Construction
The starter is a variable resistance, divided into number of sections. The contact points of the sections are called studs and shown as OFF, 1, 2… upto RUN.
The three main points of the starter are
1. L- Line terminal to be connected to positive of supply.
2. A- Armature terminal to be connected to armature winding
3. F- Field terminal to be connected to the field winding
‘L’ is further connected to an electromagnet called overload release coil (OLR). The other end of ‘OLR’ is connected to a point where handle of the starter is pivoted. The handle is free to move from its other side against the force of the spring.
Under the influence of its own force, this spring brings back the handle to the OFF position. Another parallel path is from stud ‘1’ connected to another electromagnet called No Volt Coil (NVC). The NVC is further connected to terminal ‘F’. The starting resistance is in series with the armature. The two protecting devices of the starter are OLR and NVC.
Working
The handle is in the OFF position initially. The DC supply to the motor is switched ON. To start the motor, after closing the supply switch, the starter handle is moved clockwise from OFF position to make contact with stud No.1. At this point, field winding gets supply through the parallel path through NVC. When full resistance is in series with the armature, the armature current reduces. As the handle is moved further, it goes on making contact with studs 2, 3, etc., cutting out the starting resistance. When the starter handle is in ‘RUN’ position, the entire starting resistance gets removed and the motor develops back emf when it gathers speed.
No Voltage Release Coil (NVR)
It is an electromagnet. The coil is in series with field winding. When the handle is in ON position, the No volt coil is magnetized and attracts the soft iron and keeps the handle in ON position against the spring tension. In the case of a break in the field circuit the NVR coil is de-energized thereby releasing the arm and prevents the damage of the motor..
Over Load Release Coil (OLR)
It consists of an electromagnet connected in the supply line. When the motor becomes overloaded beyond a certain predetermined value, the armature current increases and hence the attracting power of electromagnet increases and the movable arm is lifted and short circuits the electromagnet (NVR). Thus the arm is released and returns to OFF position.
Disadvantages
The NVC and the field winding are connected in series. When controlling the speed of the motor above rated, field current is reducing by adding an extra resistance in series with field winding. Because of this, the current through NVC reduces, thereby reducing the magnetism produced by NVC. It may release the handle from its RUN position switching OFF the motor. To overcome this, four point starter is used in which NVC and the field winding are connected in parallel.
See More: Working Principle of D.C Generator
See More: Construction of a D.C Machine
See More: OperationalAmplifier:(Different Stages)
