On cooling towers, Variable Frequency Drives (VFDs) eliminate many of the drawbacks associated with starter-controlled fans. There are many benefits, including reduced energy consumption, resulting in lower utility costs; reduced maintenance requirements which decreases personnel & equipment replacement costs; and process water temperature stabilization.
The fan may be spinning when a VFD is commanded to start. A VFD must correctly identify motor rotation, slow the motor down to zero speed (when opposite rotation is detected), accelerate the motor in the correct direction and not trip on an over-voltage or over-current condition. Mechanical brakes or anti-ratcheting devices can be used to ensure that a fan doesn’t rotate in the wrong direction. A VFD eliminates mechanical and electrical brakes as well as anti-ratcheting devices, time delay relays, etc. In extremely cold weather, tower icing can be averted by running the fan more slowly than required, raising the tower and process water temperatures. It is also common to reverse a cooling tower fan, keeping the heat in the tower.
VFDs accomplish this function & eliminate reversing starters. Likewise, on hot days, when the air is thinner, fans can be run above 60 Hz, providing additional cooling capacity. The VFDs current and/or torque limit function will limit the current of the motor such that the nameplate FLA rating is not exceeded. This is impossible without a VFD.
On fan loads, the HP requirement varies as the cube of the speed, so the slower the fan speed–the less energy required. A fan running at 80% speed will consume only 50% of the power of a fan running at full speed. At 50% fan speed, power consumption is only 16%. A minimum speed of 20-25% is usually possible on an existing motor. For direct connected or belted cooling tower fans, this minimum speed is usually not a concern. However, when a gearbox is used, the minimum speed is more critical, as the gearbox may depend on an internal oil slinger for lubrication. Consult the manufacturer of the cooling tower for minimum speed requirements.
VFD controlled cooling tower fans operate over many speeds as opposed to the fans on a single or two-speed motor starter. As such, it is a good practice to perform a vibration analysis on the fan and tower assembly, as a mechanical resonance may develop at certain speeds. Identified problem speeds may be programmed into the drive and “locked out”. Vibration switches are often wired into a drive or control system, shutting down the cooling tower if a problem arises.
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