Soft Starter vs Star-Delta Starter: Cost, Current & Torque

Quick answer:

A star-delta starter cuts starting current and starting torque to roughly one-third of a direct-on-line start, and none of it is adjustable. A soft starter ramps the voltage instead, so you can set a current limit, keep built-in motor protection and stop the load smoothly – at a higher device price. Choose star-delta for light, low-torque loads; choose a soft starter when the load needs more starting torque, protection or a soft stop. Simply Buy supplies both routes as a multi-brand distributor and supply-chain integrator, with genuine stock and DDP delivery.

This guide is for B2B distributors, system integrators, EPC buyers and plant maintenance teams choosing a motor starter for a three-phase motor. It answers three buying questions in order: how much starting current each option draws, how much starting torque it leaves you, and what each one really costs to buy, run and maintain. Then it gives you a selection table and the information to send for a quote.

Scope: low-voltage three-phase induction motors up to 690 V, started at fixed speed. Every motor starter here is a fixed-speed device. Not medium voltage, and not variable-speed drives – when a VFD is the answer, it is usually because you need speed control rather than a gentler start.

Two terms first. Direct-on-line (DOL) starting means switching the motor straight onto the supply – the cheapest method and the harshest. A contactor is a heavy-duty relay that switches motor current. Compare a soft starter vs star delta starter on price alone and the star-delta starter wins almost every time; our motor starter range covers both routes.

Which Motor Starter Reduces Starting Current, and By How Much?

A cage induction motor started direct-on-line typically draws 6 to 8 times its full-load current while it accelerates – a figure stated plainly in Nidec’s engineering guide to soft starters. That surge is why both the star-delta starter and the soft starter exist, and both work the same way: they reduce the voltage the motor winding sees.

A star-delta starter does it in one fixed step. In the star connection, each winding sees 1/√3 of the line voltage, about 58%, so winding current falls by the same ratio, and torque – which follows the square of voltage – drops to one third. Because line current in star equals winding current, the current at the terminals lands near one third of a DOL start. On a motor that draws 6 to 8 times In at DOL, that is roughly 2 to 2.7 times In. Low, and free of electronics – but fixed.

A soft starter spreads the same reduction across a controlled ramp. Schneider Electric’s Altistart 48 (ATS48) lets you set the maximum starting current from 1.5 to 7 times motor In, capped at 5 times the starter’s nominal current, with ramp-up times from 1 to 60 seconds, per the Altistart 48 catalogue. For fans, compressors and crushers, the usual working range is 300% to 450% of full-load current.

Internally, they differ: a soft starter uses thyristors – solid-state switches with no moving parts – while a star-delta starter uses three contactors and a timer. What you pay for with a soft starter is control, not extra torque: both motor starter types reduce voltage, and only one lets you choose how much.

Two Things a Star-Delta Starter Does That a Soft Starter Does Not

First, a fixed low current for very light loads. For light, repetitive starting duty, a star-delta starter can hold starting current low and costs less than a soft starter, and the Nidec guide makes that argument directly. Fair – provided you accept the second one.

Second, the transition. At the star-to-delta changeover, there is a current transient and a torque shock, described in the same guide. That is the moment a star-delta starter loads the motor, the coupling and the driven machine. A soft starter ramps continuously, with no changeover step.

Why Does Starting Torque Decide More Than Starting Current?

Because the load, not the motor starter, sets the limit. Ask one question: can this load reach full speed on one third of the motor’s DOL starting torque?

Motors declare locked-rotor and starting torque under IEC 60034-12. For a common design-N motor, the figure typically falls between about 1.5 and 2.5 times rated torque, so one third lands near 0.5 to 0.85 times rated torque. Read the datasheet rather than assuming: if it declares 2 times rated torque at DOL, a star-delta starter leaves you about 0.67 times rated torque, and the load either accelerates on that or it does not.

Loads that start comfortably at low torque:

• Centrifugal pumps and fans, where load torque rises with the square of speed and is low at standstill.
• Conveyors and compressors started empty or lightly loaded.

Loads that do not:

• Positive-displacement pumps, screw compressors and extruders, which need roughly constant torque from zero speed.
• Loaded belt conveyors, crushers, mills and mixers, with high breakaway torque.
• High-inertia loads such as large fans, where a fixed torque ceiling stretches acceleration time and heats the motor.

Where the duty is a pump or a fan, worked selections sit on the fan and pump VFD control E air compressor control pages. Where the load is heavy from zero speed, a VFD for elevator and crane duty is often the honest answer rather than any motor starter.

The Retrofit Trap That Cancels the Most Star-Delta Plans

A star-delta starter needs a motor with six terminals and a winding rated for delta at the line voltage. A motor whose nameplate shows a single connection, or a winding configured star for the line, cannot be star-delta started without rewinding. Check the terminal box before you request a quotation; this one detail cancels more star-delta plans than torque does.

No motor starter manufactures torque. If the load will not accelerate at the current limit, a different setting will not fix it. Your options are a larger motor, a different starting method or a VFD.

What Does Each Motor Starter Really Cost to Buy and Run?

Compare the two motor starter options as bills of material, not as unit prices.

A star-delta starter assembly contains:

• Three contactors, and not three of the same size. The main contactor carries full line current; the delta contactor carries winding current, about 58% of line current; the star contactor is typically rated at about one third of the motor rating. Schemes vary, so take the contactor schedule from the manufacturer’s selection table. A range such as TeSys Deca covers 9 A to 150 A AC-3, with frame selection following those ratios.
• UN relè di sovraccarico. Where it sits changes the setting: in the winding circuit it is set near 58% of motor full-load current; in the line it is set to full-load current.
• A timer relay, or PLC logic, for the changeover.
• Six-core cable to the motor, or two three-core runs, plus the extra terminals and panel space.
• Commissioning time: timer setting, transition verification, and a re-check whenever load conditions change.

Avviatore motore TeSys GV2

Produttore: Schneider Electric
Modello: TeSys GV2
Tipo: Interruttore automatico del motore
Applicazione Relay: 0,06 kW a 75 kW Protezione motore
Compatibilità del prodotto: LC1D

Visualizza prodotto

A soft starter assembly contains:

• The soft starter itself. The ATS48 covers 4-900 kW (5-1200 hp) on 230-415 V supplies with IP20 protection, Torque Control System and Modbus RTU, per the Simply Buy listing. The wider soft starter category covers other brands and ratings.
• A bypass contactor – a contactor that shorts out the starter’s power section once the ramp has finished, so it stops generating heat. On the ATS48, this bypass is external, so budget for it.
• Protection inside the device: motor thermal protection with selectable class (2 to 30, class 10A, sub-class 2, or off), underload, excessive starting time, current overload, line phase inversion, phase loss threshold, PTC probe monitoring and starter thermal protection. On a star-delta panel, each of those is either a separate purchase or a gap.
• Three-core cable in the standard line connection.
• One part number spanning a range of motor ratings, which cuts the spare lines you carry.

The crossover point depends on labour rates, cable runs and panel practice, so treat any universal kW threshold as a rough pattern, not a rule. At small ratings, the two assemblies land close together and installation decides it. As the motor grows, the star-delta starter side grows in copper, terminals and panel space, while the soft starter side grows mainly in the device. We quote line by line, so the comparison that means anything is a full BOM.

The Cost That Never Appears on the Quote

Three contactors contain moving parts and wear on every start. TeSys Deca LC1D contactors are rated for up to 1 million electrical operations in AC-3 per the product listing – a large number, but still a wear item. A soft starter’s main power path has no moving parts apart from the bypass contactor, and soft stopping removes the water hammer that damages pump pipework, check valves and seals. On a pump starting several times a day, that is a maintenance line item.

Which Motor Starter Fits Your Application?

Table 1 – Head-to-head comparison

Table 2 – Which motor starter to specify

Get a BOM, Not a Unit Price

Send us the application and we will price both motor starter routes on one sheet. Useful for the first reply: motor nameplate (kW, voltage, full-load current, terminal count), load type, starts per hour, supply arrangement, cable distance, destination and Incoterm, plus any documentation your project requires. Send it through the contact page or straight to our team, and we normally return a quote within 12 hours.

When Is Neither Starter the Right Answer?

Not every motor needs a reduced-voltage motor starter. A small motor on a stiff supply often belongs on a direct-on-line starter, and forcing a soft starter onto it adds cost and a failure mode.

Where motors are spread across a machine, an electronic motor starter mounted in the I/O rack beats a panel full of contactors. The Siemens SIMATIC ET 200SP 3RK1308-0CE00-0CP0 fail-safe direct-on-line motor starter covers 4.0 to 12 A and motors up to 5.5 kW at 400 V, with electronic switching, electronic overload protection and PROFIenergy, and it brings safety-related switching into the same rack as the control system. It is a DOL device, not a reduced-voltage one: it solves motor protection and safety integration, not inrush. Siemens alternatives sit in the Siemens range.

When the process needs speed control, or real torque control below base speed, neither motor starter in this comparison is right. That is a VFD conversation.

How Do You Buy a Motor Starter Without Getting Burned?

Simply Buy is a multi-brand distributor and supply-chain integrator. We do not manufacture motor starters; we source them across more than 100 industrial brands and a network of over 300 suppliers, and deliver to more than 50 countries. That positioning matters in four ways.

Genuine-product verification. Motor starters are a counterfeit-prone category, and a relabelled unit or a refurbished frame is not obvious in a photograph. Ask for country-of-origin documentation and, where the line carries it, distributor documentation, and record serial numbers at dispatch. Our own quality system is ISO 9001:2015 certified. For the wider picture on this risk, see our guide to sourcing ABB products without counterfeit risk.

Certificates. Tell us which ones your project requires – a CE declaration, a UL file number, a country of origin – and we confirm what the motor starter carries before the quote is final. Our IEC-certified and UL-listed ranges are a useful starting point.

Model matching. This is where motor starter orders most often go wrong. An ATS48 part number encodes the current frame and the voltage range, and the Q and Y variants do not interchange: a Q-range unit works on 230-415 V supplies, while the 208-690 V families are different articles. A contactor part number encodes both the AC-3 rating and the coil voltage. Send the full order code and the motor nameplate: the correct frame depends on the motor’s current and the starting duty, not on the kW alone.

Lead time, stock and delivery. The biggest swing in any quote is stock versus factory order, so ask which one you are being quoted, in writing. Express courier suits urgent single-unit replacements, air freight suits heavy urgent consignments, and sea freight suits bulk orders with calendar slack. For imported panels and projects, DDP delivery – where we arrange export, import, duty and delivery to your named place – is available to more than 50 countries, alongside local dispatch from warehouses in the United States, Europe and Australia.

Two related reads if you are comparing brands for the rest of the panel: Schneider vs Siemens vs ABB contactors and our motor control brand comparison. If the starter sits behind a moulded-case breaker, the trip unit guide explains which protection type suits a motor circuit.

Send Us the Motor Nameplate, Not Just a Part Number

The fastest route to a decision is a nameplate photo and two sentences about the load. Message us on WhatsApp at +86 18616369656, email sales@simplying.com, or use the pagina dei contatti. Include the motor’s kW and full-load current, how many leads it has, what it drives, how often it starts, your supply, and where it needs to land.

We will come back with the star-delta starter and the soft starter priced as one BOM, with stock status, lead time, warranty terms and delivery terms on one sheet – usually within 12 hours. If the honest answer is that your load needs a VFD, we will say so.

Domande frequenti

1. Is a star-delta starter always cheaper than a soft starter?

On the device, usually; on the complete assembly, not always. A star-delta starter adds two more contactors, a timer, an overload relay, a six-core cable and the labour to commission and maintain it. Compare full BOMs, not catalogue prices.

2. Can I use a star-delta starter on any motor?

No. This motor starter needs six terminals and a winding rated for delta at line voltage. A three-terminal motor, or one star-configured for the line, cannot be star-delta started without rewinding, so check the terminal box first.

3. Does a soft starter reduce starting current more than a star-delta starter?

Not in a simple contest. Star-delta delivers a fixed one third of DOL current. The ATS48 adjusts from 150% to 700% of motor In, capped at 500% of its own nominal current, so the achievable reduction depends on the load. For very light duty, star-delta holds current low at lower cost.

4. Do I need a bypass contactor with a soft starter?

On the ATS48 the bypass is external. It carries running current after the ramp, which removes the starter’s heat from the circuit during normal operation, so budget for it in the panel.

5. Can a soft starter replace an existing star-delta starter?

Often yes. The six motor leads you already have can be useful, because ATS48 Q-range units can be wired inside delta, where the starter carries only about 58% of line current. Confirm the stop behaviour you need, because inside-delta allows freewheeling stopping only.

6. Which motor starter suits a pump – a soft starter or a star-delta starter?

Both can start a centrifugal pump, because pump torque is low at standstill. Choose the soft starter if you also need to stop the pump smoothly, protect the motor, or report start data over Modbus. Choose a star-delta starter if the start is light, infrequent and the budget is tight.

What Should You Do Next?

The decision usually comes down to one number on a datasheet. Find the largest motor on your next panel, read its declared locked-rotor torque, and divide by three. If the load cannot start on that figure, star-delta is off the table and the soft starter is already decided. If it can, and the start is light and infrequent, a star-delta starter assembly remains a sound, cheap answer.

Either way, buy your motor starter as a BOM from a supplier who will confirm the model, the documents and the delivery terms before you pay. Send us the nameplate on WhatsApp at +86 18616369656 or by email at sales@simplying.com, and we will price both routes on one sheet, usually within 12 hours.