DESIGN OF ELECTRICAL MACHINES EE 6604 FACULY NAME SHRI.N.B.RAJESH M.E. AP-II/EEED VCET,MADURAI

1. DESIGN OF ELECTRICAL MACHINES EE 6604 FACULY NAME SHRI.N.B.RAJESH M.E. AP-II/EEED VCET,MADURAI

2. DESIGN OF DC MACHINESUNIT –II

3. Introduction: (DC Machines) • Used as generator or motor. • Applications • Power generation as small back up system. • Traction: DC series motor. • Drives for process industry: As a high precision drive. DC shunt motor. • Battery driven vehicles: Fork trucks, delivery vans, tram cars. Uses chopper fed DC series motor. • Machine tools: Wide range of speed control with precision. DC shunt motor. • Appliances: Shaving razors, tape recorders, cameras. Miniature DC motor.

4. Automatic control: Feedback control system. High energy PMDC motor with electronic commutation. • DC control system: DC servo motor. • Digital control system: DC stepper motor.

5. Classification of D.C. motors based on Applications Industrial motor Small motor Traction motor Miniature motor Rolling mills Cranes Machine tools Industrial drives Domestic gadgets Hand tools Starters for vehicles Locomotives Multiple unit trains Battery driven road vehicles Intermittent duty application Control motor Special motor Actuators Eddy current brakes PCB machine Servo applications i.e. loop control systems

6. Constructional Details of D.C. Machine: May be either Homopolar or Heteropolar Homopolar: Excitation is provided through permanent magnet. Used for special applications. Heteropolar: Excitation is provided through electromagnets. Widely used for industrial applications.

7. Construction Field System Armature Commutator Brush gear • Stationary part • Produces useful flux. • Includes: • Main Poles • Inter poles • Frame Rotating part Energy conversion takes place. Separated from field by air gap. Consists of: 1. Armature core with slots. 2. Armature winding carried by slots. Mounted on armature. Acts as converter for electrical current and voltage. • Consists of • Brush holders • Brush rockers • Classification: • Natural graphite. • Hard carbon • Electro graphitic • Metal graphite.

8. Physical structure of DC machines

9. Physical structure of DC machines

10. DC Machine Construction Figure 1: General arrangement of a DC machine

11. Construction of DC Machines • The DC machines used for industrial electric drives have three major parts. They are field system, armature and commutator. • The field system is located on the stationary part of the machine called stator and consists of main poles, interpoles and frame or yoke. • The main poles are designed to produce the magnetic flux. The inter-poles are placed in between the main poles. They are employed to improve the commutation condition . The frame provides mechanical support to machine and also serve as a path for flux. • The armature is the rotating part ( or rotor ) of a DC machine and consists of armature core with slots and armature winding accommodated in slots.

12. The conversion of energy from mechanical to electrical or vice-versa takes place in armature. • The commutator is mounted on the rotor of a DC machine . the commutator and brush arrangement works like a mechanical dual converter. • In case of generator it rectifies the induced ac to dc. In case of motor it inverts the DC supply to AC. ( In motor the commutator reverses the current through the armature conductors to get unidirectional torque).

13. Figure 2: DC machine stator with poles visible.

14. Figure 3 : Rotor of a DC machine

15. Figure 4: Cutaway view of a DC machine

16. Materials for Electrical Machines • The main material characteristics of relevance to electrical machines are those associated with conductors for electric circuit, the insulation system necessary to isolate the circuits, and with the specialized steels and permanent magnets used for the magnetic circuit. Conducting materials Commonly used conducting materials are copper and aluminum. Some of the desirable properties a good conductor should possess are listed below. • 1. Low value of resistivity or high conductivity • 2. Low value of temperature coefficient of resistance • 3. High tensile strength • 4. High melting point • 5. High resistance to corrosion

17. 6. Allow brazing, soldering or welding so that the joints are reliable • 7. Highly malleable and ductile • 8. Durable and cheap by cost

18. Magnetic materials The magnetic properties of a magnetic material depend on the orientation of the crystals of the material and decide the size of the machine or equipment for a given rating, excitation required, efficiency of operation etc. The some of the properties that a good magnetic material should possess are listed below. • 1. Low reluctance or should be highly permeable or should have a high value of relative permeability μr. • 2. High saturation induction (to minimize weight and volume of iron parts) • 3. High electrical resistivity so that the eddy emf and the hence eddy current loss is less • 4. Narrow hysteresis loop or low Coercivity so that hysteresis loss is less and efficiency of operation is high

19. 5. A high curie point. (Above Curie point or temperature the material loses the magnetic property or becomes paramagnetic, that is effectively non-magnetic) • 6. Should have a high value of energy product (expressed in joules / m3). • Magnetic materials can broadly be classified as Diamagnetic, Paramagnetic, Ferromagnetic, Antiferromagnetic and Ferri-magnetic materials. • Only ferromagnetic materials have properties that are well suitable for electrical machines. • Ferromagnetic properties are confined almost entirely to iron, nickel and cobalt and their alloys. The only exceptions are some alloys of manganese and some of the rare earth elements.

20. The relative permeability μr of ferromagnetic material is far greater than 1.0. When ferromagnetic materials are subjected to the magnetic field, the dipoles align themselves in the direction of the applied field and get strongly magnetized. Insulating materials To avoid any electrical activity between parts at different potentials, insulation is used. An ideal insulating material should possess the following properties. • 1) Should have high dielectric strength. • 2) Should with stand high temperature. • 3) Should have good thermal conductivity • 4) Should not undergo thermal oxidation • 5) Should not deteriorate due to higher temperature and repeated heat cycle • 6) Should have high value of resistivity ( like 1018 Ω-cm)

21. 7) Should not consume any power or should have a low dielectric loss angle δ • 8) Should withstand stresses due to centrifugal forces ( as in rotating machines), electro dynamic or mechanical forces ( as in transformers) • 9) Should withstand vibration, abrasion, bending • 10) Should not absorb moisture • 11) Should be flexible and cheap • 12) Liquid insulators should not evaporate or volatilize. Insulating materials can be classified as Solid, Liquid and Gas, and vacuum. The term insulting material is sometimes used in a broader sense to designate also insulating liquids, gas and vacuum.

22. Classification of insulating materials based on thermal consideration:

23. GENERAL DESIGN PROCEDURE • In general any electrical machine has two windings. • The DC machine and synchronous machine has armature and field winding. • The induction machine has stator and rotor winding. The basic principle of operation of all electrical machine is governed by Faraday’s law of induction. • Also in every electrical machine the energy is transferred through the magnetic field. Hence a general design procedure can be developed for the design of electrical machines.

24. The general design procedure is to relate the main dimensions of the machine to its rated power output. • An electrical machine is designed to deliver a certain amount of power called rated power. • The rated power output of a machine is defined as the maximum power that can be delivered by the machine safely. • In DC machine the power rating is expressed in kW and in AC machine in kVA. In case of motor the output power is expressed in HP.

25. In electrical machine the core and winding of the machine are together called active part. ( Because the energy conversion takes place only in the active part of the machine). • A general output equation can be developed for DC machine which relate the power output to volume of active part (D2 L), speed, magnetic and electric loading. • Similarly a general output equation can be developed for AC machine which relates kVA rating to volume of active part (D2 L), speed, magnetic and electric loading.

26. General Cryptogram Used for Designing a Rotating Machine: D = Armature diameter of stator bore, m L = Stator core length, m n = Speed, rps; ns = Synchronous speed, rps p = No. of poles; a = No. of parallel paths Z = Total no. of armature or stator conductors τ = Pole pitch, m; Tph = Turns per phase Iz = Current in each conductor, Amp Kw = Winding factor; Ia = Armature current, Amp Iph = Current per phase, Amp; E = Back EMF, Volt Eph = Induced EMF per phase, Volt; Q = kVA rating of machine P = Rating of machine, kW; Pa = Power developed by armature, kW

27. In rotating machine the active part is cylindrical in shape. The volume of the cylinder is given by the product of area of cross section and length. If D is the diameter and L is the length of cylinder, then the volume is given by πD2 L/4. Therefore D and l are specified as main dimension. In case of DC machine, D represent the diameter of armature and L represent the length of armature. In case of AC machine, D represent the inner diameter of stator and L represent the length of stator core. HereDr = Diameter of rotor lg = Length of air-gap Main Dimensions of a Rotating Machine:

28. Main Dimensions of a Rotating Machine: L D Rotor Stator Air Gap

29. MAGNETIC AND ELECTRIC LOADINGS • In rotating machines the conductors are placed in armature. In one revolution of the armature, each conductor moves through a total flux of ф webers, where ф is flux per pole and p is number of poles. If Z is the total number of armature conductors then the work done in one revolution is given by Work is p ф x Iz Z • The term pф represent the total flux entering and leaving the armature and so it is called total magnetic loading (or total flux). • The term Iz Z represents the sum of currents in all the conductors on the armature and so it is called total electric loading (or total current volume or total ampere conductors on the armature). • Total magnetic loading = pф • Total electric loading = IzZ • The work done in one complete revolution is given by the product of total magnetic loading ad total electric loading.

30. Different Loadings to be Considered while Designing a Rotating Machine: Total Magnetic Loading: “It is the total flux around the armature periphery at the air gap.” Total Electrical Loading: “It is the total no. of ampere conductors around the armature periphery.”

31. SPECIFIC MAGNETIC LOADING Each unit area of armature surface is capable of receiving a certain magnetic flux. Hence the flux per unit area is an important parameter to estimate the intensity of magnetic loading and it is also criterion to decide the volume of active material. This flux per unit area is expressed as the average value of the flux density at the armature surface or specific magnetic loading (by the assuming that the armature is smooth). It is denoted by Bav The average flux density, Bavis given by the ratio of flux per pole and area under a pole. Bav= Flux per pole = Flux per pole . Area under a pole Pole Pitch x Length of armature = ф =Pф. πD x L πDL P Pole pitch, τ = πD/p

32. Specific Loadings: Specific Magnetic Loading: “It is the average flux density over the air gap of a machine.” (Maximum Flux density in iron parts of machine, magnetizing Current and Core losses)

33. SPECIFIC ELECTRIC LOADING • Every section of armature is capable carrying certain amount of current. Hence ampere-turn per unit section of armature periphery (circumference) is an important parameter to estimate the intensity of electric loading and it is also a criterion to decide the volume of active material. • This ampere –turn per unit section of armature periphery is expressed as the specific electric loading . It is denoted by ac.

34. Specific Loadings: Specific Electric Loading: “It is the no. of armature ampere conductors per meter of armature periphery at the air gap.” (Current density, Applied voltage, Size of machine and permissible temperature rise)

35. Advantages due to Higher Specific Loadings: • Reduction in the Volume of the machine. • Reduction in the size of the machine. • Lower cost of the material required. • Lower weight of the machine. • Lower over all cost of the machine. To produce a cheaper machine with reduction in its size, the values of specific loadings must be pushed to the largest possible.

36. Disadvantage due to Higher Specific Magnetic Loading, Bav • Increased iron losses. • Larger requirement of m.m.f. • Higher field copper losses (D.C. Machine, Syn.machines) • Higher tooth density. • Tendency of saturation of magnetic parts. • Increased magnetizing current and poorer power factor (Induction Motor) • Reduced leakage reactance and larger initial current on sudden short circuit (syn. m/c). • Increased temperature rise due to higher losses. • Increased noise.

37. Disadvantage due to Higher Specific Electric Loading, ac: • Increased armature copper losses. • Increased leakage reactance because of larger turns per phase (Ind. & Sy. Machine) • Increased temperature rise because of higher copper losses. • Increased reactance voltage and inferior commutation (DC m/c) • Increased field excitation causing more field copper losses (D.C. m/c) • Poorer regulation and stability impaired (syn. m/c) • Reduction in over load capacity.

38. Specific magnetic and electric loadings

39. Output Equation: “Expressed in terms of its main dimensions, specific loadings and speed.” Direct Current Machines

40. In DC generator the electrical power generated in the armature is given by the product of induced emf and armature current. • In case of DC motor the mechanical equivalent of electrical power in armature is given by the product induced emf (back emf) and armature current. Hence, Pa= (total magnetic loading) (total electric loading) (speed in rps) x 10-3

41. By substituting these values we get, [Output co-efficient ]

42. D.C. Machine as D.C. Generator: • Prime mover supplies friction, windage and iron losses which do not exist in absence of rotation. • Armature supplies its own copper loss and the field copper loss.

43. Total losses for D.C. generator The friction, windage and iron losses for a small machine may be taken as ⅓rd of the total losses.

44. Similarly for D.C. motors Power developed by armature of a D.C. machine is given as: Generator Motor

46. Alternating Current Machines Consider, m = number of phases a = number of parallel paths/circuits = 1 kVA rating of machine: Q = no. of phases×o/p voltage/phase×current/phase×10-3 = mEphIph×10-3 Terminal voltage/phase may be taken as equal to induced EMF/phase. We have, Eph= 4.44fTphKw. Q=m×4.44fTphKw×Iph ×10-3 But, f = p.ns/2

47. Hence, Q = 1.11 Kw(p)(IZZ)ns×10-3 = 1.11Kw(total magnetic loading) (total electric loading) (synchronous speed) ×10-3

48. [Output co-efficient]

49. SEPARATION OF D AND L In DC machine the separation of D and L depends on • Pole proportions • Peripheral speed • Moment of inertia • Voltage between adjacent commutator segments

50. bp L (core length) b (pole arc) Pole proportions • The dimensions of the machine or decided by the square pole criterion. This states that for a given flux and cross-section area of pole, the length of mean turn of field winding is minimum, when the periphery forms a square. This means that the length L must be approximately equal to pole arc or L=b= ψ τ • The value of ψ is usually between 0.64 to 0.72 ( the ratio L/ τ = 0.64 to 0.72 ). However in practice L is slightly greater than pole arc , b and L/ τ is usually between 0.7 to 0.9. For square pole criterion choose L/ τ as 0.7. bp= width of pole body b = pole arc L = core length τ = pole pitch = π D/p ψ = ratio of pole arc to pole pitch = b/ τ