I feel compelled to clear up a few myths here (dons Myth Busters cap).
1) Reducing startup current will
NOT, repeat
NOT save
ANY energy! Current, yes. Energy, as in kWH, NO! You cannot fool mother nature. It takes a given finite amount of energy to make something move from one place to another, including turning a cylinder at rest into a rotating one. There is NO WAY AROUND THAT! When you start Across-the-Line (DOL for your IEC types), the time/energy curve is very steep, but very short. When you start with a soft starter or VFD, it is very low, but very long. No matter what, the area under the curve, representing the amount of energy consumed to move it from a stand-still to the desired speed, is exactly the same total. There is a minute amount of theoretical savings in using a VFD because of what DickDV said, the VFD allows the motor to start at a higher power factor, so more of the energy being used is doing useful work, but at the same time, the VFD has conversion and switching losses, so in general is evens out, maybe a little to the plus side for savings, but nothing to get excited about.
2) Starting current is, 99.9999999999% of the time, NOT seen by the utility demand meter on something like a conveyor system, or any other load that gets to speed in under 15 MINUTES! This is because the demand meter has what is called a 15 minute sliding demand window. Any change in
power (not current, because they don't bill on current) is not recorded until it has remained changed for 15 minutes. Some utilities even use a 30 minute demand window. There are a very few, and by that I mean under 50, small secondary utilities across the US that have what are called "ratcheting instantaneous demand meters" installed. These are second tier utilities who buy their power from larger ones and resell it, usually in rural areas. There are so few that it is not worth considering for the most part. Unfortunately, the snake-oil sellers who try to sell you "energy savers" seize upon the existence of these few utilities as a reason why you should be concerned about starting current, but the reality is that for 99.9999999% of us, it makes absolutely NO difference.
3) Any AC motor pulls whatever energy it needs to do the work required of it, plus a little bit in losses. Energy efficient motors can
ONLY deal with the little bit of losses, they CANNOT change the fundamental amount of energy it takes to do the work. The same holds true for VFDs, except for one very important difference.
If the application
CAN require less work
and that lower workload can be done at a slower motor speed, then using the VFD to change speed instead of a gearbox will end up modifying the motor's energy consumption to match the load requirements. These are a lot of "ifs" and cannot be taken flippantly, but in theory, if you can slow down your conveyors and still get the same total amount of work done, you
might save energy.
