The most common solution for driving the fan in
current cooling tower designs utilizes an induction
motor, driveshaft, disc coupling, and gearbox arrangement,
as shown in Figure 2. Few changes to this design
have been made in the last 20 years.
The motor employed has traditionally been a National
Electric Manufacturers Association (NEMA) induction
motor. For reduced energy consumption, two-speed
motors have been used when full fan speed is not required
due to decreased heat load. As the horsepower required to
drive the fan varies with the fan speed, it is advantageous
to reduce fan speed when possible. When the heat load
decreases enough, the two-speed drive motor can be run at
half speed. This lowers the horsepower required to only
12.5% of the rated value [2]. When the heat load requires
air flow in excess of that provided by half-speed operation,
a two-
speed motor must be run at full speed and full
horsepower as there is no other speed available. Two-speed
motors do provide some energy savings but must be
cycled on and off to maintain the desired water temperature.
This cycling involves many “across-the-line” starts,
drawing high amps and placing unnecessary strain on the
mechanical components of the system [3]. While providing
some flexibility in the tower control logic, two-speed
motors are not optimal when it comes to maximizing
energy savings during instances of reduced heat load.