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ELECTRIC DRIVESINTRODUCTION TO ELECTRIC DRIVES MODULE 1 Dr. Nik Rumzi Nik Idris Dept. of Energy Conversion, UTM 2006

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 1522 will be covering variable speed drives

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

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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Conventional electric drives (variable speed) • Bulky • Inefficient • inflexible

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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Components in electric drives e.g. Single drive - sensorless vector control from Hitachi

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Components in electric drives e.g. Multidrives system from ABB

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 Coupling between torque and flux – variable spatial angle between rotor and stator flux

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 • High performance applications dominated by DC motors – tractions, elevators, servos, etc

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics v x Newton’s law Fm d( Mv ) M Fm − Ff = Ff dt Linear motion, constant M d( v ) d2 x Fm − Ff = M = M 2 = Ma dt dt • First order differential equation for speed • Second order differential equation for displacement

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics θ Rotational motion - Normally is the case for electrical drives Tl d( Jωm ) Te − Tl = Te , ω m dt J With constant J, d( ωm ) d2θ Te − Tl = J =J 2 dt dt • First order differential equation for angular frequency (or velocity) • Second order differential equation for angle (or position)

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics For constant J, dωm Te = Tl + J dt d( ωm ) J Torque dynamic – present during speed transient dt d( ωm ) Angular acceleration (speed) dt The larger the net torque, the faster the acceleration is. 200 100 speed (rad/s) 0 -100 -200 0.19 0.2 0.21 0.22 0.23 0.24 0.25 20 15 torque (Nm) 10 5 0 0.19 0.2 0.21 0.22 0.23 0.24 0.25

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics Combination of rotational and translational motions Fl Fe Te, ω ω r M r Tl v d( v ) Fe − Fl = M Te = r(Fe), Tl = r(Fl), v =rω dt dω Te − Tl = r 2M dt r2M - Equivalent moment inertia of the linearly moving mass

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 ωm ωm1 Motor Load 1, n1 Te Tl1 J2 ωm2 n2 Load 2, J1 Tl2

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics – effect of gearing ωm ωm1 Motor Load 1, n1 Te Tl1 J2 ωm2 n2 Load 2, J1 Tl2 J equ = J1 + a 2 J 2 2 ωm Motor Equivalent Te Load , Tlequ Tlequ = Tl1 + a2Tl2 Jequ a2 = n1/n2

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Load steady state torque-speed characteristic Frictional torque (passive load) • Exist in all motor-load drive SPEED T~ C system simultaneously T~ ω2 • In most cases, only one or two T~ ω are dominating • Exists when there is motion TORQUE Coulomb friction Viscous friction Friction due to turbulent flow

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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 Te Tl Steady state speed ωr3 ωr ωr1 ωr2 Speed

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Torque and speed profile speed Speed profile (rad/s) 100 10 25 45 60 t (ms) The system is described by: Te – Tload = J(dω/dt) + Bω 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) ?

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Torque and speed profile speed (rad/s) dω 100 Te = J + Bω + Tl dt 10 25 45 60 t (ms) 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

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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 p1 p2 Thermal capacity, C (Ws/oC) Input heat power Surface A, (m2) Emitted heat power Surface temperature, T (oC) (losses) (convection)

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Power balance: dT C = p1 − p 2 dt Heat transfer by convection: p 2 = αA (T − To ) , where α is the coefficient of heat transfer Which gives: d∆T Aα p + ∆T = 1 dt C C With ∆T(0) = 0 and p1 = ph = constant , ∆T = ph αA ( 1 − e −t / τ ) , where τ = C αA

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations ph ∆T αA ∆T = ph αA (1 − e −t / τ ) Heating transient t τ ∆T ∆T = ∆T(0) ⋅ e − t / τ ∆T(0) Cooling transient τ t

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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Short time intermittent duty p1s p1 p1n ∆T p1s αA p1n αA ∆Tmax τ t t1

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations p1s p p1 τ Short time intermittent duty ≤ 1n ≥=p 1(s1 −1eτ e −t )/ τ ) p ( ≈t − t 1 /− − t1 / τ 1 ∆T p1n αA − αA 1 e 1n 1s 1 ∆T = p1s αA (1 − e −t / τ ) p1n αA ∆Tmax τ t t1

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Periodic intermittent duty Example of a simple case – p1 rectangular periodic pattern pn = 100kW, nominal power M = 800kg η= 0.92, nominal efficiency ∆T∞= 50oC, steady state temperature rise due to pn 1  p1 9000 p1 = p n  − 1 = 9kW η  Also, αA = = = 180 W / o C   ∆T∞ 50 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 τ, thermal time constant = 384000/180 = 35 minutes

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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, 35 30 25 20 15 10 5 0 0 0.5 1 1.5 2 2.5 4 x10

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

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

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

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Steady-state stability