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ELECTRIC DRIVES

ELECTRIC DRIVES. INTRODUCTION TO ELECTRIC DRIVES MODULE 1 Dr. Nik Rumzi Nik Idris Dept. of Energy Conversion, UTM 2013. Electrical Drives. Drives are systems employed for motion control. Require prime movers. Drives that employ electric motors as

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ELECTRIC DRIVES

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  1. ELECTRIC DRIVES INTRODUCTION TO ELECTRIC DRIVES MODULE 1 Dr. Nik Rumzi Nik Idris Dept. of Energy Conversion, UTM 2013

  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 INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  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) • MEP 1523 will be covering variable speed drives INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  4. Power In Power out Power loss Mainly in valve INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Example on VSD application Variable Speed Drives Constant speed valve Supply motor pump

  5. Power In Power In Power out Power loss INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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) INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 • Bulky • Inefficient • inflexible

  8. Modern electric drives (With power electronic converters) INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 • Small • Efficient • Flexible

  9. Machine design Speed sensorless Machine Theory Utility interface Renewable energy Non-linear control Real-time control DSP application PFC Speed sensorless Power electronic converters Modern electric drives INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 • Inter-disciplinary • Several research area • Expanding

  10. Components in electric drives • Motors • DC motors - permanent magnet – wound field • AC motors – induction, synchronous (IPMSM, SMPSM), brushless DC • Applications, cost, environment • Natural speed-torque characteristic is not compatible with load requirements • Power sources • DC – batteries, fuel cell, photovoltaic - unregulated • AC – Single- three- phase utility, wind generator - unregulated INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 • 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 • Electrical isolation between control circuit and power circuit is needed: • Malfuction in power circuit may damage control circuit • Safety for the operator • Avoid conduction of harmonic to control circuit INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  12. Components in electric drives • Sensors • Sensors (voltage, current, speed or torque) is normally required for closed-loop operation or protection • Electrical isolation between sensors and control circuit is needed for the reasons previously explained • The term ‘sensorless drives’ is normally referred to the drive system where the speed is estimated rather than measured. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  13. Overview of AC and DC drives INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Extracted from Boldea & Nasar

  14. 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

  15. 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

  16. Overview of AC and DC drives After semiconductor devices were introduced (1950s) INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  17. 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

  18. Classification of IM drives (Buja, Kamierkowski, “Direct torque control of PWM inverter-fed AC motors - a survey”, IEEE Transactions on Industrial Electronics, 2004. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  19. Newton’s law Linear motion, constant M • First order differential equation for speed • Second order differential equation for displacement Elementary principles of mechanics v x Fm M Ff INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  20. Tl Te , m With constant J, • First order differential equation for angular frequency (or velocity) • Second order differential equation for angle (or position) Elementary principles of mechanics Rotational motion - Normally is the case for electrical drives J INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  21. Elementary principles of mechanics For constant J, Torque dynamic – present during speed transient Angular acceleration Larger net torque and smaller J gives faster acceleration INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  22. Elementary principles of mechanics A drive system that require fast acceleration must have • large motor torque capability • small overall moment of inertia As the motor speed increases, the kinetic energy also increases. During deceleration, the dynamic torque changes its sign and thus helps motor to maintain the speed. This energy is extracted from the stored kinetic energy: INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 J is purposely increased to do this job !

  23. Fe Fl Te,   r r Tl v INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics Combination of rotational and translational motions M Te = r(Fe), Tl = r(Fl), v =r r2M - Equivalent moment inertia of the linearly moving mass

  24. m1 m Motor Te n1 Load 1, Tl1 J2 m2 Load 2, Tl2 n2 J1 Elementary principles of mechanics – effect of gearing Motors designed for high speed are smaller in size and volume Low speed applications use gear to utilize high speed motors INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

  25. m1 m Motor Te Load 1, Tl1 n1 J2 m2 Load 2, Tl2 n2 J1 Elementary principles of mechanics – effect of gearing INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 m Motor Te Equivalent Load , Tlequ Tlequ = Tl1 + a2Tl2 Jequ a2 = n1/n2=2/1

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

  27. 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

  28. 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 

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

  30. 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

  31. speed (rad/s) 100 25 60 10 45 t (ms) Torque and speed profile Speed profile INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 The system is described by: Te – Tload = J(d/dt) + B J = 0.01 kg-m2, B = 0.01 Nm/rads-1 and Tload = 5 Nm. What is the torque profile (torque needed to be produced) ?

  32. speed (rad/s) 100 25 60 10 45 t (ms) Torque and speed profile INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 0 < t <10 ms Te = 0.01(0) + 0.01(0) + 5 Nm = 5 Nm 10ms < t <25 ms Te = 0.01(100/0.015) +0.01(-66.67 + 6666.67t) + 5 = (71 + 66.67t) Nm 25ms < t< 45ms Te = 0.01(0) + 0.01(100) + 5 = 6 Nm 45ms < t < 60ms Te = 0.01(-100/0.015) + 0.01(400 -6666.67t) + 5 = -57.67 – 66.67t

  33. Torque and speed profile speed (rad/s) 100 Speed profile 25 60 t (ms) 10 45 Torque (Nm) 72.67 torque profile 71.67 INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 6 5 45 25 10 60 t (ms) -60.67 -61.67

  34. Torque and speed profile Torque (Nm) 70 J = 0.001 kg-m2, B = 0.1 Nm/rads-1 and Tload = 5 Nm. 6 45 25 10 60 t (ms) -65 INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 For the same system and with the motor torque profile given above, what would be the speed profile?

  35. 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

  36. 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)

  37. 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

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

  39. 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

  40. 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

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

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

  43. 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

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

  45. 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

  46. 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

  47. T Torque-speed quadrant of operation 1 2 • T -ve • +ve Pm -ve • T +ve • +ve Pm +ve INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 3 4 • T -ve • -ve Pm +ve • T +ve • -ve Pm -ve

  48. Te m m Te  Te T Te m m 4-quadrant operation • Direction of positive (forward) speed is arbitrary chosen • Direction of positive torque will produce positive (forward) speed Quadrant 1 Forward motoring Quadrant 2 Forward braking INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Quadrant 3 Reverse motoring Quadrant 4 Reverse braking

  49. Transient torque limit Power limit for transient torque Ratings of converters and motors Torque Continuous torque limit Power limit for continuous torque Maximum speed limit INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Speed

  50. Steady-state stability INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

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