Comparison

Star-Delta vs Direct-on-Line Motor Starting

Star-delta starting cuts line current to about one third and torque to about one third. Compare it with direct-on-line starting and know when each fits.

Electrical & Power · Last reviewed October 15, 2026

Short answer: direct-on-line is the simplest and gives full starting torque, but draws the highest inrush. Star-delta cuts the inrush and the torque to about one third each, and only suits motors that start with a light load.

	Direct-on-line (DOL)	Star-delta
How it starts	Full voltage across the windings immediately	Starts with windings in star, then switches to delta
Starting current	Typically about 6 to 8 times full-load current	About one third of the DOL value
Starting torque	Full (roughly 1.5 to 2.5 times full-load torque, depends on the motor)	About one third of the DOL value
Motor requirement	Any standard motor	Six terminals brought out, and a winding rated for the line voltage in delta
Equipment	One contactor and overload	Three contactors, a timer and an overload
Switchover	None	Brief current and torque transient at the star-to-delta change
Best for	Small motors and strong supplies	Larger motors starting unloaded or lightly loaded (fans, some pumps, machine tools)

How to choose

Check the utility or generator limit on starting current. If DOL is allowed, it is the simplest choice.
Check the load torque at standstill. Star-delta gives about one third of full starting torque, so a loaded conveyor or compressor may not start.
Confirm the motor is wound for delta at the supply voltage and has all six leads available.
If you need a smoother start than star-delta gives, or the switchover transient is a problem, compare a soft starter or a VFD.

Common mistakes

Using star-delta on a load that needs high torque to break away.
Setting the star time too short, so the motor switches to delta before it has reached speed and the second current peak is almost as high as DOL.
Specifying it for a motor with only three terminals.

Two scenarios

A 55 kW exhaust fan that starts with the damper closed, on a generator that cannot take a 7 times starting current. Star-delta cuts the line current to about a third and the fan needs little torque at the start, so it suits this load.

A loaded screw conveyor that needs high breakaway torque. Star-delta gives only a third of full-voltage torque, so the motor may stall in star. Use direct-on-line if the supply allows, or a soft starter or a VFD if it does not.

Cost and life cycle

Equipment: star-delta needs three contactors, a timer and six motor leads. DOL needs one contactor.
Wiring: the six-lead motor connection costs more in cable and installation time.
Maintenance: contactors and the timer wear. Many sites replace star-delta starters with soft starters when they fail.

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Questions

Why does star connection reduce current to one third?

In star each winding sees the line voltage divided by √3. Winding current falls by √3 and the line current by a further √3 against delta, so the line current is about one third.

Is star-delta still common?

Yes, though soft starters and VFDs have replaced it in many new installations because they avoid the switchover transient.

Can I use star-delta on any motor?

No. The motor must be rated for delta operation at the supply voltage, with all six leads brought out.

What causes the second current peak?

When the starter switches from star to delta, the motor is briefly disconnected and then reconnected at full voltage. If it has not reached near full speed by then, the second peak can approach the DOL value.

How is the star time set?

Set it long enough for the motor to reach close to full speed in star, but short enough that it does not overheat. Typical values are a few seconds up to about 15 seconds for large fans.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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