1. DC Machine CHAPTER 5 EKT 103 By: Dr Rosemizi Abd Rahim Click here to watch the dc machine animation video http://rmz4567.blogspot.my/2013/02/electrical-engineering.html

2. Contents –Overview of Direct Current Machines –Construction –Principle of Operation –Types of DC Machine –Power Flow Diagram –Speed Control

3. LEARNING OBJECTIVES Upon completion of the chapter the student should be able to: –State the principle by which machines convert mechanical energy to electrical energy. –Discuss the operating differences between different types of generators –Understand the principle of DC generator as it represents a logical behavior of dc motors.

4. Overview of Direct Current Machines Direct-current (DC) machines are divided into dc generators and dc motors. Most DC machines are similar to AC machines: i.e. they have AC voltages and current within them. DC machines have DC outputs just because they have a mechanism converting AC voltages to DC voltages at their terminals. This mechanism is called a commutator; therefore, DC machines are also called commutating machines. DC generators are not as common as they used to be, because direct current, when required, is mainly produced by electronic rectifiers. While dc motors are widely used, such automobile, aircraft, and portable electronics, in speed control applications…

5. DC Generator A dc generator is a machine that converts mechanical energy into electrical energy (dc voltage and current) by using the principle of magnetic induction. In this example, the ends of the wire loop have been connected to two slip rings mounted on the shaft, while brushes are used to carry the current from the loop to the outside of the circuit. Principle of magnetic induction in DC machine

6. DC Motor DC motors are everywhere! In a house, almost every mechanical movement that you see around you is caused by an DC (direct current) motor. An dc motor is a machine that converts electrical energy into mechanical energy by supplying a dc power (voltage and current). An advantage of DC motors is that it is easy to control their speed in a wide diapason.

7. Cutaway view of a dc motor Stator with poles visible. Construction of DC machine

8. segments brushes Construction of DC machine Rotor of a dc motor.

9. Construction of DC machine Stator: non-moving coil Rotor: rotating part Armature coil Brushes Rotor is the rotating part - armature Stator is the stationary part - field

10. ARMATURE More loops of wire = higher rectified voltage In practical, loops are generally placed in slots of an iron core The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux to increase the inductance of the loops and provide a higher induced voltage. The commutator is connected to the slotted iron core. The entire assembly of iron core, commutator, and windings is called the armature. The windings of armatures are connected in different ways depending on the requirements of the machine. Loops of wire are wound around slot in a metal coreDC machine armature

11. ARMATURE WINDINGS Lap Wound Armatures –are used in machines designed for low voltage and high current –armatures are constructed with large wire because of high current –Eg: - are used is in the starter motor of almost all automobiles –The windings of a lap wound armature are connected in parallel. This permits the current capacity of each winding to be added and provides a higher operating current –No of current path, C=2p ; p=no of poles

12. ARMATURE WINDINGS (Cont) Wave Wound Armatures –are used in machines designed for high voltage and low current –their windings connected in series –When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same –are used is in the small generator in hand-cranked megohmmeters –No of current path, C=2

13. ARMATURE WINDINGS (Cont) Frogleg Wound Armatures –the most used in practical nowadays –designed for use with moderate current and moderate armatures voltage –the windings are connected in series parallel. –Most large DC machines use frogleg wound armatures. Frogleg wound armatures

14. FIELD WINDINGS Most DC machines use wound electromagnets to provide the magnetic field. Two types of field windings are used : –series field –shunt field

15. FIELD WINDINGS (Cont) Series field windings –are so named because they are connected in series with the armature –are made with relatively few windings turns of very large wire and have a very low resistance – usually found in large horsepower machines wound with square or rectangular wire. –The use of square wire permits the windings to be laid closer together, which increases the number of turns that can be wound in a particular space

16. FIELD WINDINGS (Cont) Square wire permits more turns than round wire in the same area Square wire contains more surface than round wire –Square and rectangular wire can also be made physically smaller than round wire and still contain the same surface area

17. FIELD WINDINGS (Cont) Shunt field windings –is constructed with relatively many turns of small wire, thus, it has a much higher resistance than the series field. –is intended to be connected in parallel with, or shunt, the armature. –high resistance is used to limit current flow through the field.

18. FIELD WINDINGS (Cont) When a DC machine uses both series and shunt fields, each pole piece will contain both windings. The windings are wound on the pole pieces in such a manner that when current flows through the winding it will produce alternate magnetic polarities.

19. MACHINE WINDINGS OVERVIEW Winding Lap C=2p Wave C=2 Separately Excited Frogleg Self excited armature field seriesshuntcompound

20. Principle operation of Generator Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor. The AMOUNT of voltage generated depends on: i.the strength of the magnetic field, ii.the angle at which the conductor cuts the magnetic field, iii.the speed at which the conductor is moved, and iv.the length of the conductor within the magnetic field

21. Principle of operation (Cont)

22. Fleming’s Right hand rule (Generator Rule) Use: To determine the direction of the induced emf/current of a conductor moving in a magnetic field. The POLARITY of the voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor.

23. THE ELEMENTARY GENERATOR The simplest elementary generator that can be built is an ac generator. easily explained Basic generating principles are most easily explained through the use of the elementary ac generator. For this reason, the ac generator will be discussed first. The dc generator will be discussed later. consists of a wire loop mounted on the shaft, so that it can be rotated in a stationary magnetic fieldAn elementary generator consists of a wire loop mounted on the shaft, so that it can be rotated in a stationary magnetic field. produce an induced emf in the loopThis will produce an induced emf in the loop. (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf.Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf. Elementary Generator

24. provide the magnetic fieldThe pole pieces (marked N and S) provide the magnetic field. The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop. The loop of wire that rotates through the field is called the ARMATUREThe ends of the armature loop are connected to rings called SLIP RINGSThe loop of wire that rotates through the field is called the ARMATURE. The ends of the armature loop are connected to rings called SLIP RINGS. They rotate with the armature. The generated voltage appears across these brushes. (These brushes transfer power from the battery to the commutator as the motor spins – discussed later in dc elementary generator).The brushes, usually made of carbon, with wires attached to them, ride against the rings. The generated voltage appears across these brushes. (These brushes transfer power from the battery to the commutator as the motor spins – discussed later in dc elementary generator). THE ELEMENTARY GENERATOR (Cont)

25. THE ELEMENTARY DC GENERATOR Since DC generators must produce DC current instead of AC current, a device must be used to change the AC voltage produced in the armature windings into DC voltage. commutator.This job is performed by the commutator. The commutator is constructed from a copper ring split into segments with insulating material between the segments (See next page). Brushes riding against the commutator segments carry the power to the outside circuit. The commutator in a dc generator replaces the slip rings of the ac generator. This is the main difference in their construction. The commutator mechanically reverses the armature loop connections to the external circuit.

26. THE ELEMENTARY DC GENERATOR The loop is parallel to the magnetic lines of flux, and no voltage is induced in the loop Note that the brushes make contact with both of the commutator segments at this time. The position is called neutral plane. 0 0 Position (DC Neutral Plane)

27. THE ELEMENTARY DC GENERATOR As the loop rotates, the conductors begin to cut through the magnetic lines of flux. The conductor cutting through the south magnetic field is connected to the positive brush, and the conductor cutting through the north magnetic field is connected to the negative brush. Since the loop is cutting lines of flux, a voltage is induced into the loop. After 90 0 of rotation, the voltage reaches its most positive point. 90 0 Position (DC)

28. THE ELEMENTARY DC GENERATOR As the loop continues to rotate, the voltage decreases to zero. After 180 0 of rotation, the conductors are again parallel to the lines of flux, and no voltage is induced in the loop. Note that the brushes again make contact with both segments of the commutator at the time when there is no induced voltage in the conductors 180 0 Position (DC)

29. THE ELEMENTARY DC GENERATOR During the next 90 0 of rotation, the conductors again cut through the magnetic lines of flux. This time, however, the conductor that previously cut through the south magnetic field is now cutting the flux lines of the north field, and vice-versa.. Since these conductors are cutting the lines of flux of opposite magnetic polarities, the polarity of induced voltage is different for each of the conductors. The commutator, however, maintains the correct polarity to each brush. The conductor cutting through the north magnetic field will always be connected to the negative brush, and the conductor cutting through the south field will always be connected to the positive brush. Since the polarity at the brushes has remained constant, the voltage will increase to its peak value in the same direction.Since the polarity at the brushes has remained constant, the voltage will increase to its peak value in the same direction. 270 0 Position (DC)

30. THE ELEMENTARY DC GENERATOR As the loop continues to rotate, the induced voltage again decreases to zero when the conductors become parallel to the magnetic lines of flux. Notice that during this 360 0 rotation of the loop the polarity of voltage remained the same for both halves of the waveform. This is called rectified DC voltage. The voltage is pulsating. It does turn on and off, but it never reverses polarity. Since the polarity for each brush remains constant, the output voltage is DC. 0 0 Position (DC Neutral Plane)

31. Effects of additional turns To increase the amount of output voltage, it is common practice to increase the number of turns of wire for each loop. If a loop contains 20 turns of wire, the induced voltage will be 20 times greater than that for a single-loop conductor. The reason for this is that each loop is connected in series with the other loops. Since the loops form a series path, the voltage induced in the loops will add. In this example, if each loop has an induced voltage of 2V, the total voltage for this winding would be 40V (2V x 20 loops = 40 V). Effects of additional turns

32. Effects of additional coils When more than one loop is used, the average output voltage is higher and there is less pulsation of the rectified voltage. Since there are four segments in the commutator, a new segment passes each brush every 90 0 instead of every 180 0. Since there are now four commutator segments in the commutator and only two brushes, the voltage cannot fall any lower than at point A. Therefore, the ripple is limited to the rise and fall between points A and B on the graph. By adding more armature coils, the ripple effect can be further reduced. Decreasing ripple in this way increases the effective voltage of the output. Effects of additional coils

33. The Practical DC Generator The actual construction and operation of a practical dc generator differs somewhat from our elementary generators Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator The main advantages of using electromagnetic poles are: (1) increased field strength and (2) possible to control the strength of the fields. By varying the input voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can be controlled. Four-pole generator (without armature)

34. DC Machine Equivalent Circuit

35. The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the generator is rotated. This induced voltage is represented by a voltage source. The stator coil has resistance, which is connected in series. The pole flux is produced by the DC excitation/field current, which is magnetically coupled to the rotor The field circuit has resistance and a source The voltage drop on the brushes represented by a battery

36. DC Machine Equivalent Circuit 1.Permanent magnet 2.Separately excited 3.Self-excited

37. DC Machine Equivalent Circuit 1.Permanent magnet The poles are made of permanent magnets. No field winding required. Small size. Disadvantage is low flux density, so low torque.

38. DC Machine Equivalent Circuit 2.Separately excited The field flux is derived from a separate power source independent of the generator itself. B Field winding Armature winding

39. DC Machine Equivalent Circuit 3.Self-excited Shunt machine The field flux is derived by connecting the field directly across the terminals of the generator. B

40. DC Machine Equivalent Circuit 3.Self-excited Series machine field are connected in series with armature B

41. DC Machine Equivalent Circuit 3.Self-excited Cumulatively compounded Differentially compounded B B B B

42. DC Machine Equivalent Circuit 3.Self-excited Compounded dc generator both a shunt and a series field are present

43. DC Machine Equivalent Circuit Compounded dc motor both a shunt and a series field are present 3.Self-excited

44. Equivalent circuit of a DC motor The armature circuit (the entire rotor structure) is represented by an ideal voltage source E A and a resistor R A. A battery V brush in the opposite to a current flow in the machine direction indicates brush voltage drop. The field coils producing the magnetic flux are represented by inductor L F and resistor R F. The resistor R adj represents an external variable resistor (sometimes lumped together with the field coil resistance) used to control the amount of current in the field circuit.

45.  The armature is represented by an ideal voltage source E A and a resistor R A.  The brush voltage drop is represented by a small battery V brush opposing the direction of the current flow in the machine.  The field coils, which produce the magnetic flux, are represented by inductor L F and R F.  The separate resistor R adj represents an external variable resistor used to control the amount of current in the field circuit. DC Motor Equivalent Circuit. Equivalent Circuit of a DC Motor.

46.  The brush drop voltage is often only a very tiny fraction of the generated voltage in the motor.  Therefore, in cases where it is not critical, the brush drop voltage may be left out or approximately included in the value of R A.  Also, the internal resistance of the filed coils is sometimes lumped together with the variable resistor, and the total is called R F, Figure below. A Simplified Equivalent Circuit eliminating the Brush Voltage Drop and Combining R adj with the Field Resistance.

47. The Equivalent Circuit of Separately Excited dc Motor. From the above figure, Motor types: Separately Excited DC motors. Separately excited DC motor: a field circuit is supplied from a separate constant voltage power source.

48. The Equivalent Circuit of a Shunt dc Motor.  From the above figure, Motor types: Shunt DC motors. Shunt DC motor: a field circuit gets its power from the armature terminals of the motor.

49. Motor types: The permanent-magnet DC motor A permanent magnet DC (PMDC) motor is a motor whose poles are made out of permanent magnets. Advantages: 1.Since no external field circuit is needed, there are no field circuit copper losses; 2.Since no field windings are needed, these motors can be considerable smaller. Disadvantages: 1.Since permanent magnets produces weaker flux densities then externally supported shunt fields, such motors have lower induced torque. 2.There is always a risk of demagnetization from extensive heating or from armature reaction effects (via armature mmf).

50. Motor types: The series DC motor A series DC motor is a DC motor whose field windings consists of a relatively few turns connected in series with armature circuit. Therefore:

51. Motor types: Compounded DC motor A compounded DC motor is a motor with both a shunt and a series field. Long-shunt connection Short-shunt connection Current flowing into a dotted end of a coil (shunt or series) produces a positive mmf. If current flows into the dotted ends of both coils, the resulting mmfs add to produce a larger total mmf – cumulative compounding. If current flows into the dotted end of one coil and out of the dotted end of another coil, the resulting mmfs subtract – differential compounding.

52. Motor types: Compounded DC motor The Kirchhoff’s voltage law equation for a compounded DC motor is The currents in a compounded DC motor are The mmf of a compounded DC motor: Cumulatively compounded Differentially compounded The effective shunt field current in a compounded DC motor: Number of turns (5.85.1) (5.85.2) (5.85.3) (5.85.4) (5.85.5)

53. Torque Equation T = torque of armature (N-m) k A = geometry constant  = flux/pole (Wb) I A = armature current (A)

54. Geometry Constant p = number of field poles N = number of active conductors on armature M = number of parallel paths in armature winding (=p for lap winding, =2 for wave winding)

55. Power Equation P=power (W) – not counting losses E = EMF induced in armature (back EMF) I A = armature current (A) T = torque of armature (N-m)  = speed of rotation (rad/s) Note that P in = V L I L which will be higher than P because of loss in the field and armature windings as well as rotational (friction) losses.

56. EMF Equation E = EMF induced in armature (V) k A = geometry constant  = flux/pole (Wb)  = speed of rotation (rad/s) n = speed of rotation of armature (rpm)

57. Terminal Voltage Equation V T = voltage at motor terminals E = EMF induced in armature (V) I A = armature current (A) R A = armature resistance E VTVT + - + - RARA

58. Speed Equation (applies to shunt connected motor only) Note that  can also be written as k f I f where k f is  /I f (normally a constant ratio) Ratio Equation

59. Speed-Torque Torque Speed Series Shunt Cumulative Compound Differential Compound

60. Power flow and losses in DC machines Unfortunately, not all electrical power is converted to mechanical power by a motor and not all mechanical power is converted to electrical power by a generator… The efficiency of a DC machine is: or

61. The losses in DC machines There are five categories of losses occurring in DC machines. 1. Electrical or copper losses – the resistive losses in the armature and field windings of the machine. Armature loss: Field loss: Where I A and I F are armature and field currents and R A and R F are armature and field (winding) resistances usually measured at normal operating temperature.

62. The losses in DC machines 2. Brush (drop) losses – the power lost across the contact potential at the brushes of the machine. Where I A is the armature current and V BD is the brush voltage drop. The voltage drop across the set of brushes is approximately constant over a large range of armature currents and it is usually assumed to be about 2 V. Other losses are exactly the same as in AC machines…

63. The losses in DC machines 4. Mechanical losses – losses associated with mechanical effects: friction (friction of the bearings) and windage (friction between the moving parts of the machine and the air inside the casing). These losses vary as the cube of rotation speed n 3. 3. Core losses – hysteresis losses and eddy current losses. They vary as B 2 (square of flux density) and as n 1.5 (speed of rotation of the magnetic field). 5. Stray (Miscellaneous) losses – losses that cannot be classified in any of the previous categories. They are usually due to inaccuracies in modeling. For many machines, stray losses are assumed as 1% of full load.

64. The power-flow diagram On of the most convenient technique to account for power losses in a machine is the power-flow diagram. For a DC motor: Electrical power is input to the machine, and the electrical and brush losses must be subtracted. The remaining power is ideally converted from electrical to mechanical form at the point labeled as P conv.

65. The power-flow diagram The electrical power that is converted is And the resulting mechanical power is After the power is converted to mechanical form, the stray losses, mechanical losses, and core losses are subtracted, and the remaining mechanical power is output to the load.

66. Example 1 A 6 pole, 3.0 hp 120V DC lap-wound shunt motor has 960 conductors in the armature. It takes 25.0 A from the supply at full load. Armature resistance is 0.75 , flux/pole=10.0 mWb, field winding current is 1.20A. Find the speed and torque.

67. Example 2 A 10hp, 115V Dc series motor takes 40A at its full load speed of 1800rpm. What is the torque at 30A?

68. Example 3 (a) A 220V DC shunt motor draws 10A at 1800rpm. The armature resistance is 0.2  and field winding resistance is 440 . (a) What is the torque?

69. Example 3 (b) A 220V DC shunt motor draws 10A at 1800rpm. The armature resistance is 0.2  and field winding resistance is 440 . (b) What will be the speed and line current at a torque of 20 N-m (if field current is constant)? (shunt is constant speed)