Also known as Inverters, AC Drives, and Adjustable Frequency Drives (AFD’s), VFD’s operate by varying the frequency and voltage to AC motors. The frequency of the applied power to an AC motor determines the motor speed. The variable speed can be used in certain applications for improved efficiency, energy savings, and to optimize the motor-driven process. The energy savings can be dramatic.
For example, the speed of a centrifugal fan reduced by only 1/3 will save over 70% in energy!
VFD’s can also extend the speed range of a motor by applying adjustable frequencies up to 120 Hertz providing twice the speed in a 60 Hertz motor. At speeds below 60 Hertz, torque is constant and HP diminishes proportionately to speed. At speeds above 60Hertz, HP is constant and torque gradually diminishes to roughly 66% of maximum at 120 Hertz.
Operation of motors at speeds above their rated (nameplate) speeds must be checked and approved by the motor manufacturer. Motor cooling is usually not a concern but mechanical balance, rotational stresses and bearing life are important considerations. Inverter duty motors are in many cases recommended.
Most drives are programmable and have internal adjustments which allow the user to select operating conditions most suitable for his application. Some common applications for VFD’s are pumps, fans, mixers, blenders, air handlers, and conveyor systems.
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