Soft Starters vs VFDs: An Energy Comparison
When a motor needs gentler starting than direct connection provides, two devices are commonly considered: the soft starter and the variable frequency drive. Both ease the strain of starting, but they differ fundamentally in what they offer during running, and understanding this difference is key to choosing correctly and to realising energy savings where they exist.
What each device does at start
Both a soft starter and a variable frequency drive reduce the harsh inrush and mechanical shock of starting a motor directly. A soft starter gradually increases the voltage to the motor, ramping it up to full speed smoothly before stepping aside and letting it run directly. A variable frequency drive instead controls the frequency and voltage together, accelerating the motor gently to speed. In terms of starting alone, both achieve a smooth, controlled start.
The crucial difference during running
The decisive distinction appears once the motor is up to speed. A soft starter, having done its job, allows the motor to run at full speed thereafter; it does not control speed during operation and offers no energy saving from speed reduction. A variable frequency drive continues to control the motor throughout operation, able to vary its speed to match demand. This ongoing control is where the drive’s energy-saving potential lies.
Energy implications
For applications where the load always needs full speed, a soft starter provides gentle starting without the cost and complexity of a drive, and there is little energy to be saved by varying speed. For applications where the demand varies and could be met at reduced speed, a variable frequency drive can save substantial energy by slowing the motor, especially on pump and fan loads. Here the drive’s continuous control pays for itself; the soft starter cannot.
Choosing between them
The choice therefore hinges on whether variable speed offers value. If the goal is simply to start a motor gently and it will always run at full speed, a soft starter is simpler and cheaper. If the load varies and could benefit from speed control, a variable frequency drive offers both gentle starting and ongoing energy savings. Matching the device to the actual operating profile avoids both overspending and missed savings.
Soft starters and variable frequency drives both tame the shock of starting, but only the drive controls speed during running and can save energy where the load varies. Recognising that a soft starter offers no running energy saving, while a drive can, ensures the right device is chosen: the soft starter for gentle starting alone, the drive where varying speed to match demand turns into real energy savings.
