-: Three Point Starter :-
or
[Three point D.C. Shunt Motor Starter]
The three point starter with its electrical connections and protective devices is shown in figure -
Construction of Three Point Starter :- It consists of a series starting resistance divided into several section and connected to brass studs , brass arc by which the connection to the shunt field , no-volt release and overload release is made . Since only three terminals are available from the starter , so it is called three-point starter .
Working of Three Point Starter :- For starting the motor the handle H is moved manually and when it makes contact with the resistance , stud 1 is in the start position . The starter handle is then gradually moved from stud to stud , allowing the speed of the motor to build up until it reaches the RUN position .
In this position , the motor attains full speed , the supply is directly across both the windings of the motor , and the resistance R is completely cutout . The NVC is called no volt or under voltage protection of motor .
The other protective device incorporated in the starter is the overload protection . Overload protection is provided by the overload trip coil OLC and the NVC . When the armature current exceeds the normal rated value , P is attracted by the electromagnet of OLC and closes the contacts aa . Thus , NVC is short circuited . This results in the release of the handle H , which returns to the OFF position and the motor supply is cut off .
Drawbacks of Three - Point Starter :- The three -point starter suffers from a serious drawback for motors with large variation of speed by adjusting of the field resistance should be increased . Therefore the current through the shunt field is reduced . A very low field current will make the holding electromagnet too weak to overcome the force exerted by the spring . The holding magnet may release the arm of the starter during the normal operation of the motor and thus disconnect the motor from the line . This is not desirable . A four point starter is used to overcome this difficulty .
-: Four Point Starter :-
The schematic connection diagram of 4-point starter is shown in figure -
The basic difference in the circuit of a 4-point starter as compared to a 3-point starter is that , the holding coil is removed from the shunt field circuit and is connected directly across the line with a current limiting resistance R in series.
In the above arrangement , a change in field current for variation of speed of the motor , does not affect the current through the holding coil , because the two circuits are independent of each other .
The above arrangement forms three parallel circuits -
(i)- Armature , starting resistance and overload resistance .
(ii)- A variable resistance and shunt field winding .
(iii)- Holding coil and current limiting resistance .
or
[Three point D.C. Shunt Motor Starter]
The three point starter with its electrical connections and protective devices is shown in figure -
 |
| Three-Four-Point-Starter-Construction-Working-Three-Four-Point-Starter |
Working of Three Point Starter :- For starting the motor the handle H is moved manually and when it makes contact with the resistance , stud 1 is in the start position . The starter handle is then gradually moved from stud to stud , allowing the speed of the motor to build up until it reaches the RUN position .
In this position , the motor attains full speed , the supply is directly across both the windings of the motor , and the resistance R is completely cutout . The NVC is called no volt or under voltage protection of motor .
The other protective device incorporated in the starter is the overload protection . Overload protection is provided by the overload trip coil OLC and the NVC . When the armature current exceeds the normal rated value , P is attracted by the electromagnet of OLC and closes the contacts aa . Thus , NVC is short circuited . This results in the release of the handle H , which returns to the OFF position and the motor supply is cut off .
Drawbacks of Three - Point Starter :- The three -point starter suffers from a serious drawback for motors with large variation of speed by adjusting of the field resistance should be increased . Therefore the current through the shunt field is reduced . A very low field current will make the holding electromagnet too weak to overcome the force exerted by the spring . The holding magnet may release the arm of the starter during the normal operation of the motor and thus disconnect the motor from the line . This is not desirable . A four point starter is used to overcome this difficulty .
-: Four Point Starter :-
The schematic connection diagram of 4-point starter is shown in figure -
 |
| Three-Four-Point-Starter-Construction-Working-Three-Four-Point-Starter |
The basic difference in the circuit of a 4-point starter as compared to a 3-point starter is that , the holding coil is removed from the shunt field circuit and is connected directly across the line with a current limiting resistance R in series.
In the above arrangement , a change in field current for variation of speed of the motor , does not affect the current through the holding coil , because the two circuits are independent of each other .
The above arrangement forms three parallel circuits -
(i)- Armature , starting resistance and overload resistance .
(ii)- A variable resistance and shunt field winding .
(iii)- Holding coil and current limiting resistance .
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