Let’s consider an illustration of a typical three-phase direct-on-line (DOL) starter below:
When the start button is pressed, the 415V contactor coil is energized, the main and auxiliary contacts close, and the motor starts. The auxiliary contact in parallel with the start button holds the coil on.
Overload or overcurrent protection is provided by either thermal or magnetic trips. Thermal overload protection relies on the heating effect of the load current to heat thermal coils which in turn cause movement of a bimetallic strip. This trips a spring-loaded contact in the control circuit. The speed at which the tripping takes place is adjusted to allow for normal starting currents, which may be four or five times as large as running currents. Another form of thermal protection is given by the use of a thermistor, which is a temperature-sensitive semiconductor. It is embedded in the stator winding and activates a control circuit if the winding temperature becomes excessive. Magnetic protection uses the principle of the solenoid to operate the tripping mechanism. The time lag in this case is achieved by the use of an oil or air dashpot which slows down the action of the solenoid plunger.
Also read: Electrical Motors
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