3-POINT STARTER

                                    3-POINT STARTER  

3-POINT STARTER [DC SHUNT MOTOR]

A 3-POINT STARTER IS EXTENSIVELY USED TO START DC SHUNT MOTOR. IT ALSO PROVIDES ADDITIONAL PROTECTIVE FEATURES SUCH AS OVERLOAD PROTECTION AND NO-VOLT PROTECTION.

THE JUNCTION OF THE RESISTANCE IS BROUGHT AS A TERMINAL CALLED STUDS.

WORKING OF 3-PINT STATOR:-

1. INITIALLY, THE HANDLE IS IN OFF CONDITION. THE SUPPLY IS NOT CONNECTED TO THE ARM NOR THE FIELD OF THE MOTOR.

2. NO AFTER CLOSING THE SWITCH S MOVE THE HANDLE TO STUD 1. AT THIS POSITION ARM AND ALL THE RESISTANCES GET CONNECTED TO THE SUPPLY.

3. THE ONE CURRENT FLOWS FROM SUPPLY:-

OLRC-RESISTANCE-ARMATURE [A1-A2] AND THEN TO THE POSITIVE TERMINAL OF THE SUPPLY.

4. AS THE RECTANGULAR STRIP MAKES CONTACT WITH ARM CU-STRIP, THE FIELD COIL GETS FULLY SUPPLY AS FROM S-L-OLRC- THROUGH CU-STRIP-NVRC-F1-F2- THROUGH RHEOSTAT GET BACK TO NEGATIVE TERMINAL OF SUPPLY.

5. AS THE MACHINE PICKS UP SPEED, THE HANDLE IS MOVED FURTHER TO STUD 2. AT THIS POSITION THE EXTERNAL RESISTANCE IN THE ARMATURE CIRCUIT IS LESS AS THE FIRST RESISTANCE IS LEFT CUT. HOWEVER, FIELD CONTINUES TO GET FULL VOLTAGE BY MEANS OF CU STRIP.

6. CONTINUING IN THIS WAY, ALL RESISTANCES WILL BE LEFT OUT [1-2-3-4_ _ _ _]. WHEN STUD-g[ON] IS REACHED, IN THIS POSITION, THE ELECTROMAGNET WILL ATTRACT THE SOFT IRON PIECE ATTACHED TO THE HANDLE.

NOW THE PATH OF THE CURRENT ARE:-

S-L-OLRC HANDLE TO STUD G[ON] A1-A1-A2 THEN NEGATIVE TERMINAL.

S-L-OLRC- RECTANGULAR STRIP TO NVRC COIL1-F-F1F2-RHEOSTAT-THEN NEGATIVE TERMINAL.

SO NVRC CARRIES THE SAME CURRENT AS THAT OF FIELD COIL.

FUNCTION OF NVRC [NO VOLT RELEASE COIL]

IN CASE SUPPLY GOES OFF, IF TO ZERO HENCE NVRC WILL BE DE-ENERGISED AND WILL NOT BE ABLE TO EXERT ANY FORCE ON THE SOFT IRON PIECE OF THE HANDLE.

IN THE OFF POSITION, RESTORING FORCE OF THE SPRING WILL BRING THE HANDLE BACK TO THE MEAN POSITION.

FUNCTION OF OLRC [OVER LOAD RELEASE COIL]

NOT LET US ASSUME THAT THE HANDLE IS IN ON CONDITION.

IN CASE OVERLOAD IS APPLIED ON DC MOTOR, THE LOAD IS MORE THAN THE RATED VALUE. THEN IT REACHES A CERTAIN SPECIFIED VALUE, OLRC WILL ENERGIZE.

OLRC ATTRACTS SOFT IRON PIECES AND POINTS WILL CONNECT EACH OTHER.

DISADVANTAGES OF 3-POINT STATOR

1. THE FIELD CIRCUIT AND HOLD COIL ARE IN SERIES

2. TO INCREASE THE SPEED OF THE MOTOR, FIELD RESISTANCE SHOULD BE INCREASES OR Ip DECREASED, HENCE THE SHUNT FIELD CURRENT IS REDUCED.

A VERY LOW FIELD CURRENT MAKES THE HOLDING ELECTROMAGNET TOO WEAK TO OVERCOME THE FORCE EXERTED BY THE SPRING AND HANDLE GET BACK TO OFF POSITION.

THUS THE 3-POINT STATOR CAN'T BE USED FOR A WIDE RANGE OF SPEED CONTROL BY SHUNT FIELD CONTROL IS REQUIRED.