A Comparison of Variable Frequency Drives & Soft Starters | IConEST

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A Comparison of Variable Frequency Drives & Soft Starters


Lecturer Ahmad Alhaji, Kuwait Oil Company, Kwait


Despite the applications in which electrical motors are used for, the common challenge encountered in operating such equipment would be overcoming the Start-Up period. Such activity, Starting-Up a motor, involves a demand of high torque and large amount of energy in order to accelerate the motor’s rotor to full speed. The subject energy required by standard motors is (5-6) times theFull Load Amps(FLA) in what’s defined as anIn-Rush Currentphenomena. These excessive forces imprint undesired mechanical and electrical stress on motor’s rotors causing reduction of operational life time and higher maintenance expenditures. Therefore, a necessity to control / overcome such challenges was answered by two (2) Operational / Technical methodologies which are Soft-Starters (SS) and Variable Frequency Drives (VFD).Both technologies provide slow torque and low current starts, however, the basic foundations and theory of operations differs. Variable Frequency Drives controls electrical motors speed during start-ups to shut-downs by rectifying the input AC supply through the Rectifier, then cleaning / transferring the DC supply to the Invertor using DC Links and Filters. The Invertor works on reforming an AC sine wave using a serious of transistors in a formation known as Pulse Width Modulation Technology. The subject technology serves electrical motors commonly used for dispatch pumps, multispeed conveyors and ESP’s. However, Soft Starters controls the current and torque of electrical motors by varying its input voltage. Thyristors or Silicon Controlled Rectifiers installed parallel to the motor’s phases is used to serve poster pumps, fans and continuous speed motor applications. In conclusion, both applications have advantages and disadvantages in terms of cost, ease of operation, energy saving and enhancing equipment life time; yet, defining the requirement of an application is the key to choose between them (graphs, schematics and a comparison table shall be included in the manuscript).


vfd, softstarters, drives, variable frequency drives  


Alhaji, A. (2019). A Comparison of Variable Frequency Drives & Soft Starters. In M. Shelley & V. Akerson (Eds.), Proceedings of IConEST 2019--International Conference on Engineering, Science and Technology (pp. 28-30). Monument, CO, USA: ISTES Organization. Retrieved 29 February 2020 from www.2019.iconest.net/proceedings/26/.


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