November 2026 · 10 min read

Motor starters compared: direct on line, star delta, soft starters and variable speed drives, and the motor control centre they sit in

How each way of starting a three phase induction motor trades starting current against torque, from direct on line and star delta to soft starters and variable speed drives, the questions that point toward one, what a drive adds and costs, the motor control centre the starters sit in, and what fails in each. With a drawing of the starting current curves.

EngineeringMaintenance Strategy

Line drawing of starting current against motor speed for direct on line, star delta, soft starter and variable speed drive starting

Key takeaways

Every starting method is a trade between the current drawn from the supply and the torque delivered to the load. Direct on line gives the most of both. Every reduced voltage method gives less of both.
Star delta and soft starters cut torque as well as current, so they suit loads that start light. A loaded conveyor or a full crusher may not start on them at all.
A variable speed drive is the only method that cuts the starting current while keeping full torque, and the only one that controls speed in running. It costs more, runs hotter and puts harmonics on the supply.
The choice comes from a few questions: what the supply allows, what the load needs from standstill, whether speed control is wanted, and how often the motor starts.
Starters live in a motor control centre built and verified to IEC 61439. How its units are separated and withdrawn decides how quickly a failed starter can be replaced with the rest of the plant running.

A plant upgrades a conveyor drive and the new motor will not start the belt when it is loaded after a trip. The starter is a soft starter with its current limit set low to protect a weak supply, and at that current the torque is less than the loaded belt needs to break away. Nothing is faulty. The starting method was chosen for the supply and not for the load, and the two needed to be considered together.

This guide compares the ways a three phase cage induction motor can be started: direct on line, star delta, autotransformer, soft starters and variable speed drives. It sets out what each does to the current and the torque, the questions that point toward one, the motor control centre they sit in, and what fails in each kind of starter. Values are the typical ranges published for these methods. The motor data sheet, the load and the supply decide the real ones, and the standards named are reference points.

Current against torque

A cage induction motor at standstill draws several times its full load current, because nothing opposes the supply until the rotor turns. As it accelerates the current falls, then drops sharply near full speed. Every starting method changes that curve. Direct on line accepts it as it is. Star delta, autotransformers and soft starters lower the voltage at the motor, which lowers the current, but the torque falls with the square of the voltage, so it falls faster than the current does.

A variable speed drive works differently. By lowering the frequency and the voltage together, it keeps the motor near its rated flux and slip, so the motor can deliver rated torque from standstill while drawing close to full load current. That is why a drive can start a heavy load on a supply that would not tolerate a direct on line start.

Fig. 1 · Indicative starting current against speed for one motor started four ways, from direct on line to a variable speed drive

Halve the voltage at the motor and the starting torque falls to a quarter. Reduced voltage starting is only gentle if the load can live with it.

The methods side by side

Five methods cover almost every fixed plant motor. Direct on line, star delta and soft starters account for most of them, with drives on the motors that need speed control. Autotransformer starters still appear on older large motors and are rarely specified new.

Method	How it starts	Starting current	Starting torque	Suits	What to watch
Direct on line (DOL)	A contactor connects the motor straight to full supply voltage	About 6 to 8 times full load current	Full locked rotor torque, often 1.5 to 2.5 times rated	Small and medium motors on a stiff supply, and loads that need torque from standstill	The starting current dips the supply and the full torque shocks belts, couplings and gearboxes
Star delta	The windings start in star at reduced voltage, then a timer switches them to delta	About a third of DOL, so 2 to 3 times full load current	About a third of DOL torque	Loads that start light, such as unloaded compressors and fans, where the supply limits current	Needs six motor leads. Open transition gives a current and torque spike at the changeover
Autotransformer	A tapped transformer starts the motor at 50 to 80 per cent voltage, then bypasses	Reduced by the square of the tap, so about 1.5 to 4.5 times	Reduced by the square of the tap	Large motors on a limited supply where more torque is needed than star delta gives	Large and costly. Now mostly replaced by soft starters and drives
Soft starter	Thyristors ramp the voltage up, usually to a set current limit, then a bypass contactor closes	Set by the current limit, commonly 2.5 to 4 times full load current	Falls with the square of the voltage, so it is low at the start	Pumps, fans and conveyors that need a smooth start or a soft stop, with no speed control in running	No speed control. Heat limits the starts per hour. Too low a torque can stall a loaded start
Variable speed drive (VSD)	A rectifier, DC bus and inverter control frequency and voltage together, from zero to full speed	Close to full load current, up to about 1.5 times for a short time	Full rated torque from standstill	Loads that need speed control in running, high torque starts on a weak supply, or energy savings on centrifugal loads	Cost, heat, harmonics on the supply, and voltage stress on the motor insulation with long cables

Choosing a method

The choice follows from a few questions about the supply, the load and the duty, asked in roughly this order. The supply question comes first because it is often fixed by someone else: a network service provider's service and installation rules, a site generator, or a long feeder can limit the starting current a motor may draw. The load question comes next, because a method that meets the supply limit but cannot start the load is no method at all.

Question	What it means	Points toward
Does the supply limit the starting current?	A network service provider's rules, a site generator or a long feeder can all set a limit	Star delta, soft starter or VSD
Is speed control needed when running?	Flow, tension or throughput controlled by speed rather than by a valve or damper	VSD
Does the load need high torque from standstill?	A loaded conveyor, a crusher restarted full, a positive displacement pump	DOL where the supply allows it, otherwise a VSD. Star delta and soft starters cut torque
Is the shock of starting a problem?	Belt splices, couplings and gearboxes, or water hammer when a pump starts and stops	Soft starter or VSD
How often does it start?	Many starts an hour heat the motor and the starter	DOL or VSD rated for the duty. Check a soft starter's starts per hour
None of the above?	A small motor on a stiff supply with a light load	DOL, the simplest and cheapest, with the fewest parts to fail

What a drive adds, and what it costs

A variable speed drive is more than a starter. It controls speed in running, which on a centrifugal pump or fan can save a large share of the energy compared with throttling, because the power falls with the cube of the speed. It gives controlled acceleration and deceleration, holds torque at low speed, and reports current, torque and faults that are useful condition data.

It also brings costs that a contactor does not. The rectifier draws harmonic currents from the supply, which may need filtering to meet the network's limits, with IEEE 519 and the AS/NZS 61000 series as the usual references. The drive gives off heat and needs a cooled space. Fast switching puts voltage stress on the motor insulation and can drive current through the bearings, which matters with long cables and older motors not rated for inverter duty. Drives are specified to IEC 61800, and the motor should be checked as suitable for the drive.

The motor control centre

Starters are grouped in a motor control centre, a switchboard assembly with one functional unit per motor. It is built and verified to IEC 61439, adopted in Australia as AS/NZS 61439, which replaced the older idea of a type tested assembly with design verification. For maintenance, two choices matter most. Whether units are withdrawable decides whether a failed starter is swapped in minutes or rewired in hours. The form of separation decides whether one unit can be worked on with its neighbours and the busbars live, which is a question for the permit and isolation arrangements as much as for the electrical design.

Modern MCCs often carry motor protection relays networked to the control system, which report current, thermal load and the cause of every trip. That data belongs in the historian and in front of the people who maintain the motors. Most sites with an intelligent MCC never look at it.

Part	What it does	What to note
Incoming section	The supply connection, main switch or breaker, and metering	Rated for the fault level of the supply
Busbars	Carry power along the MCC to every functional unit	Rated current and short circuit withstand, verified to IEC 61439
Functional units	One starter per motor, each with its isolator, protection, contactor or starter, and control	Fixed, plug-in or withdrawable, which decides how quickly a failed unit can be swapped
Forms of separation	Barriers between busbars, functional units and terminals, numbered Form 1 to Form 4	Higher forms let one unit be worked on with its neighbours live
Control and communications	Control wiring or a network link from each starter to the PLC, with motor protection relays that report current and trips	An intelligent MCC gives the condition data, if someone reads it

What fails, and the preventive task

Starters fail in predictable ways, and the tasks that find the failures early are cheap. They form part of the electrical preventive maintenance program, along the power path from the supply to the motor. Thermography under load reads every joint in the MCC without an outage and is the most valuable single survey.

Starter	Common failures	Preventive task
Contactors	Worn or welded contacts, coil failure, loose terminations	Thermography under load, contact inspection by operations count, torque checks on terminals
Overload and protection relays	Wrong setting for the motor, failed to trip when needed	Settings checked against the motor nameplate, trip tests on a schedule
Star delta timers and interlocks	Timer drift or failure, a stuck contactor leaving the motor in star	Changeover time checked against the run up, interlock test
Soft starters	Thyristor failure, a bypass contactor that does not close, overheating	Cooling path inspection, the event log read, bypass operation confirmed
Variable speed drives	Blocked filters and failed fans, ageing DC bus capacitors, lost parameters	Filters and fans every six months, capacitors replaced by age, parameters backed up after every change

Where starter choices go wrong

A reduced voltage starter chosen for the supply without checking that the reduced torque can start the load.
Star delta specified for a motor whose windings are not rated for delta at the supply voltage.
A soft starter set to a low current limit and long ramp, so the motor overheats on every start.
A drive fitted to an existing motor without checking its insulation, bearings and cooling at low speed.
Harmonics from several drives added to a supply without a study, then found by the network operator.
Fixed starter units in an MCC that cannot be isolated with the neighbours live, so one failed starter stops a whole section.
Drive parameters never backed up, so a failed drive becomes a day of reconfiguration.

Common questions

What is a DOL starter?

A direct on line starter connects a motor straight to the full supply voltage through a contactor, with an overload relay or motor protection relay to trip it. It is the simplest and cheapest starter and gives the motor's full starting torque. The cost is a starting current of roughly six to eight times full load current, which dips the supply voltage, and a mechanical shock to the drive train. It suits small and medium motors on a stiff supply.

What is the difference between DOL and star delta starting?

DOL applies full voltage to the motor from standstill. Star delta starts the motor with its windings connected in star, which puts about 58 per cent of line voltage across each winding, then switches them to delta for running. Starting current and starting torque both fall to about a third of the DOL values. Star delta needs a motor with six leads rated for delta at the supply voltage, and the changeover from star to delta causes a short current and torque spike unless a closed transition starter is used.

What is the difference between a soft starter and a VSD?

A soft starter only controls starting and stopping. Thyristors ramp the voltage up to full, usually holding a set current limit, and a bypass contactor then carries the motor at full speed. A variable speed drive controls both frequency and voltage, so it can run the motor at any speed and deliver full torque from standstill at close to full load current. A soft starter is smaller, cheaper and cooler. A drive is the choice when speed control is needed or the load needs high torque on a weak supply.

Why does a motor draw so much current when it starts?

At standstill a cage induction motor behaves like a transformer with its secondary short circuited. There is no back EMF to oppose the supply until the rotor is turning, so the current is limited only by the winding impedance. As the motor accelerates, the slip falls and the current drops to its running value. The motor maker publishes the locked rotor current and torque on the data sheet, and IEC 60034-12 sets the starting performance classes.

What is a motor control centre?

A motor control centre, or MCC, is a switchboard assembly that groups the starters for many motors in one place. It has an incoming section, busbars along its length and a functional unit for each motor with its isolator, protection, starter and control wiring. MCCs are built and verified to IEC 61439, which in Australia is adopted as AS/NZS 61439. The units may be fixed, plug-in or withdrawable, and the form of separation sets which parts can be worked on with the rest live.

When should a soft starter be used?

When the start needs to be gentler than direct on line, the motor runs at fixed speed, and the load can still accelerate with the reduced torque the soft starter gives. Centrifugal pumps, where a soft stop also prevents water hammer, fans and lightly loaded conveyors are typical. A soft starter is not the choice when the load needs high torque from standstill or speed control in running.

Key terms

Plain-language definitions from our glossary for the concepts this article leans on.

Direct on line (DOL) starter Star delta starter Soft starter Variable speed drive (VSD)Motor control centre (MCC)Single line diagram

Standards and further reading

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