequency.