Professor Mohammed Zeki Khedher Electric Drives Professor Mohammed Zeki Khedher Lecture One

Introduction In some countries nearly 65% of the total electric energy produced is consumed by electric motors.

Some Applications of Electric Drives Electric Propulsion Pumps, fans, compressors Plant automation Flexible manufacturing systems Spindles and servos Appliances and power tools Cement kilns Paper and pulp mills; textile mills Automotive applications Conveyors, elevators, escalators, lifts

1. ENERGY CONVERSION IN ELECTRIC DRIVES 1.1. ELECTRIC DRIVES - A DEFINITION About 50% of electrical energy produced is used in electric drives today. Electric drives may run at constant speed (figure 1.1) or at variable speed (figure 1.2). Figure 1.1. Constant speed electric drive

Figure 1.2. Variable speed electric drive

1.2. APPLICATION RANGE OF ELECTRIC DRIVES A summary of main industrial applications and power range of electric drives is shown on figure 1.3. Figure 1.3. Electric drives - variable speed applications

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Overview of AC and DC drives 7

Energy/Cost Savings System efficiency can be increased from 15% to 27% by introducing variable-speed drive operation in place of constant-speed operation. For a large pump variable-speed drive, payback period ~ 3-5 years whereas operating life is ~ 20 years.

Electric Machines “An engineer designing a high-performance drive system must have intimate knowledge about machine performance and Power Electronics”

Electric Machines (cont’d) DC Machines - shunt, series, compound, separately excited dc motors and switched reluctance machines AC Machines - Induction, wound rotor synchronous, permanent magnet synchronous, synchronous reluctance, and switched reluctance machines. Special Machines - switched reluctance machines

Electric Machines (cont’d) All of the above machines are commercially available in fractional kW to MW ranges except permanent-magnet, synchronous, synchronous reluctance, and switched reluctance which are available up to 150 kW level.

Selection Criteria for Electric Machines Cost Thermal Capacity Efficiency Torque-speed profile Acceleration Power density, volume of motor Ripple, cogging torques Peak torque capability

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) 13

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 17

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Modern electric drives (With power electronic converters) Small Efficient Flexible 18

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 19

Controllers Controllers embody the control laws governing the load and motor characteristics and their interaction. Controller Torque/speed/ position commands Vc, fc, start, shut-out, signals, etc. Torque/speed/ position feedback Thermal and other feedback

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Components in electric drives e.g. Single drive - sensorless vector control from Hitachi 22

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Components in electric drives e.g. Multidrives system from ABB 23

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Components in electric drives Motors DC motors - permanent magnet – wound field AC motors – induction, synchronous , 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 24

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 25

DC Motors Advantage: simple torque and speed control without sophisticated electronics Limitations: Regular Maintenance Expensive motor Heavy motor Sparking

DC DRIVES Vs AC DRIVES DC drives: Advantage in control unit Disadvantage in motor AC Drives: Advantage in motor Disadvantage in control unit

1.3. ENERGY SAVINGS PAYS OFF RAPIDLY Consider a real case when a motor pump system of 15kW works 300 days a year, 24 hours a day and pumps 1200m3 of water per day. By on/off and throttling control, only, the system uses 0.36kWh/m3 of pumped water to keep the pressure rather constant for variable flow rate. Adding a P.E.C., in the same conditions, the energy consumption is 0.28kWh/m3 of pumped water, with a refined pressure control. Let us consider that the cost of electrical energy is 40fils/kWh. The energy savings per year S is: S = 1200 * 300 * (0.36 -0.28) * 0.04 /year = 1152 JD/year Now the costs of a 15kW PWM - P.E.C. for an induction motor is less than 4000JD. Thus, to a first approximation, the loss savings only pay off the extra investment in less than 4 years.

Costs Power Electronics Controller costs approximately 2 to 5 times AC motor Cost decreases with bigger size

1.4. GLOBAL ENERGY SAVINGS THROUGH P.E.C. DRIVES So far the energy savings produced by the P.E.C. in variable speed drives have been calculated for the drive only - P.E.C. and motor. Figure 1.5. Primary energy consumption for throttle / motor / pump system

Figure 1. 6. Primary energy consumption for P. E. C Figure 1.6. Primary energy consumption for P.E.C. / motor / pump systems

Power consumption with flow

Load The motor drives a load that has a characteristic torque vs. speed requirement. In general, load torque is a function of speed and can be written as: Tl  mx x=1 for frictional systems (e.g. feed drives) x=2 for fans and pumps

General Torque Equation Translational (linear) motion: F : Force (Nm) M : Mass (Kg ) v : velocity (m/s) Rotational motion: T : Torque (Nm) J : Moment of Inertia (Kgm2 ) w : angular velocity ( rad/s )

Torque Equation: Motor drives Te : motor torque (Nm) TL : Load torque (Nm) Acceleration Deceleration Constant speed

…continue Drive accelerates or decelerates depending on whether Te is greater or less than TL During acceleration, motor must supply not only the load torque but also dynamic torque, ( Jdw/dt ). During deceleration, the dynamic torque, ( Jdw/dt ), has a negative sign. Therefore, it assists the motor torque, Te.

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 37

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, 38

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Elementary principles of mechanics For constant J, Torque dynamic – present during speed transient Angular acceleration (speed) The larger the net torque, the faster the acceleration is. 39

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 40

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 41

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 42

1.6. MOTION / TIME PROFILE MATCH Figure 1.8. Motion / time profile a.) speed b.) position c.) required load torque

Figure 1.9. Required speed / time profile Example 1.2. The direct drive torque / time curve A direct drive has to provide a speed / time curve such as in figure 1.9. against a constant load torque of TL = 10Nm, for a motor load inertia J = 0.02 kgm2. Figure 1.9. Required speed / time profile Neglecting the mechanical losses let us calculate the motor torque (Te) / time requirements. The motion equation for a direct drive is:

For the linear speed / time (acceleration - deceleration) zones the speed derivative is: For the constant speed (cruising) zone . Consequently the torque requirements from the motor for the three zones are:

Figure 1.10. Motor torque / time requirements The motor torque / time requirements are shown on figure 1.10. Figure 1.10. Motor torque / time requirements

Example 1.3. gear - box drive torque / time curve Let us consider an electric drive for an elevator with the data shown in figure 1.11. Figure 1.11. Elevator electric drive with multiple mechanical transmissions and counterweight

The motor rated speed nn = 1550rpm The motor rated speed nn = 1550rpm. The efficiency of gearing system is h = 0.8. Let us calculate the total inertia (reduced to motor shaft), torque and power without and with counterweight. First the motor angular speed wm is: (1.12) The gear ratios may be defined as speed ratios - Wt /wm for J4+J5 and Wd /wm for J6 (figure 1.11). Consequently the inertia of all rotating parts Jr, reduced to the motor shaft, (figure 1.11), is: (1.13)

For the cabin and the counterweight, the inertia, reduced to motor shaft (Je) is: (1.14) Thus the total inertia Jt is: (1.15) In absence of counterweight the la of energy conservation leads to: (1.16) Consequently the motor torque, Tem, yields: (1.17)

The motor electromagnetic power Pem is: (1.18) On the other hand in presence of counterweight (1.16) becomes: (1.19) (1.20) So the motor electromagnetic P’em is: (1.21)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Motor steady state torque-speed 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 51

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 52

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  53

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Load steady state torque-speed characteristic Hoist drive Speed Torque Gravitational torque 54

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 55

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) ? 56

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 57

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 58

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? 59

Torque Equation: Graphical Te Speed Forward running Forward braking Reverse acc. Reverse running Reverse braking Forward acc.

Load Torque Load torque, TL, is complex, depending on applications. In general: TORQUE TL = k TL = kw TL = kw2 SPEED

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 62

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Steady-state stability 63

Figure 1.7. Typical load speed / torque, speed / power curves 1.5.1. Typical load torque / speed curves Typical load torque / speed curves are shown on figure 1.7. They give a strong indication of the variety of torque / speed characteristics. Along such curves the mechanical power required from the motor varies with speed. Figure 1.7. Typical load speed / torque, speed / power curves

1.7. LOAD DYNAMICS AND STABILITY (1.22) (1.23) where TS is the static friction torque (at zero speed); TC is Coulomb friction torque (constant with speed); TV is viscous friction torque (proportional to speed) and TW is windage friction (including the ventilator braking torque, proportional to speed squared): (1.24) (1.25)

Figure 1.12. Components of friction torque, Tfriction Figure 1.13. Mechanical characteristics: a.) d.c. brush motor with separate excitation b.) induction motor c.) synchronous motor

Various Motor Characteristics

Example 1.4. D.C. brush motor drive stability. A permanent magnet d.c. brush motor with the torque speed curve: drives a d.c. generator which supplies a resistive load such that the generator torque / speed equation is Wr = 2TL. We calculate the speed and torque for the steady state point and find out if that point is stable. Solution: Let us first draw the motor and load (generator) torque speed curves on figure 1.14. Figure 1.14. D.C. brush motor load match

The steady state point, A, corresponds to constant speed and B = 0 in (1.27). Simply the motor torque counteracts the generator braking torque: (1.37) Using the two torque speed curves we find: (1.38) and thus: (1.39) and (1.40) The static stability is met if: (1.41) In our case from the two torque / speed curves: (1.42) and thus, as expected, point A represents a situation of static equilibrium.

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) 70

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 71

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Heating transient  t Cooling transient t  72

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  73

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 74

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Short time intermittent duty p1s p1 p1n  t1 t 75

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Short time intermittent duty  t1 t 76

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 77

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Periodic intermittent duty p1 heating coolling t 78

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 79

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, 80

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 81

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 82

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 83

1.8. MULTIQUADRANT OPERATION These possibilities are summarised in Table 1.1 and in figure 1.16. Table 1.1.

4Q OPERATION FB FM RB RM F: FORWARD R: REVERSE M : MOTORING B: BRAKING SPEED w w Te Te FB FM II I TORQUE RB III IV w w RM Te Te

4Q OPERATION: LIFT SYSTEM Positive speed Motor Negative torque Counterweight Cage

4Q OPERATION: LIFT SYSTEM Convention: Upward motion of the cage: Positive speed Weight of the empty cage < Counterweight Weight of the full-loaded cage > Counterweight Principle: What causes the motion? Motor : motoring P =Tw = +ve Load (counterweight) : braking P =Tw = -ve

4Q OPERATION: LIFT SYSTEM Speed You are at 10th floor, calling empty cage from gnd floor You are at 10th floor, calling fully-loaded cage from gnd floor FB FM Torque RM RB You are at gnd floor, calling empty cage from 10th floor You are at gnd floor, calling Fully-loaded cage from 10th floor

DC MOTOR DRIVES Principle of operation Torque-speed characteristic Methods of speed control Armature voltage control Variable voltage source Phase-controlled Rectifier Switch-mode converter (Chopper) 1Q-Converter 2Q-Converter 4Q-Converter

Figure 1.16. Electric drives with four quadrant operation