Soft Starter for Fans and Blowers

What does a soft starter do for a fan or blower? It ramps motor terminal voltage up over a set time using back-to-back SCRs on each phase, holding start current to roughly 3-4x full-load current (FLC) instead of the 6-8x FLC a direct-on-line (DOL) start pulls. On a fan this mainly protects the belt, the wheel bearings, and the upstream supply from the current spike, since fan load torque itself is low at zero speed. This article covers inertia sizing, the right control mode for a fan curve, belt-drive slip, damper-assisted starting, and bypass duty for fans that run continuously.

Why Fan Load Torque Behaves Differently From a Pump

A centrifugal fan or blower is a variable-torque load: torque rises with the square of speed and power rises with the cube of speed (the fan affinity laws). At zero speed the wheel needs almost no torque to start turning — the air column offers little resistance until the impeller is moving. That is the opposite of a positive-displacement blower or a loaded conveyor, which need torque from the first turn.

This changes the sizing conversation. A pump on a closed valve or a loaded conveyor belt is limited by breakaway torque; a fan is limited by inertia and by how long it takes to accelerate a large wheel through its speed range. The soft starter has to supply enough torque to overcome bearing friction and windage at low speed, then ride the affinity curve up. Undersize the current limit and the ramp stalls partway up; oversize it and you lose the benefit of a soft start entirely.

What we see in the field: fan starting problems are almost never "not enough torque." They are almost always "ramp finishes before the wheel reaches speed," which reads as a stall fault or a thermal trip, not a torque fault.

Sizing a Soft Starter for High-Inertia Fan Wheels

Large fans — induced-draft (ID), forced-draft (FD), cooling tower, and big HVAC air-handler wheels — carry high rotational inertia (WK²) relative to their motor kW. A 4-foot diameter steel wheel takes meaningfully longer to spin up than a pump impeller of the same horsepower. The starter's SCRs conduct current the whole time the ramp is in progress, so a long acceleration time is a thermal problem for the starter, not just a mechanical one for the fan.

Two settings absorb this: ramp time and current limit. Ramp times for fans often sit at the upper end of the typical 5-30 s range, and some large ID/FD fans need longer settings than that — check that the starter model supports an extended ramp before specifying it for a big wheel. Current limit is normally set toward the top of the typical 300-400% FLC band so the starter has torque in reserve through the whole affinity curve, not just at the top end.

Voltage Ramp, Current Limit or Torque Control for Fans?

A plain voltage ramp works on small, direct-driven fans with modest inertia — the affinity curve is forgiving enough that an open-loop ramp rarely stalls. Current limit adds a ceiling so the ramp does not draw more than the supply or the upstream protection can tolerate, which matters more on fans sharing a bus with sensitive equipment than it does for torque control on the fan itself.

Torque control earns its keep on large or belt-driven fans, where a smooth, linear speed rise reduces mechanical shock on the wheel keyway, the belts, and the bearings. See voltage ramp, current limit and torque control soft starters for how the three modes differ mechanically. In practice, a fan rarely needs the aggressive torque limiting a high-inertia crusher or mixer needs — but it benefits from the same closed-loop smoothness a pump gets, for a different reason: less belt and coupling wear over years of starts, not water hammer.

Belt-Driven Fans: Avoiding Slip During the Ramp

Most mid-size industrial and HVAC fans are belt-driven off the motor shaft rather than direct-coupled. A belt transmits torque by friction; if the starter applies torque faster than the belt tension is designed to hold, the belt slips on the sheave. Slip shows up as a squeal on start, accelerated belt wear, and — over time — glazed, hardened belt surfaces that slip even worse.

A DOL start or an aggressive voltage-ramp with a strong kick-start pulse is the usual cause of chronic belt slip on fan drives. Extending the ramp time and lowering the kick-start pulse (or removing it, since fan breakaway torque is low) usually cures it without touching the belts or sheave alignment. This depends on belt tension being correct in the first place — a soft starter cannot compensate for a loose belt.

Damper Control and Reduced-Load Starting

Closing the inlet or discharge damper before start is standard practice on large fans regardless of starter type, because it reduces the air volume the wheel has to move and cuts starting torque and current further. Combined with a soft starter, a damper-assisted start lets the current limit be set lower than it would need to be on a wide-open start, which reduces SCR heating on every single start over the life of the unit.

Interlock the damper end-switch into the starter's start-permissive input where the control system allows it. Starting into a wide-open damper on a large ID fan is one of the more common causes of nuisance current-limit trips during commissioning, and it is usually solved by fixing the interlock logic, not by re-tuning the starter.

Bypass Contactor and Duty Cycle for Continuous-Running Fans

Fans and blowers typically run continuously for hours once started — cooling towers, ID/FD fans, and HVAC air handlers are not intermittent duty. SCRs dissipate roughly 1-1.5 W per amp per phase while conducting, so running a fan through the SCRs for hours at a time means real, continuous heat in the enclosure. A bypass contactor that closes once the fan reaches full speed removes that heat load entirely and lets the starter be sized to the ramp, not to the full run current.

Most flagship units built for this duty — ABB PSTX, Schneider Altistart ATS480, Siemens SIRIUS 3RW55 — bring the bypass contactor built in as standard, rather than as a separate field-wired component. Fans with frequent start-stop cycling (some HVAC control sequences, VAV-driven supply fans) also benefit from checking the AC-53a/AC-53b duty rating against actual starts-per-hour; see AC-53a and AC-53b duty sizing for how starts-per-hour and start time drive thermal capacity independent of motor kW.

Common Fan and Blower Applications

Induced-draft (ID) and forced-draft (FD) fans on boilers and furnaces are usually the largest, highest-inertia fans on a site, and the ones most likely to need an extended ramp and heavy-duty bypass. Cooling tower fans add a second complication: they are frequently mounted outdoors, so enclosure thermal design and starter derating for ambient temperature matter as much as the ramp settings. HVAC supply and return fans are smaller and lower-inertia, and often the target for VFD-based speed control rather than a soft starter — where the goal is throttling airflow rather than just reducing inrush, see VFD vs soft starter for that comparison rather than repeating it here. Dust-collection and material-handling blowers sit closer to the pump end of the spectrum, since they often start against a partially loaded duct system.

Not every fan needs the flagship torque-control unit. A small direct-driven exhaust fan on voltage ramp is adequate; a large belt-driven ID fan is where the torque-control, built-in-bypass tier earns its price. Match the starter tier to the wheel, not to the motor nameplate alone: see how to select and size a soft starter for a motor for the full sizing sequence, and the soft starter selection guide for how fan duty fits alongside pump, conveyor, and compressor selection. Fan sizing shares more with soft starters for pumps than with high-breakaway loads — both are variable-torque and inertia-limited rather than breakaway-limited, though pumps add static head and water-hammer concerns that fans do not have.

Browse ABB, Schneider and Siemens soft starters sized for fan and blower duty, from small direct-driven exhaust fans to bypass-equipped ID/FD units.

Conclusion

Fan and blower starting is an inertia problem more than a torque problem: low breakaway torque, torque rising with the square of speed, and — on large wheels — enough WK² to make acceleration time the limiting factor for both the fan and the starter's SCRs. Size the current limit and ramp time to the wheel's inertia, favor torque control and a built-in bypass on large or belt-driven units, and use damper interlocks to cut starting load before the ramp even begins. Get those settings right and belt life, bearing life, and starter temperature all improve together.