As far as I know, frequency conversion operation has existed in the form of alternators since the advent of automatic induction motors. Changing the generator's speed alters its output frequency. Before the advent of high-speed transistors, this was one of the main ways to change motor speed, but frequency variation was limited because the generator speed reduced the output frequency rather than the voltage. Therefore, let's look at the components of a frequency converter to see how they actually work together to change the frequency and motor speed. Frequency converter components: Rectifier. Because it's difficult to change the frequency of an AC sine wave in AC mode, the first job of a frequency converter is to convert the waveform to DC. To make it look like AC, operating DC is relatively easy. The first component of all frequency converters is a device called a rectifier or converter, as shown in the diagram below.
A variable frequency rectifier (VFD) converts alternating current (AC) to direct current (DC), operating much like a battery charger or arc welder. It uses a diode bridge to limit the AC sine wave to travel in only one direction. The result is that the fully rectified AC waveform is interpreted as a local DC waveform by the DC circuit. A three-phase VFD accepts three independent AC input phases and converts them into a single DC output. Most three-phase VFDs can also accept single-phase (230V or 460V) power, but since there are only two input branches, the VFD output (HP) must be dated because the resulting DC current is proportionally reduced. On the other hand, a true single-phase VFD (a single-phase VFD that controls a single-phase motor) utilizes a single-phase input and produces a DC output proportional to the input. Three-phase motors are more commonly used than single-phase VFDs when it comes to variable speed operation for two reasons. First, they have a wider power range. Second, single-phase motors typically require some external intervention to start rotating. The second component, the DC bus (shown in the diagram), is not visible in all VFDs because it does not directly affect VFD operation. However, it is always present in high-quality general-purpose inverters. The DC bus uses capacitors and inductors to filter out the AC “ripple” voltage in the converted DC power before it enters the inverter section. It also includes filters to prevent harmonic distortion, which can be fed back to the inverter power supply. Older inverters require separate line filters to complete this process. The right side of the inverter illustration shows the “internals” of the inverter. The inverter uses three sets of high-speed switching transistors to create DC “pulses” for all three phases of an analog AC sine wave. These pulses determine not only the voltage of the wave but also its frequency. The term inverter, or simply inverter, means “reverse,” which, simply put, is the up-and-down movement of the generated waveform. Modern inverters use a technique called “pulse width modulation” (PWM) to regulate voltage and frequency. Then we talk about IGBTs, which stands for “Insulated Gate Bipolar Transistor,” and are the switching (or pulse) elements of the inverter. Transistors (replacing vacuum tubes) serve two roles in our electronic world. They can act as amplifiers, amplifying signals, or they can act as switches, simply turning signals on and off. IGBTs are a modern version that offers higher switching speeds (3000-16000Hz) and reduced heat generation. Higher switching speeds improve the accuracy of AC wave simulation and reduce motor noise. Reduced heat generation means smaller heat sinks, resulting in a smaller inverter footprint. The following diagram shows the waveform produced by 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 narrow sets at the beginning and end of the positive and negative portions of the AC cycle. The sum of the areas of the pulses equals the effective voltage of the true AC wave. If you were to cut off the portion of the pulses above (or below) the true AC waveform and fill the blank area below the curve with them, you would find that they match almost perfectly. This is how the inverter controls the motor voltage.
The sum of the pulse width and the gap width between them determines the frequency of the waveform the motor sees (hence PWM or pulse width modulation). If the pulses were continuous (i.e., without gaps), the frequency would still be correct, but the voltage would be much larger than a true AC sine wave. Depending on the required voltage and frequency, the inverter will change the pulse height and width, as well as the gap width between them. Some might wonder how this "fake" AC (actually DC) runs an AC induction motor. After all, isn't an alternating current needed to "induct" the current in the motor rotor and its corresponding magnetic field? Well, AC would naturally induce induction because it is constantly changing direction. On the other hand, DC doesn't function properly once the circuit is activated. However, if a DC is turned on and off, it can induce current. For those older readers, car ignition systems (before solid-state ignition) used to have a set of points in the distributor. The purpose of these points was to "pulse" from the battery to the coil (transformer). This induced a charge in the coil and then raised the voltage to a level that allowed the spark plugs to ignite. The wide DC pulse seen in the image above is actually composed of hundreds of individual pulses, and this on-off motion of the inverter output is allowed to occur through DC induction.
One factor that complicates alternating current (AC) 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 again. How do we determine the actual voltage applied to a circuit? The illustration on the left is a 60Hz, 120V sine wave. But note 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 falls back to 0. It continues to fall to -170V and then rises back to 0. The area of the green rectangle at the original upper boundary of 120V is equal to the sum of the areas of the positive and negative parts of the curve. So, is 120V the average level? Well, if we were to average all the voltage values at every point throughout the entire cycle, the result would be approximately 108V, so that's not the answer. So why is this value 120V as measured by the VOM? It relates to what we call "effective voltage."
If you were to measure the heat generated by a direct current flowing through a resistor, you would find that it is greater than the heat generated by an equivalent alternating current. This is because AC does not remain constant throughout the cycle. In a laboratory setting, under controlled conditions, if a specific direct current produces a 100-degree increase in heat, its AC equivalent would produce a 70.7-degree increase, or 70.7% of the DC value. Therefore, the effective value of AC is 70.7% of DC. It can also be seen that the effective value of 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 you want to measure voltages from 0 to 180 degrees, the effective voltage will be the peak voltage of 0-707. In the graph, 0.707 times the peak voltage of 170 equals 120V. This effective voltage is also called the root mean square (RMS) voltage. Therefore, the peak voltage is always 1.414 of the effective voltage. A 230V AC current has a peak voltage of 325V, while a 460V AC current has a peak voltage of 650V. Even though the voltage is independent of the AC motor's operating speed, the inverter must change the voltage, except for frequency variations. The graph shows two 460V AC sine waves. The red one is the 60Hz curve, and the blue one is the 50Hz one. Both have a peak voltage of 650V, but the 50Hz curve is much wider. You can easily see that the area under the curve (0-10ms) of the 50Hz curve is larger than the area under the curve (0-8.3ms). Furthermore, since the area under the curve is proportional to the effective voltage, its effective voltage is higher. As the frequency decreases, the increase in effective voltage becomes more drastic. Allowing a 460V motor to operate at these higher voltages could significantly reduce its lifespan. Therefore, the inverter 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 how inverters work and how motor speed is controlled. Most inverters allow users to manually set the motor speed via multi-position switches or a keypad, or automate the process using sensors (pressure, flow, temperature, level, etc.).
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