The soft starting process avoids something called inrush current. When a standard motor first turns on, it can draw potentially damaging currents that are 5 to 10 times its normal running load. These extreme currents cause voltage fluctuations and place unnecessary strain on the windings and moving parts. Gentle start mitigate inrush current by applying voltage in small increments over time rather than all at once.
Components and Soft Starters
All gentle start contain some basic components: a contractor or thyristor, a reactor or resistor, and a control unit. When power is supplied, the control unit energizes the contractor in short bursts. This allows voltage to slowly build up in the reactor/resistor before fully conducting to the motor.
Soft Starter contractor acts like a switch, connecting and disconnecting the motor from power in a controlled sequence. It is usually a thyristor that can be triggered into conduction and switched off through varying pulse widths. Each time the thyristor conducts for a bit longer, it feeds slightly more voltage to the load in a step-like manner.
As voltage rises incrementally, current also increases gradually rather than spiking at switch-on. The inductive reactor or resistor helps regulate this process by absorbing and releasing energy in a precisely metered flow. It maintains a steady increase in voltage regardless of load conditions. After a programmed time period, usually 4-10 seconds, the soft starter turns the contractor fully on and runs the motor at operating speed.
Applications
There are several common applications where gentle start deliver significant benefits:
Pumps: Starting water pumps or centrifugal pumps without a soft starter causes dangerous surges. This can damage pipes and joint connections over time due to hydraulic hammer effects. Gentle start overcome this issue.
Fans: The intense starting loads of industrial fans and blowers put serious stresses on bearings, shafts and housing components. Gentle start lengthen equipment life spans.
Conveyors: Belt conveyors, roller conveyors and screw conveyors all experience high starting torques. Gentle start safeguard gearboxes and drive mechanisms from impact loads during acceleration.
Compressors: Compressors of any type, whether centrifugal or reciprocating, demand careful voltage control on startup. Without it, compressors strain internal parts and risk trip failures from excessive locked rotor amps.
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