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Choosing The Right Variable Frequency Drive For Your Application

Updated: Aug 25, 2020

Another one of our frequently asked questions' is "How do I decide on a Variable Frequency Drive (VFD) for my application?". The selection criteria detailed below is an essential prerequisite before you make your final decision on which VFD will be the most suitable.

Examine the Motor and VFD Technical Specifications

To ensure high performance of the VFD, the motor data must be known. The VFD has to be selected based on the rated motor current or the motor horsepower(HP) rating where the VFD's rated output current (or HP rating) must be greater than or equal to the motor's rated current (or HP rating). Motor data can be obtained from the motor nameplate which offers a great deal of information such as:

  • Type of Motor (Three-Phase Asynchronous Motor)

  • Rated Motor Voltage

  • Rated Motor Current

  • KW / HP Rating

  • Rated Motor Speed

If several motors are to be connected in parallel to the output of the VFD, the motor currents should be added geometrically, i.e. separately by active and reactive current components. A VFD with a sufficient rating that is capable of supplying the total required current should hence be selected.

Research your Application Requirements

Understanding what type of control your application needs is the next step in determining an appropriate solution. There are several types of VFD's available in the market, each built to serve a specific purpose and provide a distinct type of control. The following briefly summarizes the different types of VFD's available and when these type of drives should be considered in an application:


I. V/Hz (Volts per Hertz) Variable Frequency Drives

These drives are used very frequently and are applicable for general-purpose applications that do not require a large starting torque, full motor torque at low speeds, and speed feedback loop control, i.e. pumps, blowers, conveyors and others. A single V/Hz drive can command several motors and come equipped with multiple digital and analog I/O's along with several programming features that can be adapted to suit one's application.

II. Sensorless (Open-Loop) Vector Variable Frequency Drives

These open-loop vector control drives are way more complex than V/Hz drives in terms of programming and are useful for high accuracy applications where a large torque at low RPM is required. One VFD will drive only one motor, i.e. machine tools, lathe, centrifugal machine, injection molding machine, and others. If the requirements of the application exceed the programming capabilities of a sensorless vector control VFD, then the next alternative would be a closed-loop vector control VFD.

III. Closed-Loop Vector Variable Frequency Drives

This category of high-performance VFDs provides very precise speed regulation and superior torque performance with extremely versatile programming features. Closed-loop vector control VFD's are used in instances where a pulse encoder is installed to monitor feedback from the motor and can operate exceptionally well when properly utilized for the suitable application.

Consult your VFD distributor or manufacturer prior to making a firm decision about your VFD purchase and they will assist you in your inquiries and provide you with a good drive recommendation.

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