We all know that the frequency converter is engaged in electrical work should master a technology, the use of frequency converters to control the motor is a more common method of electrical control; some also require that must be skillfully used. Today, I will organize and summarize the relevant knowledge points with a shallow knowledge, content or repetition, aiming to share with you those wonderful relationship between the frequency converter and the motor.
First of all, why use the inverter to control the motor?
Let's start with a brief understanding of these two devices.
The motor is an inductive load, which impedes the change of current and produces a large change in current during startup.
Frequency converter, is the use of power semiconductor devices on and off action of the power frequency power supply will be converted to another frequency of electrical energy control device. 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 motors with higher power, the higher the power, the higher the starting current, the excessive starting current will bring a larger burden to the power supply and distribution network, and the frequency converter can solve this start-up problem, allowing the motor to start smoothly without causing excessive starting current.
Another function of using frequency converter is to regulate the speed of the motor, many occasions need to control the speed of the motor in order to get better productivity, and frequency converter speed control has been its biggest highlight, frequency converter by changing the frequency of the power supply in order to achieve the purpose of controlling the speed of the motor.
What are the frequency converter control methods?
The five most commonly used ways of inverter motor control are as follows:
Low-voltage general-purpose inverter output voltage is 380-650V, output power is 0.75-400kW, working frequency is 0-400Hz, and its main circuits all adopt AC-DC-AC circuit. Its control mode has gone through the following four generations.
1U/f=C sinusoidal pulse width modulation (SPWM) control mode
Characterized by a simple control circuit structure, lower cost, mechanical characteristics of the hardness is also better to meet the general transmission of smooth speed requirements, has been widely used in various fields of industry.
However, this control method at low frequency, due to the lower output voltage, torque by the stator resistance voltage drop is more significant, so that the output maximum torque is reduced.
In addition, its mechanical characteristics are not as hard as DC motor, the dynamic torque capacity and static speed performance are not satisfactory, and the system performance is not high, the control curve will change with the load, the torque response is slow, the motor torque utilization is not high, the low speed due to the stator resistance and the existence of the inverter deadband effect and performance degradation, stability deterioration, and so on. Therefore, vector control frequency conversion speed regulation has been studied.
Voltage space vector (SVPWM) control method
It is based on the premise of the overall generation effect of three-phase waveforms, in order to approximate the ideal circular rotating magnetic field trajectory of the motor air gap for the purpose of generating three-phase modulated waveforms at a time, and control in the way of the inner polygon approximation of the circle.
It has been improved after practical use, i.e., frequency compensation is introduced, which can eliminate the error of speed control; the magnetic chain amplitude is estimated by feedback, which eliminates the influence of the stator resistance at low speeds; and the output voltage and current are closed-looped to improve the accuracy and stability of the dynamics. However, the control circuit has more links and does not introduce torque regulation, so the system performance is not fundamentally improved.
Vector control (VC) method
The practice of vector control frequency control is to convert the stator current Ia, Ib, Ic, of an 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 the rotary transformation according to the rotor magnetic field orientation, which is equivalent to the synchronous rotating coordinate system into the DC current Im1, It1 (Im1 is equal to the (Im1 is equivalent to the excitation current of DC motor; It1 is equivalent to the armature current which is proportional to the torque), and then imitate the control method of DC motor to get the control quantity of DC motor, and realize the control of asynchronous motor after the corresponding inverse transformation of coordinates.
In essence, the AC motor is equivalent to a DC motor, and the two components of speed and magnetic field are controlled independently. By controlling the rotor magnetic chain, and then decomposing the stator current to obtain the torque and magnetic field components, through the coordinate transformation, to realize orthogonal or decoupled control. The proposed vector control method is of epoch-making significance. However, in practical applications, due to the rotor magnetic chain is difficult to accurately observe, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the control process of the equivalent DC motor is more complicated, making it difficult for the actual control effect to achieve the results of the ideal analysis.
Direct torque control (DTC) method
In 1985, Prof. DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency conversion technology. This technology has largely solved the shortcomings of the above vector control, and has been rapidly developed with 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 drives for electric locomotive traction. Direct torque control analyzes the mathematical model of AC motor directly in the stator coordinate system to control the magnetic chain 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 mimic the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.
Matrix AC-AC control method
VVVF inverter, vector control inverter, and direct torque control inverter are all types of AC-DC-AC inverter. Their common disadvantages are low input power factor, high harmonic currents, the need for large energy storage capacitors in the DC circuit, and the regenerative energy cannot be fed back to the grid, i.e., four-quadrant operation is not possible.
For this reason, the matrix AC-AC inverter came into being. As the matrix AC-AC inverter eliminates the intermediate DC link, thus eliminating the large size, expensive electrolytic capacitors. It can realize the power factor of l, the input current is sinusoidal and can operate in four quadrants, the power density of the system is large. The technology is not yet mature, but still attracts many scholars to study in depth. Its essence is not to indirectly control the current, magnetic chain and other quantities, but to realize the torque directly as the controlled quantity.
The specific method is:
Controlling the stator magnetic chain introduces the stator magnetic chain observer to realize the speed sensor-less method;
Automatic identification (ID) relies on an accurate mathematical model of the motor to automatically identify the motor parameters;
Calculate the actual values corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. Calculate the actual torque, stator magnetic chain, rotor speed for real-time control;
Realization of Band-Band control generates PWM signals according to the Band-Band control of magnetic chain and torque to control the inverter switching state.
Matrix AC-AC inverter has fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (<+3%); it also has high starting torque and high torque accuracy, especially at low speeds (including 0 speeds), and it can output 150% to 200% torque.
How to control the motor by frequency converter? How are they wired?
Frequency converter control motor wiring is relatively simple, with the wiring of the contactor is almost the same, three mains power into the line, and then out of the line to the motor, but one of the settings on the said, the control of the frequency converter is more than a different way.
First of all, let's take a look at the inverter terminals, although the brand is more, the wiring is also different, but most of the inverter terminals are not too much. Generally divided into positive and negative switching inputs, used to control the motor more than the start of positive and negative. Feedback terminal, used to feedback the running status of the motor, including running frequency, speed, fault status and so on. Speed setting control, some frequency converter is used potentiometer, some directly using the key, are not accessible.
Through the physical wiring to control the way, there is another way is to go to the communication network, a lot of frequency converter now support communication control, you can control the motor through the communication line to start and stop, forward and reverse, adjust the speed, etc., at the same time, the feedback information is also transmitted through the communication.
What happens to the output torque when the rotational speed (frequency) of the motor is changed?
The starting torque and maximum torque of an inverter drive is less than that of a direct drive with an industrial frequency power supply.
Motors have large starting and acceleration shocks when powered by an industrial-frequency power supply, but these shocks are weaker when powered by an inverter. Direct starting at industrial frequency produces 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 motor starting current and impact is smaller.
Usually, the torque produced by the motor decreases with the frequency (speed reduction). The actual data for the reduction is given in some inverter manuals for illustration.
By using an inverter with flux vector control, the lack of torque at low motor speeds will be improved, and the motor will produce sufficient torque even in the low speed zone.
When the frequency converter is speed-controlled to a frequency greater than 50 Hz, the output torque of the motor will be reduced.
Normally motors are designed and manufactured for 50Hz voltage, and their rated torque is also given in this voltage range. Therefore speed regulation below the rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)
When the output frequency of the inverter is greater than 50Hz frequency, the torque produced by the motor has to decrease in a linear relationship inversely proportional to the frequency.
When the motor is operated at a speed greater than 50Hz frequency, the size of the motor load must be given consideration to prevent a lack of motor output torque.
For example, the torque produced by a motor at 100 Hz is reduced to approximately 1/2 of the torque produced at 50 Hz.
Therefore, speed control above the rated frequency is called constant power speed control. (P=Ue*Ie)
Application of frequency converter above 50Hz
As you know, for a particular motor, its rated voltage and rated current are constant.
For example, inverter and motor rated values are: 15kW/380V/30A, the motor can work above 50Hz.
When the speed of 50Hz, the output voltage of the inverter is 380V, the current is 30A, at this time, if you increase the output frequency to 60Hz, the maximum output voltage and current of the inverter can only be 380V/30A, it is clear that the output power remains unchanged, so we call it constant power speed control.
What is the torque situation at this time?
Because P = wT (w; angular velocity, T: torque), because P is unchanged, w increased, so the torque will be reduced accordingly.
We can also look at it another way:
The stator voltage of the motor U = E + I * R (I is the current, R is the electronic resistance, E is the induced potential)
It can be seen that when U and I are constant, E is also constant.
And E=k*f*X (k: constant; f: frequency; X: magnetic flux), so when f from 50 --> 60Hz, X will be reduced accordingly
For a motor T=K*I*X (k: constant; I: current; X: flux), so the torque T will decrease with the flux X.
Meanwhile, less than 50Hz, the flux (X) is constant when U/f=E/f is constant because I*R is very small. Torque T is proportional to current. This is why the overcurrent capability of an inverter is usually used to describe its overload (torque) capability and is called constant torque speed regulation (constant rated current --> constant maximum torque)
Conclusion: The output torque of the motor decreases when the output frequency of the inverter is increased from 50Hz or more.
Other factors related to output torque
Heat generation and heat dissipation ability determines the output current ability of inverter, thus affecting the output torque ability of inverter.
Carrier frequency: the rated current marked by the general inverter is the highest carrier frequency, the highest ambient temperature can ensure the continuous output value, reduce the carrier frequency, the motor current will not be affected. But the heating of the components will be reduced.
Ambient temperature: Just as it will not increase the inverter protection current value when the surrounding temperature is detected to be lower.
Altitude: Increased altitude has an effect on heat dissipation and insulation performance. Generally below 1000m can be disregarded, above every 1000 meters to reduce the capacity of 5% can be.