What is a Frequency Converter? And how it functions?

From the induction motor's inception, variable frequency operation has existed in the form of the alternating current (AC) generator. The frequency produced by a generator may be altered by adjusting its rotating speed. One of the few ways to adjust motor speed before the development of high-speed transistors, frequency modifications were restricted since slowing the generator decreased the output frequency but not the voltage. A little down the road, we'll examine why this matter. Variable speed pumping applications used to be far more difficult to implement in our field than they are now. One of the more straightforward approaches was to use a multi-pole motor with switches that could control the proportion of the stator's poles that were actually being used. The switches allowed both manual and sensor-based control of the rotational speed. This technique is still widely used in many variable flow pumping applications. Pool pumps, cooling tower fan and pump systems, and hot and cold-water circulators are all good examples. Some residential booster pumps include a pressure diaphragm valve that communicates with a fluid drive or variable belt drive system (essentially an automatic gearbox) to regulate pump speed. And there were a few more that were much more intricate.

The current frequency converter is a game-changer (pun intended) in the variable speed pumping environment because it eliminates the need for the hoop-jumping of the past. Changing the speed of a pump may now be done manually or automatically with the installation of a small electronic box (which commonly replaces more sophisticated starting equipment) at the site of application. Let's look at the parts that make up a frequency inverter and how they work together to change the frequency and, consequently, the motor's rotational speed.

Parts of a frequency converter

The Rectifier

A frequency converter's primary function is to transform an alternating current (AC) sine wave into a direct current (DC) before attempting to alter its frequency. As we'll see in a moment, AC can be faked with minimal effort via DC manipulation. Frequency converters begin with a device called a rectifier or converter.

The rectifier circuit functions similarly to that of a battery charger or arc welder, converting alternating current (AC) to direct current (DC). A diode bridge is used to restrict the alternating current sine wave to a single direction. A DC circuit will recognize the resultant completely rectified AC waveform as an authentic DC waveform in order to produce a DC output; three-phase frequency converters take in three AC input phases. As there are only two incoming legs, the frequency converter's output (HP) must be derated since the DC current generated is reduced correspondingly. Nevertheless, most three-phase frequency converters can also receive single-phase (230V or 460V) power. Nevertheless, real single-phase frequency converters (the kind used to regulate single-phase motors) take in just a single phase of power and output a DC voltage that is directly proportionate to the input voltage.

When it comes to variable speed operation, three-phase motors have two advantages over their single-phase counterparts. They have a far broader power range, for starters. Yet, the capacity to initiate rotation independently is also crucial. In contrast, starting a single-phase motor often needs an external force. 

The DC Bus

As it does not immediately aid in variable frequency operation, the DC Bus (shown in the middle) is not included in all frequency converters. However, high-quality, multipurpose frequency converters will always have this issue (those manufactured by Standard frequency converter manufacturers). Before the converted DC reaches the inverter portion, it is filtered by capacitors and an inductor to remove the AC "ripple" voltage. It may also use filters to prevent harmonic distortion from resonating back into the supply voltage of the frequency converter. Separate line filters are necessary for certain older frequency converters and for some pump-specific frequency converters.

The Inverter

The "guts" of the frequency converter are shown in the figure on the right. The three sets of high-speed switching transistors in the inverter generate DC "pulses" that are analogous to the three phases of an AC sine wave. The wave's voltage and frequency are both determined by these pulses. The word "inverter" is derived from the word "inversion," meaning "reversal," and describes the simple up-and-down motion of the produced wave shape. The voltage and frequency of a contemporary frequency converter inverter are controlled using pulse width modulation (PWM).