Motors arranged for star-delta starting have six terminals - the two ends of each phase winding being brought out to terminals marked U1, V1, W1 and U2, V2, W2. These terminals are connected to similarly-marked terminals in the starter.
The basic circuit of a typical hand-operated air-break or oil-immersed starter is shown in the diagram, the incoming supply being controlled by a line contactor. With the change-over switch in the start position, the motor windings are connected in star (U1, V1 and W1 together) and in the running position in delta (U2 to W1, V2 to U1 and W2 to V1).
In starting the motor, the handle of the change-over switch is put into the start position, as indicated, and the ‘start’ button is pressed. This energizes the contactor coil which closes the triple-pole main switch and auxiliary switch (1). Note that the contactor coil cannot be energized unless the changeover switch has been placed in the ‘start’ position.
When the motor has reached full speed, which is noticeable by sound, the handle of the change-over switch is moved to the ‘run’ position and the ‘start’ button is released. The motor is now directly connected to the line.
In some star-delta starters, the overload units are by-passed in the ‘start’ position. A complete connection diagram of a hand-operated star-delta starter with this feature is also shown. Apart from the fact that the over-load units are brought into circuit only in the ~run~ position, the circuit is the same at the basic circuit.
A fully-automatic star-delta starter has two contactors and a triple-pole line contactor with time-delay relay between ‘start’ and ‘run’ connections.