VFD Optimization

Hello, this is Saurabh Gupta, Product Marketing Engineer for Baltimore Aircoil Company. Let’s talk about VFDs and maximize energy savings using VFDs.

VFD stands for variable frequency drive. The VFD is a device that controls the speed and power drawn by various electrical components in a mechanical system. To optimize the system, we need to consider a few critical points including overall cooling capacity, VFD cycling and size and start-up sequence. Keep in mind that the system will be operating below design conditions 99% of the total hours in a year. Proper system design and application of VFD can result in significant savings during these “part load hours”.

The most important factor in optimizing a system using a VFD is to understand your application and the specific desired characteristics of the system. If a cooling tower is designed for a 900 ton application with a 60 HP motor, the VFD should be selected based on the exact motor parameters. Any mismatch will lead to an imperfect system.

The second set of factors are control strategy and system loading schedule. To optimize the system, we need to have smooth electrical loading. This will ensure lower electrical consumption at lower load levels when compared with erratic loading as with fan cycling control sequence.

In evaporative condensers, the control strategy should optimize energy consumption between fan motor horsepower, pump horsepower and floating head pressure control. As the head pressure drops, the system efficiency increases. However, running the condenser fans consumes electricity. The system designer needs to determine what is a minimum acceptable head pressure and condensing temperature. Once you know these two parameters, then sequencing the fans on the VFD becomes a simple exercise.

In cooling towers, once again the system designer needs to provide exact system parameters so that one does not shock the system with overcooled water from the tower.

Finally, we need to follow a “parallel” orientation to ensure equal loading. “Parallel” implies that all fan motors or multiple units should be wired to accept equal loading. For example, if the installation has two units and the current requires 45% load, one fan motor should not run at 90% while the other fan motor remains idle, but instead each fan motor should see 45% loading. This will improve efficiency and increase lifetime of the critical components. The same principle applies to multi-cell closed circuit cooling towers or condensers.

These are major factors that determine the operational efficiency of a unit using a VFD. If you have other ideas or have employed other methods to increase efficiency, we would love to hear about them.

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