Concept Of Variable Frequency Drive

The way that a VFD saves energy in a centrifugal load, such as a centrifugal fan, is by reducing the LOSSES in the system. So IF you are VARYING the flow to match the process you are using this for, then you have two basic options on varying that flow: mechanical restriction, such as dampers, and changing the motor speed such as with a VFD. Both methods reduce energy when flow is reduced, but a VFD reduces the energy MORE than the dampers will.

If you use a damper to reduce flow, there are losses associated with that method; fixed energy losses in the motor, friction losses as the air moves over the dampers, turbulence losses that are being converted to heat in the air stream, noise in the air (which, believe it or not, represents energy input), etc. But still, as you have observed, restricting the flow to 30% nets an energy reduction (we don't know what the energy input is at full flow, so we can't say how much).

When you reduce the flow by reducing speed, you are doing so without adding those comparative losses, plus you also reduce some of the fixed losses inside of the motor itself. The general formula always used is that the energy required by the fan reduces at the CUBE of the speed change. So for example at 50% speed, the energy input required by the fan from the motor is dropped to 12.5% (.5 x .5 x .5 = .125) of what it would have been at full speed. But speed does not directly equate to flow, so you can't assume that 50% speed = 50% flow, it doesn't work that way because you must overcome static pressure first.

Still, the vast majority of situations where a VFD replaces a damper system result in significant energy savings, but only IF there is VARIATION in flow rates. In other words you must have sometimes when you need 100% flow, other times when you need 75% flow, and times when you need only 30% flow. If you ALWAYS need the SAME amount of flow, then a VFD will NOT save you any energy compared to just using a smaller motor.

What we DON'T know in your situation is what your entire flow profile looks like and what your energy consumption is for that flow profile. Without that, nobody can help you determine in advance whether or not you will save energy.