Soft starting motors

[lesmcw]

Hi:
Soft starting is the Method of getting a motor running with less current and less demand on everything.
It was very common to start a motor in wye (connection configuration)and switch to delta after it was up to a certain speed. Other common ways were reduced voltage starting and autotransformer starting.
Now its usually done with a premade package of thyristors (SCR's) that often look like a v.f.d. (variable frequendy Drive) in fact, most v.f.d. are also soft starts. They basically limit the availaable current to a motor and allow a "ramp up to speed" time. They are used on the bigger motors because the starting contactor and switchgear would have to be huge to take the otherwise high starting current. they are also used to reduce the high initial starting torque that motors produce so that it doesnt break things like shafts and couplings, or on loads like conveyors where the conveyor would take off under the load and spill something etc. Most soft starts also have braking capabilities as well to slowly bring things to a stop. Almost every manufacturer makes them, like Toshiba, Mitsibushi, Cutler Hammer, Allan Bradley, ABB, Seimens etc.
Hope this helps...Les

[Alectric]

What is 'soft starting' a large... say 500HP motor?

[Alectric]

YES... it helps loads. Thanks Les