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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.

Wye-delta (star-delta) starting

X1X21StopStartMOLM1, 2Main2MTR2, 2on-delay 4 sT.O.TRDS2Star (wye)T.C.TRSD2Delta

Turn on JavaScript to press the buttons and run this circuit.

Do this

Start the motor connected in star (lower voltage per winding, lower inrush), then switch to delta for full power.

Follow the changeover

  1. Start latches the main contactor M on rung 1 (Stop, Start, seal-in, overload — lesson 3 again).
  2. 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.
  3. When TR times out, its timed-open contact opens and S drops out.
  4. 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.
  5. 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.

Questions people ask

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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