Voltage Ramp vs Current Ramp vs Torque Control Soft Starters
What separates voltage ramp, current ramp, and torque control on a soft starter? All three regulate SCR firing angle to ramp motor terminal voltage during start under IEC 60947-4-2, but voltage ramp follows a fixed time-based curve open loop, current limit clamps start current to roughly 300-400% of full-load current (FLC), and torque control calculates motor torque in real time to hold a linear speed rise. That difference decides how repeatable the start is from one supply dip to the next, and whether a pump ramps smoothly enough to avoid check-valve slam. This article breaks down the control logic behind each method, the torque-vs-voltage square law that limits all three, how ABB, Schneider, and Siemens implement them across their ranges, and which method fits pumps, conveyors, and high-inertia fans.
Voltage Ramp: The Open-Loop Baseline
Voltage ramp is the simplest of the three modes. The starter fires the SCRs at a rising angle over a fixed ramp time, typically 5-30 seconds, starting from an initial voltage or pedestal set at 30-50% of line voltage. It never measures motor current or torque during the ramp — the curve runs the same whether the motor is lightly loaded or near breakaway. Phase-angle firing on the SCRs is what produces the ramp itself, and that mechanism is identical across all three control modes described here. Voltage ramp is simply the plainest way to schedule it.
Open loop means no feedback path corrects the curve mid-start. If the supply sags 10% during the ramp, or the load is heavier than the panel builder assumed, actual current draw shifts and the starter does not compensate. This is why voltage ramp units sit at the bottom of the range: ABB PSR and Schneider Altistart ATS01 use it exclusively, with no built-in bypass and no current or torque feedback. Fine for a fan or a lightly loaded pump. Not for a load with variable breakaway torque.
Current Limit and Current Ramp: Capping Inrush
Current limit adds one feedback variable: current. The starter measures RMS current on each phase and holds the firing angle wherever it needs to be to keep current at or below a set ceiling, commonly 300-400% FLC, until the motor accelerates past the ceiling on its own. Current ramp is a close variant that lets the ceiling rise gradually rather than jumping straight to the limit — gentler on the supply, slightly slower to reach full speed.
Formula: Current-Limited Start Current — Source: IEC 60947-4-2, current-limiting starting mode
Istart = CL × IFLC
| Symbol | Description | Unit |
|---|---|---|
| Istart | Actual current drawn during the current-limited portion of the start | A |
| CL | Current-limit setting, expressed as a multiple of FLC | ratio |
| IFLC | Motor full-load current from the nameplate | A |
This is the mode most panel builders reach for when a site carries a strict utility inrush limit, because it gives a predictable current ceiling regardless of load variation. Direct-on-line draws 6-8x FLC uncontrolled; current limit holds it to whatever the setting allows. ABB PSE, Schneider Altistart ATS22, and Siemens SIRIUS 3RW50 all support current limit with built-in bypass on most references. What current limit does not do is regulate torque directly, so a pump can still see an uneven acceleration through the ramp even with current held flat — torque and current are not linearly related once the rotor is turning.
Torque Control: Closed-Loop Regulation for Sensitive Loads
Torque control closes the loop on the variable that actually matters to the mechanical system: torque. The starter continuously calculates developed motor torque from measured current, voltage, and phase angle, then adjusts firing angle to hold a linear torque or speed rise through the whole start. ABB PSTX, Schneider Altistart ATS480 (branded TCS, Torque Control System), and Siemens SIRIUS 3RW55 implement this as their flagship mode, alongside detachable HMI keypads and fieldbus options that the lower lines skip.
What we see in the field: torque control earns its extra cost almost entirely on pumps and conveyors, where a plain voltage ramp or current limit still produces a jerky first second or two of motion — visible in the piping, audible in the belt. On a fan or a lightly loaded compressor, the gap between current limit and torque control is often not worth the cost difference. Torque control also enables the soft-stop function many pump applications need, ramping torque down at the end of a cycle to reduce check-valve slam, which the other two modes cannot manage symmetrically.
The Square Law Behind All Three Methods
Every one of these control modes runs into the same physical limit: starting torque falls with the square of applied voltage, not linearly. Drop the terminal voltage to 50% of line and the motor produces roughly 25% of its DOL locked-rotor torque, not 50%.
Formula: Torque vs. Voltage Ratio — Source: NEMA MG-1 / IEC 60034 motor torque-voltage relationship
T / TDOL = (V / Vline)2
| Symbol | Description | Unit |
|---|---|---|
| T | Motor torque at the reduced voltage | N·m or % |
| TDOL | Locked-rotor torque at full line voltage (DOL) | N·m or % |
| V | Applied motor terminal voltage during the ramp | V |
| Vline | Full line voltage | V |
This is why the choice of control method never removes the #1 sizing trap: whatever curve the starter runs, the load's breakaway torque must stay below the reduced motor torque at every point in the ramp, or the motor stalls partway through. Torque control manages this more precisely than voltage ramp or current limit, but it cannot start a load that genuinely needs more torque than the motor produces at reduced voltage. Sizing still has to account for the worst-case breakaway point, not the average. For the full sizing walkthrough, see how to select and size a soft starter for the motor.
Bypass and Heat: Independent of Control Method
Whichever of the three modes runs the ramp, the SCRs dissipate roughly 1-1.5 W per amp per phase while conducting. A bypass contactor closes once the motor reaches full speed, shorting the SCRs so they stop conducting and run cool — this happens the same way after a voltage-ramp start, a current-limit start, or a torque-controlled start. See how a soft starter bypass contactor works for the enclosure and derating implications. What changes with control mode is only how the ramp itself behaves, not what happens once the motor reaches speed.
Matching Control Method to the Load
The table below is a starting point, not a substitute for checking the load's actual breakaway torque and duty cycle against the soft starter selection guide.
| Criteria | Voltage Ramp | Current Limit / Ramp | Torque Control |
|---|---|---|---|
| Feedback variable | None (open loop) | Current (closed loop on current only) | Calculated torque (closed loop) |
| Start repeatability under load variation | Low | Medium | High |
| Typical products | ABB PSR, Altistart ATS01 | ABB PSE, Altistart ATS22, SIRIUS 3RW50 | ABB PSTX, Altistart ATS480, SIRIUS 3RW55 |
| Best-fit load | Fans, lightly loaded pumps | Sites with a strict utility inrush cap | Pumps (soft stop), conveyors, high-inertia loads |
| Built-in bypass | Rarely | Most references | Standard |
| Relative price tier | Lowest | Mid | Highest |
Pumps deserve a specific note: soft starters sized for pump duty generally justify the added cost of torque control because water-hammer damage from an abrupt stop costs more than the price gap between a current-limit and a torque-control unit. A workshop compressor with no piping to protect rarely needs the same spec — this depends on the pump's static head and check-valve type, so it is not a universal rule.
Frequently Asked Questions
Is torque control worth the extra cost over current limit?
On pumps and conveyors, usually yes — the smoother ramp and soft-stop capability reduce mechanical wear that current limit alone cannot address. On fans or lightly loaded machines, current limit is often adequate and cheaper.
Can a soft starter combine voltage ramp and current limit?
Yes. Most mid-range units, including ABB PSE and Siemens SIRIUS 3RW50, run a voltage ramp as the base curve with a current limit acting as a ceiling that overrides the ramp if current would otherwise exceed the set value.
Does current limit protect against locked rotor better than voltage ramp?
Both rely on the starter's separate locked-rotor and stall protection functions rather than the ramp mode itself. Current limit does cap the current a stalled motor draws more predictably than an open-loop voltage ramp.
Which control method suits a centrifugal pump?
Torque control, where the budget allows it, because the soft-stop function ramps torque down symmetrically at shutoff and reduces check-valve slam. This depends on the pump's static head and pipe layout, so it is not universal.
Do all soft starter brands offer torque control?
No. It is reserved for the flagship line in each brand — ABB PSTX, Schneider Altistart ATS480, Siemens SIRIUS 3RW55 — while economy and mid-range lines stop at voltage ramp or current limit.
Conclusion
Voltage ramp, current limit, and torque control are three answers to the same question: how much feedback does the ramp need? Voltage ramp needs none and costs the least. Current limit adds a current ceiling and suits sites with a hard inrush cap. Torque control closes the loop on the variable the mechanical system actually feels, and earns its price on pumps, conveyors, and other loads sensitive to how the start unfolds — not on every motor that needs one. None of the three removes the square-law limit on starting torque, so sizing against breakaway torque still comes first and control mode second. Browse the full range of soft starters to compare current-limit and torque-control models by frame size and rating.
