Soft Stop, Pump Control and Braking on a Soft Starter

What does a soft starter actually do when it stops a motor? A soft-stop function ramps the SCR output voltage down over an adjustable time instead of opening the line contactor, so the motor decelerates on its own inertia and the load's drag, using the same phase-angle control that IEC 60947-4-2 defines for starting. The practical consequence shows up hardest in pump systems: soft stop lets flow decay slowly enough for a check valve to close before it slams shut, while a straight contactor trip lets the impeller free-wheel at whatever rate the piping and static head dictate. This article covers soft stop versus plain coasting, the check-valve-slam problem in pump systems, the torque-voltage relationship the ramp-down depends on, the SCR-to-bypass handback that has to happen before a stop can begin, and where true DC-injection braking differs from soft stop.

Soft Stop vs. Coasting to a Stop

Open the line contactor on a running motor and it coasts: deceleration rate is set entirely by the load's inertia, friction and drag, with zero control from the starter. A soft-stop function keeps the SCRs conducting and ramps the output voltage down over a set time, typically a few seconds up to a minute, instead of cutting power in one step. That sounds like it slows the stop down, and for most loads it does, but there is a limit worth stating plainly: removing torque gradually cannot make a motor decelerate faster than a full coast would. It can only stretch the stop out or shape it more evenly. Soft stop is a deceleration profile, not a brake.

The setting itself is usually a simple stop time plus, on torque-control units, a curve shape. A voltage-ramp-only starter just linearly reduces output over the set seconds; a closed-loop torque-control unit (ABB PSTX, Schneider ATS480, Siemens 3RW55 class) can instead hold a target torque or speed profile through the whole deceleration, which matters once the load has its own dynamics, as most pumps do.

Why Pumps Need Soft Stop: Check-Valve Slam and Water Hammer

A centrifugal pump discharging into a system with a check valve is the textbook case. Trip the contactor and the impeller free-wheels down while the column of water in the discharge line keeps moving on its own momentum, then reverses through the pump as it slows past the point where it can still push flow forward. The check valve, sensing reverse flow, slams shut. That slam is a pressure transient, not a gentle event, and repeated water hammer loosens pipe joints, damages valve seats and stresses pump seals over time.

Soft stop attacks the cause, not the symptom. By pulling motor torque down gradually instead of removing it in one step, the starter lets flow decay toward zero before the pump can drive it backward, so the check valve closes on near-zero flow instead of a reversing column. This is exactly the reason torque-control soft starters get specified for pump duty over the plain voltage-ramp economy units: a linear voltage ramp shapes torque loosely, while torque control tracks the actual deceleration the pump curve demands.

What we see in the field is that plants add soft stop after a check valve failure, not before. Nobody budgets for water hammer until a coupling cracks at 2 a.m.

The Torque-Voltage Relationship Behind the Ramp-Down

The ramp-down works on the same physics as the ramp-up, just run in reverse. Motor torque falls with the square of applied voltage, so a modest voltage reduction produces a much bigger torque cut. That non-linearity is why the last portion of a stop ramp barely does anything electrically: by the time output voltage is down to 20-30%, available torque is already under 10% of full value, and the load's own drag is doing most of the remaining work.

This is exactly why a plain voltage-ramp stop cannot guarantee a smooth linear speed decay on its own: the torque removal is non-linear even though the voltage command is linear. A closed-loop torque-control starter compensates by adjusting voltage output faster or slower through the ramp to track a real torque or speed target, rather than following a fixed voltage slope regardless of what the load is doing.

SCR-to-Bypass Handback Before a Soft Stop Can Begin

Most feature-class soft starters run bypassed once up to speed: a bypass contactor closes across the SCRs so they carry no current and generate no heat during the run. That is fine for steady running but it is a problem the moment a stop is commanded, because the SCRs cannot ramp voltage down while they are shorted out. The starter has to open the bypass contactor and hand conduction back to the SCRs before the voltage ramp-down logic can do anything at all.

On built-in-bypass units this handback is managed internally and happens fast enough that the motor never sees a torque gap. On starters without a built-in bypass, the SCRs stay in the circuit for the entire run, so there is no handback needed for a stop, but that comes at the cost of continuous conduction losses and a larger heatsink for the same current rating. Either way, a soft-stop command on a bypassed unit is not instant; there is a brief internal transfer before the ramp actually starts.

Soft Stop vs. True Braking (DC Injection)

Soft stop and coasting both end the same way: the motor slows down because torque is removed, not because anything is actively opposing rotation. An SCR-based starter, run in its normal forward mode, cannot push torque backward. It can only reduce or remove forward torque. That distinction matters for anyone expecting a soft starter to stop a motor quickly the way a mechanical brake or a VFD with a braking resistor would.

Some feature-class units add DC-injection braking as a separate, bolted-on function: after (or instead of) the AC voltage ramp, a DC current is injected into the stator to produce a fixed retarding torque and bring the motor down faster than a coast or soft-stop ramp alone would. That torque comes at a cost, extra heating in the windings concentrated in a short burst, and it typically needs its own contactor logic and thermal limit separate from the SCR ramp function. Not every soft starter offers it, and not every application needs it; a pump with a check valve usually wants a slower, controlled stop, not a fast one.

For applications where speed needs to be actively controlled through deceleration rather than just paced, a VFD versus soft starter comparison is worth reading before settling on either device, since a drive's regenerative or resistor braking behaves fundamentally differently from an SCR soft stop.

Settings That Shape the Soft-Stop and Pump-Stop Profile

Three settings do most of the work. Stop time sets the overall ramp duration, typically a few seconds for small pumps up to 60 seconds or more on large discharge systems with long pipe runs. A minimum voltage or torque threshold, where fitted, stops the ramp short of zero and lets the contactor open once torque has dropped far enough that the load is essentially freewheeling anyway, avoiding a long tail where the SCRs are doing nothing useful. On torque-control units, a stop-curve selection (linear, S-curve, or a pump-tuned profile) changes how torque is apportioned across the ramp rather than just how long it lasts.

This depends on the pump's static head and pipe length as much as on the starter's settings; a short, high-head system may slam in two seconds regardless of stop time, while a long low-head line gives more room to tune. Frequent stop-ramp cycles also add heat on top of whatever the starts-per-hour duty rating already accounts for, so a unit sized only against AC-53a and AC-53b duty ratings for starting can still run hot if every stop is a long, high-current ramp-down as well.

Choosing between voltage-ramp-only, current-limit, and torque-control soft starters up front — covered in more depth in the article on voltage ramp, current ramp and torque control — determines how much of this stop-side tuning is even possible, since a basic economy unit gives you a stop timer and nothing else. Sizing decisions made at selection time, detailed in the soft starter sizing guide, also carry over directly to how a stop behaves, because a starter running near its thermal limit on starts has less margin for long, hot ramp-downs.

Conclusion

Soft stop is a torque-removal profile, not a brake, and the distinction decides what it can and cannot fix. For pump systems with a check valve, that is usually enough: pacing torque down avoids the reverse-flow slam that causes water hammer, without needing any active braking hardware. Where a faster, forced stop is genuinely required, DC-injection braking or a different device altogether is the answer, not a shorter stop timer. Getting the stop side right starts with the same selection questions covered in the soft starter selection guide and applies just as much to the soft starters chosen for pump duty as to any other application: know what the bypass has to do before a stop can start, and size the stop, not just the start.