The Energy Efficiency Factor when deciding between VSD or Soft Starter.
This is where the comparison becomes most important for running costs.
A soft starter saves energy only during the brief starting period. Once the motor is running, it draws the same power it would on a DOL starter. If your application runs a pump or fan at full speed all day, a soft starter does nothing for your electricity bill during those running hours.
A VSD, by contrast, can deliver dramatic energy savings continuously. For centrifugal loads (pumps, fans, blowers), power consumption follows the cube law — meaning small reductions in speed produce large reductions in energy use. A fan running at 80% speed uses only about 51% of the energy it would at full speed. Over a year of continuous operation, this saving typically pays back the higher cost of a VSD within 12 to 24 months.
In South Africa’s current energy cost environment, this payback case is stronger than ever.
The Decision in Plain Terms
| VFD/VSD | Soft Starter | |
|---|---|---|
| Controls starting | ✓ | ✓ |
| Controls speed during operation | ✓ | ✗ |
| Energy savings at startup | Moderate | Moderate |
| Upfront cost | Higher | Lower |
| Long-term running cost | Lower | Higher |
| Process / Speed control | Yes | No |
| Best for | Variable-speed loads | Fixed-speed loads |
