A soft start for motor applications is a solid-state device that reduces inrush current and mechanical shock by ramping up voltage to an AC induction motor over a set time (typically 2 to 30 seconds). If you need a direct sizing answer: for standard centrifugal loads (fans, pumps), size your soft starter at 1.5 to 2 times the motor’s Full Load Amps (FLA). For high-inertia or high-breakaway-torque loads (crushers, compressors, reciprocating pumps), size it at 3 to 4 times the FLA. Never size strictly by horsepower without checking the nameplate amperage and starting duty cycle.
Which Motor Types Fit the Soft Start Profile?
Soft starters are almost exclusively paired with 3-phase AC squirrel-cage induction motors. They work by using back-to-back thyristors (SCRs) on each phase to chop the voltage waveform during startup. They do not control speed during steady-state operation; once the motor reaches full speed, an internal bypass contactor closes to shunt the SCRs and prevent overheating.
It is a common mistake to confuse soft starters with variable frequency drives (VFDs) or to assume they can be used on precision positioning motors. Here is how the major motor types break down regarding control needs:
| Motor Type | Torque Curve & Starting Profile | Required Controller | Relative Cost |
|---|---|---|---|
| AC Induction (Squirrel Cage) | High starting torque, massive inrush current (600% of FLA). Needs current limiting. | Soft Starter (for starting only) or VFD (for speed control). | Low / Medium |
| Wound Rotor Induction | Adjustable starting torque via external slip-ring resistance. | External liquid or metallic resistor bank. | High |
| PMSM / AC Servo | High precision, dynamic torque, variable speed. Requires rotor position feedback. | Dedicated Servo Drive (Field Oriented Control). | High |
| Stepper Motor | Open-loop positioning, high holding torque, low-speed vibration. | Stepper Driver (Chopper/Constant Current). | Low |
Note: Stepper and servo motors are never paired with traditional soft starters. They require dedicated pulse/direction or field-oriented control (FOC) drives that manage current vectors, not just voltage ramps.
Sizing Rule of Thumb and Worked Load Example
Sizing a soft starter requires looking at the motor's Full Load Amps (FLA), the locked rotor current (LRC), and the number of starts per hour. Do not rely on blind HP/kW conversions, as a 10 HP motor at 230V draws roughly 28A, while a 10 HP motor at 460V draws roughly 14A. The SCRs inside the soft starter only care about amperage and thermal dissipation.
Worked Example: 15 HP Centrifugal Water Pump
- Nameplate Data: 15 HP, 460V, 3-Phase, FLA = 19.5A, Code Letter J (LRC roughly 6x FLA = 117A).
- Application: Standard centrifugal pump (low breakaway torque, ramps up smoothly).
- Duty Cycle: Maximum 10 starts per hour.
The Math: For a standard centrifugal load with a normal duty cycle, we use a 1.5x to 2x multiplier on the FLA to account for the thermal stress of the starting ramp.
19.5A (FLA) × 1.5 = 29.25A minimum continuous rating.
19.5A (FLA) × 2.0 = 39.0A for a comfortable safety margin.
Concrete Pick: You would select a soft starter rated for at least 39A at 460V. The Schneider Electric Altivar ATS22D47Q is rated for 47A at 460V and handles standard-duty pumping applications perfectly. Expect to pay between $850 and $1,100 for this specific frame size in 2026. If this were a high-inertia rock crusher requiring 30 starts an hour, you would use a 3x to 4x multiplier (19.5 × 3 = 58.5A) and step up to a heavy-duty frame like the ATS22D62Q (62A).
Wiring and Terminal Identification: Inline vs. Inside-the-Delta
When wiring a soft start for motor control, you have two topological choices: standard inline (3-wire) or inside-the-delta (6-wire). Identifying the terminals correctly is critical to avoid blowing the SCRs on the first start command.
Standard Inline (3-Wire) Wiring
This is the most common and foolproof method. The soft starter is placed in series with the motor.
- L1, L2, L3 (Line In): Connects to the upstream line-side contactor or breaker.
- T1, T2, T3 (Load Out): Connects directly to the motor's U, V, W terminals.
- Control Circuit: Usually requires a 24VDC or 110VAC source for the logic board, plus dry-contact inputs for RUN and STOP commands.
Inside-the-Delta (6-Wire) Wiring
If the motor is wired in a Delta configuration, you can place the soft starter inside the delta loop. This is done to downsize the soft starter, as the current flowing through the delta windings is exactly $1/\sqrt{3}$ (about 58%) of the line current.
- Terminals Used: You will use U1/V1/W1 and U2/V2/W2 on the motor, breaking the factory delta links.
- Sizing Advantage: For our 19.5A pump motor, the line current is 19.5A, but the delta winding current is only 11.2A. You can buy a soft starter rated for just 11.2A (multiplied by your safety factor), saving significant panel space and money.
- The Catch: Requires 6 wires pulled to the motor instead of 3, and the motor must be capable of delta wiring (not all Wye-start/Delta-run or specific voltage motors allow this). Consult the motor nameplate diagram.
Failure Signatures: Hum, Overheat, and Stall
When a soft start for motor setup fails, it rarely does so silently. The physical symptoms at the motor and the fault codes on the starter's LED display will tell you exactly what went wrong.
| Symptom | Probable Cause | Diagnostic & Fix |
|---|---|---|
| Hum & Vibration (No Rotation) | Phase loss or severe phase imbalance. One SCR failed open, or a terminal lug is loose, causing single-phasing. | Measure phase-to-phase voltage at T1-T2, T2-T3, and T1-T3 during the start ramp. All three should be balanced within 2%. Tighten lugs to manufacturer torque specs or replace the starter if an SCR is blown. |
| Overheat (Thermal Trip) | Exceeding the starts-per-hour limit, or the starter is undersized for the load's actual inertia. The internal $I^2t$ thermal model trips to save the silicon. | Check the fault log for "Motor Thermal" or "SCR Overtemp". If you are doing 15 starts/hr on a Class 10 rated starter, you must upgrade to a heavy-duty frame, add a line-side bypass contactor, or reduce the cycle rate. |
| Stall (Groaning Shaft) | Initial pedestal voltage (breakaway torque) is set too low. The soft starter ramps voltage, but never reaches the threshold required to break the load's static friction. | Access the parameter menu and increase the "Initial Voltage" or "Pedestal Torque" setting. Bump it up in 5% increments (e.g., from 25% to 30%) until the shaft cleanly breaks static friction without jerking. |
Decision Path: Selecting Your Exact Controller
Do not get paralyzed by catalog options. Use this decision matrix to lock in the exact hardware you need based on your load profile and operational requirements. This path terminates in specific, purchasable part numbers.
| Load Profile & Requirement | Starts per Hour | Speed Control Needed? | Concrete Hardware Pick (460V Class) |
|---|---|---|---|
| Standard Centrifugal (Pumps, Fans, Blowers) | < 10 | No (Fixed speed) | ABB PSR Series (e.g., PSR30-600-70). Compact, analog dial setup, built-in bypass. ~$600. |
| High Inertia / High Torque (Compressors, Crushers, Conveyors) | 10 to 30 | No (Fixed speed) | Schneider Altivar ATS22 (e.g., ATS22D62Q). Digital keypad, advanced torque control, heavy-duty thermal mass. ~$1,200. |
| Variable Flow / Energy Saving (HVAC pumps, process fans) | Continuous modulation | Yes | ABB ACS580 VFD (e.g., ACS580-01-032A). A soft starter cannot do this; you must use a VFD to vary frequency and save energy. ~$1,800. |
The Final Verdict: If your goal is strictly to eliminate the 600% inrush current spike and reduce mechanical water-hammer or belt-slap on a standard 3-phase induction motor, buy an ABB PSR (for simple loads) or a Schneider ATS22 (for tough loads). If you realize during this selection process that you actually need to slow the motor down to 80% speed to save on your monthly electricity bill, abandon the soft starter entirely and purchase a VFD. Soft starters manage the first 10 seconds of operation; VFDs manage the entire runtime.
For deeper technical specifications on thyristor firing angles and coordination with upstream short-circuit protection (Type 1 vs Type 2 coordination), refer to the ABB Softstarter Application Guides or the Schneider Electric Altistart 22 documentation.
