1. ACTIVE LEARNING ASSIGNMENT TOPIC NAME :- DC MOTOR STARTERS ELEMENTS OF ELECTRICAL DESIGN (2150904 ) AY-2015-16 Course co-ordinator:- SAMIR B PATEL PREPARED BY:- DEEP CHOTAI (130110109007) SHREYANSH JETHWA (130110109017) KHILAN JOSHI (130110109018)

2. THREE POINT STARTER  It consists of graded starting resistances, r s which is connected in series with armature.  It also consists of two protective devices namely no volt and overload release, starting arm and the brass arc segment s.  Generally three terminals of starter L,F & A are brought out, which are connected to positive line terminal, shunt field & armature terminal of the motor respectively.

3. CONTINUE

4. FUNCTION OF NO VOLT RELEASE COIL (NVC)  When the handle is in the 'RUN' position, soft iron piece connected to the handle gets attracted by the magnetic force produced by NVC. Design of NVC is such that it holds the handle in 'RUN' position against the force of the spring as long as supply to the motor is proper. Thus NVC holds the handle in the 'RUN' position and hence also called hold on coil.  NVC consists of a thin wire of many turns & is connected in series with the shunt field winding of motor. The coil is magnetized when the current flows through the shunt field winding.  NVC performs the similar action under low voltage conditions and It releases the handle and goes to OFF position by spring tension due to the failure of supply and disconnects the motor from supply.

5. FUNCTION OF OVERLOAD RELAY COIL (OLC)  It’s function is to demagnetize the NVC in case of fault or overload of the machine OR protect the motor from overload.  It consists of a few turns of thick wire & is connected in series to armature. This coil is magnetized when excessive current flows through armature due to overload or some fault & it attracts the tripping plunger C, which is short circuited the terminal of NVC.  NVC will be demagnetized & release the starting arm which will come to OFF position due to spring tension & thus motor stops.

6. DISADVANTAGE OF 3 POINT STARTER  In this starter, the NVC and the field winding are in series. So while controlling the speed of the motor above rated, field current is reduced by adding an extra resistance in series with the field winding. Due to this, the current through NVC also reduced. Due to this, magnetism produced by NVC also reduces. This may release the handle from its RUN position switching off the motor. To avoid the dependency of NVC and the field winding, four point starter is used, in which NVC and the field winding are connected parallel.

7. FOUR POINT STARTER

8. 4 POINT STARTER  Figure shows a wiring diagram of it which is connected to a long shunt compound motor.  In this starter NVC is connected directly to the line through protective resistance. when starting arm touches stud no. 1 line current divides into 3 parts: 1)one part passes through starting resistance Rs, series field & armature 2)Second part passes through shunt field winding. 3)Current passes through the NVC & protective resistance r.

9. CONTINUE  With this arrangement, NVC is independent of shunt field circuit, therefore it will not be affected by the change of current in shunt field circuit, it means electromagnetic pull exerted by NVC will always be sufficient & will prevent the spring from restoring the starting arm to OFF position.  The possibility of accidentally opening the field circuit is quite remote; hence greater acceptance of 4 point starter over 3 point starter.

10. BACK EMF STARTER  Figure shows a wiring diagram and control circuit of a back emf type automatic starter used for starting d.c. shunt and compound motors.  It consists of start and stop push buttons PB1 and PB2, M and M A are normally open contacts, which are actuated by the main operating coil C M, a,b, and c are normally open contacts, which are actuated by the operating coils Ca,C b and Cc respectively. r1,r2 and r3 are starting resistances.  M A is the auxiliary or maintaining contact which is used to keep the operating coil C M energized after the finger is removed from the ‘START’ push button PB1.

11. BACK EMF STARTER  When the START push button PB1 is pressed, the circuit is completed from positive terminal through L, normally closed contact OL,PB2,PB1, main operating coil C M to the negative terminal of the line.  I.e. Positive terminal(+) – L – OL –PB2- PB1- C M –Negative terminal(-)

12. BACK EMF STARER  Thus, the operating coil C M is energized and closes its contacts M and M A. Closing of contacts the armature of the motor to the supply through full starting resistance in series and field circuit directly across the supply, thus the motor accelerate.  As the speed of the motor increases, its back emf increases, which is available across the coils Ca,Cb and Cc. When the back emf become equal to the operating voltage of the coil C A, it is energized and closes its contact ‘a’, thus cutting down the starting resistance r1 of the first section. The motor further accelerates causing the back emf to further increase.

13. BACK EMF STARTER  When the back emf reaches the operating voltage of the coil Cb, it is energized and closes its contacts b, cutting down r2. similarly contact ‘c’ get closed after sufficient increase in back emf is achieved. Now, all the starting resistances are cut down and the motor armature is connected directly accross the supply.  When the STOP push button PB2 is pressed, the supply through the contactor coil C M is disconnected. Sinnce the operating coil C M is de-energized, the contacts M and M A are opened, the supply to the motor is disconnected and the motor stops.  Also in case of an overload on the motor, overload relay coil OLC is energized and opens the contact OL. Thus the coil C M is disconnected from the supply and therefore disconnect the motor from the supply.

14. DISADVANTAGE OF BACK EMF STARTER  The disadvantage of back emf type starter is that the closing of contacts a,b and c connected across starting resistances r1, r2 and r3 depends upon the amount of back emf developed by the motor.  Hence, in case, if the accelerations of the motor and hence building of back emf is delayed due to load condition, the starting resistance will continue to be in line for a prolonged period and may get damaged due to overheating as they are not designed for continupous operation.

15. TIME DELAY STARTER  This starter overcomes the drawback of the back emf starter. In this section resistances are cut out at definite time intervals. Figure shows the wiring diagram and control circuit of automatic time delay type d.c. starter.  When the START push button PB1 is pressed, the operating coil C M is energized through the following path: Posive terminal (+) – OL – PB2 – PB1 – C M – Negative terminal (-)  Thus, closes its contacts M and M A. The closing of contacts M connect the armature of the motor across the supply, thus the motor across main supply through full starting resistances and field circuit directly across the supply, thus the motor starts rotating.

16. TIME DELAY STARTER  The closing of contacts MA energizes the coil Ca which closes its contacts ‘a’ after a pre-set time delay. Hence the first section resistance r1 is cut out. At the same time, coil Cb is energized and after a pre-determined time delay, closes its contact ‘b’, short circuiting the starting resistance r2 and also energizing the coil Cc.  As the coil Cc is energized, the contact ‘c’ is closed and cut out the starting resistance r3 after pre-set time delay. In this way the entire starting resistance is cut out and the motor armature is connected directly across the supply.

17. TIME DELAY STARTER  The protection against overload is provided by the overload relay OLC which is connected in series with the motor armature. In case of an overload on the motor, overload relay coil OLC is energized and opens the contact OL. Thus disconnect the motor from the supply.  Pressing the STOP push button PB2, de-energizes all contactors and thus the supply to the motor is disconnected and the motor stops.