Basic VFD Selection Criteria

Nov. 2, 2008
Choosing a VFD for a centrifugal fan application is a complex decision that should take into account a number of factors:• Motor type. Is it a new or retrofit installation? Some older motors don’t have the proper insulation to withstand the pulse-width-modulation output of a drive. Select an inverter-duty motor if possible. Determine if an output line reactor and filter is required to protect the motor.• Distance limitations between the drive and motor.• Motor data — voltage, HP or kW, full load amps (always make certain that rated drive amps exceed motor full load amps), and base speed data.• Control method — local control (keypad), remote control via terminal strip, or serial communications.• Minimum and maximum allowable speed.• Acceleration and deceleration time.• May the load be reversed? (Does the unit require reverse lockout?)• Will the VFD need to “catch” a spinning fan when it starts?• Will DC injection braking be required to stop the fan before starting or at the end of the stop ramp? • Environment. A VFD is designed to withstand a standard industrial environment. Sometimes, however, extra precautions must be taken — such as use of an enclosure that includes climate control based on worst-case environmental conditions. Protection of the drive isn’t the place to cut corners if you want to ensure long, trouble-free life of the drive system.• Power quality. It’s good practice to perform a harmonics analysis to ensure that sensitive electronic equipment elsewhere in the building isn’t affected. VFDs and other non-linear loads (e.g., uninterruptible power supplies and electronic ballasts) can cause harmonic line distortion that could impact this equipment. A number of harmonics estimation programs are available on the Internet or from your vendor. If the analysis reveals high levels of harmonic distortion, you can install devices such as input line reactors, harmonics filters or isolation transformers directly before the drive.Trained fan professionals and VFD vendors can provide guidance on these matters.Source: TECO-Westinghouse Motor Co.

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