How Do Frequency Converters Work?

Since the advent of the induction motor, variable frequency converters have been around in the form of AC generators. When you change the rotational speed of a generator, you can change the output frequency. In fact, frequency converters were used until high speed transistors were invented in order to vary motor speed. Unfortunately, frequency changes were limited due to generator speed reduction lowering the output frequency but not the voltage.

Frequency converters have revolutionized the way a variety of applications function. This is especially the case for the variable speed pumping environment which previously employed multi-pole motors. To better understand how frequency converters work, the next section will cover their various components and how they work together to vary frequency and motor speed.

The Rectifier

As it is difficult to change the frequency of an AC sine wave in AC mode, the frequency converter must convert the wave to DC; thus, the first component we will be looking at is the rectifier, or converter. The rectifier circuit converts AC to DC by utilizing a diode bridge to limit the travel of the AC sine wave to one direction only. This results in a fully rectified AC waveform that is interpreted by a DC circuit as a native DC waveform. Moreover, three phase frequency converters accept three separate AC input phases and convert them into a single DC output. True single phase frequency converters, on the other hand, use a single phase input and produce a DC output that is proportional to the input.

The DC Bus

The second major component of a frequency converter is the DC Bus, though it does not contribute directly to variable frequency operation and is not always available. If it is present, then the frequency converter is usually of high quality. In terms of operation, the DC Bus uses capacitors and an inductor to filter the AC “ripple” voltage from the converted DC before the voltage makes its way into the inverter section. In some cases, it may also have filters that prevent harmonic distortion that can feed back into the power source supplying the frequency converter. Keep in mind that older versions and those of which are pump specific necessitate separate line filters to do this.

The Inverter

The inverter utilizes three sets of high speed switching transistors to produce DC “pulse” that mimics all three phases of the AC sine wave. These pulses determine the voltage of the wave and its frequency. The term “inverter” means “reversal” and usually refers to the up and down motion of the generated waveform.  Generally, modern frequency converters take advantage of a Pulse Width Modulation (PWM) technique to regulate voltage and frequency. You should also become familiar with the term “Insulated Gate, Bipolar Transistor” (IGBT) which is a switching or pulsing component of the inverter. IGBTs serve as modern versions of transistors that provide higher switching speeds and reduced heat generation.

Inverter Output

Generally, the inverter output consists of a series of rectangular pulses with fixed height and adjustable width, and the sum of the areas of the pulses equals the effective voltage of a true AC wave. Moreover, if you chop off portions of the pulses above or below the true AC wave and use them to fill in any blank spaces above or below the curve, they match almost perfectly. This is how a frequency converter controls the voltage going to the motor.

Effective Voltage

If you measured the heat generated by a DC flowing through a resistance, you would find that it is greater than the heat produced by an equivalent AC. This is because the AC does not maintain a constant value throughout its cycle. For example, under the controlled conditions of a laboratory setting, a DC with a heat rise of 100 deg would be equivalent to an AC with a heat rise of 70.7 deg, or 70.7% of the DC value; thus, the effective value of AC is 70.7% of that of DC. Furthermore, the effective value of an AC voltage is equal to the square root of the sum of the squares of the voltage across the first half of the waveform.

Conclusion

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February 28, 2024

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