Soft Starter for High-Inertia Loads: Sizing and Start Profile

What changes when a soft starter drives a high-inertia load? A high-inertia load — a large fan, centrifuge, ball mill, or big pump with a heavy impeller — needs several seconds to several tens of seconds to reach full speed, and during that whole window the motor must produce more torque than the load at every point on the ramp. Get the current limit or ramp time wrong and the motor stalls partway up, the SCRs sit at full conduction far longer than the thermal model expects, and the starter trips or cooks itself. This article covers the torque-vs-voltage limit that catches most sizing mistakes, how moment of inertia (WK²) drives acceleration time, which control mode holds up on long starts, and how duty ratings and thermal protection have to be set for these loads.

What makes a load "high-inertia" for starter sizing

There is no single inertia number that flips a load into the high-inertia category — it is a ratio. Applications engineers usually flag a load as high-inertia when the reflected load inertia is more than about 3-5x the motor's own rotor inertia, or when the calculated acceleration time at reduced voltage exceeds roughly 10-15 seconds. Large centrifugal fans, crushers, ball and rod mills, centrifuges, and large pumps with high static head all fall into this bracket. So does anything with a flywheel, a large disc, or a long belt-and-pulley system with significant reflected inertia.

Two things happen at once with these loads: the motor needs sustained torque above the load curve for longer, and the SCRs conduct at high current for longer. A pump that reaches speed in 2 seconds barely warms the semiconductors. A mill that takes 25 seconds does not behave the same way thermally — this is where undersized starters fail in the field, not on paper.

Why torque falls with the square of voltage — the sizing trap

A soft starter reduces motor terminal voltage to cut inrush current, but motor torque does not scale linearly with voltage — it scales with the square of it.

At 70% voltage the motor gives about 49% of its DOL torque. At 50% voltage it gives about 25%. That drop-off is brutal for a high-inertia load, because the load torque curve does not fall to match it — a fan or mill still needs close to its own breakaway torque at low speed. If the reduced motor torque dips below the load torque curve anywhere on the ramp, the motor stalls at that speed, current stays pinned at the limit, and the starter's overload or stall protection eventually trips it. This is the single most common soft starter sizing error on high-inertia jobs: picking a starter (or a pedestal/current-limit setting) that looks fine on average current but never checking whether torque clears the load curve at every speed point, not just at zero and full speed.

Moment of inertia and how it sets acceleration time

Acceleration time depends directly on the difference between average motor torque and average load torque across the ramp, and on the total inertia reflected to the motor shaft — motor rotor plus load, corrected for gear ratio where the load is not directly coupled. Double the reflected inertia and, for the same net accelerating torque, acceleration time roughly doubles. That is why a direct-drive ball mill with a heavy trommel behaves nothing like a direct-coupled pump of the same motor kW — the mill can take 10-20x longer to reach speed.

What we see in the field: two motors of identical frame size and kW rating, one driving a pump and one driving a crusher, often need completely different soft starter current-limit and ramp-time settings, even though the nameplate current is the same. The starter does not know the load — it only knows the setpoints given to it.

Practical inertia check before ordering

Ask for (or calculate) the reflected WK² at the motor shaft, the required breakaway torque, and the load's speed-torque curve before selecting a current-limit percentage. Motor and starter manufacturers publish sizing software that takes these inputs and outputs an estimated start time and SCR heating — use it rather than a rule-of-thumb current-limit percentage on anything with a flywheel, mill, or crusher.

Setting current limit and ramp time for long accelerations

Current limit is typically adjustable from about 200% to 500% of FLC, and ramp time from a few seconds to several minutes on flagship units. For a high-inertia load, the practical range narrows: the current limit needs to sit high enough that the corresponding torque clears the load curve (per the formula above), and the ramp time needs to be long enough that the current limit is not the only thing doing the work — a starter set to ramp in 10 seconds but held at its current ceiling for 25 seconds because the load will not accelerate faster is not really "ramping," it is current-limiting the whole start.

Initial voltage (pedestal) also matters more here than on light loads. Too low a pedestal on a high-inertia start wastes time at near-zero torque before the ramp even starts contributing; too high a pedestal produces a current spike at t=0 that undercuts the point of soft starting. A kick-start (breakaway) pulse — a brief boost above the pedestal — helps unstick static friction on mills and conveyors, but it should last only long enough to break away, not bleed into the main ramp.

Torque control vs. current limit on long, heavy starts

A basic voltage-ramp starter (ABB PSR, Schneider Altistart ATS01, Siemens 3RW30-class) has no adjustable current limit at all and is a poor fit here — it ramps voltage on a timer regardless of what the motor is actually doing, which on a high-inertia load means either a stall (ramp too short) or an unnecessarily long, hot start (ramp set long to be safe). A current-limit starter (ABB PSE, Schneider ATS22, Siemens 3RW50/3RW52) is a real improvement: it holds current, and therefore roughly holds torque, at a chosen ceiling through the whole start. That is workable for most high-inertia loads if the ceiling is set correctly.

Closed-loop torque control (ABB PSTX, Schneider ATS480, Siemens 3RW55) goes a step further: it targets a torque profile directly rather than inferring it from current, which tracks the load's actual speed-torque curve more closely and avoids the current-limit starter's tendency to over- or under-torque at different points on the ramp. On a mill, crusher, or large fan with a non-linear load curve, that difference is not cosmetic — it shows up as lower peak heating for the same acceleration time.

This depends on the load's torque curve shape as much as its inertia — a load with a flat, low breakaway torque but a long ramp (some large fans) can do fine on current limit alone, while a load with a high breakaway spike (some crushers) benefits more from torque control even at moderate inertia. See the full comparison of voltage ramp, current ramp, and torque control for the underlying mechanics.

Thermal protection and duty rating for extended starts

A starter rated for a 10-second start does not automatically handle a 25-second high-inertia start at the same current multiple — the SCR junction temperature rise is roughly proportional to current squared times time, so a 2.5x longer start at the same current pushes far more heat into the same silicon. Two paths fix this: derate the current limit (accepting a longer, gentler start) or step up to a starter frame size rated for the actual AC-53a duty the application needs. Manufacturers publish extended-duty tables for exactly this reason — pulling the wrong line off a generic selection chart and skipping the duty-rating check is a repeat sizing failure on high-inertia jobs.

Starts-per-hour compounds this. A mill re-started twice in ten minutes after a trip does not get a full thermal reset between starts, so the starter's thermal model and the motor's own I²t overload class (10/20/30) need to reflect the real duty cycle, not one worst-case start. Stall/locked-rotor trip time also needs margin above the calculated acceleration time, or the protection trips a normal, if slow, start.

For the full duty-rating math and how AC-53a differs from AC-53b, see soft starter duty rating sizing. For the broader step-by-step selection process, start with how to select and size a soft starter.

Bypass, cooling and enclosure notes for long starts

Because the SCRs conduct longer on a high-inertia start, the bypass contactor matters even more here — once the motor is at speed, bypass removes the SCR heat entirely rather than leaving it in circuit at run current. Panels housing these applications should budget more enclosure airflow than the same-frame starter on a quick-starting load, since heat generation duty differs even at the same nameplate rating. Not always obvious from a datasheet ambient rating alone: some panel builders assume "same frame, same heat," which does not hold once start time triples.

Conclusion

Sizing a soft starter for a high-inertia load comes down to three checks: does motor torque clear the load's torque curve at every speed point given the planned voltage/current-limit setting, does the reflected WK² and load curve produce an acceleration time the starter's AC-53a duty rating can actually absorb, and does the control mode (current limit vs. torque control) match how non-linear the load's torque curve is. Skip any of the three and the failure shows up as a nuisance trip, a stalled start, or a starter that runs hot every cycle. For load-specific guidance, see soft starter sizing for pumps, and for the full selection framework, the soft starter selection guide covers how these checks fit into an overall specification alongside the soft starters range.