DOL Starter of 3 Phase Induction Motor:
Function of NOC NVC:
The No-Volt-Coil (NVC) is a contactor coil (coil C) placed on an iron core. When this coil is connected across two lines, it is energized and current flows through it. This current magnetizes the core and therefore either a part of core or some soft iron piece, called armature is attracted to the core against the spring tension. This movement is used to close the contacts. The contacts remain closed so far the core is magnetized. However, when supply fails, supply voltage becomes zero, called no-volt condition, the coil current falls to zero, the core is demagnetized, attractive force becomes zero and the armature is send back to its original position due to spring mechanism. Thus the contacts are opened & motor is disconnected from the supply. On recovery of supply, motor can not start because start push button normally open (NO) contact appears in series with the coil. Thus accidental starting on recovery of supply is prevented and motor, operator, driven machine are protected.
Function of OLC:
The overload coil (OLC) is a heating coil which carries the motor current & heats up the thermal bimetallic strip. The heat developed by this coil is proportional to the heat developed in the motor by its copper loss. During overload condition, if the motor heating is about to damage the motor, the bi-metal
strip bends and opens its normally closed (NC) contact in series with the contactor coil. Therefore, the contactor coil current is interrupted, contactor is tripped and motor is disconnected from the supply. Thus motor is protected from severe over-load conditions.
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