SEPARATELY EXCITED & SHUNT DC MACHINES

HOW IT WORKS

1. PREPARED BY: TASHFEEN SHARF SHABBIR 1

2. Separately excited & shunt dc machines Presented by: TASHFEEN SHARF SHABBIR BEE-F16-001 SHERYAR BASHIR BEE-F16-065

3. CONTENT Introduction Construction Circuit diagram Working Operating Characteristics Speed Control Methods Applications PREPARED BY: TASHFEEN SHARF SHABBIR 3

4. INTRODUCTION Separately Excited DC Motor Motor whose field circuit is supplied from a separate constant voltage power supply. Shunt DC Motor Motor whose field circuit gets its power directly across the armature terminals of the motor. When the supply voltage to a motor is assumed constant. PREPARED BY: TASHFEEN SHARF SHABBIR 4

5. INTRO… CONTNUE Separately excited DC motor have both stator and rotor. Stator refers to the static part of motor, which consists of the field windings And the rotor is the moving armature which contains armature windings or coils.  Separately excited dc motor has field coils similar to that of shunt wound dc motor. In separately excited DC motor, separate supply Provided for excitation of both field coil and armature coil. [1] PREPARED BY: TASHFEEN SHARF SHABBIR 5

6. INTRO… CONTNUE In Shunt wound DC motor , the field windings are shunted to, or are connected in parallel to the armature winding of the motors. For this reason both the armature winding and the field winding are exposed to the same supply voltage . There are separate branches for the flow of armature current and the field current. PREPARED BY: TASHFEEN SHARF SHABBIR 6

7. CONSTRUCTION Shunt DC Motor The shunt field winding made of several turns on the coil and those turns are made of the thin wire, and shunt winding size is quite small and can’t carry high current. The armature can support higher current because it has higher gauge thus higher current passes through the armature during starting up of the motor or when it runs a lower speed. PREPARED BY: TASHFEEN SHARF SHABBIR 7

8. CONSTRUCTION… CONTINUE SEPARATELY EXCITED DC MOTOR   Separately excited dc motor has field coils similar to that of shunt wound dc motor. In separately excited DC motor, separate supply Provided for excitation of both field coil and armature coil.  PREPARED BY: TASHFEEN SHARF SHABBIR 8

9. Circuit Diagrams PREPARED BY: TASHFEEN SHARF SHABBIR 9

10. CIRCUIT DIAGRAM… CONTINUE PREPARED BY: TASHFEEN SHARF SHABBIR 10

11. WORKING Figure below shows the separately excited dc motor. [1] PREPARED BY: TASHFEEN SHARF SHABBIR 11

12. PREPARED BY: TASHFEEN SHARF SHABBIR 12

13. Separately excited motor have separate field winding means supply to armature is different and supplied to field is different. So, any change in armature current ( Ia ) has no effect on field current (If) Direction of rotation of motor can be reverse by reversing voltage. Separately excited DC motor can operate above base speed. Armature is the rotatory part and field winding is stationary. [3 ] PREPARED BY: TASHFEEN SHARF SHABBIR 13

14. RESPONSE TO LOAD When load increased induced torque will exceed. Motor will start to slow down. Its internal generated voltage drops. Armature current in the motor increases. If motor has armature reaction its load increases. PREPARED BY: TASHFEEN SHARF SHABBIR 14

15. RESPONSE TO LOAD… CONTINUE The flux weakening effect reduce its flux. This cause increase in motor speed at any given load. The induced torque also increases. Finally induced and load torque will be equal at lower speed of rotation. It has straight line with – ve slope. PREPARED BY: TASHFEEN SHARF SHABBIR 15

16. TERMINAL CHARACTERISTIC OF SHUNT MOTOR It is plot of machines output quantities versus each other. The output quantities are shaft, torque and speed. It is plot of its output torque versus speed. PREPARED BY: TASHFEEN SHARF SHABBIR 16

17. Operating characteristics of Separately excited dc motor [1] Both in shunt wound dc motor and separately excited dc motor field is supplied from constant voltage so that the field current is constant. Therefore these two motors have similar speed -armature current and torque – armature current characteristics. In this type of motor flux is assumed to be constant. PREPARED BY: TASHFEEN SHARF SHABBIR 17

18. • Speed – armature current (N – Ia ) characteristics:   Speed of dc motor is proportional to back emf / flux i.e Eb / φ . When load is increased back emf Eb and φ flux decrease due to armature resistance drop and armature reaction respectively .However back emf decreases more than φ so that the speed of the motor slightly decreases with load. PREPARED BY: TASHFEEN SHARF SHABBIR 18

19. • Torque – armature current ( τ – Ia ) characteristics :   Here torque is proportional to the flux and armature current . Neglecting armature reaction, flux φ is constant and torque is proportional to the armature current Ia . τ – Ia characteristics is a straight lien passing through the origin. From the curve we can see that huge current is needed to start heavy loads. So this type of motor do not starts on heavy loads PREPARED BY: TASHFEEN SHARF SHABBIR 19

20. Speed control of separately excited DC motor [1] I. Field rheostat control:   Here a variable resistance is connected in series with the field coil. Thus the speed is controlled by means of flux variation. Reluctance control involving variation of reluctance of magnetic circuit of motor. Field voltage control by varying the voltage at field circuit while keeping armature terminal voltage constant. PREPARED BY: TASHFEEN SHARF SHABBIR 20

21. III. Armature resistance control:   Here, the speed is controlled by varying the source voltage to armature. Generally, a variable resistance is provided with the armature to vary the armature resistance. IV. Armature terminal voltage control Involving variation of voltage in armature circuit. PREPARED BY: TASHFEEN SHARF SHABBIR 21

22. SPEED CONTROL OF SHUNT DC MOTOR Adjusting the field resistance and thus field flux. Adjusting the terminal voltage applied to armature. Inserting a resistor in series with armature circuit. PREPARED BY: TASHFEEN SHARF SHABBIR 22

23. APPLICATIONS Separately excited DC motors are often used as actuators in trains and automotive traction applications. For their constant speed characteristics, shunt DC motors are used in fixed speed applications such as fans. PREPARED BY: TASHFEEN SHARF SHABBIR 23

24. Since the series motors can give high torque per ampere (since their toque is directly proportional to the square of armature current), they can be used in applications that require high starting torque. Examples of these applications include; starter motors in cars, and elevator motors. PREPARED BY: TASHFEEN SHARF SHABBIR 24

25. REFRENCES http://www.electronics-tutorial.net/electronic-systems/separately-excited-dc-motor/index.html https://www.electrical4u.com/shunt-wound-dc-motor-dc-shunt-motor/ https://www.youtube.com/watch?v=dJxTjJ3XBVE PREPARED BY: TASHFEEN SHARF SHABBIR 25