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A motor soft
starter is a device used with AC electric
motors to temporarily reduce the load and torque in
the powertrain of the motor during startup. This reduces
the mechanical stress on the motor and shaft, as well
as the electrodynamic stresses on the attached power
cables and electrical distribution network, extending
the lifespan of the system.
Motor soft starters can
consist of mechanical or electrical devices, or a combination of both. Mechanical
soft starters include clutches and several types of couplings using a fluid,
magnetic forces, or steel shot to transmit torque, similar to other forms
of torque limiter. Electrical soft starters can be any control system that
reduces the torque by temporarily reducing the voltage or current input, or
a device that temporarily alters how the motor is connected in the electric
circuit.
Electrical soft starters
can utilize solid state devices to control the current flow and therefore
the voltage applied to the motor. They can be connected in series with the
line voltage applied to the motor, or can be connected inside the delta loop
of a delta-connected motor, controlling the voltage applied to each winding.
Solid state soft starters can control one or more phases of the voltage applied
to the induction motor with the best results achieved by three-phase control.
Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled
rectifiers (thyristors), but in some circumstances with three-phase control,
the control elements can be a reverse-parallel-connected SCR and diode.
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