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1. Why a DC motor starter is called 3 point starter
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Answer Posted / hima teja
3 POINT DC STARTER
3 point DC starter is a unit intended to limit the starting current in a DC circuit. This starter is a must for any DC motor applications. This starter has three functions.
(a) To provide series resistance in a DC circuit when this starter is in START position. I.e. In STARTposition full resistance is included, and is reduced to zero ohms when the handle is moved to ON position, in discrete steps. The starter is held in ON position by an electromagnet. At this ON position the DC circuit gets its full current from the DC Rectifier unit. In the case of a DC motor, this is
connected in series with the armature circuit, to limit the starting current and eventually this resistance is CUTOUT, from the circuit. There by enabling the motor to run at it’s full armature current. Therefore this starter acts as a first order protection circuit, in between a DC rectifier unit and the DC circuit.
(b) No Volt Coil (NVC) – This is normally connected to Field circuit in a DC motor. In case of a failure in current in this circuit, for any reason, namely accidental tripping, or unintentional break in field circuit, this coil will provide a signal to the electromechanically held arm (handle) to release, and the starter is switched to OFF position, thereby providing safety to the system.
(c) Over Load (OL) release switch – This is connected in series with the armature circuit. In case of current beyond the rated current of this coil in the circuit, this automatically trips the electromagnet, which is holding the handle in ON position. Thereby tripping the starter from being over loaded
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