The starting method for a three-phase induction motor is chosen from the driven load’s breakaway torque first and the supply’s tolerance for inrush second, not the other way round. Direct on line gives full torque and draws 6 to 8 times full load current. Star delta cuts that current to roughly a third, and cuts starting torque to a third with it. A soft starter ramps voltage with a settable current limit. A variable frequency drive is the only one of the four that reduces starting current while increasing available torque, because it reduces frequency rather than voltage. Pick wrong and the motor either stalls in star, trips the incomer, or burns out in six weeks.
TL;DR
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Read the torque column before the current column. Every reduced-voltage method buys current reduction by paying in torque, as a square law.
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Star delta gives about a third of the current and about a third of the torque. On a loaded conveyor, crusher, screw compressor or positive displacement pump, that is a stall, not a saving.
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Three standard families, not one: IS/IEC 60947-4-1 for electromechanical starters, IEC 60947-4-2 for soft starters, IEC 61800 for drives.
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BIS records IS/IEC 60947-4-1:2012 as withdrawn. The concurrent standard is IS/IEC 60947 Part 4 Sec 1: 2018.
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The overload relay in a star delta panel is set wrong more often than any other item in a starter audit.
What are the four ways to start a three-phase motor?
Every method trades the current the supply must deliver against the torque the shaft receives. Three of the four buy current reduction by cutting voltage, and voltage costs torque as a square law. Only the drive changes frequency.
Figures below are indicative for a 90 kW four-pole motor at 415 V drawing 153 A full load. The locked rotor current on the manufacturer’s datasheet is the only number to design on.
| Method | What it varies | Starting current | Starting torque | 90 kW at 415 V |
|---|---|---|---|---|
| Direct on line | Nothing, full voltage | 6 to 8 × FLA | 100% | 920 to 1,225 A |
| Star delta | Winding connection | 2 to 2.6 × FLA | About 33% | 306 to 400 A |
| Soft starter | Voltage, ramped by thyristors | 2.5 to 4 × FLA, settable | 25 to 64% | 380 to 615 A |
| VFD | Frequency and voltage together | 1 to 1.5 × FLA | 150 to 200% | 153 to 230 A |
A 90 kW motor on star delta delivers roughly a third of rated torque until changeover. If the load needs more than that to break away, the motor sits in star, draws locked rotor current, heats, and never transitions.
How does a soft starter actually work?
A soft starter reduces the voltage reaching the motor using phase-angle control across back-to-back thyristors, two per phase, wired in antiparallel so each conducts on one half of the AC cycle.
The gate of each thyristor is fired at a delay after the voltage zero crossing. Fire late in the half-cycle and only a small slice of each half-wave reaches the motor, giving low RMS voltage. Advance the firing point progressively and more of each half-wave passes through, so voltage climbs along the ramp the user has set. At full conduction the thyristors are effectively closed switches and the motor sees full line voltage.
Three settings follow from that mechanism and they are what you actually commission:
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Initial voltage or pedestal, the starting point of the ramp, set just high enough to break the load away.
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Ramp time, how long the climb to full voltage takes.
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Current limit, a ceiling the controller will not exceed, holding the ramp if it would be breached.
Because torque follows the square of applied voltage, a soft starter set to 70% voltage delivers about 49% of DOL torque. That relationship is the whole design problem: the current limit you want and the breakaway torque you need to pull in opposite directions.
Once at speed, most units close a bypass contactor across the thyristors, so conduction losses and heat disappear for the rest of the run. A soft starter left permanently conducting dissipates roughly 3 to 5 watts per ampere and needs the panel heat budget to allow for it.
What are the types of soft starter?
Three distinctions matter when you specify one.
By phases controlled. Two-phase controlled units switch two phases and leave the third connected directly. They cost less and suit light loads, but produce unbalanced current during the ramp. Three-phase controlled units switch all three, give balanced acceleration, and are the correct choice for anything loaded.
By bypass arrangement. Bypassed units close a contactor across the thyristors at full speed, eliminating running losses. Non-bypassed units conduct continuously and must be derated for ambient.
By control mode. The voltage ramp is the simplest. The current limit holds a ceiling. Torque control shapes the ramp against a torque profile rather than a voltage profile, which is what pump applications need to kill the pressure surge on start and stop.
For pumps, the stop ramp matters as much as the start. A soft stop decelerates the motor over a set time instead of dropping it, which is what removes the water hammer that shakes pipework and cracks joints.
Why does star delta stall loaded conveyors and crushers?
Because the torque reduction is not a side effect. It is the same physics that gives the current reduction.
Star delta is safe on unloaded or low inertia starts. It is a trap on high breakaway loads: loaded conveyors, crushers, positive displacement pumps and screw compressors routinely need more than 33% of rated torque to move. IEC 60034-12:2024 sets starting performance parameters for single speed three phase cage induction motors up to 1,000 V on duty type S1 intended for direct on line or star delta starting. The fourth edition was published 22 May 2024 and cancels the 2016 edition that many BIS-aligned specifications still quote.
Two wiring details cause more field failures than the torque question itself.
Contactor sizing. In delta, winding current is line current divided by √3. On a 90 kW motor at 415 V the line sees 153 A while each delta contactor sees 88.5 A, so main and delta contactors are rated at 58% of FLA. The star contactor is lighter still, at about a third of FLA, because it only carries current during the star period. Specifying full-FLA contractors across all three is money spent for nothing.
Overload relay placement. If the relay sits in the motor leads inside the delta, set it to FLA divided by √3, which is 88.5 A on that motor. If it sits in the line, set it to 153 A. A lead-mounted relay set to 153 A leaves the motor with no overload protection whatsoever. This is the single most common finding in a starter panel audit, and it is invisible until the motor fails.
There is also a second current peak at changeover. On a motor that has not reached close to full speed in star, that peak can approach the DOL value, producing exactly the inrush you were avoiding, just later.
When is DOL still the right answer?
DOL is correct whenever the motor is small enough that its inrush does not disturb the board, and it is then the cheapest and most robust option available.
The supply-side test is the transformer. As a working rule we apply on site, the largest DOL motor should not exceed about 15 to 20% of transformer kVA. Above that, the voltage dip on start drops out contactors elsewhere on the same board, and the fault you chase is never the motor that started.
There is a thermal ceiling too. During a DOL start the motor draws 6 to 8 times FLA, and heating goes as current squared, so a single start generates 36 to 64 times the heat of running at full load for that duration. A motor asked to start 150 times an hour spends most of its thermal budget starting rather than working. That is why crane duty motors are built with deliberately low rotor inertia and 200 to 300% starting torque: a shorter, more forceful start is a cooler start.
Soft starter or VFD: which do you actually need?
A soft starter controls how the motor gets to speed. A VFD controls what speed it runs at. If the process never needs a speed other than nameplate, the drive is buying something you will not use.
| Question | Soft starter | VFD |
|---|---|---|
| Controls running speed | No | Yes |
| Starting torque | Reduced, 25 to 64% | Increased, 150 to 200% |
| Losses once at speed | Near zero if bypassed | Continuous drive losses |
| Energy saving, centrifugal load | None | Substantial |
| Energy saving, constant torque load | None | Limited |
| Motor implications | None beyond normal duty | Converter duty considerations |
| Governing standard | IEC 60947-4-2:2020 | IEC 61800 series |
| Typical NIEL application | Fire water and process pumps, fans | Mill drives, DRI fans, cooling water pumps |
The energy case rests entirely on load type. On centrifugal fans and pumps with no static lift, input power varies as the cube of the speed ratio, so a modest speed reduction cuts power disproportionately. On constant torque loads such as conveyors, hoists, extruders, positive displacement pumps and reciprocating compressors, input power varies linearly with speed and the cube law does not apply. Sell a drive into a conveyor line on the strength of the cube law and the savings will not appear.
A drive also changes the motor. IEC TS 60034-25:2022 distinguishes a converter capable motor from a converter duty motor, and its Annex D defines derating requirements. Insulation sees faster voltage rise times, bearings can see shaft currents that pit the raceways, and the motor’s own fan slows with the motor, so sustained low-speed running needs derating or forced ventilation.
Which standard covers which starter?
Three families govern the four methods, and a specification naming only one is incomplete.
Electromechanical starters fall under IS/IEC 60947-4-1. Ministry of Heavy Industries guidelines under the Electrical Equipment (Quality Control) Order classify equipment to Clause 5.2 as contactor, DOL starter, star delta starter, two-step auto transformer starter, rheostatic rotor starter, combination or protected starter, and motor protective switching device. Soft starters and drives are not in that list.
Soft starters fall under IEC 60947-4-2:2020, fourth edition, published 26 June 2020, covering semiconductor motor controllers and soft starters up to 1,000 V AC. It replaced the 2011 third edition.
Drives fall under the IEC 61800 series, with IEC 61800-9-2 defining IE and IES classes and the loss determination methodology.
Two Indian currency checks belong in every enquiry. BIS records IS/IEC 60947-4-1:2012 as withdrawn, with IS/IEC 60947 Part 4 Sec 1: 2018 as concurrent, under committee ETD 07. Separately, IS 12615 came under mandatory certification from 1 October 2017. If motor and starter are quoted against dead editions, you are comparing bids on a standard that no longer exists.
What should a starting method specification say?
Most enquiries state a kW rating and a starter type and stop. Seven lines are what a panel builder needs.
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Driven load and breakaway torque, named machine and torque as a percentage of motor rated torque. Without it every reduced-voltage method is a guess.
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Duty class, CDF and starts per hour. Two motors both marked S4-40% are not interchangeable if one is rated 150 starts per hour and the other 300.
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Locked rotor current from the datasheet, as an actual ratio, not a 6× assumption. IS 12615 specifies it in terms of full load current.
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Fault level and transformer kVA at the point of connection. This decides whether DOL is permissible at all.
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Standard and edition by starter type. IS/IEC 60947-4-1, IEC 60947-4-2 or IEC 61800, with the year.
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Overload relay trip class and coordination type. Class 2, 3, 5, 10 or 10A, and Type 1 or Type 2 coordination with the short circuit protective device.
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Ambient, dust, humidity and IP rating. Soft starters and drives are semiconductor equipment and lose rating with ambient in a way contactors do not.
And one commercial line that is not a specification item at all: where the motor, the starter and the panel come from three vendors, nobody owns the stall.
What New India Electricals supplies across the four methods
Our low voltage panels are fully modular Power and Motor Control Centres, with each compartment variable in width, height and depth. A single MCC can carry Direct on Line, Star Delta, Soft Start and Auto Transformer starters together with their controls and protections, and the design supports intelligent MCCs integrating programmable devices on DeviceNet and ProfiBus.
Drive feeders are built into the same line-up using ABB drives across the low voltage AC range: ACS 550 for pump, fan and conveyor duty, ACS 355 for machine building, and ACS 800 modules for building into customer cabinets, plus medium voltage AC drives and DC drives for mill motor systems.
On the motor side, the starting method feeds back into selection. Crane duty and slipring motors use rotor resistance starting, which holds starting current to roughly 2 to 3 times full load current while delivering 200 to 300% starting torque. On a weak grid that is a different answer from any of the four LV methods above, and it is the reason slipring machines persist in crane and crusher duty.
Frequently asked questions
- What is the difference between a soft starter and a VFD?
- A soft starter controls the motor only during acceleration and deceleration, ramping applied voltage through thyristors and typically limiting starting current to 2.5 to 4 times full load. It does not control running speed. A VFD varies frequency and voltage continuously, so it sets running speed as well as starting behaviour. If the process needs a speed other than nameplate at any point, you need a drive. If it only needs a gentler start, a soft starter costs less, takes less panel space and adds no continuous losses once bypassed.
- How does a soft starter work?
- A soft starter uses phase-angle control across back-to-back thyristors, two per phase in antiparallel. Firing each gate at a delay after the voltage zero crossing passes only part of each half-wave to the motor, reducing RMS voltage. Advancing the firing point progressively raises voltage along a set ramp until the thyristors conduct fully. Most units then close a bypass contactor, removing conduction losses for the rest of the run.
- What are the types of soft starter?
- They divide three ways: two-phase controlled versus three-phase controlled, bypassed versus continuously conducting, and by control mode, meaning voltage ramp, current limit or torque control. Three-phase controlled and bypassed is the standard industrial choice. Torque control matters on pumps, where shaping the ramp is what removes the pressure surge on start and stop.
- Which is better, star delta or a soft starter?
- Neither in the abstract; the driven load’s breakaway torque decides. Star delta is a fixed two-step reduction, roughly a third of current and a third of torque, with an abrupt step at changeover. A soft starter gives an adjustable ramp, so current limit and ramp time can be set to the load, and it removes the mechanical shock of transition. On a pump that shock reduction is the point. On an unloaded fan starting twice a day, star delta is cheaper and entirely adequate.
- Up to what kW can you use a DOL starter?
- There is no single kW limit, because the constraint is the supply. As a working rule on Indian plant boards, the largest DOL motor should not exceed about 15 to 20% of feeding transformer kVA, or the voltage dip on start will drop out contactors on other feeders. Check three things: locked rotor current from the datasheet, transformer rating and fault level at the point of connection, and whether other voltage-sensitive loads share the bus.
- Which standard applies to motor starters in India?
- For electromechanical contactors and starters, IS/IEC 60947 Part 4 Sec 1. BIS records IS/IEC 60947-4-1:2012 as withdrawn with the 2018 edition concurrently, under committee ETD 07, and the product carries mandatory certification under the Electrical Equipment (Quality Control) Order, 2020. Soft starters sit under IEC 60947-4-2 and drive under IEC 61800. A supplier certified for one is not automatically certified for another.
