The biggest "green lie" of all is that VFD's "save energy". When the pumps speed is reduced using a VFD, the Affinity Law states, "horse power is reduced by the cube of the RPM." Many people use this to try and show energy is being saved, by reducing the RPM. This is absolutely not true (See figure #3)).

Figure 3
When running the pump at full speed and maximum flow, the energy used by the VFD itself, along with motor losses from the VFD's sub-standard voltage wave form, causes more energy to be used per gallon. //

The only real energy savings, comes from the system that will last the longest. It takes a lot of energy to mine, manufacture, transport, and install pumps and controls. The longer a system last, the more energy and money is saved. It is very harmful to the environment to install a system that was designed to only last a short time. Our landfills are being filled with electronic components like VFD's, and non-recyclable pumps and motors. This waste a lot of energy and natural resources, leaches heavy metals into our water supplies, and drains money out of your bank account.

Create or De-rate?
When the flow is reduced, the amps drawn by a pump controlled by a VFD, and a pump controlled by a CSV are almost the same. The difference is that a VFD creates a smaller motor from a larger one, and a CSV de-rates the motor load.

When a VFD slows down a 10 HP motor, until it is only drawing a 5 HP load, it has "created" a 5 HP motor from the 10 HP. Even though the motor is only drawing half of a 10 HP load, the harmonic content produced by the VFD increases heat in the motor. The motor must be cooled as if it were a fully loaded 5 HP, which still requires 1/2 a foot per second flow. According to the submersible motor cooling chart, a 6" motor in an 8" casing needs a minimum of 45 GPM to remain cool. So a 10 HP pump and motor in 8" casing, cannot be slowed to less than 45 GPM, or the motor will overheat and be destroyed.

The same 10 HP motor controlled by a CSV doesn't need as much flow to remain cool. This is because the 10 HP motor has been "de-rated" to a 5 HP load. When a CSV restricts a 10 HP pump, the amps can also drop to a 5 HP load.