A soft start device eliminates the many problems arising from across the line starts of a large motor. So does an electronic motor drive. Why choose one over the other?
The decision to install a soft start is often based on sparing the distribution system the ravages of the high inrush currents of large motors. And you might use soft starts for smaller motors just so they last longer. Motor life extension is also a compelling reason to buy an electronic motor drive.
But a soft start won’t give you speed control. A motor drive will, with precision across a wide range of speeds.
Before the advent of electronic drives, you had a constant motor speed but used mechanical means at the output to adjust the speed of the load. Belt-driven fans in HVAC systems are examples of this speed control method.
Changing speed in a belt drive system is not variable or gradual; it’s a step change. If you need only 2 or 3 speeds and don’t mind shutting the system down to change belts and/or pulleys, this is acceptable. A gear drive also has this limitation, unless it’s mated with a transmission box that allows speed changes.
Unlike belt drives, gear drives don’t slip if torque is anything but low. Consequently, they are often used to multiply the torque a given motor produces. For this reason, many applications involve combining an electronic motor drive (on the input) with a gear drive (on the output).