Soft starters protect AC electric motors at startup by reducing inrush currents and limiting torque. After the motor is up to speed, the soft starter transitions control over to a bypass contactor. The bypass contactor acts as “cruise control” because it allows the motor to continuously run at full speed.

A blog from Rockwell Automation explains the differences between internal and external bypass contactors and their applications.

A soft-start bypass allows a motor to run continuously after the start in a closed contactor mode. Image: Rockwell AutomationA soft-start bypass allows a motor to run continuously after the start in a closed contactor mode. Image: Rockwell AutomationMany of today’s soft starters feature an integral bypass contactor, which requires only power and control wiring. Integral bypass soft starters are suitable for conveyors, fans, pumps, and other applications where the current and speed do not need to change while the motor is at speed.

Advantages of integrated bypass control include smaller footprint, reduced wiring, less heat dissipated and lower total installation cost, the blog says. Internal bypass contactors also allow the soft starter to operate cooler with the motor at speed compared to their external bypass counterparts.

Designed for harsh, vibration-prone environments, external bypass contactors allow for specialized control and have higher operations per hour. These solid-state starters also have no mechanical life limitations, the blog says.

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