Star-delta starting: why and how it works

Star-delta starting reduces the starting current of a three-phase motor. The motor starts in star (lower voltage on each winding, about one third of the current and torque) and, once up to speed, switches to delta (full voltage, full power). It is used on larger motors such as pumps, compressors and fans, where direct-on-line starting would trip protection or dip the supply voltage.

Star-delta (wye-delta) motor starting explained, with terminal box diagram

For 230/400 V installations to IEC standards (Europe, UK and most of the world). North America uses different voltages, colours and rules: see the “Around the world” notes.

Why not start at full power?

An induction motor started direct on line draws roughly 5 to 7 times its rated current for the first seconds. For a small motor that is fine. For a bigger one (from a few kW) it means:

nuisance tripping of protection,
voltage dips (lights flicker across the building),
a mechanical jolt through the gearbox or belts.

Star-delta cuts the starting current to about one third.

How it works

Stage	Winding connection	Voltage per winding (400 V supply)	Effect
Start	star (Y)	about 230 V (√3 lower)	current and torque about 1/3
Run	delta (Δ)	400 V (full)	full power

A star-delta starter uses three contactors (main, star, delta) and a timer that switches Y to Δ after a few seconds. The star and delta contactors are interlocked so they can never close at the same time, which would be a short circuit.

Terminal box: star and delta differ only in how the links are placed.

When it is not used

Small motors (up to about 3 to 4 kW) usually start direct on line.
Drives with a frequency inverter (VFD): the inverter ramps the motor up by itself.
Soft starters: an electronic alternative to star-delta.

Around the world: in North America the same method is called wye-delta starting. The principle is identical, but supply voltages differ (for example 480 V at 60 Hz), so the rating plate must match your network.

Note: the motor must be designed to run in delta at your supply voltage. On a 400 V network, a plate reading 400/690 V means delta is fine; 230/400 V means star only. Selecting contactors, timer and protection and building the control panel is work for a qualified person.