Why the motor starting method matters
Starting a three-phase induction motor directly on the supply can draw several times its rated current and apply a sudden mechanical torque that stresses belts, pumps and bearings. Motor control devices reduce that shock, and the two most common choices are the soft starter and the variable frequency drive (VFD).
Soft starter
A soft starter gradually raises the voltage applied to the motor at start-up and then bypasses once the motor reaches speed, limiting inrush current and mechanical shock.
- Lower cost and simpler to install and maintain.
- Well suited to pumps, fans and conveyors that run at a fixed speed.
- It does not change running speed; it only manages starting and stopping.
Variable frequency drive (VFD)
A VFD changes the frequency and voltage supplied to the motor, so speed can be regulated continuously while it runs, in addition to controlled starting and stopping.
- Precise control of speed and torque to match the process.
- Significant energy savings on pump and fan loads running at partial speed.
- Higher cost, and it needs attention to harmonics, cooling and earthing.
How to choose
- If the load runs at constant speed and you only need to limit starting current, a soft starter is the economical choice.
- If the process needs speed variation, energy savings or precise torque control, a VFD is the better fit.
- Always confirm motor ratings, operating environment and applicable standards with a qualified engineer before the final selection.
This article is general technical information and does not replace an engineering study of your application.
