What Is a Soft Starter? How Soft Starters Work and When to Use One
A soft starter explained — how it reduces motor inrush and mechanical shock with controlled voltage ramping, its components, benefits, limits, and PLC control.
A soft starter is a solid-state motor control device that gradually increases the voltage applied to an AC induction motor during starting, then holds the motor at full voltage once it reaches operating speed. By controlling how quickly voltage climbs from near-zero to line voltage, the soft starter limits the surge of current that a motor draws at the instant of energisation — and limits the mechanical shock that surge delivers to the driven load.
That definition is the answer to the most common search on this topic. The rest of this article explains the engineering behind it and, critically, how a PLC uses a soft starter in a real control scheme.
What a Soft Starter Is
A soft starter sits electrically between the supply contactor and the motor terminals. Internally it uses three pairs of silicon controlled rectifiers (SCRs) — one pair per phase — to chop the AC waveform. By delaying the firing angle of each SCR within every half-cycle, the device delivers a reduced effective voltage to the motor. As the ramp timer counts up, the firing angle advances progressively until the SCRs fire early enough in each half-cycle to pass virtually the full sine wave. At that point, a bypass contactor closes across the SCRs and the motor runs at full line voltage from the supply, with the SCRs out of the current path entirely.
The key takeaway for a controls engineer: a soft starter does not change frequency. It reduces voltage only, during starting and stopping. That is both its strength (simplicity, low cost, low heat at run) and its limitation (no speed control at steady state).
The Problem It Solves
Inrush Current
When an AC induction motor is connected directly to line voltage at rest, it draws a starting current that can be five to eight times its full-load rated current. For a 75 kW motor with a 140 A full-load rating, that is potentially 700–1100 A for the first few seconds of acceleration. This inrush:
- Causes voltage sags on the supply bus that affect other equipment
- Stresses motor windings with high magnetic forces on every start
- Trips upstream protection devices that are not sized for surge current
- Increases peak demand charges from the utility on metered sites
Mechanical and Torque Shock
A direct-on-line (DOL) start also delivers full locked-rotor torque to the driven load instantaneously. On a conveyor, pump, or compressor, this can:
- Snap belts and couplings
- Shift product off a conveyor belt
- Water-hammer a pipeline when a pump suddenly accelerates
- Accelerate wear on gearboxes and mechanical seals
A soft starter ramps voltage — and therefore torque — up gradually. The acceleration profile is controllable, which is what makes the technology useful across such a wide range of applications.
How a Soft Starter Works
SCR Phase-Angle Control
SCRs are unidirectional, so a soft starter uses antiparallel pairs — one SCR handling the positive half of the sine wave, the other handling the negative half. The firing circuit delays the gate pulse relative to the zero-crossing of each half-cycle. A 90-degree delay fires the SCR at the midpoint of the half-cycle, delivering roughly half the peak voltage. A 0-degree delay fires immediately after the zero crossing, passing nearly the full half-cycle. The relationship between firing angle and delivered voltage is approximately sinusoidal, not linear, which the soft starter's control algorithm compensates for.
The result is a continuously variable effective RMS voltage delivered to the motor stator, without changing the supply frequency.
Voltage Ramp
The most common starting mode is a linear voltage ramp: the soft starter begins at a configurable initial voltage (typically 30–50 % of line voltage) and ramps linearly to full voltage over a configurable ramp time (commonly 3–30 seconds). The initial voltage must be high enough to produce sufficient torque to break the load away from rest — too low an initial voltage and the motor stalls on the ramp.
Kick Start (Current Boost)
Many soft starters include a kick start or boost function: a short pulse of higher initial voltage (sometimes 80–90 % line voltage for 0.1–0.5 seconds) before the voltage ramp begins. This is used for high-static-friction loads — loaded conveyors, screw compressors, mixers with settled product — where the motor needs a torque impulse to break loose before the controlled ramp takes over.
Soft Stop
On de-energisation, a soft starter can perform a soft stop: ramping voltage back down over a configurable time rather than cutting power abruptly. This is particularly useful for:
- Pumping systems — reducing water hammer and pressure transients when a pump stops
- Conveyors — preventing product from piling up at the discharge end due to sudden deceleration
- Mixers — reducing mechanical shock to agitator blades and seals
Soft stop is not regenerative braking. The motor coasts down under reduced voltage. For loads requiring precise stop positioning or rapid deceleration, a VFD with dynamic braking is the correct technology.
Components of a Soft Starter
| Component | Function |
|---|---|
| SCR pairs (x3) | Phase-angle voltage control on each phase |
| Gate drive / firing board | Generates firing pulses at the correct angle for each SCR |
| Control board / microprocessor | Executes the ramp algorithm, reads feedback, manages I/O |
| Current transformers (CTs) | Monitor motor current for protection and ramp feedback |
| Bypass contactor | Closes at end of ramp to remove SCRs from the current path |
| Thermistor / heat sink monitoring | Protects SCRs from overtemperature |
| Terminal block I/O | Digital inputs, relay outputs, and (on some models) analog I/O |
Modern soft starters integrate electronic overload protection so a separate thermal overload relay is not required, though local regulations and safety standards may still require a separate device in some jurisdictions.
The Bypass Contactor
Once the motor reaches full voltage, the SCRs have done their job. Leaving SCRs in the current path at full conduction would generate unnecessary heat (SCRs have a forward voltage drop of roughly 1–1.5 V per device, which at 140 A is 140–210 W of dissipation per phase). The internal bypass contactor closes across the SCR pairs when the soft starter determines that full voltage is reached, routing current directly through the copper contacts. At this point the SCRs are cold and the motor runs at full efficiency from the supply.
Some smaller soft starters omit the internal bypass contactor to reduce cost. These must be paired with an external bypass contactor, or the SCR heat dissipation must be accommodated in the panel thermal design.
The bypass contactor is what creates the characteristic click you hear at the end of a successful soft start — and it is what the PLC looks for via the run/end-of-ramp feedback relay before releasing the motor to normal speed-demand control.
Benefits and Limitations
Benefits
- Reduced inrush current — typically limits starting current to 2–4 × full-load current versus 5–8 × for DOL
- Reduced mechanical shock — extends service life of couplings, belts, gearboxes, and mechanical seals
- Lower peak demand charges — relevant on sites billed for kVA demand peaks
- Compact, low-maintenance — no moving parts except the bypass contactor; much smaller than a star-delta starter for equivalent duty
- Integrated protection — overload, phase loss, phase imbalance, thermistor input, and under-voltage protection in one device
- Soft stop capability — reduces water hammer and conveyor shock on stop
Limitations
- No speed control at steady state — once the bypass contactor closes, the soft starter is simply a contactor. Speed regulation requires a VFD.
- Heat generation during starting — the SCRs dissipate heat during the ramp. Frequent starting cycles require careful thermal derating per the manufacturer's duty cycle tables.
- Reduced torque at reduced voltage — torque is proportional to the square of voltage. A 70 % voltage start delivers approximately 49 % of full-load torque, which may be insufficient for high-inertia loads.
- Harmonic injection — SCR phase-angle control generates low-order harmonics during the ramp. For sensitive supply buses, a dedicated line reactor or harmonic filter may be required.
- Not suitable for variable speed — if the process requires speed regulation (pump flow trimming, fan speed control, variable conveyor speed), specify a VFD instead.
Soft Starter vs DOL vs VFD
| Starting Method | Inrush Reduction | Speed Control | Cost | Typical Application |
|---|---|---|---|---|
| Direct-on-line (DOL) | None (5–8 × FLA) | No | Lowest | Small motors, short-start applications |
| Star-delta starter | Moderate (reduces to ~33 % torque at start) | No | Low | Fixed-speed pumps, fans |
| Soft starter | Good (2–4 × FLA typical) | No | Medium | Pumps, conveyors, compressors |
| VFD | Excellent (programmable) | Yes | Highest | Variable speed, precise control |
The choice is almost always between a soft starter and a VFD. The soft starter wins when:
- The process runs at one fixed speed and does not need regulation
- Budget and panel space are constrained
- Frequent soft-start cycles are not required (VFDs handle high-cycle-count applications better with regenerative braking)
The VFD wins when:
- The process benefits from variable speed (energy savings on centrifugal loads, process flow control)
- Precise acceleration profiles or torque control are required
- The load has high inertia that a soft starter's reduced torque cannot overcome reliably
For a detailed comparison, see the dedicated VFD vs soft starter guide.
The Controls View: PLC Integration
This is where soft starters move from motor-drive catalogue items into a control engineering problem.
Digital I/O Interface
Most soft starters provide a straightforward digital I/O interface that a PLC drives directly:
| Soft Starter Terminal | PLC Signal | Description |
|---|---|---|
| RUN / START input | Digital output | Initiates the voltage ramp. Held ON to keep motor running. |
| STOP input | Digital output (or de-energise RUN) | Initiates soft stop ramp if configured; otherwise coast. |
| RESET input | Digital output | Clears a latched fault and prepares the unit for next start. |
| RUN relay output (NO) | Digital input | Closes when the bypass contactor engages — confirms motor is at speed. |
| FAULT relay output (NC) | Digital input | Opens on any protection trip — overload, phase loss, thermistor, over-temperature. |
The PLC motor start/stop rung structure for a soft-started motor follows the same motor start/stop ladder logic principles as a standard motor starter, with two important additions: the end-of-ramp interlock and the fault latch with reset.
Analog Interface
Higher-specification soft starters provide an analog output — typically 0–10 V or 4–20 mA — representing actual motor current as a percentage of full-load current. The PLC reads this as a motor load signal, which is useful for:
- Load monitoring and trending — catching gradual mechanical degradation before it causes a trip
- Process protection — detecting a jammed conveyor, a blocked pump, or a lost-coupling condition through unexpected current rise
- Demand management — staggering starts to avoid simultaneous peak demand events
Some soft starters also accept an analog ramp time input, allowing the PLC to set the acceleration ramp dynamically — for example, ramping faster on an empty conveyor, more slowly when loaded.
Start/Stop and Ramp Control from the PLC
A typical PLC control sequence for a soft-started motor:
- PLC receives a Run command (operator pushbutton, HMI, or upstream process interlock).
- PLC checks that the FAULT input is healthy (fault relay NC = closed = healthy) and that any process interlocks are satisfied.
- PLC energises the RUN digital output to the soft starter.
- Soft starter begins the voltage ramp. Motor current rises to the current limit and then the motor accelerates.
- PLC monitors the RUN relay feedback input. When the bypass contactor closes, this input energises, confirming end-of-ramp. The PLC records a timestamp and sets a "Motor Running" status bit.
- During run, the PLC monitors the FAULT input. Any trip causes the FAULT relay to open, the PLC de-energises the RUN output, sets a fault bit, and logs the event.
- To stop: PLC de-energises the RUN output (or pulses a separate STOP input if wired). Soft stop ramp executes if configured.
- To reset after a fault: operator acknowledges on HMI, PLC pulses the RESET digital output, fault latch clears, and the soft starter is ready for the next start.
End-of-Ramp Bypass Logic
The end-of-ramp feedback is more than a status signal — it is a process interlock point. In food processing, for example, a conveyor motor must not be considered "running" until the bypass contactor has confirmed full voltage. If the PLC starts a downstream process the instant it issues the RUN command — rather than waiting for the bypass confirmation — it may attempt to process product while the motor is still accelerating.
The standard pattern in ladder logic is a timer interlock with bypass confirmation:
- Start the RUN output and start a watchdog timer (typically 1.5× the configured ramp time).
- If the bypass confirmation input does not energise before the watchdog timer expires, the PLC de-energises the RUN output and raises a "Soft Starter Ramp Timeout" fault.
- Only when bypass confirmation is active does the PLC release downstream process interlocks.
This logic catches a failed bypass contactor, a misconfigured ramp time, or a load that stalled during the ramp — conditions that would otherwise leave the system in an ambiguous state.
For the foundational PLC programming patterns used to build these routines, see the PLC programming basics fundamentals guide.
Frequently Asked Questions
What is a soft starter?
A soft starter is a solid-state device that controls the voltage applied to an AC induction motor during starting and stopping. It uses silicon controlled rectifiers (SCRs) to ramp voltage from a reduced initial level up to full line voltage over a configurable time period, limiting inrush current and mechanical shock. Once the motor reaches full speed, an internal bypass contactor closes and the motor runs directly from the supply.
How does a soft starter work?
A soft starter works by phase-angle firing control of SCR pairs on each phase of the supply. By delaying the gate pulse of each SCR within every half-cycle of the AC waveform, the effective RMS voltage delivered to the motor is reduced. As the ramp timer progresses, the firing delay decreases, progressively increasing the voltage until full conduction is reached and the bypass contactor closes.
What is the difference between a soft starter and a VFD?
A soft starter controls voltage only during starting and stopping. Once the bypass contactor closes at the end of the ramp, it is electrically equivalent to a closed contactor — the motor runs at fixed speed from the supply. A VFD (variable frequency drive) controls both voltage and frequency continuously, enabling true variable speed control throughout the entire operating range. A VFD costs more and is larger, but is the correct choice for any application requiring speed regulation. See VFD vs soft starter for a full comparison.
Does a soft starter control speed?
No. A soft starter controls speed only during the acceleration ramp — it reduces voltage to limit torque and current while the motor accelerates from standstill to full speed. Once the bypass contactor closes, the motor operates at the fixed speed determined by supply frequency and motor pole count. For variable speed control, a variable frequency drive (VFD) is required.
When should I use a soft starter instead of a VFD?
Use a soft starter when the driven load operates at a single fixed speed and the primary requirements are reduced inrush current, reduced mechanical shock on starting, or soft stop to prevent water hammer. Common applications include fixed-speed pumps, centrifugal fans, conveyors, and compressors. The soft starter is the correct choice when budget, panel space, and thermal constraints rule out a VFD, and when variable speed control is not needed by the process.
Does a soft starter save energy?
During starting, a soft starter reduces peak demand by limiting inrush current, which can reduce utility demand charges on metered sites. However, once at full speed with the bypass contactor closed, a soft starter introduces no energy savings compared to a direct-on-line starter — the motor draws the same current as it would with a plain contactor. Energy savings at partial load require a VFD, which can reduce motor speed and exploit the cubic relationship between speed and power on centrifugal loads.
