Main Components of a Soft Starter: SCRs, Bypass and Control
What are the main components of a soft starter? A soft starter's construction centers on six back-to-back SCRs (thyristors), two per phase, wired either in-line with the motor or inside the delta per IEC 60947-4-2, plus a control board that fires each pair at a calculated angle to ramp terminal voltage. Get any one of these wrong at spec time — undersized SCRs, no bypass on a continuous-duty pump, a control board without current limit — and the starter nuisance-trips or overheats on the first hot shift. This article covers the SCR power stage, the firing and control board, the bypass contactor and heat sink, current and voltage sensing, protection electronics, and the terminal layout that decides in-line versus inside-delta connection.
The SCR Power Stage
Every soft starter has six SCRs (silicon-controlled rectifiers), two per phase in back-to-back pairs. One SCR of each pair conducts the positive half-cycle, the other the negative half-cycle, so together they pass full AC current while letting the control board delay the firing angle within each half-cycle. Delay the firing angle further and less of the sine wave reaches the motor; advance it and more does. That is the entire mechanism behind a voltage ramp — no moving parts, no taps, just timing.
SCR current rating sets the starter's frame size, and that rating is not the motor's full-load current alone — it is FLC times the duty class. A frame built for AC-53a 3.0-10:50 has to survive 3x FLC for 10 seconds, 50% duty, without derating; a frame spec'd against a lighter duty table will run hot on the same motor. See AC-53a and AC-53b duty sizing for how that table drives frame selection. For the phase-angle firing sequence itself, how SCR phase-angle control works covers the half-cycle timing in more detail.
Firing Circuit and Control Board
The control board reads line voltage and current on every half-cycle — 10 ms at 50 Hz, 8.3 ms at 60 Hz — and recalculates the firing angle for the next one. On an economy unit this loop only tracks a fixed voltage ramp: pedestal voltage rising to full voltage over a set time, open loop, no feedback from the motor. On a feature unit the same board runs current-limit or closed-loop torque control instead, holding start current at a ceiling or shaping torque directly. Voltage ramp, current limit and torque control compares the three methods motor by motor.
What we see in the field: a plant will buy the cheapest voltage-ramp unit for a pump, then complain about check-valve slam on stop. The control board was never the problem — it simply never had a soft-stop or torque-control mode to sell them in the first place. That is a spec-sheet issue, not a wiring issue.
Bypass Contactor and Heat Sink
An SCR pair dissipates heat only while conducting, and it conducts on every cycle the motor runs — not just during the start ramp. Left in the circuit at full speed, the SCR stack keeps shedding that heat into the enclosure for as long as the motor runs, which is why continuous-duty starters size a heat sink and forced-air path around it. A bypass contactor removes that problem: once the motor reaches full speed, the contactor closes across the SCR pairs and carries the run current instead, at near-zero conduction loss.
Formula: SCR Conduction Loss (per phase, unbypassed) — Source: typical manufacturer thermal design data
PSCR = k × Iline
| Symbol | Description | Unit |
|---|---|---|
| P_SCR | Heat dissipated per conducting SCR pair | W |
| k | Loss coefficient, roughly 1 to 1.5 per phase (device-dependent) | W/A |
| I_line | Steady-state line current during run | A |
Bypassed, a starter needs a smaller enclosure and no derating for continuous run current at high ambient. Unbypassed, the SCRs stay in the circuit permanently and the enclosure has to shed that heat all day — fine for an intermittent conveyor start, worse for a cooling-tower fan that runs 24/7 in summer. Why a bypass contactor is used goes through the sizing math for both cases.
Current and Voltage Sensing, Protection Electronics
Three current transformers, one per phase, feed the control board the real-time current signal it needs for current-limit and current-ramp control — and, on the same signal, for overload protection sized to an I²t class (10, 20 or 30). Voltage sensing on the line side does the rest: phase loss, phase imbalance, and phase sequence checks all run off the same three-phase voltage measurement, before the motor is ever allowed to start.
Feature-tier boards add shorted-SCR detection (comparing expected versus actual current at a given firing angle) and undercurrent detection, which on a pump is the only electronic sign an impeller is spinning dry. That single protection function is the difference between a starter that just starts a motor and one built for pump duty, where dry-running is the failure mode that actually damages equipment.
Terminal Layout: In-Line vs Inside-Delta Wiring
The last component decision is not electronic at all — it is how the SCRs connect to the motor terminals. In-line (3-wire) wiring puts the starter in series with the supply line, so each SCR pair carries the full line current. Inside-delta (6-wire) wiring places the SCR pairs inside the motor's delta winding instead, so each pair only sees the phase current, which is line current divided by √3 — about 58% of it.
That lower per-device current lets a smaller starter frame run a bigger motor, which is why inside-delta shows up on high-current retrofits where enclosure space is fixed. The trade-off: it needs all six motor leads brought to the starter, correct phase matching between SCR and winding, and a motor terminal box built for six leads in the first place — not every existing installation has that. In-line vs inside-delta connection walks through when the smaller frame is worth the extra wiring.
HMI, Keypad and Communication Modules
The visible control-board interface tracks the same tiering as the electronics behind it. Economy units (ABB PSR, Schneider Altistart ATS01, Siemens 3RW30/3RW40 class) set ramp time and current limit with analog trimmers or DIP switches — no display, no comms. Mid-range units add a basic keypad and built-in bypass but stop short of torque control. Flagship units (ABB PSTX, Schneider Altistart ATS480, Siemens SIRIUS 3RW55) add a detachable LCD keypad, closed-loop torque control, and a communication port for Modbus RTU as standard, with PROFIBUS, PROFINET or EtherCAT available through a gateway module.
This depends on what the panel actually needs to report back to a PLC or SCADA system. A stand-alone pump skid rarely needs fieldbus; a multi-motor process line usually does, if only to pull start current and fault status into the same dashboard as the drives next to it. ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 lines up the flagship-tier electronics side by side. Browse current stock of all three families in the soft starters collection.
Frequently Asked Questions
Does every soft starter have a built-in bypass contactor?
No. Economy units such as ABB PSR or Schneider Altistart ATS01 are typically voltage-ramp only, with no built-in bypass. Bypass is standard on most mid-range and flagship units (ABB PSE/PSTX, Schneider ATS22/ATS480, Siemens 3RW50/3RW55), but always check the specific reference before assuming.
How many SCRs does a soft starter have?
Six, arranged as three back-to-back pairs, one pair per phase. Each pair blocks current until the control board fires it at the calculated angle, letting the board control conduction on both the positive and negative half-cycles.
What is the difference between in-line and inside-delta wiring?
In-line (3-wire) puts the SCRs in series with the supply, so each pair carries full line current. Inside-delta (6-wire) places the SCRs inside the motor's delta winding, so each pair carries only phase current — about 58% of line current — which allows a smaller starter frame on the same motor, at the cost of needing all six motor leads.
Why does a soft starter need current and voltage sensing if it already has SCRs?
The SCRs only do the switching; the control board needs a live current and voltage signal to know what to switch. Current transformers feed current-limit control and overload protection, and voltage sensing on the line side catches phase loss, phase imbalance and incorrect phase sequence before the motor starts.
What does the keypad or HMI on a soft starter actually control?
On flagship units it sets and displays ramp time, current limit, torque-control parameters, protection thresholds and fault history, and often configures the fieldbus address. On economy units without a keypad, the same parameters are set with DIP switches or analog trimmers instead.
Can a soft starter run continuously without a bypass contactor?
It can, but the SCR stack keeps dissipating heat the entire time it runs, not only during the start ramp. For continuous or near-continuous duty (cooling tower fans, some compressors), an unbypassed unit needs a larger heat sink and enclosure, or a bypass contactor should be specified instead.
Conclusion
A soft starter is not one part — it is an SCR power stage sized to a duty class, a control board that turns that stage into a voltage ramp, current-limit or torque-control profile, a bypass contactor that removes SCR heat once the motor is up to speed, sensing and protection electronics behind the scenes, and a terminal layout that decides in-line versus inside-delta wiring. Missing any one of them on the spec sheet shows up later as a nuisance trip, an oversized enclosure, or a starter that cannot talk to the rest of the panel. Check all five against the actual duty cycle and motor terminal box before ordering — the soft starter selection guide walks through that process end to end.
