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Since the advent of automatic induction motors, inverter operation has existed in the form of alternators. Change the speed of the generator and change its output frequency. Before the advent of high-speed transistors, this was one of the main ways to change the speed of a motor, but because the generator speed reduced the output frequency rather than the voltage, the frequency change was limited.

So let's take a look at the components of the inverter and see how they actually work together to change the frequency and motor speed.

01 Inverter component - rectifier

Since it is difficult to change the frequency of the AC sine wave in AC mode, the first job of the inverter 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 below:

A rectifier circuit converts alternating current into direct current and works in much the same way as a battery charger or arc welder. It uses a diode bridge to limit the AC sine wave to move 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 inverter takes three separate AC input phases and converts them into a single DC output.

Most three-phase inverters can also accept a single-phase (230V or 460V) power supply, but since there are only two input branches, the inverter output (HP) must be derated because the DC current generated is reduced proportionally. On the other hand, a true single-phase inverter (the single-phase inverter that controls a single-phase motor) utilizes a single-phase input and produces a DC output proportional to the input.

When it comes to variable speed operation, three-phase motors are more commonly used than single-phase counter components for two reasons. First, they have a wider power range. Single-phase motors, on the other hand, usually require some external intervention to start spinning.

02 Inverter components - DC bus

The second component of the DC bus (shown by the DC bus in the figure) is not visible in all converters because it does not directly affect the frequency conversion operation. However, it is always present in high quality general purpose drives. The DC bus uses capacitors and inductors to filter out the AC "ripple" voltage in the converted DC and then into the inverter section. It also includes a filter that blocks harmonic distortion and can be fed back to the inverter power supply. Older inverters and separate line filters are required to complete this process.

03 Inverter components - Inverters

To the right of the illustration is the "guts" of the inverter (shown in the inverter). The inverter uses three sets of high-speed switching transistors (shown in the IGBT) to create a DC "pulse" that simulates all three phases of the AC sine wave. These pulses determine not only the voltage of the wave, but also its frequency. The term inverter or inverter means "inversion" and is simply the up and down motion of the resulting waveform. Modern inverter inverters use a technique called "pulse width modulation" (PWM) to regulate voltage and frequency.

Then we talk about the IGBT, which stands for "insulated-gate bipolar transistor," which is the switching (or pulse) component of the inverter. Transistors (instead of vacuum tubes) serve two functions in our electronic world. It can act as an amplifier like an amplifier and increase the signal, or it can act as a switch by simply turning the signal on and off. The IGBT is a modern version that offers higher switching speeds (3000-16000 Hz) and reduced heat generation. Higher switching speed can improve the accuracy of AC wave simulation and reduce motor noise. The reduced heat generated means that the heat sink is smaller, so the frequency converter has a smaller footprint.

04 Inverter PWM waveform

The figure below shows the waveform produced by an inverter with a PWM converter compared to a true AC sine wave. The inverter output consists of a series of rectangular pulses with a 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 real AC wave. If you were to cut out the pulses above (or below) the real AC waveform and fill in the blank space below the curve with them, you would find that they match almost perfectly. It is in this way that the inverter can control the voltage of the motor.

The sum of the pulse widths and the blank widths 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., there are no gaps), the frequency will still be correct, but the voltage will be much larger than a true AC sine wave. Depending on the desired voltage and frequency, the frequency converter will change the height and width of the pulse and the width of the blank space between them.

Some people may wonder how this "fake" AC (actually DC) runs an AC induction motor. After all, do you need an alternating current to "sense" the current in the rotor of a motor and its corresponding magnetic field? Then, AC will naturally cause induction, because it is constantly changing direction, on the other hand, DC will not act normally once the circuit is activated.

However, if the DC is on and off, the DC can induce current. For those older, car ignition systems (before solid state ignition) used to have a set of points in the distributor. The purpose of these points is to "pulse" from the battery to the coil (transformer). This induces an electric charge in the coil, which then increases the voltage to a level that allows the spark plug to fire. The wide DC pulses seen in the figure above are actually made up of hundreds of individual pulses, and this on and off motion of the inverter output allows for this to occur through DC induction.

05 Effective voltage

One factor that makes AC complicated is that it constantly changes the voltage, from zero to some maximum positive voltage, then back to zero, then to some maximum negative voltage, and then back to zero again. How to determine the actual voltage applied to the circuit? The illustration below is a 60Hz, 120V sine wave. Note, however, 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 to 0 again. It continues down to -170 and then rises again to 0. The area of the original green rectangle with the upper boundary at 120V is equal to the sum of the areas of the positive and negative parts of the curve. So 120V is the average?

Well, if you were to average all the voltage values at each point throughout the cycle, the result would be about 108V, so it can't be the answer. So why is this value 120V as measured by VOM? It has to do with what we call the "effective voltage."

If you were to measure the heat generated by the DC current flowing through the resistor, you would find that it is greater than the heat generated by the equivalent AC current. This is due to the fact that AC does not maintain a constant value throughout the cycle. If performed in the laboratory, under controlled conditions, it is found that a particular DC current produces a 100 degree increase in heat, its AC equivalent will produce a 70.7 degree increase or 70.7% of the 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 squared voltages of the first half of the curve.

If the peak voltage is 1 and the individual voltages from 0 degrees to 180 degrees are to be measured, the effective voltage will be the peak voltage of 0-707. 0.707 times the peak voltage of 170 in the figure is equal to 120V. This effective voltage is also known as the root mean square or RMS voltage. Therefore, the peak voltage is always 1.414 of the effective voltage. The 230V AC current has a peak voltage of 325V while the 460 has a peak voltage of 650V.

In addition to frequency changes, even if the voltage is independent of the operating speed of the AC motor, the inverter must also change the voltage.

The diagram shows two 460V AC sine waves. Red is a 60hz curve, blue is 50hz. Both have a peak voltage of 650V, however, 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, since the area under the curve is proportional to the effective voltage, the effective voltage is higher. As the frequency decreases, the increase in effective voltage becomes more dramatic.

If 460V motors are allowed to operate at these higher voltages, their life can be greatly reduced. Therefore, the inverter must constantly change the "peak" voltage relative to the frequency in order 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 the inverter works and how to control the motor speed. Most drives allow the user to manually set the motor speed with a multi-position switch or keyboard, or automate the process using sensors (pressure, flow, temperature, level, etc.).