To optimize effectiveness and reduce the cost of operating
a tower, the use of adjustable speed drives (ASDs)
has become much more commonplace in recent years.
Data from a noted cooling tower manufacturer indicate
that ASD s are being installed in the majority of new
towers. Additionally, most towers being upgraded or
refurbished are also equipped with ASDs. ASDs have the
advantages of a soft mechanical start, no large startingcurrent
draw, and the ability to run the fan at any desired
speed from zero to the maximum design speed for the
application [4]. The energy savings realized by using an
ASD are well recognized and documented, so no further
discussion will be introduced here [5]. Two factors that
must be considered when applying an ASD are the critical
speeds of the mechanical system and the proper lubrication
of the gearbox at slow speeds. For practical
purposes, the fan is generally not run at speeds below
30% of the nominal design speed.