Variable Frequency Drives

A variable frequency drive (VFD) is an electronic controller that adjusts the speed of an electric motor by modulating the power being delivered. These drives provide continuous control, matching motor speed to the specific energy demands. In other words, the VFD controls the rotational speed of an AC electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive has the potential to be a significant cost-saving device for water and wastewater facilities. Variable frequecy drives are an excellent choice for adjustable speed drive users because they allow operators to fine-tune processes while reducing costs for energy and equipment maintenace.

Variable frequency drives are used in a wide number of applications to control pumps, fans, hoists, conveyors, and other machinery. They are enjoying increasing popularity at water and wastewater facilities, where the greatest energy draw comes from pumping and aeration-two applications particularly suited to variable frequency drives.

For applications where flow requirements vary, mechanical devices such as flow-restricting valves or moveable air vanes are often used to control flow, which is akin to driving a car at full throttle while using the brake to control speed. This process uses excessive energy an may create punishing conditions for the mechanical equipment involved. Variable frequency drives enable pumps to accommodatd fluctuating demand by running pumps at lower speeds and drawing less energy while still meeting pumping needs.

As shown in Figure 5.3, a variable frequency drive systems generally consists of an AC motor, a controller, and an operator interface. Variable frequency drives work with most threee phase electric motors, so existing pumps and blowers that use throttling devices can be retrofit with these controls. Delta VFD can also be specified for new equipment.

A huge advantage of variable frequency drives is that they give a single-speed drive motor a “soft-start” capability, gradually increasing the motor to high torque and current surges up to 10 times the full-load current. If normal operating load current is 10 apms, for example, that motor could draw 100 amps at start. The VFD soft-start capability lessens mechanical and electrical stress on the motor system, can reduce maintenance and repair costs, and can extend motor life.

In addition to their soft-start capability, variable frequency drives allow more precise control of process such as water distribution, aeration, and chemical feed. Pressure in water distribution systems can be maintained to closer tolerances. Wastewater treatment plants can consistently maintain desired dissolved oxygen concentrations over a wide range of flow and biological loading conditions by using automated controls to link disolved oxgen sensors to variable frequency drives on the aeration blowers.

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