Three Point Starter of DC Shunt Motor: DC Motor can be started by Three point starter or Four Point starter.
Three Point Starter of DC Shunt Motor
Three-point Starter
The connection diagram of a three-point shunt motor starter is shown in Fig. below. The starter terminals to
be connected to the motor are A (armature), F (fi eld) and L (line).
The starting resistance n steps between conducting raised studs. As the starting handle is rotated about its fulcrum, it is one stud to the next, one resistance step is cut out, and it gets added to the fi eld circuit.
There is a short time wait at each stud for the motor to build up speed. This arrangement ensures a high average starting torque.
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At start, the handle is brought to stud one. The line voltage gets applied to the armature with full starting
resistances in series with armature and to the fi eld with NVC in series.
Thus, the motor starts with maximum pick-up speed; the handle is moved from the stud to the ‘ON’ position shown in Fig. below. The starting resistance has been fully cut out and is now included in the fi eld circuit; being small it makes little difference in the fi eld current.
The resistance of NVC is small and forms part of the fi eld resistance. The voltage across the armature is the line voltage. The handle is held in this position by the electromagnet excited current fl owing through NVC.
Two protections are incorporated in the starter.
– NVC (No Volt Coil) In case of failure of fi eld current (due to accidental or otherwise open
circuiting), this coil releases the handle (held electromagnetically), which goes back to the OFF portion
under the spring action.
– OL (Overload) Release The contact of their relay at armature current above a certain value
(Overload short circuit) closes the NVC ends, again bringing the handle to OFF position.
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