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

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

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 1523 will be covering variable speed drives

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

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

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

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

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

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

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 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 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

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 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 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 Overview of AC and DC drives Extracted from Boldea & Nasar

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

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

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

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

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.

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

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

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 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: J is purposely increased to do this job !

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

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 Motor Te Load 1, Tl1 Load 2, Tl2 J1 J2 m m1 m2 n1 n2

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

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

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 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 T~ C Coulomb friction T~ 2 Friction due to turbulent flow T~  Viscous friction TORQUE

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

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

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 Tl Te Torque r2 r3 r1 Steady state speed r Speed

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Torque and speed profile 10 25 45 60 t (ms) speed (rad/s) 100 Speed profile 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) ?

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Torque and speed profile 10 25 45 60 t (ms) speed (rad/s) 100 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

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

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?

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

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 Thermal capacity, C (Ws/oC) Surface A, (m2) Surface temperature, T (oC) p2 Emitted heat power (convection) Input heat power (losses)

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

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

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 Steady state temperature reached Nominal output power chosen equals or exceeds continuous load Continuous duty Short time intermittent duty Periodic intermittent duty Losses due to continuous load t p1n 

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

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

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

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

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 = 0.92, nominal efficiency T= 50oC, steady state temperature rise due to pn Also, 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

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,

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

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

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

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Steady-state stability