Compare the starting currents of three methods and see how much energy slowing the motor down saves.
1️⃣ Starting currents — DOL · Soft Starter · VFD
Current as % of rated, the first seconds after start
Soft Starter: —
VFD: —
2️⃣ Start the motor — feel the difference
Press START and watch how hard each method “hits” the grid
3️⃣ Speed control with a VFD — saving energy
Lower the speed and watch the consumption drop
VFD — frequency control
* Starting currents are typical values: DOL ≈ 7×In, Soft Starter ≈ 3.5×In, VFD ≈ 1.2×In. The energy calculation is based on the affinity laws — the actual effect depends on your load profile. Our engineers will do the exact calculation for you.
Why does it matter how you start a motor?
Starting is the hardest moment in an electric motor's life. With direct-on-line starting (DOL) the motor draws 7 times its rated current from the grid — that is ~300 amps for a 22 kW motor, and over 1,700 for a 132 kW one. The consequences repeat on every single start: voltage dips across the whole shop, flickering lights, damaged contactors, overheated windings, and a mechanical shock to gearboxes, belts and shafts. The more often a motor starts, the more expensive "free" DOL starting becomes.
DOL, Soft Starter or a Variable Frequency Drive?
Direct-on-line (DOL) — the simplest and cheapest scheme. Justified only for small motors (≤7.5 kW) that start rarely and cannot shake the grid.
Soft Starter — reduces the starting current to ~3.5× and softens the mechanical shock. A good middle-ground solution, but after start-up the motor still runs at constant speed — it saves no energy.
Variable Frequency Drive (VFD) — the only method that solves both problems: the starting current is just ~1.2× (the grid barely notices), and while running, the speed follows the actual demand. That is where the real savings begin.
How much does a VFD really save?
Fans and pumps obey the affinity laws: power consumption follows the cube of speed (P ∝ n³). Slowing down by just 20% saves nearly half the energy — while dampers and throttles simply throw that energy away. On screw compressors a VFD removes the hidden losses of Load/Unload control: idling, the blow-down on every cycle, and an unnecessarily elevated pressure band. Use the simulator above to calculate your yearly savings with your own motor and your own tariff.
Frequently asked questions
Can a VFD be installed on an old motor?
In most cases — yes. Standard asynchronous motors work well with a VFD; on older motors we check the insulation condition and add a du/dt or sine filter where needed.
What size VFD do I need?
The basic rule: the VFD's rated current must exceed the motor's rated current. For heavy-starting machinery (e.g. a fully loaded conveyor) we go one size up. Our engineers will help you with the exact selection free of charge.
How fast does a VFD pay for itself?
On variable-load equipment (fans, pumps, compressors) the typical payback is 6-18 months — the simulator above will show you the exact figure. On top of that you gain extended motor and mechanical life.
👉 Browse variable frequency drives in the IESCO catalog — INOVANCE, DANFOSS and other leading brands, with local service.
