Fuji Electric - How exactly does a frequency converter work?

Since the advent of automatic induction motors, variable frequency operation has existed in the form of alternators. Change the rotational 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 the frequency change was limited because the generator speed reduced the output frequency instead of the voltage.

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

01 Inverter Element - 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 an AC, it is relatively easy to operate the DC. The most important component of all frequency converters is a device known as a rectifier or converter

The 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 restrict the AC sine wave from moving in only one direction. The result is a fully rectified AC waveform interpreted by the DC circuit as a local DC waveform. A three-phase inverter accepts three independent AC input phases and converts them into a single DC output.

Most three-phase drives can also accept single-phase (230V or 460V) power, but since there are only two input branches, the converter output (HP) must be derated because the DC current generated 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 produces a DC output proportional to the input.

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

02 Inverter Element - DC Bus

The second component of the DC bus, which is not visible in all drives, does not directly affect the inverter operation. However, it is always present in high-quality universal drives. The DC bus uses capacitors and inductors to filter out the AC "ripple" voltage from the converted DC and then into the inverter section. It also includes a filter that prevents harmonic distortion and can be fed back to the inverter power. Older drives and separate line filters are required to complete this process.

03 Inverter Element - Inverter

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

Then let's talk about IGBTs, which refer to "insulated-gate bipolar transistors", which are the switching (or pulsing) elements of an inverter. Transistors (instead of vacuum tubes) serve 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 and simply turn the signal on and off. The IGBT is a modern version that offers higher switching speeds (3000 - 16000 Hz) and reduced heat generation. The higher switching speed can improve the degree of AC radio wave simulation and reduce the motor noise. Less heat generated means smaller heat sinks and therefore a smaller drive footprint.

04 PWM waveform of inverter

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 true AC wave. If you were to cut out the part of the pulse above (or below) the true AC waveform and fill in the empty space below the curve with them, you would see that they almost *** matched. It is in this way that the inverter can control the voltage of the motor.

The sum of the pulse widths 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 pulse is continuous (i.e. there are no gaps), the frequency is still correct, but the voltage will be much larger than a true AC sine wave. Depending on the voltage and frequency required, the inverter will change the height and width of the pulse and the width of the gap between the two.

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

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

05 Effective voltage

One factor that complicates alternating current is that it constantly changes the voltage, from zero to some *** large positive voltage, then back to zero, then to some *** large negative voltage, and then back to zero.

If you were to measure the heat generated by a DC current flowing through a resistor, you would find that it was 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 it is carried out in the laboratory, under controlled conditions, and it is found that a particular DC current produces a 100 degree heat rise, its AC equivalent will produce a 70.7 degree rise or 70.7% DC value. So the RMS value of AC is 70.7% of DC. It can also be seen that the RMS 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.

You should now have a good understanding of how a drive works and how to control the speed of your motor. Most drives allow the user to manually set the motor speed via a multi-position switch or keypad, or to automate the process using sensors (pressure, flow, temperature, level, etc.).

Fuji Electric - How exactly does a frequency converter work?

Fuji Electric - How exactly does a frequency converter work?

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