Feedback unit supplier reminds you that since the emergence of automatic induction motors, the form of AC generators has already undergone variable frequency operation. Change the speed of the generator and adjust its output frequency. Before the emergence of high-speed transistors, this was one of the main ways to change the motor speed, but due to the generator speed reducing the output frequency instead of voltage, the frequency variation was limited.
Therefore, let's take a look at the components of the frequency converter and see how they actually work together to change the frequency and motor speed.
Inverter components - rectifier
Due to the difficulty in changing the frequency of AC sine waves in AC mode, the first task of a frequency converter is to convert the waveform into DC. To make it look like AC, it is relatively easy to operate DC. The first component of all frequency converters is a device called a rectifier or converter. The rectifier circuit of the frequency converter converts alternating current into direct current, and its working mode is roughly the same as that of a battery charger or arc welding machine. It uses a diode bridge to restrict the AC sine wave from moving in only one direction. The result is that the fully rectified AC waveform is interpreted by the DC circuit as a local DC waveform. A three-phase frequency converter accepts three independent AC input phases and converts them into a single DC output.
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.
If 460V motors are allowed to operate at these higher voltages, their lifespan can be greatly reduced. Therefore, the frequency converter must constantly change the "peak" voltage relative to the frequency to maintain a constant effective voltage. The lower the operating frequency, the lower the peak voltage, and vice versa. You should now have a good understanding of the working principle of the frequency converter and how to control the motor speed. Most frequency converters allow users to manually set motor speed through multi position switches or keyboards, or use sensors (pressure, flow, temperature, liquid level, etc.) to automate the process.