Starting an electric motor or any electrical device requires the most amount of energy. As accelerating from zero to the required speed consumes a lot of power, there is the requirement of a large starting current when accelerating an AC motor to full speed using a high voltage connection. The uncontrolled torque from an AC motor could shock the connected equipment and could cause potential damage. Thus, using a soft starter or a variable speed drive (VSD)/ variable frequency drive (VFD) depends on what output is desirable, safe, and cost-efficient.
Soft start drives are used to limit the current inrush associated with the start-up cycle of an electric motor. By adding solid state series impedance, soft start drives lower the initial voltage into a motor. There are plenty of applications for soft starters. Desired torque control and speed ramping when stopping or starting. To avoid supply network issues or penalty charges, high starting currents to start large motors must be limited. Gradual and controlled start-up is required to prevent torque spikes and tension in mechanical systems such as conveyors, gears, belt-driven systems, and coupling. Soft starters avoid hammering and pressure surges in piping systems. Benefits to soft starters are, they are the more economical when only the starting condition when speed and torque control is considered. But when taking the space occupied into consideration, they usually occupy less space than VSDs do.
A variable speed drive is an electric motor control device which controls the speed and protects an AC induction motor, and also reduces the power consumption during start-up, and gives total control of engine speed during all the phases of a motor. The more precise the control, the less stress and strain it has on the system, providing longevity to the devices due to less wear and tear. It is applied when custom control is significant. Benefits of VSDs are that they give fully adjustable speed for pumps, fans, and conveyors. It has dynamic control with advanced overload protection; it can also relay outputs, perform self-diagnostics and communications.
VSDs are significantly large and expensive when compared to soft starters. Thus, not all manufacturers can invest in VSDs, but a soft starter is a good start in energy efficiency.
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