Soft Starter Bypass Contactor: Why and When to Use One

Why does a soft starter need a bypass contactor? A soft starter's SCRs dissipate roughly 1-1.5 W per amp per phase while conducting, and once the motor reaches full speed that heat has nowhere useful to go unless it leaves the current path. A bypass contactor closes across the SCRs after the ramp finishes, shorting them out so run current flows through metal contacts instead of silicon junctions, which cuts heat, shrinks the enclosure, and removes the derate a non-bypassed starter needs for continuous duty. This article covers why SCR conduction losses force the issue, how the bypass sequence and interlocking work, built-in versus external bypass, the sizing and enclosure payoff, and where the bypass contactor stops being the whole story.

What the Bypass Contactor Actually Does

A soft starter ramps voltage (or current, or torque) during start using back-to-back SCR pairs on each phase. Those SCRs stay in the circuit for the whole start, but they do not need to stay there once the motor is at speed and drawing steady-state current. The bypass contactor is wired in parallel across each SCR pair. When the control logic decides the ramp is complete, it closes the bypass contacts, giving run current a metal path with near-zero voltage drop instead of a semiconductor path that always drops 1-2 V per SCR and dissipates the difference as heat.

Two details matter here. First, the contactor has to be rated for the motor's full-load current on a continuous-duty basis, same as any line contactor — it is not a light-duty accessory. Second, the SCRs are not physically removed from the circuit; they just stop conducting once bypassed, on units that also cut or hold off the firing pulses. Some designs keep firing at 100% instead of removing pulses, using the bypass purely as a heat management path; check the manufacturer's sequence before assuming.

Why SCRs Run Hot: Conduction Losses

Every SCR pair that is conducting drops roughly 1-1.5 V across the junction, and that drop times the current is heat, not useful work. Across three phases at real motor currents, that adds up fast. A 200 A motor running non-bypassed might dissipate 300-900 W continuously in the SCRs alone, all of which has to be removed by a heatsink and forced or natural air ventilation inside the enclosure.

Multiply that per-phase figure by three and the total starts to matter for panel design, not just the starter itself. Bypass it, and the same 200 A motor drops to the contactor's own contact resistance losses — typically under 5% of the SCR figure — which is why bypassed starters run noticeably cooler to the touch after a few hours of continuous operation.

Bypass Sequence and Interlocking

The bypass does not close the instant the ramp timer expires. On a decent unit, the logic waits until the motor is confirmed at line frequency (voltage ramp complete, or current has settled below a threshold on current-limit and torque-control units), then closes the bypass contactor, then either removes or reduces SCR firing. This order matters: closing the bypass before the SCRs are fully conducting can cause a current spike as the contactor suddenly shorts a partially-fired SCR pair. Reversing the order on stop matters too — for a plain stop, the bypass has to open before the SCRs take back control of the shutdown, and for a soft-stop or ramp-down profile, the bypass must open before the ramp-down begins, or the motor just gets disconnected abruptly at full voltage instead of ramping down.

Mechanical or electrical interlocking between the bypass contactor and the SCR firing circuit is not optional on a well-built starter. Without it, a control fault could leave both paths trying to carry current at once, which stresses the SCRs and can weld or damage the bypass contacts.

Built-In vs External Bypass

Flagship units (ABB PSTX, Schneider Altistart ATS480, Siemens 3RW55) build the bypass contactor into the starter housing, sized and interlocked by the manufacturer, with the sequencing handled in firmware. Economy units (ABB PSR, Schneider ATS01, Siemens 3RW50 in some configurations) either omit the bypass entirely or leave it as a field-added external contactor that the panel builder wires and interlocks separately, usually driven by an auxiliary "at speed" or "run" output relay from the starter.

External bypass costs more panel space and more engineering time — you are now sizing, wiring, and interlocking a second device — but it lets you retrofit bypass capability onto a starter that did not ship with one. It also means the failure mode is different: with a built-in bypass, a failure is a starter fault; with an external bypass, it can be a wiring or interlock fault outside the starter's own diagnostics, which is harder to troubleshoot from the keypad alone. For a detailed brand-by-brand comparison of which lines bypass what, see ABB PSTX vs Schneider ATS480 vs Siemens 3RW55.

Sizing and Enclosure Impact of a Bypassed Starter

A bypassed starter changes three things in the panel design. The heatsink and any forced-air fan can be sized for start-duration heat only, not continuous run heat, because the SCRs are not conducting once bypassed — this is the single biggest driver of the smaller footprint bypassed units have compared to non-bypassed models at the same current rating. The enclosure ventilation budget for the whole panel drops with it, which matters more in outdoor or NEMA-rated enclosures with limited airflow. And the starter's own current rating no longer needs a continuous-duty derate; a non-bypassed unit run continuously at its nameplate current will run hotter than the same physical device operated in bypass, so manufacturers publish a lower continuous rating for non-bypassed use.

When the Bypass Isn't the Whole Story

What we see in the field: panel builders sometimes treat "has a bypass contactor" as equivalent to "runs cool, no more thinking required," and that skips two things worth checking. The bypass contactor itself has a mechanical and electrical life — normal contactor wear applies, and a bypass contactor that welds shut from repeated hard starts will fail exactly like any other contactor, just less visibly because it is buried inside a starter enclosure. And a bypassed starter that also needs soft-stop, ramp-down, or braking still has to open the bypass and hand control back to the SCRs for that stop event — if the interlock logic is misconfigured or a spare-parts contactor was swapped in without matching the original coil voltage or auxiliary contacts, the ramp-down feature simply stops working while everything else looks normal.

Common Bypass Wiring and Sequencing Mistakes

Undersizing the external bypass contactor to the starter's current-limit rating instead of the motor's actual full-load current is the most common error on retrofit jobs — the bypass has to carry continuous run current, not the reduced start current. Skipping the auxiliary interlock between bypass and SCR firing, on units that expect the panel builder to wire it, is the second most common; it works fine until a firing-circuit fault coincides with a bypass closure and stresses both devices. Third: not checking whether the "at speed" signal driving an external bypass is based on a timer (fires regardless of whether the motor actually reached speed) or on actual current/speed feedback — a timer-based signal on a load with variable starting time under different conditions (a pump against varying head, for example) will close the bypass too early or too late depending on the day.

Conclusion

The bypass contactor is not a convenience feature — it is what lets a soft starter run continuously at its full nameplate rating without derating for SCR heat. Built-in bypass on flagship units (ABB PSTX, Schneider ATS480, Siemens 3RW55) removes the field-engineering burden of sizing, wiring, and interlocking an external contactor, at the cost of a higher unit price. Economy units without a built-in bypass are a reasonable choice for light, intermittent duty, but check the non-bypassed continuous rating against the actual duty cycle before assuming the smaller unit is a fair swap. For the broader picture on how these ramp and control methods interact with the bypass decision, see the soft starter selection guide, the breakdown of voltage ramp, current ramp and torque control, and how SCR phase-angle control actually fires the thyristors that the bypass eventually shorts out. Stoklink stocks bypassed and non-bypassed soft starters across all three brands for both retrofit and new-panel builds.