Soft Starter Selector
Match your motor to the right soft starter — the AutomationDirect soft starter selector sizes the smallest suitable model across seven families and derates automatically for heat, altitude, and duty cycle.
Soft Starter Sizing
Potential Uprating Factors
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About soft starters
A soft starter gradually ramps voltage to an AC induction motor, limiting the inrush current and mechanical shock of an across-the-line start. That protects the motor, the driven load, and everything upstream — and it costs less than a variable frequency drive when you only need controlled starting and stopping.
This selector sizes the smallest suitable model across eight families — SR22, SR35, SR55, SSW05, SSW07, SSW900, VSII, and Dold MINISTART — from your supply voltage, phase, motor full-load current, and trip class. It automatically derates for high ambient temperature, altitude, and frequent starts, and flags when a cooling fan is required.
Soft starter FAQ
What is a soft starter?
A soft starter is an electronic motor control that ramps voltage up at start-up and down at stop, cutting the inrush current — typically 6–8× full-load current on an across-the-line start — and the mechanical shock that wears belts, gears, couplings, and pumps.
Soft starter or VFD — which do I need?
Choose a soft starter when you only need smooth starting and stopping plus overload protection. Choose a variable frequency drive (VFD) when the process needs continuous speed or torque control while running. A soft starter is smaller and lower cost when variable speed isn’t required.
What is a trip class?
Trip class describes how long the overload protection tolerates starting current before it trips — higher classes (20, 30) allow longer, harder starts. Pick the class that matches your load’s starting time, or choose an application and the selector maps it to a class for you.
When does a soft starter need derating?
Allowable current drops in hot rooms (typically above 40–50°C), at high altitude (above 1000 m, where thinner air cools less), and with frequent starts per hour. The selector applies these uprating factors automatically and steps up to a larger model when needed.
