The soft-stop should be achieved "injecting" a dc component. So the rotor is affected by a static flux that brakes it.
Probably since the star wiring doesn't allow the dc component to flow if the neutral is isolated, then the soft-stop is not possible.
Soft stop is the opposite of soft start in that the voltage applied to the motor is gradually ramped down from line voltage to zero or a preset low point and cut. The effecto of this is to extend the stopping time of the motor and it is useful in applications where the motor normally stops quickly when power is removed. An example of an application where soft stop provides a worthwhile benefit is for pumping where soft stop allows the pump to slow rather than stopping dead. This reduces/elliminates water hammer when the pump stops. In effect, soft stop adds inertia to the load.
If the starter can soft start the motor in star, it should also be able to soft stop the motor in star. If the motor takes a reasonable time to stop with out soft stop, you may not see the effect of the soft stop.
Alex68 What you describe is DC braking which is the opposite of soft stoping. DC braking is used to make the motor stop quicker and is a means of reducing the effect of load inertia. DC braking should be abel to be applied to a star connected motor provided that the neutral is not connected, i.e. a three wire connection. It is possible to design a DC brake for a four wire installation, but most motor installations are three wire, not four wire.
Ls12, Where did you hear that anyway?
Marke is right, soft stopping is the opposite of DC injection braking, but neither should be restriced by the motor connection method.
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