What is a Soft Starter? from AutomationDirect

So, what is a soft-starter? A soft starter—also known as a motor soft-start controller or reduced-voltage starter—is an electronic device that helps an AC motor start and stop smoothly. Instead of slamming the motor with full voltage the instant it’s turned on, a soft starter gradually increases the voltage based on pre-defined or user-programmed curves. Soft starters come in a wide range of sizes to match everything from small motors to large industrial machines. They can also be very simple or equipped with advanced features like current limiting, pump control modes, built-in protection, and communications. They work through solid-state components—typically thyristors or SCRs—that control how much voltage reaches the motor during startup. As the voltage slowly ramps up, the motor accelerates gently, avoiding the harsh mechanical stresses you get from a direct across-the-line start. This controlled acceleration also reduces inrush current. Typical motors draw multiple times their full-load amps when started across the line, and that surge can stress wiring, breakers, and the entire electrical system. By limiting this high current spike, a soft starter helps keep your electrical infrastructure running cleaner and more stable. At the same time, it reduces mechanical stresses on couplings, belts, shafts, and gearboxes—helping to protect both the motor and the equipment it drives. The result is less downtime, fewer repairs, and longer equipment life. Once the motor reaches full speed, the soft starter usually bypasses itself, allowing power to flow directly to the motor for maximum efficiency. Many soft starters also offer soft stop, gradually reducing voltage so the motor slows down smoothly instead of stopping abruptly or coasting to a stop. This is especially valuable on pumps, where a sudden stop can cause water hammer, or on conveyors, where controlled deceleration keeps material moving evenly. You’ll find soft starters in all kinds of applications—pumps, fans, compressors, mixers, conveyors, and crushers—anywhere smooth motor control is essential for performance and reliability. And to wrap up, here’s how soft starters compare to other common motor-starting methods. An across-the-line starter hits the motor with full voltage the moment you start it. It’s simple and inexpensive, but it creates a very high inrush current and can put a lot of stress on mechanical components. A soft starter reduces that inrush by gradually ramping the voltage, giving you a smooth, controlled start. It cuts mechanical stress, protects your equipment, and will keep things simple. But once the motor reaches full speed, it’s running across the line—there’s no speed control during normal operation. A VFD, or variable frequency drive, goes further by controlling both the voltage and frequency. That means smooth starts, low inrush, and full variable-speed control. It’s the most capable option, but also the most complex and costly. AutomationDirect - The #1 Value in Automation To find soft starters for your system, click here. If you need help with soft starters or any other product, please see our free support options here. Click here to subscribe to be notified of new products and solutions.