The survey of installed motors typically identifies a number of instances where motors are oversized or where a constant-speed motor is used to drive a load that varies. These applications offer an greater potential for energy savings through the application of a variable-frequency drive (VFD), which varies motor speed in relation to its load.
The use of partly loaded induction motors makes these units effective in reducing motor energy use. When operated at a constant speed, motor energy requirements fall off only slightly as the load on the motor decreases. But if the speed of the motor is reduced along with the load, motor energy requirements fall off significantly.
Since most motors in facilities operate most of the time under part-load conditions, annual savings from the use of a VFD can pay for the unit in less than one year.
VFDs also offer the advantage of reducing stress on the motor. When a motor starts, it is subjected to a high starting torque and current. When a motor connected to a VFD starts, it gradually ramps up to operating speed, reducing mechanical and electrical stresses.
James Piper, P.E., is a national consultant based in Bowie, Md., with more than 25 years of experience with facilities maintenance and management issues.
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