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Concept
Motor soft starter
Summary
A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the powertrain and electric current surge of the motor during start-up. 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.
It 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.
Whenever the armature of an electric motor is moving, both the motor action and generator action are occurring simultaneously; the electromagnetic force produced by generator action opposes the desired motor action and effectively creates a variable motor resistance which increases with motor speed. When a voltage is applied to the motor, this resistance dictates the current drawn by the motor. At rest, the resistance is relatively low, so the starting or inrush current can be high if the full line voltage is applied to the motor. Compared to DC motors, AC motors tend to have significantly higher stator resistance and correspondingly lower inrush current.
Nevertheless, across-the line starting of induction motors is accompanied by inrush currents up to 7-10 times higher than running current, and higher efficiency motors can experience inrush currents 10-15 times running current. In addition, starting torque can be up to 3 times higher than running torque. The starting torque transient can create a sudden mechanical stress on the machine, which leads to a reduced service life. Moreover, the high inrush current stresses the power supply, which may lead to voltage dips.
Official source
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