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Star–delta starting explained
Why star–delta starting reduces starting current, how the three contactors and the timer work, and the interlocks that keep it safe.
The idea behind star–delta
A motor wound for delta operation at the supply voltage can be started in star. In star, each winding sees the phase voltage instead of the line voltage, which is about 58 % (1/√3) of normal. The starting current drawn from the line, and the starting torque, fall to roughly one third of their direct-on-line values.
After a set time, when the motor has accelerated, the starter switches the windings to delta and the motor runs at full voltage.
Three contactors and a timer
A star–delta starter uses a main contactor (KM1), a star contactor (KM3) and a delta contactor (KM2). On START, KM1 and KM3 close and the motor runs in star. An on-delay timer then opens the star circuit and closes the delta circuit.
The motor has six terminals: U1, V1, W1 and U2, V2, W2. The main contactor feeds U1, V1 and W1. The star contactor links U2, V2 and W2 together; the delta contactor connects each winding end to the next phase.
Interlocks are essential
KM2 and KM3 must never be closed at the same time: that would short-circuit the supply through the two contactors. Each one therefore has a normally-closed auxiliary contact of the other in its coil circuit (electrical interlock), and many panels add a mechanical interlock as well.
Good designs also make sure the star contactor has fully opened before delta closes, so that there is a short open-transition gap.
Things to check
Because the windings carry the phase current in delta, the overload relay in the usual position (in the motor leads after KM1) is set to the phase current, about 58 % of the line current. The star–delta changeover time should match the time the machine needs to accelerate, and the motor must be suitable for delta connection at the supply voltage.
Try it yourself
Load the Star–Delta starter and start the motor. Watch the timer change the connection from STAR to DELTA. In Learn, run the lesson that removes an interlock and see what the inspector says.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.