Soft Starter for Compressors: Selection and Setup

04 Aug, 2026
Posted by: Muhammet KUL

How do you size a soft starter for a compressor? A soft starter ramps motor terminal voltage over a set time, typically 5-30 s, to hold starting current to about 3-4x FLC instead of the 6-8x FLC a direct-on-line start draws, per IEC 60947-4-2. On a reciprocating compressor the sizing depends almost entirely on whether the unloading valve dumps head pressure before the motor is asked to turn; on a screw or centrifugal machine, an unloaded start behaves closer to a fan load and is easier to soft-start. This article covers compressor load types and torque demand, why screw and centrifugal machines are the easy case, why reciprocating compressors are the hard case, ramp and current-limit settings, starts-per-hour sizing for cycling units, and when a bypass contactor pays for itself.

Compressor Types and Their Starting Torque Profile

Three compressor architectures show up in industrial plants, and each asks something different of the motor at zero speed. A reciprocating compressor has pistons compressing gas against valve resistance from the first degree of rotation unless something vents that resistance first — locked-rotor torque can run close to what the motor needs at full load, not the 150-200% a fan needs. A rotary screw compressor has meshing helical rotors; with the inlet closed and the bypass or slide valve open, compression work at zero speed is near zero, so the torque curve looks far gentler. A centrifugal compressor spins an impeller that does almost no work until it reaches enough speed to build head — torque rises with speed roughly like a fan curve, which is the friendliest profile a soft starter sees.

Unloading valve is a valve (or bypass/slide valve on a screw machine) that vents compressed gas or opens the suction path so the compressor turns against little or no gas resistance during start, per typical compressor manufacturer commissioning practice.

The practical consequence: two compressors with the same motor kW can need completely different soft starter settings depending on whether they start loaded or unloaded. Nameplate horsepower alone does not tell you the start profile — you need to know the machine type and whether it unloads.

Screw and Centrifugal Compressors — the Easy Case

With the bypass valve open or the inlet guide vanes closed, a screw or centrifugal compressor starts against a load curve a soft starter handles well. Current limit set to roughly 300-350% FLC and a ramp time of 5-10 s is usually enough — no torque control needed for a properly unloaded start. Voltage-ramp-only units (ABB PSR, Schneider Altistart ATS01) can cover small screw compressors on this duty; current-limit units add margin when suction pressure at start is not fully zero.

Key takeaway: Confirm the unloading valve actually opens before power is applied — a stuck-closed bypass on a screw compressor turns an easy fan-like start into a stalled-motor call.

What we see in the field: unloader solenoids seize over a maintenance interval and nobody notices until a start trips on overcurrent. A soft starter with shorted-SCR and stall detection catches this faster than a plain contactor-and-overload setup, because the current-limit trip happens before winding temperature climbs.

Reciprocating Compressors — Why the Unloader Decides the Sizing

A reciprocating compressor without a working unloader asks the motor to produce near-full compression torque from standstill. Starting torque falls with the square of applied voltage, so a soft starter set to reduce voltage without confirming the machine is unloaded can stall the motor mid-ramp rather than protect it.

Formula: Torque vs Voltage — Source: induction motor theory, T ∝ V²

T_x = T_DOL x (V_x / V_line)²

Symbol	Description	Unit
T_x	Motor torque at reduced voltage V_x	N·m (or % of DOL)
T_DOL	Motor locked-rotor torque at full line voltage	N·m (or %)
V_x	Motor terminal voltage during ramp	V
V_line	Full rated line voltage	V

At 70% voltage a motor gives about 49% of its DOL torque; at 50% voltage, about 25%. If the reciprocating compressor still needs 60-70% of its running torque to turn over because the unloader has not vented, a mid-ramp voltage of 50-70% simply is not enough — the motor stalls, current pins at the limit, and the soft starter trips on stall protection rather than damaging anything, which is the correct failure mode but the wrong outcome for the process.

Locked-rotor torque is the torque a motor produces at zero speed and rated voltage — the reference point every reduced-voltage torque calculation scales from.

This depends on how completely the unloader vents head pressure, not just whether it exists. A worn unloader that only partially opens leaves residual compression torque on the crankshaft, and that residual load is what actually sets the minimum pedestal voltage a soft starter needs.

Setting Up a Soft Starter for a Compressor Start

Initial voltage (pedestal), typically 30-50%, should sit just above whatever residual torque the unloaded compressor still presents — too low and the shaft does not break away; too high and the reduced-voltage benefit disappears. Ramp time, typically 5-30 s, should be tuned to how fast the unloading valve vents, not to an arbitrary "slow start" preference — ramping faster than the unloader vents just means the motor is fighting full compression torque at low voltage for part of the ramp. Current limit, typically 300-400% FLC, sets the ceiling; torque control mode (ABB PSTX, Schneider ATS480, Siemens 3RW55) tracks the actual load torque curve through the ramp rather than following a fixed voltage or current profile, which matters more on reciprocating machines with an uneven torque-vs-crank-angle curve than on the smoother screw or centrifugal case.

Key takeaway: Set pedestal voltage and ramp time from the unloader's actual venting behavior on the specific machine, not from a generic "pump-style" default — a reciprocating compressor's residual torque does not follow a pump's speed-squared curve.

See our soft starter sizing guide for the full motor-side calculation (FLC, service factor, ambient derating) that this compressor-side torque check sits on top of.

Starts-Per-Hour and Duty Cycle for Pressure-Switch-Controlled Compressors

Small reciprocating and screw compressors on a pressure switch cycle far more often than a continuous-duty pump or fan — some cut in and out every few minutes under light demand. Sizing here is not just about start current; it is about how many starts per hour the SCRs can absorb before their thermal model calls a lockout. IEC 60947-4-2 duty ratings such as AC-53a (e.g. 3.0-10:50, meaning 3x FLC for 10 s at 50% duty) specify exactly this, and a soft starter picked only on FLC without checking the duty rating against actual cycling frequency will nuisance-trip on a compressor that cycles more than the rating assumes.

Refer to our AC-53a / AC-53b duty sizing guide for how to read the rating string and match it to a real cycling schedule.

Bypass Contactor for Continuous-Duty Compressors

Process air, refrigeration, and instrument-air compressors that run for hours once started are a different case from cycling units. Here the SCR conduction losses, roughly 1-1.5 W per amp per phase, matter over the run, not just the start — a built-in bypass contactor closes once the compressor reaches full speed and shorts the SCRs so they carry no current at all during the run. This drops heat in the enclosure, removes the need to derate the starter for continuous run current, and lets a smaller enclosure handle the same motor.

Key takeaway: On a compressor that runs more than a few minutes per cycle, an internal-bypass unit (PSTX, ATS480, 3RW55) earns back its cost in reduced enclosure cooling and run-current derating versus a non-bypass economy unit.

Full mechanism and wiring detail is in our soft starter bypass contactor guide.

Voltage Ramp, Current Limit or Torque Control — Which Fits Which Compressor

A screw or centrifugal compressor that starts fully unloaded rarely needs more than voltage ramp with a current-limit ceiling as a safety net. A reciprocating compressor with an unloader that does not fully vent, or one where the unloader timing is not synchronized with the starter ramp, benefits from torque control because it adjusts voltage continuously to match the actual (uneven) torque demand rather than following a fixed profile. Some plants run centrifugal compressors on a VFD instead of a soft starter for speed control during the process, not just for starting — see our soft starter vs VFD comparison for when that trade-off makes sense. For a brand-level comparison of the torque-control flagships, see our ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 comparison. Browse the full range of soft starters across all three brands.

Not every reciprocating compressor needs the flagship unit, though. A small single-stage unit with a well-maintained unloader and light duty cycle is often fine on a mid-range current-limit starter; torque control pays off on larger, harder-to-unload machines and on units running high starts-per-hour. Start with our soft starter selection guide if you have not narrowed the family yet.

Frequently Asked Questions

Can a soft starter start a reciprocating compressor without an unloader valve?

Only if the motor's locked-rotor torque comfortably exceeds the full compression torque at zero speed, which is uncommon on anything but a small unit. Most reciprocating compressors rely on the unloader to make a reduced-voltage start viable at all.

What current limit should I set for a screw compressor soft starter?

300-350% FLC is a common starting point for a properly unloaded screw compressor. Confirm against the motor's actual locked-rotor current and the starter's duty rating rather than using this as a fixed value.

Does a soft starter replace the need for an unloading valve?

No. A soft starter reduces electrical inrush and mechanical shock during the ramp; it does not reduce the gas compression work the piston or rotors must do. The unloader is what makes that work small enough for a reduced-voltage start to succeed.

How many starts per hour can a soft starter handle on a cycling compressor?

It depends on the unit's AC-53a duty rating, not a fixed number — a rating like 3.0-10:50 implies a specific starts-per-hour ceiling at that current and time. Check the rating against the compressor's actual pressure-switch cycling frequency before final selection.

Is torque control worth it for a compressor soft starter?

It is worth it on reciprocating compressors with an uneven or imperfectly unloaded torque curve, and on any compressor where nuisance stalling has been a problem. On a cleanly unloaded screw or centrifugal machine, current limit alone is often adequate.

Should a compressor soft starter have a built-in bypass contactor?

Yes if the compressor runs continuously or for extended cycles — the bypass removes SCR conduction heat during the run and avoids derating. On a short-cycle, frequently-stopping unit the benefit is smaller since the SCRs rarely stay in conduction long.

Conclusion

Compressor soft starter sizing is a torque problem before it is a current problem. Screw and centrifugal machines that start unloaded behave like a fan and are the easy case for any soft starter family. Reciprocating compressors live or die on the unloading valve — a partially open or slow-venting unloader is what actually sets the pedestal voltage and ramp time, and no soft starter setting compensates for an unloader that is not doing its job. Add starts-per-hour duty checking for pressure-switch-cycled units and a bypass contactor for continuous-duty machines, and the selection comes down to matching the torque-control feature set to how uneven the real load curve is.