Lecture_11.pptxbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

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1. Electrical engineering (MED) – u.e.t. taxila 11 Engr. M. Mansoor ashraf

2. DC machines The broad classification of DC machines include DC generator and DC motor. DC Generator is rotary DC machine which converts the rotational mechanical energy into electrical energy. DC Motor is rotary DC machine which converts electrical energy into rotational mechanical energy. There are four types of DC machines w.r.t. construction; Shunt Machine Separately Excited Machine Series Machine Compound Machine

3. DC motor The current carrying conductors placed in the magnetic field experience force. The DC voltage is applied by connecting a DC source. The field may be established either by permanent magnets or electromagnets.

4. DC motor - cemf Counter Electromotive Force (cemf) Consider a DC machine initially at rest, is supplied by a DC source E s . The current will flow through all the armature conductors. The individual armature conductors will be subjected to force.

5. DC motor - cemf The force on armature conductors will be applied because these are placed within magnetic field. These forces add up to produce a powerful torque, causing the armature to rotate. As armature rotates, the voltage E o is induced in the armature conductors.

6. DC motor - cemf In case of motor, the induced voltage E o is called Counter-Electromotive force (cemf) because its polarity is always acts against the source voltage E s . So the net/resultant acting voltage will be equal to ( E s -E o ) volts.

7. Acceleration of dc motor The net voltage in the armature circuit is ( E s -E o ) volts. The resulting armature current flowing will be: When the motor is at rest, the induced voltage is E o =0, and starting current will be: The starting current may be 20 to 30 times greater than nominal full load current of the motor.

8. Acceleration of dc motor As the speed increases, the counter-emf E o increases, with the result that the value of ( E s -E o ) decreases. Thus the armature current drops progressively as the speed increases. At no-load, the counter-emf E o is slightly less than the source voltage E s . At load, there is a difference between counter-emf and source voltage. The speed of motor becomes constant when developed torque is equal to load torque.

9. problems The armature of a permanent magnet DC generator has a resistance of 1 Ω and generates a voltage of 50 V when the speed is 500 r/min. If the armature is connected to a source of 150 V, calculate the following: The starting current. The counter-emf when the motor runs at 1000 r/min and 1460 r/min. The armature current at 1000 r/min and 1460 r/min. (150 A, 100 V, 146 V, 50 A, 4 A)

10. Mechanical power and torque The counter-emf induced in armature is given by: Electrical power supplied by source is given by: Source voltage is sum of induced voltage and drop: Putting value;

11. Mechanical power and torque The I 2 R term represents the loss in terms of heat dissipated in the armature. The mechanical power of motor is therefore exactly equal to product of cemf and armature current. The mechanical torque and power relationship of motor is given in 3.5 (Chapter 3).

12. Mechanical power and torque Equating both power equations: Thus T is the torque developed by the motor.

13. Speed of rotation When a DC motor drives a load between no-load and full-load, the IR drop due to armature resistance is always small compared to the supply voltage E s . This means that counter-emf is very nearly equal to E s .

14. speed control of dc shunt motor Speed of rotation of armature is given by: Speed of rotation is directly proportional to the source voltage and inversely proportional to the magnetic flux of the poles. There are two well-known speed control methods: armature speed control method and field speed control method.

15. Armature speed control method In this method, a rheostat is placed in series with armature of motor. By adjusting rheostat, E s voltage can be varied accordingly.

16. field speed control method In this method, speed of motor is varied by varying field flux with help of rheostat connected in series with field winding. The flux is inversely proportional to speed.

17. Load behavior of dc shunt motor

18. problems A shunt motor rotating at 1500 r/min is fed by a 120 V source. The line current is 51 A and shunt field resistance is 120 Ω . If armature resistance is 0.1 Ω , calculate; Armature current. Counter emf. Mechanical power developed by motor. (50 A, 115 V, 5750 W (7.7 hp ))

19. dc series motor A series motor is identical in construction to a shunt motor except for the field. The series field is connected in series with armature winding. The series field is composed of a few turns of wire having large cross sectional area. In series motor, the value of field flux depends upon the armature current. When the current is large, the flux is large and vice versa.

20. Dc series motor

21. dc series motor When series motor starts up, the armature current is higher than normal, field flux is higher. At this time, the starting torque will also be higher than shunt motor. On the other hand, if the motor operates at less than full-load, the armature current and field flux is smaller than normal. The weaker flux causes the speed of motor to rise as shunt motor.

22. Dc series motor If the load is small, the speed may rise to dangerously high values. For the reason we never permit a series to operate at no-load. DC series motors are mostly used for high torque applications.

23. Dc compound motor The schematic diagram of DC compound motor is shown in figure. A compound DC motor carries both series field and shunt field. In cumulative compound motor, the series and shunt fields support each other and add up.

24. Dc compound motor When motor runs at no-load, the armature current is low and series field is weak. The shunt field of motor is fully excited by exciting current. As the load on the motor increases, the series field becomes stronger.

25. Dc compound motor And the shunt field remains constant. The total field of motor thus increases than before. This causes the motor speed to decrease according to following.

26. Differential compound motor If the series field is connected so that it opposes the shunt field, a differential compound motor is obtained. In such motor, the total field decreases with increasing load, thus speed of motor rises according to following.

27. Load behaviors of dc motors

28. Reversing direction of rotation To reverse the direction of rotation of a DC compound motor: Either the armature connections are reversed. Or both shunt and series field connections are reversed.

29. Reversing direction of rotation

30. Reversing direction of rotation

31. Starting DC shunt motor If full voltage is applied to stationary shunt motor, the starting current in the armature will be very high and following risks are highlighted. Burning out the armature Damaging the commutator and brushes Overloading of motor All DC motors must, therefore, be provided with means to limit the starting current to reasonable values. The resistance is gradually reduced as the motor accelerates and is eventually eliminated entirely at full speed.

32. Face plate starter The schematic diagram of manual face-plate starter is shown below. Bare copper contacts are connected to current limiting resistors.

33. Face plate starter Conducting arm 1 sweeps across the contacts when it is pulled to the right by means of insulated handle 2. The conducting arm 1 is at dead contact M, and motor circuit is open.

34. Face plate starter As handle is moved to right, the conducting arm 1 first touches fixed contact N. The supply voltage immediately causes the full field current I x to flow but armature current I is limited.

35. Face plate starter The motor begins to turns and cemf E o builds up which reduces the armature current. When the motor speed rises to some extent, the arm is pulled to next contact and so on.

36. Face plate starter At last contact, the arm is held at this position by a small electromagnet 4 which is in series with shunt field.

37. Face plate starter If the supply voltage is suddenly interrupted or field is accidently cut, the electromagnet releases the arm, allowing it to return to its dead position, under pull of spring 3. This is safety of motor.

38. references Electrical Machines, Drives and Power Systems (5 th Edition) Theodore Wildi Chapter 05 – DC Motors (5.1 – 5.8, 5.10 – 5.14)