How does the inverter convert frequency?

Variable frequency operation has existed in the form of alternators since the advent of the automatic induction motor. Changing the speed of the generator changes its output frequency. Before the advent of high-speed transistors, this was one of the main ways to change motor speed, but frequency changes were limited because the generator speed reduced the output frequency rather than the voltage.

So let’s look at the components of a frequency converter and see how they actually work together to vary the frequency and motor speed.

01 Inverter Components-Rectifier

Since it is difficult to change the frequency of an AC sine wave in AC mode, the first job of the frequency converter is to convert the waveform to DC. It is relatively easy to manipulate DC in order to make it look like AC. The first component of all frequency converters is a device called a rectifier or converter, as shown in the figure below:

Frequency conversion rectifier

A rectifier circuit converts AC power to DC power in much the same way as a battery charger or arc welder. It uses a diode bridge to restrict the AC sine wave to move in only one direction. The result is a fully rectified AC waveform that is interpreted by the DC circuit as a native DC waveform. A three-phase inverter accepts three separate AC input phases and converts them to a single DC output.

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

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

02 Inverter Components-DC Bus

The second component, the DC bus (shown as DC bus in the diagram), is not seen in all VFDs because it does not directly affect VFD operation. However, it is always present in high-quality general-purpose VFDs. The DC bus uses capacitors and inductors to filter the AC "ripple" voltage from the converted DC power before it enters the inverter section. It also includes filters to block harmonic distortion that can feed back into the VFD power supply. Older VFDs and require separate line filters to complete this process.

03 Frequency Converter Components - Inverter

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

Then we come to the IGBT, which stands for "Insulated Gate Bipolar Transistor" and is the switching (or pulse) element of the inverter. The transistor (which replaced the vacuum tube) plays two roles in our electronic world. It can act as an amplifier and increase the signal, like an amplifier, or it can act as a switch, simply turning the signal on and off. The IGBT is a modern version that offers higher switching speeds (3000 - 16000 Hz) and reduces heat generation. Higher switching speeds increase the accuracy of the AC wave simulation and reduce motor noise. Less heat generated means smaller heat sinks and therefore a smaller inverter footprint.

04 Inverter PWM waveform

The figure below shows the waveform produced by the inverter of a PWM frequency converter 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 equals the effective voltage of the true AC wave. If you were to cut off the pulses above (or below) the true AC waveform and use them to fill the empty space below the curve, you would find that they match almost perfectly. This is how the inverter can control the voltage to the motor.

The sum of the width of the pulses and the width of the blanks between them determines the frequency of the waveform seen by the motor (hence PWM or pulse width modulation). If the pulses were continuous (i.e. no blanks) the frequency would still be correct, but the voltage would be much greater than a true AC sine wave. Depending on the desired voltage and frequency, the frequency converter will vary the height and width of the pulses and the width of the blanks between them.

Some of you may be wondering how this "fake" AC (which is actually DC) can run an AC induction motor. After all, doesn't an AC need to be present to "induce" current in the motor's rotor and its corresponding magnetic field? Well, AC will naturally cause induction because it is constantly changing direction. DC, on the other hand, will not because it will not operate normally once the circuit is activated.

However, DC can induce a current if it is switched on and off. For those of you old enough, automotive 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 which then raised the voltage to a level that allowed the spark plug to fire. The wide DC pulses seen in the image above are actually made up of hundreds of individual pulses, and this on and off motion of the inverter output allows induction to occur via DC.

05 Effective voltage

One factor that makes AC complicated is that it is constantly changing voltage, from zero to some maximum positive voltage, then back to zero, then to some maximum negative voltage, then back to zero again. How do you determine the actual voltage applied to a circuit? The illustration below is a 60Hz, 120V sine wave. But notice that its peak voltage is 170V. How can we possibly call it a 120V wave if its actual voltage is 170V?

In one cycle, it starts at 0V, rises to 170V, and then drops back down to 0. It continues to drop to -170, and then rises back up to 0 again. It turns out that the area of ​​the 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 level?

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

If you were to measure the heat generated by a DC current flowing through a 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, in a laboratory, under controlled conditions, a particular DC current was found to produce a 100 degree heat rise, its AC equivalent would produce a 70.7 degree rise, 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 an 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 are measuring the voltage at each angle from 0 to 180 degrees, the effective voltage will be 0-707 of the peak voltage. 0.707 times the peak voltage of 170 in the diagram equals 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. 230V AC current has a peak voltage of 325V, while 460 has a peak voltage of 650V.

In addition to the frequency change, the VFD must also vary the voltage even though the voltage has nothing to do with the speed at which the AC motor is running.

This graph shows two 460V AC sine waves. The red one is the 60hz curve and the blue one is the 50hz. Both have a peak voltage of 650V, however, the 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). And, 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 dramatic.

If a 460V motor is allowed to operate at these higher voltages, its life 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 how VFDs work and how they control motor speed. Most VFDs allow the user to manually set the motor speed via a multi-position switch or keypad, or use sensors (pressure, flow, temperature, level, etc.) to automate the process.