Chapter 1-Electrical Drives.pptx

Engineering

1. Chapter 1: Electrical Drives By Mr. Harshal Vaidya Assistant Professor Department of Electrical Engineering DIT, Pune

2. Definition of Electrical Drives Drives – system employed for motion control Motion control requires prime movers Electrical Drives – Drives that employ Electric Motors as prime movers 2

3. Advantages of Electrical Drives Flexible control characteristic particularly when power electronic converters are employed Wide range of speed, torque and power High efficiency – low no load losses Low noise Low maintenance requirements, cleaner operation Electric energy easily transported Adaptable to most operating conditions Available operation in all four torque-speed quadrants 3

4. Conventional Electric Drives Ward-Leonard system – introduced in 1890s Disadvantage : Bulky Expensive Inefficient Complex 4

5. Modern Electric Drives Small (compact) Efficient Flexible Interdisciplinary 5 feedback

6. Electric Drives Application Line Shaft Drives Oldest form Single motor, multiple loads Common line shaft or belt Inflexible Inefficient Rarely used 6

7. Electric Drives Application Single-Motor, Single-Load Drives Most common Eg : electric saws, drills, fans, washers, blenders, disk-drives, electric cars. 7

8. Electric Drives Application Multi-motor Drives Several motors, single mechanical load Complex drive functions Eg : assembly lines, robotics, military airplane actuation. 8

9. Basic Components of Electric Drives Power Source Motor Power Processing Unit (Electronic Converter) Control Unit Mechanical Load 9 feedback

10. Basic Components of Electric Drives - Motor Obtain power from electrical sources DC motors - Permanent Magnet or wound-field (shunt, separately excited, compound, series) AC motors – Induction, Synchronous (wound –rotor, IPMSM, SPMSM), brushless DC Selection of machines depends on many factors, e.g.: 10 Electrical energy Mechanical energy Motor application cost efficiency environment type of source available

11. Basic Components of Electric Drives – Power Source Provides energy to electric motors Regulated ( e.g : utility) or Unregulated (e.g. : renewable energy) Unregulated power sources must be regulated for high efficiency – use power electronic converters DC source batteries fuel cell photovoltaic AC source single- or three- phase utility wind generator 11

12. Basic Components of Electric Drives – Power Processing Unit Provides a regulated power supply to motor Enables motor operation in reverse, braking and variable speeds Combination of power electronic converters Controlled rectifiers, inverters –treated as ‘black boxes’ with certain transfer function More efficient – ideally no losses occur Flexible - voltage and current easily shaped through switching control Compact Several conversions possible : AC-DC , DC-DC, DC-AC, AC-AC 12

13. Basic Components of Electric Drives – Power Processing Unit DC to AC: 13

14. Basic Components of Electric Drives – Power Processing Unit DC to DC: 14

15. Basic Components of Electric Drives – Power Processing Unit AC to DC: 15

16. Basic Components of Electric Drives – Power Processing Unit AC to AC: 16

17. Basic Components of Electric Drives – Control Unit Supervise operation Enhance overall performance and stability Complexity depends on performance requirement Analog Control – noisy, inflexible, ideally infinite bandwidth Digital Control – immune to noise, configurable, smaller bandwidth (depends on sampling frequency) DSP/microprocessor – flexible, lower bandwidth, real-time DSPs perform faster operation than microprocessors (multiplication in single cycle), complex estimations and observers easily implemented 17

18. Basic Components of Electric Drives – Component Selection Several factors affecting drive selection: Steady-state operation requirements nature of torque-speed profile , speed regulation, speed range, efficiency, quadrants of operations , converter ratings Transient operation requirements values of acceleration and deceleration, starting, braking and reversing performance Power source requirements Type, capacity, voltage magnitude, voltage fluctuations, power factor, harmonics and its effect on loads , ability to accept regenerated power Capital & running costs Space and weight restrictions Environment and location Efficiency and reliability 18

19. DC or AC Drives? DC Drives AC Drives (particularly Induction Motor) Motor requires maintenance heavy, expensive limited speed (due to mechanical construction) less maintenance light, cheaper high speeds achievable (squirrel-cage IM) robust Control Unit Simple & cheap control even for high performance drives decoupled torque and flux control Possible implementation using single analog circuit Depends on required drive performance complexity & costs increase with performance DSPs or fast processors required in high performance drives Performance Fast torque and flux control Scalar control – satisfactory in some applications Vector control – similar to DC drives 19

20. Torque Equation for Rotating Systems Motor drives a load through a transmission system ( eg. gears, V-belts, crankshaft and pulleys) Load may rotate or undergo translational motion Load speed may be different from motor speed Can also have multiple loads each having different speeds, some may rotate and some have translational motion 20 Motor Load T e ,  m T L Represent motor-load system as equivalent rotational system

21. Torque Equation for Rotating Systems 21 First order differential equation for angular frequency (or velocity) Second order differential equation for angle (or position) With constant inertia J ,  T e ,  m T L Torque equation for equivalent motor-load system : where: J = inertia of equivalent motor-load system, kgm 2  m = angular velocity of motor shaft, rads -1 T e = motor torque, Nm T L = load torque referred to motor shaft, Nm (1) (2)

22. Torque Equation for Rotating Systems with Gears Low speed applications use gears to utilize high speed motors Motor drives two loads: Load 1 coupled directly to motor shaft Load 2 coupled via gear with n and n 1 teeth Need to obtain equivalent motor-load system 22 Motor T e Load 1, T L0 Load 2, T L1 J J 1  m  m  m1 n n 1 T L0 T L1 Motor T e J Equivalent Load , T L  m T L

23. Torque Equation for Rotating Systems with Gears Gear ratio a 1 = Neglecting losses in the transmission: Hence, equivalent motor-load inertia J is: 23 Kinetic energy due to equivalent inertia =  kinetic energy of moving parts (3) (4)

24. Torque Equation for Rotating Systems with Gears If  1 = transmission efficiency of the gears: Hence, equivalent load torque T L is: 24 Power of the equivalent motor-load system =  power at the loads (5)

25. Torque Equation for Rotating Systems with Belt Drives By neglecting slippage, equations (4) and (5) can still be used. However, where: D m = diameter of wheel driven by motor D L = diameter of wheel mounted on load shaft 25 (6)

26. Torque Equation for Rotating Systems with Translational Motion Motor drives two loads: Load 1 coupled directly to motor shaft Load 2 coupled via transmission system converting rotational to linear motion Need to obtain equivalent motor-load system 26 Motor T e J Equivalent Load , T L  m T L

27. Torque Equation for Rotating Systems with Translational Motion Neglecting losses in the transmission: Hence, equivalent motor-load inertia J is: 27 Kinetic energy due to equivalent inertia =  kinetic energy of moving parts (7)

28. Torque Equation for Rotating Systems with Translational Motion If  1 = transmission efficiency of the transmission system: Hence, equivalent load torque T L is: 28 Power of the equivalent motor-load system =  power at the loads and motor (8)

29. Relation between Translational and Rotational Motions The relationship between the torques and linear forces are: Relationship between linear and angular velocity: Hence, assuming the mass M is constant : 29

30. Components of Load Torque Load torque can be divided into: Friction torque – present at motor shaft and in various parts of load. Viscous friction torque T V – varies linearly with speed ( T v   m ) . Exists in lubricated bearings due to laminar flow of lubricant Coulomb friction torque T C – independent of speed . Exists in bearings, gears coupling and brakes. Windage torque T w – exists due to turbulent flow of air or liquid. Varies proportional to speed squared ( T w   m 2 ). Mechanical Load Torque T L - torque to do useful mechanical work. 30

31. Mechanical Load Torque Torque to do useful mechanical work T L – depends on application. Load torque is function of speed where k = integer or fraction Mechanical power of load: and 31 Angular speed in rad/s Speed in rpm

32. Torque-Speed Characteristics of Load 32 Torque independent of speed Linear rising Torque-Speed Non-Linear rising Torque-Speed Non-Linear falling Torque-Speed

33. Mechanical Load Torque Torque independent of speed , k = 0 Hoist Elevator Pumping of water or gas against constant pressure 33

34. Mechanical Load Torque Torque proportional to square of speed , k = 2 Fans Centrifugal pumps Propellers 34

35. Mechanical Load Torque Torque inversely proportional to speed , k = -1 Milling machines Electric drill Electric saw 35

36. Classification of Electrical Drives 36 Group Drive(Shaft Drive) Individual Drive Multi-Motor Drive

37. Classification of Electrical Drives 37 Group Drive(Shaft Drive) “If Several groups of Mechanisms or Machines are organized on one shaft & driven by one motor, the system is called a group drive (Shaft Drive)” Disadvantages There is no flexibility, Addition of an extra machine to the main shaft is difficult. The efficiency of the drive is low, because of the losses occurring in several transmitting mechanisms. The complete drive system requires shutdown if the motor, requires servicing or repair. The system is not very safe to operate The noise level at the work spot is very high.

38. Classification of Electrical Drives 38 Individual Drive “If a single motor is used to drive a given mechanism & it does all the jobs connected with load, the drive is called an individual drive” Examples Single Spindle drilling machine Lathe machines

39. Classification of Electrical Drives 39 M ult i - M o t or Dr i v e “In a Multi-Motor drive, each operation of the mechanism is taken care of by a separate drive motor. The system contains several individual drives, each of which is used to operate its own mechanism” Examples Metal cutting machine tool Rolling mills Travelling cranes

40. Dynamic Conditions of a drive system 40 Dynamic conditions occur in a electric drive system when operating point changes from one steady state condition to another, following a change introduced in the system variables. This variables may be mechanical such as speed, torque etc. or electrical such as voltage, current etc. These conditions generally exist during starting, braking and speed reversal of the drive. The dynamic conditions arise in a variable speed drive when transition from one speed to another is required.

41. Dynamic Conditions of a drive system 41 The drive may also have transient behavior if there are sudden changes of load, supply, voltage or frequency. The dynamic behavior of a drive has a close relation to its stability. A drive is said to be stable if it can go from one state of equilibrium to another following a disturbance in one of the parameters of the system. Stability can be identified as either steady-state or transient.

42. Dynamic Conditions of a drive system 42 The condition of stability depend on the operating point. The dynamics of the drive can be investigated using the Torque balance equation given by

43. Dynamic Conditions of a drive system 43

44. Dynamic Conditions of a drive system 44

45. Dynamic Conditions of a drive system 45

46. Dynamic Conditions of a drive system 46

47. Dynamic Conditions of a drive system 47 The load torque occurring in mechanical system may be Passive or active. Passive torque If the torque always opposes the direction of motion of drive motor it is called a passive torque. A ct i v e t o r que Load torque which have the potential to drive the motor under equilibrium condition are called active load torque.

48. Motor T-  characteristic – variation of motor torque with speed with all other variables (voltage and frequency) kept constant. Loads will have their own T-  characteristics. Steady State Operating Speed 48 Synchronous motor Induction motor Separately excited / shunt DC motor Series DC motor SPEED TORQUE

49. Steady State Operating Speed At constant speed, T e = T L Steady state speed is at point of intersection between T e and T L of the steady state torque characteristics 49 T L T e Steady state Speed,  r Torque Speed  r2  r3  r1 By using power electronic converters, the motor characteristic can be varied

50. Steady State Stability Drives operate at steady-state speed (when T e = T L ) only if the speed is of stable equilibrium . A disturbance in any part of drive causes system speed to depart from steady-state point. Steady-state speed is of stable equilibrium if:  system will return to stable equilibrium speed when subjected to a disturbance Steady-state stability evaluated using steady-state T-  characteristic of motor and load . Condition for stable equilibrium : 50 (9)

51. Steady State Stability Assume a disturbance causes s peed drop to  r ’ At the new speed  r ’, 51 T e ’ > T L ’ motor accelerates operation restored to steady-state point Steady-state speed is of stable equilibrium T e T L Steady-state point A at speed =  r  r  r ’ T e ’ T L ’  m T Evaluated using steady-state T-  characteristic of motor and load .

52. Steady State Stability Let’s look at a different condition! Assume a disturbance causes s peed drop to  r ’ At the new speed  r ’, 52 T e ’ < T L ’ motor decelerates operation point moves away from steady-state point Point B is at UNSTABLE equilibrium T e T L Steady-state point B at speed =  r  r  r ’ T L ’ T e ’  m T

53. Torque-Speed Quadrant of Operation 53 Direction of positive (forward) speed is arbitrary chosen Direction of positive torque will produce positive (forward) speed  m T e T e  m T e  m T e  m  T Quadrant 1 Forward motoring Quadrant 2 Forward braking Quadrant 3 Reverse motoring Quadrant 4 Reverse braking P = + ve P = - ve P = - ve P = + ve Electrical energy Mechanical energy MOTOR P = + ve

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