When starting a higher power 3-phase induction motor, like this example: https://cdn.automationforum.co/uploads/2020/10/Untitled-5.jpg, a Y/Δ starter can be used to limit the inrush current.
Inrush currents (Ia) can go up to 12× nominal current (In) when starting Direct OnLine (DOL).
In case of the example (with a Δ voltage of 415V), Ia is "12×In" (12×27,13A), which would be 325,56A.
In a Δ circuit the line current (supplied current of each phase) of 27,13A will devide between the 2 connected coils of each phase.
This will result in the coil current (current through a coil) and will be "line current÷√3" (27,13A÷√3)=15,66A.
The line voltage (measured between phases) and coil voltage (measured over a coil) remain 415V.
When connecting the motor in a Y circuit, the coil voltage will be "line voltage÷√3" (415V÷√3)=240V.
This is the result of connecting 1 end of all 3 coils together, creating a neutral point.
The coil current will drop parallel with √3 from 15,66A to 9,04A.
In a Y circuit, the line current and coil current are the same because the current is not devided over 2 coils.
This means that the line current will be 3× less in a Y circuit than in a Δ circuit (9,04A vs. 27,13A).
This also means that the inrush current will be 3× less when starting the motor in a Y circuit.
The DOL inrush current of 325,56A and will now be 108,52A.
In the circuit are 2 voltmeters and 2 ammeters to show how the voltage or current will be devided.
At start the Y circuit is active with:
Blue=line voltage
Green=line current
Orange=coil voltage
Red=coil current
Toggle the SPST switch to switchover to Δ circuit and see how the voltages and currents will behave
