A positioning axis driven by a brushless servo motor takes its start/stop and torque profile from the servo drive and the servo motor, not from a line-voltage soft starter, because a soft starter sits on a fixed-frequency AC bus and assumes an induction-machine load curve.
Specify a soft starter only when the actuator on that axis is a standard induction motor, where the RFQ line must carry mains voltage, full-load amps, kW, braking duty, kick current, bypass type, IO bus, and enclosure rating; missing any one of these is the most common reason a quote returns with a revised price or a requote cycle.
Why a soft starter does not fit a positioning axis
A soft starter phase-angle-ramps the line voltage on a fixed-frequency AC bus, then hands off to a bypass contactor, which only makes sense for an induction motor that needs a limited inrush to ride through a weak supply or to protect couplings [S3]. A positioning axis fed by a servo drive closes its own current loop at the motor phase using encoder or resolver feedback and applies a controlled torque profile during accel/decel, so the ramp function a soft starter would perform is already inside the drive's S-curve or trapezoidal profile generator.
Putting a soft starter upstream of a servo drive is also electrically wrong, because the drive first rectifies mains to a DC bus, then inverts it at variable frequency; the soft starter's ramped AC output does not help the DC-link pre-charge, and at low speed the drive draws discontinuous current that the soft starter's firing-angle logic misreads as a fault. If a buyer writes a soft starter onto a servo drive RFQ by mistake, the safe corrections are: delete the soft starter line, raise the drive's rated peak current to cover the axis accel pulse, and confirm the servo motor thermal model is sized for the duty cycle.
RFQ spec map: soft starter on an induction-motor positioning axis
Use this line-by-line map for the only case that is legitimate, which is a positioning axis where the prime mover is a standard induction motor (typical example: large indexing table driven through a gear reducer, or a low-dynamics rotary axis with a positioning controller on top).
Line 1, mains: 3-phase, 380-480 V AC ±10%, 50/60 Hz, with the actual plant value written in, because quoting teams price the thyristor stack by voltage class and a missing or wrong nominal adds a margin. Line 2, motor data: kW (real mechanical shaft power at the duty point), FLA at the specified voltage, and locked-rotor current multiple, with FLA in amps because the bypass contactor and the overload relay are sized off it. Line 3, duty: cycles per hour, accel time target, and the equivalent S2/S3/S6 class, because a 6 starts-per-hour rating cannot be reused on a 30 starts-per-hour application.
Line 4, ramp: adjustable initial voltage typically 30-60% of nominal, ramp-up time 1-60 s, and ramp-down time 1-60 s, with the values written as numbers rather than left as "adjustable". Line 5, kick start: a short voltage pulse of 70-100% for 0.1-0.5 s to break a high static load, declared as either "no" or a value, because the option changes the stack rating. Line 6, braking: DC injection or reverse-brake, with stop time target and the maximum braking torque, since a servo positioning axis usually needs a defined stopping distance and a soft starter without braking cannot deliver it.
Line 7, bypass: integral or external contactor with the AC-3 rating sized to FLA, and the bypass engagement window written as a percentage of nominal speed (commonly 80-110%), because the bypass heatsink drops out of the thermal budget only after the motor is up to speed. Line 8, IO: hard-wired start/stop, fault relay, analog 4-20 mA setpoint if any, plus the fieldbus option (PROFINET, EtherNet/IP, Modbus TCP), and for a positioning axis the fieldbus is the link to the servo drive controller above it. Line 9, enclosure: IP class (IP20 panel-mount, IP54 or IP66 cabinet), ambient temperature, and altitude, with derating curves referenced, since the thyristor stack is the most thermally constrained part.
Line 10, protection: electronic overload class 10/20/30, phase loss, phase sequence, and locked-rotor trip; these are the fields whose omission causes the most requote loops because they change the OL relay and the trip curves. Line 11, standards: IEC 60947-4-2 for the soft starter itself, plus the regional electrical code, and for explosive atmospheres the ATEX 2014/34/EU category or the IECEx scheme with the Ex marking written out, not just "ATEX" [S2].
Selection criteria: solid-state, bypassed, and servo-controlled options
Three options are usually offered for an induction-motor positioning axis, and they line up against four decision criteria in a way that drives the pick. Option A is the classic solid-state soft starter with internal bypass (most common in the Danfoss VLT and similar lines), priced low, but its torque control is open-loop and stops are not repeatable to a position. Option B is the soft starter with an external braking contactor and DC injection, which closes the gap on stopping distance but still has no position feedback. Option C is to delete the soft starter entirely and let the servo drive handle ramp, brake, and position, which on a true positioning axis is the engineered solution, with the trade-off of higher cost on the drive side.
On a positioning axis where the spec calls for a defined stopping distance and a known final angle, Option C is the correct pick even if it costs more, because Option A and B cannot guarantee repeatability of the index position. On a high-inertia axis where the soft starter is being used to limit inrush, Option B with DC injection is acceptable when the position is set externally by a mechanical stop. On a low-duty indexing table, Option A is acceptable when the position accuracy is held by the mechanical detent and the soft starter only handles start/stop.
Use cases, limits, and failure modes
Legitimate use cases: large crusher mills, conveyor belts with long acceleration ramps, centrifugal pumps, and low-dynamics rotary axes on packaging machinery where a servo press or soft starter is being chosen against a VFD for cost reasons [S2][S3]. A soft starter on a servo-driven axis is not a legitimate use case and is excluded from the spec map above; the encoder-driven motion control already does the ramp.
Common failure modes that show up in the field, and which the RFQ should pre-empt: thyristor failure from too-frequent starts (locked in by writing the S2/S3/S6 duty on the line), nuisance trips from the phase-sequence relay on a phase-loss-tolerant supply (declared or disabled on the line), and overheating of the bypass contactor because the engagement percentage was set above the motor's pull-up torque point (set on the commissioning sheet). The most expensive failure mode is the soft starter sized off kW without the FLA cross-check, which leaves the bypass contactor under-rated and welding shut on a stall; this is why the RFQ line carries FLA as a separate field, not just kW.
Sourcing, standards, and the comparable drive route
For an RFQ dated 2026-08-11, the spec should reference IEC 60947-4-2 for the soft starter, the regional electrical code, and any hazardous-area marking (ATEX 2014/34/EU or IECEx) with the actual Ex code, and the supplier quotes against those exact lines [S2]. For a positioning axis where the buyer is wavering between a soft starter and a VFD, a variable-frequency drive with a closed-loop position card is the next-up cost option, and the spec swap is to replace the soft starter line with a drive line carrying the same voltage, the same kW, an encoder feedback card, and a positioning profile generator. The buyer who wrote the soft starter by mistake should fix the RFQ before sending it: a positioning axis with a servo motor takes a servo drive, a positioning axis with an induction motor takes a VFD, and only the rare low-dynamics induction axis takes a soft starter.
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