EE 1403 - SOLID STATE DRIVES UNIT 1 - Fundamentals of Electric Drives

1. EE 1403 - SOLID STATE DRIVESUNIT 1 - Fundamentals of Electric Drives

2. Electrical Drives Drives are systems employed for motion control Require prime movers Drives that employ electric motors as prime movers are known as Electrical Drives

3. Electrical Drives • About 50% of electrical energy used for drives • Can be either used for fixed speed or variable speed • 75% - constant speed, 25% variable speed (expanding)

4. Power In Power out Power loss Mainly in valve Example on VSD application Variable Speed Drives Constant speed valve Supply motor pump

5. Power In Power In Power out Power loss Example on VSD application Variable Speed Drives Constant speed valve Supply Supply motor pump motor PEC pump Power out Power loss Mainly in valve

6. Power In Power out Power loss Example on VSD application Variable Speed Drives Constant speed valve Supply Supply motor pump motor PEC pump Power In Power out Power loss Mainly in valve

7. Conventional electric drives (variable speed) • Bulky • Inefficient • inflexible

8. Modern electric drives (With power electronic converters) • Small • Efficient • Flexible

9. BLOCK DIAGRAM OF ELECTRIC DRIVE

10. Components in electric drives • Motors • DC motors - permanent magnet – wound field • AC motors – induction, synchronous (IPMSM, SMPSM), brushless DC • Applications, cost, environment • Power sources • DC – batteries, fuel cell, photovoltaic - unregulated • AC – Single- three- phase utility, wind generator - unregulated • Power processor • To provide a regulated power supply • Combination of power electronic converters • More efficient • Flexible • Compact • AC-DC DC-DC DC-AC AC-AC

11. Components in electric drives • Control unit • Complexity depends on performance requirement • analog- noisy, inflexible, ideally has infinite bandwidth. • digital – immune to noise, configurable, bandwidth is smaller than the analog controller’s • DSP/microprocessor – flexible, lower bandwidth - DSPs perform faster operation than microprocessors (multiplication in single cycle), can perform complex estimations INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

12. AC-DC Converters or Rectifiers

13. AC-DC Converters or Rectifiers (Cont.)

14. AC Voltage Controller

15. VSI Controlled Inverter for IM Drive

16. CSI Controlled Drives for IM

17. DC – DC Converter (Chopper)

18. Overview of AC and DC drives Extracted from Boldea & Nasar

19. Overview of AC and DC drives DC motors: Regular maintenance, heavy, expensive, speed limit Easy control, decouple control of torque and flux AC motors: Less maintenance, light, less expensive, high speed INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Coupling between torque and flux – variable spatial angle between rotor and stator flux

20. Overview of AC and DC drives Before semiconductor devices were introduced (<1950) • AC motors for fixed speed applications • DC motors for variable speed applications After semiconductor devices were introduced (1950s) • Variable frequency sources available – AC motors in variable speed applications • Coupling between flux and torque control • Application limited to medium performance applications – fans, blowers, compressors – scalar control INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 • High performance applications dominated by DC motors – tractions, elevators, servos, etc

21. Overview of AC and DC drives After vector control drives were introduced (1980s) • AC motors used in high performance applications – elevators, tractions, servos • AC motors favorable than DC motors – however control is complex hence expensive • Cost of microprocessor/semiconductors decreasing –predicted 30 years ago AC motors would take over DC motors INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

22. SPEED Synchronous mch Induction mch Separately / shunt DC mch Series DC TORQUE Motor steady state torque-speed characteristic INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 By using power electronic converters, the motor characteristics can be changed at will

23. T~ C T~ 2 T~  Coulomb friction Viscous friction Friction due to turbulent flow Load steady state torque-speed characteristic Frictional torque (passive load) • Exist in all motor-load drive system simultaneously • In most cases, only one or two are dominating • Exists when there is motion SPEED TORQUE INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

24. SPEED Gravitational torque Vehicle drive Te TORQUE TL gM  FL Load steady state torque-speed characteristic Constant torque, e.g. gravitational torque (active load) INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 TL = rFL = r g M sin 

25. Speed Torque Gravitational torque Load steady state torque-speed characteristic Hoist drive INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

26. Te Tl Steady state speed r r2 r1 r3 Load and motor steady state torque At constant speed, Te= Tl Steady state speed is at point of intersection between Te and Tl of the steady state torque characteristics Torque INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Speed

27. Thermal considerations Unavoidable power losses causes temperature increase Insulation used in the windings are classified based on the temperature it can withstand. Motors must be operated within the allowable maximum temperature Sources of power losses (hence temperature increase): - Conductor heat losses (i2R) - Core losses – hysteresis and eddy current - Friction losses – bearings, brush windage INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

28. Thermal considerations Electrical machines can be overloaded as long their temperature does not exceed the temperature limit Accurate prediction of temperature distribution in machines is complex – hetrogeneous materials, complex geometrical shapes Simplified assuming machine as homogeneous body Ambient temperature, To INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 p1 Thermal capacity, C (Ws/oC) Surface A, (m2) Surface temperature, T (oC) p2 Emitted heat power (convection) Input heat power (losses)

29. Thermal considerations Power balance: Heat transfer by convection: , where  is the coefficient of heat transfer Which gives: INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 With T(0) = 0 and p1 = ph = constant , , where

30. Heating transient  Thermal considerations t INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Cooling transient t 

31. Continuous duty Load torque is constant over extended period multiple Steady state temperature reached Nominal output power chosen equals or exceeds continuous load Losses due to continuous load p1n t  Thermal considerations The duration of overloading depends on the modes of operation: Continuous duty Short time intermittent duty Periodic intermittent duty INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

32. Thermal considerations Short time intermittent duty Operation considerably less than time constant,  Motor allowed to cool before next cycle Motor can be overloaded until maximum temperature reached INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

33. p1s p1n  t1 Thermal considerations Short time intermittent duty p1 INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 t

34.  t1 Thermal considerations Short time intermittent duty INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 t

35. Thermal considerations Periodic intermittent duty Load cycles are repeated periodically Motors are not allowed to completely cooled Fluctuations in temperature until steady state temperature is reached INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

36. p1 heating coolling heating coolling heating coolling Thermal considerations Periodic intermittent duty INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 t

37. Thermal considerations Periodic intermittent duty Example of a simple case – p1 rectangular periodic pattern • pn = 100kW, nominal power • M = 800kg • = 0.92, nominal efficiency T= 50oC, steady state temperature rise due to pn Also, INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 If we assume motor is solid iron of specific heat cFE=0.48 kWs/kgoC, thermal capacity C is given by C = cFE M = 0.48 (800) = 384 kWs/oC Finally , thermal time constant = 384000/180 = 35 minutes

38. Thermal considerations Periodic intermittent duty Example of a simple case – p1 rectangular periodic pattern For a duty cycle of 30% (period of 20 mins), heat losses of twice the nominal, INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

39. Type of Loads • Load torque can be of two types • Active load torque:- Active torques continues to act in the same direction irrespective of the direction of the drive. e.g. gravitational force or deformation in elastic bodies. • Passive load torque:- the sense of the load torque changes with the change in the direction of motion of drive. e. g. torques due to friction, due to shear and deformation of inelastic bodies

40. Type of Loads (Cont.) • It is a passive load to the motor. • Load torque is independent of the speed of the motor. • Characterized by the requirement of an extra torque at very near zero speed. • It is also known as break away torque or stiction.

41. Type of Loads (Cont.) • Torque is directly proportional to the speed. • Calendaring machines, eddy current brakes and separately excited dc generators feeding fixed resistance loads have such characteristics. Viscous Friction Load

42. Type of Loads (Cont.)

43. Type of Loads (Cont.) • Load torque magnitude is proportional to some power of speed. • Centrifugal pumps, propeller in ships or aeroplanes, fan or blower type of load has such characteristics. • For fan, Fan type Load

44. Types of Load (Cont.) • Hyperbolic speed-torque characteristics, where load torque is inversely proportional to speed or load power is constant.Certain type of lathes, boring machines, milling machines, steel mill coilers etc are having this type of load characteristics. Constant Power Load

45. Types of Load (Cont.) • Load torques that vary with timeLoad variation with time can be periodic and repetitive in certain applications. • One cycle of the load variation is called a duty cycle. • The variation of load torque with time has a greater importance in the selection of a suitable motor.Classification of loads that vary with time:(a) Continuous, constant loads: Centrifugal pumps or fans operating for a long time under the same conditions, paper making machines etc.(b) Continuous, variable loads: Metal cutting lathes, hoisting winches, conveyors etc.

46. Type of Loads (Cont.) • (c) Pulsating loads: Reciprocating pumps and compressors, frame saws, textile looms and generally all machines having crank shaft.(d) Impact loads: Apparent, regular and repetitive load peaks or pulses which occurs in rolling mills, presses, shearing machines, forging hammers etc. Drives for such machines will have heavy fly wheels. (e) Short time intermittent loads: Almost all forms of cranes and hoisting mechanisms, excavators, roll trains etc.(f) Short time loads: Motor generator sets for charging batteries, servo motors used for remote control of clamping rods of drilling machines. Loads of the machines like stone crushers and ball mills are characterized by frequent impact of small peaks so they are classified as continuous variable loads rather than the impact loads

47. Types of Load (Cont.) • One and the same machine can be represented by a load torque which either varies with the speed or with the time. • For example, a fan load whose load torque is proportional to the square of the speed, is also a continuous, constant load. • Load torque of a crane is independent of the speed and also short time intermittent nature. • Rocking pumps for petroleum have a load which vary with angular position of the shaft, but also be classified as a pulsating load.

48. Type of Load (Cont. High speed HoistTraction Load (Constant torque; but with viscous friction)

49. Power requirement for different Loads • Mine Hoist Polishing Machine

50. Power requirement for different Loads (Cont.) • Sheering machine for cutting Textile loom