Direct current motors slides with numerical

Slides of DC Motors

1. DC Motors

2. DC Motors A DC motor or Direct Current Motor converts electrical energy into mechanical energy. A direct current (DC) motor is a fairly simple electric motor that uses electricity and a magnetic field to produce torque, which turns the rotor and hence give mechanical work. A DC motor consists of an stator, an armature, a rotor and a commutator with brushes. Opposite polarity between the two magnetic fields inside the motor cause it to turn. DC motors are the simplest type of motor and are used in household appliances, such as electric razors and in electric windows in cars.

3. Basic working principle of motor When a current carrying conductor is moved in a magnetic field, a force is produced in a direction perpendicular to the current and magnetic field directions. Lorentz’s force law, which relates force on a conductor to the current in the conductor and the external magnetic field, in vector form is: where = force vector (conductor per unit length) current vector magnetic field vector  

4. Right Hand Rule The right hand rule illustrates the relationship between all these vectors. The right hand rule suggests that if your right-hand index finger points in the direction of the current and your middle finger is aligned with the field direction, then your extended thumb (perpendicular to the index and middle fingers) will point in the direction of the force.

5. Construction of DC Motor

6. Components of DC Motor It is outer cover of dc motor also called as frame. It provides protection to the rotating and other part of the machine from moisture, dust etc. Yoke is an iron body which provides the path for the flux to complete the magnetic circuit. It provides the mechanical support for the poles. It is made up of low reluctance material such as cast iron, silicon steel, rolled steel, cast steel etc. Reluctance is the property of a magnetic circuit of opposing the passage of magnetic flux lines. Yoke

7. Components of DC Motor Poles are electromagnet, the field winding are wound on it. It produces the magnetic field when field winding is excited. The construction of pole is done using the lamination of particular shape to reduce the power loss due to eddy current. Pole shoe is an extended part of a pole. Due to its typical shape, it enlarges the area of the pole, so that more flux can pass through the air gap to armature. It also uses low reluctance magnetic material such as cast steel or cast iron is used for construction of pole and pole shoe. Poles, Pole Core and Pole Shoe

8. Components of DC Motor Poles, Pole Core and Pole Shoe

9. Components of DC Motor The coil wound on the pole core are called field coils. Field coils are connected in series to form field winding. Current is passed through the field winding in a specific direction, to magnetize the poles and pole shoes. Thus magnetic flux is produce in the air gap between the pole shoe and armature. Magnetic flux is defined as the number of magnetic field lines passing through a given closed surface. It provides the measurement of the total magnetic field that passes through a given surface area. Field winding is also called as exciting winding. Material used for conductor is copper. Due to the current flowing through the field winding alternate N and S poles are produced. Field Winding

10. Components of DC Motor Field Winding

11. Components of DC Motor Armature core is a cylindrical drum mounted on the shaft. It is provided with large number of slots all over its periphery and it is parallel to the shaft axis. Armature conductors are placed in these slots. Armature core provides low reluctance path to the flux produced by the field winding. High permeability, low reluctance cast steel or cast iron material is used. Laminated construction of iron core is used to minimize the eddy current losses. Armature Core

12. Components of DC Motor Armature conductor is placed in a armature slots present on the periphery of armature core. Armature conductor are interconnected to form the armature winding. When armature winding is connected to a voltage source, then due to current flowing in the conductor, a magnetic field is produced. This magnetic field interacts with the magnetic field of field winding and as a result, the motor rotates. Armature winding is suppose to carry the entire load current hence it should be made up of conducting material such as copper. Armature Winding

13. Components of DC Motor Armature Winding

14. Components of DC Motor A commutator is a rotatory electrical switch that reverses the direction of current between the rotor and the external circuit periodically.  It is a cylindrical drum mounted on a shaft along with the armature core. It is made up of large number of wedge shaped segments of hard drawn copper. These segments are insulated from each other by thin layer of mica. Armature winding are tapped at various points and these tapping are successively connected to various segments of the commutator. It helps to produce unidirectional torque. It is made up of copper and insulating material between the segments is mica. Commutator

15. Components of DC Motor

16. Commutator

17. Components of DC Motor Current is conducted from the armature to the external load by the carbon brushes which are held against the surface of the commutator by springs. Function of brushes: To collect the current from the commutator and apply it to the external load in generator , and vice versa in motor. Brushes are made of carbon and they are rectangular in shape. Brushes

18. Components of DC Motor In case of DC motor when the armature winding of dc motor starts rotating in the magnetic flux produced by the field winding, it cuts the lines of magnetic flux and induces the emf in the armature winding. According to Lenz’s law (The law that whenever there is an induced electromotive force (emf) in a conductor, it is always in such a direction that the current it would produce would oppose the change which causes the induced emf.), this induced emf acts in the opposite direction to the armature supply voltage. Hence this emf is called as back EMF. Back EMF

19. Components of DC Motor The back emf is defined as: where = Speed in rpm = Flux per pole = Total number of armature conductors = Number of slots x Number of Conductors/slot = number of pole = area of cross-section of conductor = back emf   Back EMF

20. Components of DC Motor The equivalent circuit of armature of DC motor is given as follows: Multiplying both sides by , we get   Voltage and Power Equations

21. Components of DC Motor where = electrical power supplied to the motor = electrical equivalent of mechanical power produced by motor = power loss in armature winding   Voltage and Power Equations

22. Components of DC Motor Mechanical power required to rotate the shaft of the motor is given as where = Torque in Newton meter = angular velocity in radian per second Gross mechanical power produced by motor on electrical side is given as . Equating electrical power and mechanical power, we get: where =   Torque and Speed Equations:

23. Components of DC Motor Rearranging the above equation for , we get: Since are constant, we get: Thus torque produce by the DC Motor is proportional to the main field flux and armature current   Torque and Speed Equations:

24. Types of DC Motors In this type of DC motor the armature and field windings are connected in series. The resistance of the series field winding is much smaller than armature resistance The flux produced is proportional to the field current but ∝ Thus flux can never become constant in dc series motor as load changes and also gets changed Thus dc series motor is not a constant flux motor.   DC Series Motor

25. Types of DC Motors DC Series Motor

26. Types of DC Motors We know that for DC motors: But since , thus , so we get and Since   DC Series Motor

27. DC Series Motor To study the performance of the DC series motor various types of characteristics are to be studied. Torque Vs Armature current characteristics. Speed Vs Armature current characteristics. Speed Vs Torque characteristics. Characteristics of DC Series Motor

28. DC Series Motor Torque developed in any dc motor is ∝ In a series motor, as field windings also carry the armature current, ∝  up to the point of magnetic saturation. Hence, before saturation, ∝   Torque Vs. Armature Current Characteristics

29. DC Series Motor After saturation, is almost independent of hence   ∝   only.  So the characteristic becomes a straight line. So we conclude that (prior to magnetic saturation) on heavy loads, a series motor exerts a torque proportional to the square of armature current.    Torque Vs. Armature Current Characteristics

30. DC Series Motor Since we know that: Change in  , for various load currents is small and hence may be neglected for the time being. With increased  , also increases. Hence, speed varies inversely as armature current.   Speed Vs. Armature Current Characteristics

31. DC Series Motor It is found from above that when speed is high, torque is low and vice-versa. It is clear that series motor develops high torque at low speed and vice-versa.  It is because an increase in torque requires an increase in armature current, which is also the field current. The result is that flux is strengthened and hence the speed drops. Speed Vs. Torque Characteristics

32. Types of DC Motors In DC Shunt motor, armature and shunt winding are connected parallel to supply voltage. The resistance of shunt winding is larger than the resistance of armature winding. Since and both remains constant the remains essentially constant, as field current is responsible for generation of flux. thus ∝ So shunt motor is also called as constant flux motor As we know that:   DC Shunt Motor

33. Types of DC Motors DC Shunt Motor

34. Types of DC Motors Since ∝ , as is constant so is also constant and we get the following relationship for DC Shunt motor: and ∝ ∝   DC Shunt Motor

35. DC Shunt Motor To study the performance of the DC shunt Motor various types of characteristics are to be studied. Torque Vs Armature current characteristics. Speed Vs Armature current characteristics. Speed Vs Torque characteristics. Characteristics of DC Shunt Motor

36. DC Shunt Motor Since we know that:   Torque Vs. Armature Current Characteristics

37. DC Shunt Motor Since we know that: After removing constant terms and rearranging, we get: ∝ ∝   Speed Vs. Armature Current Characteristics

38. DC Shunt Motor From the above two characteristics of dc shunt motor, the torque developed and speed at various armature currents of dc shunt motor may be noted. If these values are plotted, the graph representing the variation of speed with torque developed is obtained. This curve resembles the speed Vs current characteristics as the torque is directly proportional to the armature current. Speed Vs. Torque Characteristics

39. Types of DC Motors The DC compound motor is a combination of the series motor and the shunt motor. It has a series field winding that is connected in series with the armature and a shunt field that is in parallel with the armature. The combination of series and shunt winding allows the motor to have the torque characteristics of the series motor and the regulated speed characteristics of the shunt motor. Some types of compound excited motor are: Long Shunt Compound Excited Motor Short Shunt Compound Excited Motor DC Compound Excited Motor

40. Compound Excited Motors In case of long shunt compound wound DC motor, the shunt field winding is connected in parallel across the series combination of both the armature and series field coil. Long Shunt Compound Excited Motor

41. Compound Excited Motors Let E and be the total supply voltage and current supplied to the input terminals of the motor. And , , be the values of current flowing through armature resistance , series winding resistance and shunt winding resistance respectively. We know that in case of shunt motor: And in case of series motor: Therefore, the equation for current will be as follows: The voltage will be given as follows: +   Long Shunt Compound Excited Motor

42. Compound Excited Motors In case of short shunt compound wound DC motor, the shunt field winding is connected in parallel across the armature winding only. And series field coil is exposed to the entire supply current, before being split up into armature and shunt field current Short Shunt Compound Excited Motor

43. Compound Excited Motors Here also let, E and be the total supply voltage and current supplied to the input terminals of the motor. And , , be the values of current flowing through armature resistance , series winding resistance and shunt winding resistance respectively. Since the entire supply current flows through the field winding, we can say that: And in case of shunt motor: The voltage will be given as follows: +   Short Shunt Compound Excited Motor

44. Compound Excited Motors But we also know that: Therefore, the final equation will be as follows:   Short Shunt Compound Excited Motor

45. Compound Excited Motors A compound wound DC motor is said to be cumulatively compounded when the shunt field flux produced by the shunt winding assists or enhances the effect of main field flux, produced by the series winding.   Cumulative Compounding of DC Motor

46. Compound Excited Motors A compound wound DC motor is said to be differentially compounded when the flux due to the shunt field winding diminishes the effect of the main series winding. This particular trait is not really desirable, and hence does not find much of a practical application.   Differential Compounding of DC Motor

47. Speed Control of DC Motors The speed equation of dc motor is But the resistance of armature winding or series field winding in dc series motor are small. Therefore the voltage drop or across them will be negligible as compare to the external supply voltage V in above equation. Therefore: since V > > > >  

48. Speed Control of DC Motors Thus we can say Speed is inversely proportional to flux . Speed is directly proportional to armature voltage. Speed is directly proportional to applied voltage V. So by varying one of these parameters, it is possible to change the speed of a dc motor  

49. Flux Control Method

50. Example 1