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AC Frequency Converter [What is it?]

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A frequency converter or a frequency changer is an electronic device that transforms AC (Alternate Current) of a particular frequency to an AC of the other frequency. This device can also vary the voltage and it is subsidiary to its core purpose.

Initially, this device was an electromechanical machine known as a generator-motor set. With the arrival of solid-state electronics, it became possible to redesign frequency converters completely.

Frequency Converters – Basic Info

It comprises a rectifier that produces a Direct Current (DC), which is then inverted to generate AC frequency. The stated inverter can use IGBTs, IGCTs, or thyristors.

For voltage conversion, a transformer is also involved in the output circuit or in the AC input. It also isolates the AC i/p and o/p circuits.The addition of a battery can also take place, to improve outages in the i/p power.

Components of the AC frequency converter

The rectifier

As by the operation of the frequency changer, it transforms AC to DC, then transforms back to AC. So, it is the 1st main component.

Its working principle is the same as the battery charger. Diode bridges keep the sine wave traveling in a similar direction.

So, the AC wave is changed into DC completely. Likewise, the AC frequency converts into DC (zero value).

As the diagram below depicts, the converter gets 3 AC inputs and transforms them into the DC output. 3-phase converters may also receive 1-phase power, but, the o/p horsepower will be derated as the generated DC current is reduced.

Figure 1: 3-Phase bridge rectifier

DC bus

It is not really seen in all the frequency changers as it does not pay to the operation of variable frequency. Nonetheless, it will permanently be present in the general-purpose, high-quality frequency changers.

It uses inductors and capacitors to filter the ripple voltage before it gets into the inverter.

IGBT inverter

It uses converted DC into the variable frequency AC. PWM (Pulse Width Modulation) is used to control frequency and voltage in the up-to-date converters.

This inverter produces DC pulses via 3 pairs of high switching transistors, and these pulses simulate sinusoidal waves. The pulses not only determine the voltage but the frequency also.

An IGBT is a 3-terminal semiconductor device used as a switch and is included in the inverter & is used for fast switching and high efficiency. IGBT is able to produce switching speeds very high up to 16000Hz, with less heat generation.

Less heat indicates small radiators and smaller devices. Here is a good video on how PWM works in a VFD:

How Pulse Width Modulation works in a VFD

Power filter

It is a filter circuit that comprises inductance, resistance, and capacitance. This filter may filter a particular frequency, to attain a required frequency, or to reject a specific frequency.

EMI filter

It is typically a low-pass filter that is composed of shunt capacitors and series reactors. It permits the signal to come into the equipment, while working normally, rather than hinder the interference signal of high frequency.

Inverter output

Its o/p comprises rectangular pulses in series with an adjustable width and a fixed height. The addition of the area of the pulse gives the effective voltage.

The addition of the width and the blank spaces in them gives the frequency.

Figure 2: Inverter output.

Isolation transformer

Its purpose in frequency changer is to lessen the noise & to increase the S/N ratio. It is used to deliver AC power to devices.

It also isolates the load from the converter. It provides isolation of current to prevent electrical noise and electric shocks.

It permits AC components to pass and blocks DC. Isolation transformer having static shielding is used for the power source of delicate equipment like laboratory instruments, medical equipment, or computers,

Applications of the AC frequency converter

  • These converters change the AC power of one frequency to the other when two grids operate at dissimilar frequencies.
  • It is used to control the speed of AC motors like fans and pumps.
  • These are used in the airline and aerospace industries.
  • An HVDC system may assist as a frequency changer for a large number of loads.

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