# eece 259 dc motor     Post on 22-Jan-2018

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1. 1. Electric Motor M Electrical Energy Mechanical Energy DC Motor
2. 2. Model Electric Motor Beakman Motor What do you need? 1. Electric Energy 2. Coil 3. Magnetic Field
3. 3. Electric Motor
4. 4. DC Motor Principle Whenever a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force which collectively produce a driving torque that sets the armature rotating whose direction is given by Flemings Left-hand Rule.
5. 5. Motor & Generator Action
6. 6. Back EMF/Governor The term back electromotive force, or just back-EMF, is most commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the magnetic field from the motor's field magnets, or windings.
7. 7. Voltage Equation Eg = V + Ia Ra (Generator) V = Eb + Ia Ra (Motor)
8. 8. Voltage Equation Derivation
9. 9. Condition for Maximum Power Gross mechanical power developed by a motor is maximum when back e.m.f. is equal to half the applied voltage.
10. 10. Torque Equation Derivation
11. 11. Armature Torque Equation Derivation
12. 12. Armature Torque of a D.C. Motor
13. 13. Shaft Torque Equation Derivation The difference (Ta Tsh) is known as lost torque and is due to iron and friction losses of the motor.
14. 14. Speed of a D.C. Motor Speed Regulation
15. 15. D.C. Motor Characteristics 1. Torque and armature current i.e. Ta/Ia characteristic. It is known as electrical characteristic. 2. Speed and armature current i.e. N/Ia characteristic. 3. Speed and torque i.e. N/Ta characteristic. It is also known as mechanical characteristic.
16. 16. Characteristics of Series Motors 1. Torque and armature current i.e. Ta/Ia characteristic. It is known as electrical characteristic. At light loads, Ia and hence is small. But as Ia increases, Ta increases as the square of the current. Hence, Ta/Ia curve is a parabola . After saturation/heavy Loads, is almost independent of Ia hence Ta Ia only. So the characteristic becomes a straight line. The shaft torque Tsh is less than armature torque due to stray losses. As practically is equals to Ia
17. 17. Characteristics of Series Motors 2. Speed and armature current i.e. N/Ia characteristic. With increased Ia, also increases. Hence, speed varies inversely as armature current. When load is heavy, Ia is large. Hence, speed is low (this decreases Eb and allows more armature current to flow). But when load current and hence Ia falls to a small value, speed becomes dangerously high. Hence, a series motor should never be started without some mechanical (not belt-driven) load on it otherwise it may develop excessive speed and get damaged due to heavy centrifugal forces so produced. It should be noted that series motor is a variable speed motor. As practically is equals to Ia
18. 18. Characteristics of Series Motors 3. Speed and torque i.e. N/Ta characteristic. It is also known as mechanical characteristic. When speed is high, torque is low and vice-versa.
19. 19. Characteristics of Shunt Motors 1. Torque and armature current i.e. Ta/Ia characteristic. It is known as electrical characteristic. Assuming to be practically constant (though at heavy loads, decreases somewhat due to increased armature reaction) Hence, the electrical characteristic is practically a straight line through the origin. Shaft torque is shown dotted. Since a heavy starting load will need a heavy starting current, shunt motor should never be started on (heavy) load. As is practically constant
20. 20. Characteristics of Shunt Motors 2. Speed and armature current i.e. N/Ia characteristic. As is practically constant As Eb is also practically constant, speed is, for most purposes, constant But strictly speaking, both Eb and decrease with increasing load. However, Eb decreases slightly more than so that on the whole, there is some decrease in speed. The drop varies from 5 to 15% of full-load speed, being dependent on saturation, armature reaction and brush position. Hence, the actual speed curve is slightly drooping as shown by the dotted line. But, for all practical purposes, shunt motor is taken as a constant-speed motor.
21. 21. Characteristics of Shunt Motors 3. Speed and torque i.e. N/Ta characteristic. It is also known as mechanical characteristic. When speed is high, torque is almost constant. As is practically constant
22. 22. Compound Motors These motors have both series and shunt windings. If series excitation helps the shunt excitation i.e. series flux is in the same direction (a) then the motor is said to be cumulatively compounded. If on the other hand, series field opposes the shunt field (b), then the motor is said to be differentially compounded.
23. 23. Characteristics of Compound Motors
24. 24. Applications of DC Motors
25. 25. Power Stages A B = copper losses and B C = iron and friction losses
26. 26. Factors Controlling Motor Speed the speed can be controlled by varying (i) flux/pole, (Flux Control) (ii) resistance Ra of armature circuit (Rheostatic Control) (iii) applied voltage V (Voltage Control)
27. 27. Speed Control of DC motors (i) Variation of Flux or Flux Control Method It is seen from above that N 1/. By decreasing the flux, the speed can be increased and vice versa. Hence, the name flux or field control method. The flux of a d.c. motor can be changed by changing Ish with help of a shunt field rheostat. Since Ish is relatively small, shunt field rheostat has to carry only a small current, which means I2R loss is small, so that rheostat is small in size. This method is, therefore, very efficient.
28. 28. Speed Control of DC motors (ii) Armature or Rheostatic Control Method This method is used when speeds below the no-load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or resistance (called controller resistance) in series with the armature circuit as shown in Fig. (a). As controller resistance is increased, potential difference(p.d.) across the armature is decreased, thereby decreasing the armature speed. For a load constant torque, speed is approximately proportional to the p.d. across the armature. From the speed/armature current characteristic [Fig. (b)], it is seen that greater the resistance in the armature circuit, greater is the fall in the speed.
29. 29. Speed Control of DC motors (iii) Voltage Control Method a) Multiple Voltage Control b) Ward-Leonard System
30. 30. Starter For DC Motor When the motor is at rest, there is, as yet, obviously no back e.m.f. developed in the armature. If, now, full supply voltage is applied across the stationary armature, it will draw a very large current because armature resistance is relatively small. This excessive current will blow out the fuses and, prior to that, it will damage the commutator and brushes etc. To avoid this happening, a resistance is introduced in series with the armature (for the duration of starting period only, say 5 to 10 seconds) which limits the starting current to a safe value. The starting resistance is gradually cut out as the motor gains speed and develops the back e.m.f. which then regulates its speed.