How to size components of a Direct on line (DOL) Motor Control
A DOL starter is the simplest and most common method for starting and stopping electric motors under 5 hp/4Kw. It typically comprises of some form of a circuit breaker, contactor and an overload protection. It offers no way to limit the starting / inrush current consequently its components most me properly sized to accommodate he magnitude of voltage and current the system is expected to carry. Components of a DOL Starter: Fuse or circuit breaker : This device provides protection to the motor from overcurrent conditions caused by short circuits. Contactor : This is the main switching device that that is energized to connect the motor to the power supply. It is it is energized and de-energized by a start/stop pushbutton or other control signal. Overload relay : This device protects the motor from overcurrent conditions caused by motor overload. It trips and disconnects the motor from the power supply if the current exceeds a safe limit. Typical Power Circuit of a DOL Starter Sizing