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2. Inverter stage:

Electronic switches - power transistors or thyristors - switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency. The amount of distortion depends on the design of the inverter and filter.

3. Control system:

An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex control functions.

Converting direct current (DC) to variable frequency alternating current (AC) is accomplished using an inverter.

Since an induction motor rotates near synchronous speed, the most effective and energy-efficient method to change the motor speed is to change the frequency of the applied voltage. Variable frequency drives (frequency changers) convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed.

A VSD (frequency converter, variable speed drive, variable frequency drive, VFD) converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage and the inverter stage. The conversion process of frequency changers incorporates three functions:

1. Rectifier stage:

A full-wave, solid-state rectifier converts three-phase 60 Hz power from a standard 208, 460, 575 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers if higher supply voltages are used. (to be continued)

Variabl frequency drives (VFDs, variable speed drives, VSDs) are an excellent choice for adjustable-speed drive users. They allow drive users to fine-tune processes while reducing costs for energy and equipment maintenance.

At water and waste-water facilities, the applications of variable frequency drives (VFDs) are so popular.

Figure below illustrates the reduced energy consumption of Variabl frequency drives (VFDs, variable speed drives, VSDs) over valve control systems at water and waste-water facilities.

For applications where flow requirements vary, mechanical devices such as flow-restricting valves or moveable air vanes are often used to control flow, which is akin to riving a car at full throttle while using the brake to control speed.

Frequency inverter, in different countries, exists different names, including frequency changer, frequency converter, variable speed drive, variable frequency drive, adjustable speed drive, adjustable frequency drive, ac drive, variable voltage variable frequency drive, vvvf drive, even microdrive.

A frequency inverter is an electronic device, that converts alternating current (AC) of one frequency to alternating current (AC) of another frequency. Frequency inverter may also change the voltage, but if it does, that is incidental to the principal purpose.
The image of frequency inverter from GreenDriv Tech Limited

But where's the market for frequency inverter?

I think different demands exists in different areas. The requirements for frequency inverter in Japan may be different from that in India. The requirements for frequency inverters in USA are different from Mexico. So different qualities meet the demands of different countries because of the cost.

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The motive power can be reduced when the speed of the fan and pump is reduced to control the air volume and flow rate.

Example of the machine for which the frequency inverter can save energy.

Feed water pump, air conditioning pump, plant water pump, drain pump, boiler water pump, etc.
Ventilation fan, cooling tower cooling fan, air conditioning (air supply and exhaust) fan, etc.

The majority of HVAC applications where ac drives are used are fans, pumps and compressors. Applying ac drives for fans, pumps and compressors, energy saving is large. 40% of all energy in Europe and North America is consumed in buildings. Thus energy consumption by buildings is dramatically reduced with AC drives.

In heating, ventilation and air conditioning (HVAC) applications, the main processes are related to heating, cooling, drying and circulating air. Supporting processes are mostly related to taking the extra heat out of the building, or providing additional heat energy to the building.

The majority of HVAC applications where ac drives are used are fans, pumps and compressors.

Ac Drives for Fans and pumps of HVAC applications:

Using ac drives to control the fan or pump output rather than using dampers, vanes, valves or on/off control brings substantial energy savings, if the required output is less than nominal most of the time.

Energy savings are achieved by optimising the system setup of compressor, chilled water circulation and condenser water circulation. The optimum set point for chilled water temperature and condenser water temperature is based on outdoor and indoor temperature and humidity.

The types of motors that ac drives control are normally operating at constant speed. Enabling the user to control the speed of motor potentially gives ac drive various benefits in terms of process control, system stress and energy savings.

Advantage of frequency inverter for process control:

controlling the process output to match the need; synchronising different parts of the main process to secure smooth flow between subprocesses; easily changing the setup when the process requirements change.

Advantage of frequency converter in system stress:

Reducing the start-up current, which allows use of smaller fuses and supply connections and reduces peak loads on the electrical network; Reducing the mechanical shock in start and stop situations.

Advantage of ac drive for energy conservation:

One of the most important tasks in oil refinery management is cost reduction, particularly the technological component of processing costs. Power comsumption in oil refinery management is large. Application of frequency converters (ac drives) is one way to achieve this goal because the advantage of applying frequency inverters (Variable frequency drives) is for energy conservation.

Application of frequency converters helped to significantly improve the quality of air cooler control by means of regulation of the fan rotation speed by eliminating negative factors which affected the operation of air coolers due to ambient temperature fluctuations. The dynamics of air cooler power consumption has also changed.