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Basic theoretical knowledge of VFD
Jan 14, 2019

For more than 20 years, frequency converters (VFDs, also known as variable speed drives - VSD) have been used in a variety of applications. These applications include single-motor driven pumps, fans and compressors, as well as highly complex multi-drive machines.

The function of the VFD is to adjust the motor speed to better meet the requirements of the drive load. The result is better process control and reduced energy consumption. VFD systems are quite expensive, but provide excellent control over plant operations. In most cases, the cost of energy is reduced. Installing a VFD is enough to offset (if not completely) the high initial cost.

VFD Basic theoretical

The VFD converts AC power from the input power source to a DC voltage. The VFD may also include a control mechanism through which the motor and the slave load are communicated remotely.

VFD Basic theoretical2


Variable frequency drive type

There are currently three different types of VFD designs on the market. In theory, the main difference between these VFDs is the type of rectification they convert from AC power to DC power.

The most common centrifugal load applications for using frequency converters are pumps, fans and compressors. The most common constant torque load applications using VFDs are positive displacement pumps, gear pumps, reciprocating pumps, conveyors, extruders, extruder cutters, mixers and reciprocating compressors. The main considerations when choosing a VFD are: The nature of the application, Cost, and Operating environment.

VFD has many basic uses as a special process control that is not common in other types of shift control. When energy saving is the primary goal, VFD is best suited for centrifugal loads. Due to its flexibility in automatic process control systems, VFDs are also often used for constant torque loads. VFDs are very sensitive to applications, so you must thoroughly analyze application compatibility before choosing a VFD.


Voltage Source Inverter (VSI) Inverter

VSI or variable source inverters are the first solid state VFDs. Due to the voltage sent to the motor, it is sometimes referred to as a "six-step" drive.

VFD Basic theoretical3

Variable source inverter operation is relatively simple. Basically, the AC input voltage and frequency are converted to DC by the rectifier and then converted back to AC through the inverter section to produce the desired voltage and frequency to meet the voltage-to-hertz ratio of the VFD output.

VSI's advantages include a good speed range, multi-motor control of a single unit and a simple regulator design.

Some disadvantages include: As the speed decreases, the power factor decreases. Inducing harmonics, and Start/stop the action directly.

Looking closely at the displayed waveform, you can see how the "six-step" power causes the cogging effect.

VFD Basic theoretical4

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