Inverter, Star/Delta, VFD or Soft Starter

How do you expect these devices to reduce your utility bill? Are you being billed on a demand?

There may be some cases in which you could run some pumps at decreased speed and thereby save energy. But there may not be much latitude for this, because if you slow them down too much they might not maintain enough static head.

Soft starters or VFDs may lessen the starting current and eliminate the star-to-delta transients. If it were a new installation VFD's would be a clearer choice; but as a retrofit you'll want to study the economics very closely.

GA, but also there is the issue of the fact that a reduction in measured current because of a change in power factor is NOT a reduction in the energy consumed by the load!

Brelvi,

VFDs save energy by matching motor power to load requirements, but at the cost of a few percent in efficiency because of losses in the VFD itself. So the only way the save real energy is if a lot of energy is being wasted in the system and adding the VFD can reduce that waste. If a motor is required to be running at full speed, a VFD is NOT going to save energy in fact it will increase the losses, so it will use MORE energy.

The best savings on VFD applications are only for variable torque (a.k.a. quadratic) loads such as centrifugal pumps and fans, where variable flow is part of the process. Running a motor at reduced speed will consume less energy than running it at full speed and throttling the output for the variable flow requirements. Throttling the output of a pump or fan also reduces the energy used by the square of the flow reduction, but changing the motor speed reduces the energy consumed by the cube of the speed reduction. There is a significant difference in the energy consumed in the middle of those two curves, but not much as either end. So how much you can save is totally dependent on how much load reduction is involved.

Soft starters and Star-Delta starters will not save any energy in the motors. Soft starters help with voltage drop at start-up and torque reduction for mechanical wear and tear avoidance, and they can "soft stop" pumps to help reduce water hammer, but they do not save energy in pumps. Some manufacturers claim so, but those claims are nothing more than "marketing spin" and it is untrue.

Star-Delta should be avoided at all times. It is bad for the motor, bad for the mechanical equipment, bad for the power grid and bad for the control components. The only thing it is good for is reducing initial installed cost because they are slightly cheaper than soft starters.