Soft Starter for Conveyors and Crushers

What is a soft starter for conveyors and crushers? It is a reduced-voltage SCR starter selected for enough breakaway torque to move a loaded belt or restart a crusher chamber that still holds material, usually set up for torque control or a high current-limit ceiling (400-500% FLC) rather than the plain voltage ramp that works fine on a pump or fan. Undersize the torque margin and the motor stalls partway through the ramp, pulling near-DOL current for the whole stall duration, the single worst thermal event a starter sees. This article covers breakaway torque sizing, why voltage-vs-torque math catches people out, starting a crusher with material already in the chamber, current limit versus torque control for these two duty types, anti-rollback and soft-stop on inclined belts, and the protection settings that matter for conveyor and crusher applications specifically.

Why Conveyors and Crushers Load a Starter Differently Than a Pump

A centrifugal pump or fan is a quadratic-torque load: torque demand rises with the square of speed, so it needs almost nothing to get moving and ramps up smoothly as it accelerates. A loaded belt conveyor is closer to a constant-torque load. It needs close to full running torque at zero speed just to break the belt tension and overcome idler friction, then holds roughly flat torque through the whole ramp. A crusher is worse: it combines constant torque from the crushing action with a large flywheel that stores kinetic energy and must be accelerated from zero regardless of whether material is present.

What we see in the field: engineers spec a soft starter using the same current-limit setting that worked on a fan of similar kW, then wonder why the conveyor stalls at 60% speed under a full burden. The starter isn't faulty. The torque margin ran out.

Breakaway Torque and the Voltage-Squared Trap

Starting torque on a soft starter falls with the square of the applied voltage, not linearly. Halve the voltage and torque drops to roughly a quarter of the direct-on-line value. That relationship is fixed by motor physics, not by starter brand, and it's the reason a starter tuned too conservatively, low pedestal voltage, low current limit, simply can't produce enough torque to break a belt away from rest.

For a loaded belt or a crusher with settled material, the break-away point can demand 80-100% of DOL torque for a fraction of a second before the load starts moving. A starter with pedestal voltage set at 40% only delivers about 16% of DOL torque at that instant, nowhere close. This is why soft starter sizing for these applications starts from the load's breakaway torque curve, not from motor nameplate kW alone.

Starting a Crusher With Material Still in the Chamber

A jaw or cone crusher stopped mid-cycle, whether from a trip or a planned shutdown, often has rock still wedged in the crushing chamber. Restarting under that condition needs both torque, to break the jam, and enough current-limit headroom to sustain it without tripping the starter's own overload curve. Best practice is to clear the chamber before every stop where possible; where it isn't, the starter's kick-start or breakaway pulse, a brief higher-voltage boost ahead of the normal ramp, is what unsticks the rotor without resorting to a full DOL start.

Inertia is the other half of the crusher problem. A large flywheel takes real time to accelerate, and that time appears directly in the AC-53a duty rating (start current, start time, duty %, starts/hour) the starter is selected against. A crusher's start time can run well past the 10-second reference point used in most catalog duty tables, which pushes the selection toward a larger frame even when the running current alone would fit a smaller one. This is covered in more depth in the AC-53a and AC-53b duty sizing guide and the high-inertia load sizing article.

Current Limit vs. Torque Control for These Two Duties

An economy starter running open-loop voltage ramp has no feedback on what the load is actually doing. On a fan that's fine, the torque curve is predictable. On a conveyor with variable burden, or a crusher where feed rate changes minute to minute, an open-loop ramp can under- or overshoot torque as conditions shift, sometimes stalling for a moment mid-ramp before recovering, sometimes producing a jerk that stresses the gearbox and belt splice.

Closed-loop torque control changes that. It measures actual motor torque and adjusts firing angle to hold a commanded, linear torque rise regardless of load variation, the same reason it's specified for pumps to avoid fluid hammer works in reverse here, holding torque steady enough to avoid belt snap-back or a stalled crusher jaw. ABB PSTX, Schneider Altistart ATS480, and Siemens 3RW55 all offer this mode; see the ABB PSTX vs. ATS480 vs. 3RW55 comparison for the detail differences between them. Entry units (ABB PSR, Altistart ATS01, Siemens 3RW50-class) don't offer torque control at all, fine for a light, unloaded-start conveyor, wrong for a loaded overland belt or a crusher.

Some plants still run open-loop starters on heavy conveyors because that's what was specified a decade ago, and it mostly works, until a wet, overloaded belt stalls one morning and nobody can explain why. Not always predictable. That's the trade-off being made, knowingly or not.

Anti-Rollback and Soft-Stop on Inclined Belts

A soft starter controls voltage while the SCRs are conducting. Once the motor reaches zero speed and the starter stops firing, it provides no holding torque at all, it is not a brake. On an inclined conveyor, that means a mechanical backstop (ratchet, roller, or holdback device) is still required regardless of starter brand or feature set. The starter's anti-rollback logic, where fitted, only monitors for reverse rotation during the stop sequence and can flag or trip on it; it does not physically prevent the belt from running back.

What soft-stop does provide is a controlled deceleration ramp rather than an abrupt torque cutoff. On a downhill or heavily loaded belt that matters: an uncontrolled stop can let material shift or spill at the head pulley, and a hard stop transmits a shock load through the gearbox and belt splices that a ramped-down stop avoids.

Protection Settings Worth Reviewing for Conveyor and Crusher Duty

Standard overload protection (class 10) sized for a quick pump start is often too tight for a crusher's long, high-inertia ramp. Class 20 or 30 thermal curves are more common here, matched to the actual measured start time rather than a default. Stall and locked-rotor protection deserve separate attention: a crusher jam mid-cycle is a stall condition, and the starter needs to trip on it fast enough to protect the SCRs before the thermal model saturates, rather than waiting out the full overload curve.

Phase loss and phase imbalance protection matter more on crushers and long overland conveyors than on a compact pump skid, simply because the motor feeder run is often longer and more exposed to nuisance faults. Shorted-SCR detection is worth confirming is enabled on any unit doing frequent, high-torque starts. Repeated near-full-torque breakaways are exactly the duty that stresses SCR junctions over years of service, and the built-in bypass on PSTX/ATS480/3RW55-class units reduces that stress once the motor is at speed by shorting the SCRs out entirely.

Conclusion

Conveyors and crushers ask more of a soft starter than a pump or fan of the same kW: more breakaway torque, longer start times, and, for crushers, the chance of restarting under load. Size the pedestal voltage and current limit to the load's actual breakaway torque, choose torque control where feed or burden varies, keep mechanical backstop hardware on inclined belts regardless of starter features, and match overload class to the measured start duration rather than a catalog default. Get those four right and the starter runs the application it was actually bought for, not the pump-duty case the price sheet quotes it against. For the broader selection process across load types, see the soft starter selection guide, and browse the current range in the soft starters collection.