Connect a large motor straight to the line and it draws six to eight times its running current for a few seconds and slams its load with full torque. A soft starter tames both problems, ramping voltage up smoothly so the motor - and everything it drives - comes up to speed gently. This guide explains what a soft starter is, how the SCRs inside it work, how it differs from a DOL starter and a VFD, and where it earns its place on oil and gas sites.
Soft Starter in one line: A soft starter is a solid-state motor controller that gradually increases voltage to an AC induction motor during starting, reducing the inrush current and mechanical shock of a direct-on-line start. It uses back-to-back thyristors (SCRs) to phase-control the voltage, then typically switches to a bypass contactor once the motor reaches full speed.
A soft starter places pairs of back-to-back SCRs (silicon-controlled rectifiers) in series with each of the motor's three phases. By delaying when each SCR fires within every AC half-cycle, the starter controls how much of the sine wave reaches the motor, effectively lowering the applied voltage. It begins at a low initial voltage and ramps up over a set acceleration time - typically 5 to 30 seconds - until full line voltage is applied.
Because motor torque is roughly proportional to voltage squared, reducing voltage cuts both starting torque and inrush current. That softens the mechanical jolt on couplings, belts, gearboxes, and pump internals while limiting the current spike that would otherwise dip the site's voltage. Once the motor is up to speed, most soft starters close an internal or external bypass contactor so the SCRs stop dissipating heat, and many add a controlled soft-stop ramp to prevent water-hammer on pumps.
A direct-on-line (DOL) starter connects the motor to full voltage instantly - cheapest and simplest, but with maximum inrush and shock. A soft starter sits in the middle: it eases starting and stopping without the cost or complexity of a drive, but once running the motor turns at full line speed only. It does not control running speed and gives no energy savings from speed reduction.
A VFD (variable frequency drive) controls both frequency and voltage, so it soft-starts and then holds any speed across the range - ideal where throttling a pump or fan saves energy or the process needs variable output. The rule of thumb: choose a soft starter when the only pain point is hard starting or stopping of a fixed-speed load, and a VFD when you also need continuous speed control. Soft starters are common on large fixed-speed pumps, fans, compressors, and conveyors.
On production and midstream sites, soft starters are used on large electric submersible pump surface drives, water-injection and transfer pumps, and big compressor and fan motors where a hard start would stress the mechanical train or the local electrical supply, especially on weaker distribution or generator-backed systems. Soft-stop is valued on pipeline and disposal pumps to avoid the pressure surge of an abrupt stop.
Modern soft starters expose status and measured values - run, bypassed, fault, motor current, thermal capacity, and trip cause - as digital and analog signals or over a fieldbus. A cloud SCADA such as Merobix can read those tags from the controller or directly from a starter that speaks Modbus, so operators see start status and overload trips for every motor across remote sites in one browser view.
A soft starter is a solid-state device that gradually ramps voltage up to an AC motor when starting, cutting the large inrush current and mechanical shock of a full-voltage start. It also provides a controlled soft stop, reducing water-hammer on pumps. It does not control running speed.
A soft starter only smooths starting and stopping; once running, the motor turns at full line speed. A VFD controls speed continuously across the whole range and can save energy by slowing the motor. Choose a soft starter for hard-start relief on a fixed-speed load, a VFD when you need variable speed.
Not while the motor is running at full speed - it only reduces the inrush during starting. Any steady-state savings are marginal. If the goal is to save energy by running a pump or fan slower, a VFD is the right tool, because a soft starter cannot change running speed.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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