Adjustable or variable frequency drives — AFDs or VFDs — use rectifiers to convert ac powerline electricity into direct current. Then inductor-capacitor filters smooth its waveforms and pulse-width-modulation inverters turn it into shaped ac power with a frequency of 2 to 12 Hz. This output is used to drive motors for constant torque or (with the help of encoders) to control motor speed. VFDs are proliferating, as they save substantial amounts of energy during motor operation — an objective of increasing importance.

Plug-and-play motor control

Some VFDs have built-in ac and dc brake functions to help transform kinetic energy from the application into braking power.

VLT variable frequency drives from Danfoss Drives, Loves Park, Ill., are also easy to install: Users simply connect the motor and power cables, turn the control knob, and watch motor speed change. An automatic energy optimizer function makes the drive more efficient; it can temporarily supply 150% more torque (for up to a full minute) to eliminate the need to oversize the drive. An M5 frame size was recently released to cover 380 to 480 Vac, for 25 to 30 hp.

VFDs keep things cool

Cooling towers, used in commercial and industrial designs, are heat exchangers (14 to 28 ft in diameter) that remove waste heat from a system's water or other fluid. The motors that power these fans typically operate at about 1,800 rpm. However, fan speeds — determined by blade diameter — are slower to keep the blade tips subsonic, typically 90 to 230 rpm. For this reason, traditional tower fans use gearboxes, drive shafts, couplings, and impellers between fan and motor to reduce speed.

Now, variable-frequency control is used to slow motor speed directly, and eliminate inefficient mechanical parts. New cooling-tower drive controllers and RPMAC permanent-magnet motors from Baldor Electric Co., Fort Smith, Ark., are designed to fit existing gearbox mounts for retrofits. Permanent magnet control further boosts efficiency, even for a fan's light load and speed.

Baldor's cool-running electric motor and controls are the same used to power most modern hybrid vehicles: Laminated steel frame plus high flux-strength Neodymium Iron Boron salient-pole rotors output high torque at slow speed from a low-profile motor that can be mounted directly to the fan and operated at variable speed for efficiency.