Jennifer Wilson posted on February 06, 2012 16:58
This is a guest blog post from Rick Heaton, an intern at the Environmental Resources Training Center at Southern Illinois University, Edwardsville.
Variable Frequency Drive (VFD) should be seriously considered when replacing motors or updating outdated equipment. Since motor-driven centrifugal pumps, fans, and blowers run at extended periods of time at high speeds, even slight reductions of speed can result significant energy savings. Without a VFD, motors run at higher speeds continuously, which can lead to excessive and unnecessary wear on motors and pumps.
The VFD changes the frequency to the electric motor to match output needs, allowing motors to run at slower speeds when demands are lower, resulting in less energy used. Running motors at slower speeds when demands are lower has several benefits.
The first is lower energy usage. Typically, VFD motors are powered by three-phase electricity where frequency of each phase is approximately 60 Hz. Changing the frequency to something less than 60 Hz during lower demands reduces motor running speeds, resulting in lower energy usage. The logic is that it’s much better to reduce electrical usage and running speeds than using control valves to control outputs. Many VFDs pay for themselves in a matter of months.
Secondly, there is less wear to motors and blowers. Across-the-line single-speed systems start motors abruptly, subjecting the motor to a high starting torque and to current surges that are up to 8 times the full-load current. Variable speed drives instead gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment. Typically, startup will be less than 2 Hz, then ramp up to its normal operating frequency; eliminating the need for a soft starter. Stopping the motor works the same, slowly ramping the frequency down to 0 Hz. An additional advantage to slower running speeds is the reduced wear on the pump and blower bearings, and electric motor bushings. The result is less maintenance costs, fewer parts replaced, and reduced labor in repairing and replacing these components.
A third advantage is that sensors and electronics can be interfaced with a VFD to control frequency needs. An example would be a dissolved oxygen sensor in an aeration basin communicating with the VFD, which in turn adjusts frequencies to slow down or speed up the electric motor running the blower.
A fourth advantage eliminates the need for valves and dampers to control output since the output is controlled by the speed of the motor. Valves and dampers can cause damaging backpressures that are eliminated with the VFD.
It’s important to be aware that not all VFDs are the same. Motors and VFDs must be compatible with one another. There are three major designs to suit differing needs: pulse width modulation (PWM), current source inverter (CSI), and voltage source inverter (VSI). An operator should consult with the manufacturers of both the VFD and the motor to be sure they will work together effectively.
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