Motor control · lesson 11 of 12

Wye-delta starting

A wye-delta starter reduces the starting current by starting the motor with its windings in wye (star), then switching them to delta when it is up to speed. The control circuit is a timer, three contactors and two interlocks — everything from the earlier lessons at once.

Arranque estrella-delta

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Haga esto

Arranque el motor conectado en estrella (menos tensión por devanado, menos corriente de arranque) y luego pase a delta para tener plena potencia.

Follow the changeover

Start latches the main contactor M on rung 1 (Stop, Start, seal-in, overload — lesson 3 again).
An M contact feeds rung 2. The timer TR energizes, and so does S through TR's timed-open contact and D's NC interlock. The motor starts in star.
When TR times out, its timed-open contact opens and S drops out.
Only now does S's NC interlock in the D branch close. TR's timed-closed contact is already closed, so D pulls in. The motor is in delta.
Stop drops M, which takes away rung 2's supply: TR, S and D all go.

The gap between S dropping and D pulling in is the "open transition": for a moment the motor is disconnected. Real starters sometimes add a short transition timer so the star contactor's arc is out before delta closes.

En la obra

Three contactors, a timer and two interlocks. Everything in this circuit exists to make sure the star and delta contactors are never in together, and that is what you test first.

Common mistakes

Trusting the timer to keep star and delta apart. The timer sets when; the NC interlocks in each other's branches set whether. Both, always, plus the mechanical interlock on the pair.
Starting a motor that cannot be started this way. It needs six leads brought out and windings that run in delta at the supply voltage. That is a nameplate question, answered before the starter is ordered.
Landing the six motor leads in the wrong groups. Star shorts one end of the three windings together; delta connects those same ends to the lines. Get the grouping wrong and the transition is a fault, not a changeover.
Setting the transition time by feel. Too early and the motor is still drawing heavy current in delta; too late and you have lost the point of the soft start. The driven load decides, and it is measured, not guessed.
Expecting full starting torque. Reduced current means reduced torque. A load that needs torque from standstill may not be a wye-delta candidate at all.

On a real panel

The gap between star dropping out and delta pulling in is the open transition — the motor is momentarily disconnected. Some starters add a short transition step to get the star contactor's arc out before delta closes.
Overload sensing on a wye-delta starter usually sits in the motor phase legs rather than the lines, because the current in those two places is not the same. Follow the manufacturer's diagram.
Six leads plus a bond means a bigger duct and more terminals than a direct-on-line starter. Plan the enclosure entries early.

Prove it before you leave

Control energised, motor disconnect open. Press Start and watch the sequence: main and star in, timer runs, star out, delta in.
Push the star contactor in by hand and try to energise delta. It must not pull in. Then the other way round.
Check the mechanical interlock physically on the star/delta pair.
Press Stop mid-sequence. Everything must drop together and stay out.
Only then close the disconnect. Watch and listen to the transition, and measure the run current once it settles.

Preguntas frecuentes

Why start a motor in wye (star) and run it in delta?

In wye each winding sees the line voltage divided by root 3, so the starting current and starting torque are both about a third of a direct-on-line start. Once the motor is near speed it is switched to delta for full power.

Why must star and delta never be closed together?

The star contactor shorts one end of the windings together; the delta contactor connects those same ends to the supply lines. Both at once is a short across the supply. So each has a normally closed contact of the other in its coil circuit, and usually a mechanical interlock too.

Can any motor be started wye-delta?

Only a motor with six leads brought out and windings rated to run in delta at the supply voltage. That is a nameplate question.

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