Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
What it does
Star-delta starting reduces the heavy inrush current of a large motor by starting it in a star (wye) connection, then switching to delta for normal running.
How it works
Connecting the motor's windings in star applies less voltage across each winding, so the starting current (and torque) is reduced — gentle on the supply. Once the motor is near full speed, a changeover switches the windings to delta, applying full voltage for full power.
Three contactors do the work (main, star, delta) sequenced by a changeover timer, with an interlock so star and delta can never be connected together. The timer sets how long it runs in star before transferring.
Why it matters
It's a cheaper way to tame inrush than a soft starter or VSD when only the start needs reducing. If it never transfers to delta, the motor runs weak in star and may trip — a timer, delta-contactor, or interlock fault.
Safety first
Several contactors and high currents are involved; never defeat the star-delta interlock (a simultaneous closure is dangerous). Isolate before working in the starter.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Star-delta starter not transitioning to delta
A star-delta (wye-delta) starter starts the motor in star but never switches to delta — the motor runs weak/slow, or trips, because it stays in the starting connection.
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Timer relay not switching its output
A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Timer relay
A relay that switches its contacts after a set delay, enabling sequenced and timed control.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.