Le fournisseur de l'unité de retour d'information rappelle que depuis l'apparition des moteurs à induction automatiques, les générateurs de courant alternatif fonctionnent déjà à fréquence variable. On modifie la vitesse du générateur et on ajuste sa fréquence de sortie. Avant l'avènement des transistors à haute vitesse, c'était l'un des principaux moyens de faire varier la vitesse du moteur, mais comme la vitesse du générateur diminuait la fréquence de sortie plutôt que la tension, la variation de fréquence était limitée.
Examinons donc les composants du convertisseur de fréquence et voyons comment ils fonctionnent ensemble pour modifier la fréquence et la vitesse du moteur.
Composants de l'onduleur - redresseur
En raison de la difficulté à modifier la fréquence des ondes sinusoïdales alternatives en mode alternatif, la première tâche d'un convertisseur de fréquence est de convertir la forme d'onde en courant continu. Pour obtenir un signal d'apparence alternative, il est relativement simple de le convertir en courant continu. Le premier composant de tout convertisseur de fréquence est un dispositif appelé redresseur ou convertisseur. Le circuit redresseur du convertisseur de fréquence convertit le courant alternatif en courant continu, et son mode de fonctionnement est similaire à celui d'un chargeur de batterie ou d'un poste à souder à l'arc. Il utilise un pont de diodes pour empêcher la propagation de l'onde sinusoïdale alternative dans une seule direction. Ainsi, la forme d'onde alternative redressée est interprétée par le circuit continu comme une forme d'onde continue locale. Un convertisseur de fréquence triphasé accepte trois phases d'entrée alternatives indépendantes et les convertit en une seule sortie continue.
Most three-phase frequency converters can also accept single-phase (230V or 460V) power supply, but due to only two input branches, the output (HP) of the frequency converter must be derated because the generated DC current is proportionally reduced. On the other hand, a true single-phase inverter (a single-phase inverter that controls a single-phase motor) utilizes a single-phase input and generates a DC output proportional to the input.
There are two reasons why three-phase motors are more commonly used than single-phase counter components when it comes to variable speed operation. Firstly, they have a wider power range. On the other hand, single-phase motors typically require some external intervention to start rotating.
Inverter components - DC bus
The second component of the DC bus cannot be seen in any frequency converter because it does not directly affect the operation of the frequency converter. However, it always exists in high-quality general-purpose frequency converters. The DC bus uses capacitors and inductors to filter out the AC "ripple" voltage in the converted DC power, and then enters the inverter section. It also includes a filter to prevent harmonic distortion, which can be fed back to the inverter power supply. Older frequency converters require separate line filters to complete this process.
Inverter components - Inverter
On the right side of the illustration is the "internal organs" of the frequency converter. The inverter uses three sets of high-speed switching transistors to create all three-phase DC "pulses" that simulate AC sine waves. These pulses not only determine the voltage of the wave, but also its frequency. The term 'inverter' means' reversal ', which simply means the up and down movement of the generated waveform. Modern frequency converters use a technique called "pulse width modulation" (PWM) to regulate voltage and frequency.
Then let's talk about IGBT. IGBT refers to "insulated gate bipolar transistor", which is the switching (or pulse) component of the inverter. Transistors (replacing vacuum tubes) play two roles in our electronic world. It can act like an amplifier and increase the signal, or it can act as a switch by simply turning on and off the signal. IGBT is a modern version that provides higher switching speeds (3000-16000 Hz) and reduces heat generation. A higher switching speed can improve the accuracy of AC wave simulation and reduce motor noise. The reduction in heat generated means that the heat sink is smaller, so the frequency converter occupies a smaller area.
Inverter PWM waveform
The waveform generated by the inverter of a PWM inverter compared to a true AC sine wave. The inverter output consists of a series of rectangular pulses with fixed height and adjustable width.
In this particular case, there are three sets of pulses - a wide set in the middle and a narrow set at the beginning and end of the positive and negative parts of the AC cycle.
The sum of the areas of the pulses is equal to the effective voltage of the true AC wave. If you want to cut off the pulse parts above (or below) the actual communication waveform and fill the blank area below the curve with them, you will find that they almost perfectly match. It is precisely in this way that the frequency converter can control the voltage of the motor. The sum of the pulse width and the blank width between them determines the frequency of the waveform seen by the motor (hence PWM or pulse width modulation). If the pulse is continuous (i.e. without blanks), the frequency will still be correct, but the voltage will be much larger than a true AC sine wave.
According to the required voltage and frequency, the frequency converter will change the height and width of the pulse, as well as the blank width between the two. Some people may wonder how this' fake 'AC (actually DC) operates an AC induction motor.
After all, does an alternating current need to "induce" the current and corresponding magnetic field in the motor rotor? So, AC will naturally cause induction because it is a constantly changing direction, while DC will not operate normally once the circuit is activated.
However, if the DC is turned on and off, it can sense current. For those who are older, the car ignition system (before solid-state ignition) used to have a set of points in the distributor. The purpose of these points is to go from battery "pulses" to coils (transformers). This induces a charge in the coil and then raises the voltage to a level that allows the spark plug to ignite. The wide DC pulse seen in the above figure is actually composed of hundreds of individual pulses, and the opening and closing motion of the inverter output allows for DC induction to occur.
Effective voltage
One factor that makes alternating current complex is that it constantly changes voltage, from zero to a maximum positive voltage, then back to zero, then to some maximum negative voltage, and then back to zero. How to determine the actual voltage applied to the circuit? The illustration below is a 60Hz, 120V sine wave. But it should be noted that its peak voltage is 170V. If its actual voltage is 170V, how can we call it a 120V wave?
One factor that makes alternating current complex is its constant change in voltage, from zero to a maximum positive voltage, then back to zero, then to some maximum negative voltage, and then back to zero. How to determine the actual voltage applied to the circuit?
A 60Hz, 120V sine wave should be noted that its peak voltage is 170V. If its actual voltage is 170V, how can we call it a 120V wave?
In one cycle, it starts at 0V, rises to 170V, and then drops again to 0. It continues to drop to -170, and then rises again to 0. The area of the green rectangle with an upper boundary of 120V is equal to the sum of the areas of the positive and negative parts of the curve.
So 120V is the average level? Okay, if we were to average all the voltage values at each point throughout the entire cycle, the result would be approximately 108V, so it cannot be the answer. So why is this value measured by VOM at 120V? It is related to what we call 'effective voltage'.
If you want to measure the heat generated by the direct current flowing through a resistor, you will find that it is greater than the heat generated by the equivalent alternating current. This is because AC does not maintain a constant value throughout the entire cycle. If conducted under controlled conditions in the laboratory, it is found that a specific DC current produces a 100 degree heat increase, resulting in a 70.7 degree increase in AC equivalent or 70.7% DC value.
So the effective value of AC is 70.7% of DC. It can also be seen that the effective value of the AC voltage is equal to the square root of the sum of the squares of the voltages in the first half of the curve. If the peak voltage is 1 and various voltages from 0 degrees to 180 degrees need to be measured, the effective voltage will be the peak voltage of 0-707 degrees. 0.707 times the peak voltage of 170 in the figure is equal to 120V. This effective voltage is also known as root mean square or RMS voltage.
Therefore, the peak voltage is always 1.414 of the effective voltage. 230V AC current has a peak voltage of 325V, while 460 has a peak voltage of 650V. In addition to frequency variation, even if the voltage is independent of the operating speed of the AC motor, the frequency converter must also change the voltage. Two 460V AC sine waves. The red curve is 60Hz, and the blue curve is 50Hz. Both have a peak voltage of 650V, but 50Hz is much wider. You can easily see that the area within the first half of the 50Hz curve (0-10ms) is larger than the first half of the 60Hz curve (0-8.3ms). Moreover, as the area under the curve is directly proportional to the effective voltage, its effective voltage is higher. As the frequency decreases, the increase in effective voltage becomes more severe.
Si des moteurs de 460 V fonctionnent à des tensions plus élevées, leur durée de vie peut être considérablement réduite. Par conséquent, le convertisseur de fréquence doit constamment ajuster la tension de crête en fonction de la fréquence afin de maintenir une tension efficace constante. Plus la fréquence de fonctionnement est basse, plus la tension de crête est faible, et inversement. Vous devriez maintenant bien comprendre le principe de fonctionnement du convertisseur de fréquence et comment contrôler la vitesse du moteur. La plupart des convertisseurs de fréquence permettent à l'utilisateur de régler manuellement la vitesse du moteur à l'aide de commutateurs multipositions ou d'un clavier, ou d'utiliser des capteurs (pression, débit, température, niveau de liquide, etc.) pour automatiser le processus.