Fundamentals of Electric Drives and its applications.pptx

Fundamentals of Electric Drives

1. Dr. D. Y. Patil Institute of Technology, Pimpri, Pune –411018 Department of Electrical Engineering - Presented by Dr. (Mrs.) Manasi P.Deore Assistant Professor

2. 2 Unit 01 : Electrical Drives …………………………..( 08 Hrs) Definition, Advantages of electrical drives, Components of Electric drive system, Types of Electrical Drives (DC and AC). Motor-Load Dynamics, Speed Torque conventions and multi quadrant operation, Equivalent values of drive parameters. Load Torque Components, Nature and classification of Load. Constant Power operation of a Drive. Steady state stability, Numerical based on motor load dynamics . SYLLABUS CONTENT:

3. 3 e m p l o y e d f o r m o t i o n T h e s y s t e ms wh i c h a r e control are called drives. The drives employing e l e c t r ic m o t o r s f o r s u p p l y i n g m e c h a n i c al e n e r g y f o r m o t i o n c o n t r o l is called Electric Drive. Advantages: Flexible control characteristics Automatic fault detection PLC for desired sequential operation Electrical Drives:

4. Advantages: Wide range of speed, Torque and power H i g h e ffi c i e n c y , l o w n o l o a d l o ss e s, s h o r t t i m e overloading capability Longer life, lower noise, cleaner operation Pollution free Four quadrant operation No refuelling and warm up period required . 4

5. 5 Load Motor Power Modulator Control Unit Components of Electric Drive System:

6. 6 Steady state operation requirements Transient operation requirements Requirements related to source Capital and running cost Maintenance needs and lifetime Space and weight restrictions Environment and location Reliability Efficiency Choice of Electrical Drives:

7. 7 DC Drives: These are used in variable speed applications. Applications : Paper, sheet, film winding and unwinding machines Extruder screws Lathes, milling machines, boring machines. AC Drives: Used in fixed speed applications but nowadays used in variable speed applications also. Applications : Cranes, lifts, overhead cranes Pump , fans Feeders, conveyers Dispensers , mixers. Types of Electrical Drives:

8. ω 𝒎 m l m l d ω 𝒎 ω d 𝑱 dt 𝒎 dt This equation is applicable to variable inertia drives such as mine winders, reel drives, industrial robots. Motor Load Dynamics: Analogy with Newton’s Second Law The motor load systems is represented by equivalent rotational system as below: J= Polar moment of inertia of motor load. ω 𝒎 = Instantaneous angular velocity. I n s t a n t a n e o u s v al u e of d e v elo p ed m o t or 𝒎 = torque. 𝒍 = Instantaneous value of load torque referred to motor shaft. Above motor load system can be described by following fundamental torque equation: 8

9. m l For drives with constant inertia: ω 𝒎 Motor Load Dynamics: This equation shows that torque developed by motor is counter balanced by load torque l d t d ω 𝒎 and dynamic torque . (presents only during transient operations). During Acceleration : During Deceleration : m l m l here dynamic torque component will assist motor torque and maintains drive motion by extracting energy from stored kinetic energy. The stored kinetic energy is given by ω 𝒎 𝟐 9

10. Assumptions Made: Motor speed is considered positive when rotating in the forward direction. For drives which operate in only one direction, forward speed will be there normal speed. For reversible drives, forward speed is chosen arbitrarily. The rotation in opposite direction gives reverse speed which is assigned the negative speed. In loads involving up and down motions, the speed of motor which causes upward motion is considered forward motion. Positive motor torque is defined as the torque which produces acceleration. Motor torque is considered negative if it produces deceleration. Positive load torque is opposite to positive motor torque Speed Torque Conventions and Multi-quadrant operation: 10

11. 1 1 Speed Torque Conventions and Multi-quadrant operation: Motor operates in two modes: Motoring Braking Power developed by a motor is given by the product of speed and torque. Function Quadrant Speed Torque O utpu t Power Forward M o t o r in g I + + + F o r w a r d Braking II + - - Reverse M o t o r in g III - - + R e v e r se Braking IV - + -

12. 1 2 Multi-quadrant operation Example:

13. 13 Equivalent values of drive parameters: Different part of load may be coupled through different mechanisms such as gears, V- Belts and Crankshafts. These parts may have different speeds and different types of motions such as rotational and translational. 1. Loads With Rotational Motion: ELECTRICAL ENGINEERING DEPARTMENT 1

14. 14 Equivalent values of drive parameters: 1. Loads With Rotational Motion: Here Consider motor driving two loads, one directly coupled to its shaft and other through a gear with n and Gear teeth . Here, -- Moment of inertia of motor and load directly coupled to its shaft ω -- motor speed of directly coupled load -- Torque of directly coupled load Also… -- Moment of inertia of load coupled through gear ω -- speed of load coupled through gear -- Torque of load coupled through gear ELECTRICAL ENGINEERING DEPARTMENT

15. 15 Equivalent values of drive parameters: No w , ω 𝒎𝟏 ω 𝒎 n 𝟏 𝟏 ……Gear Tooth Ratio If the losses in the transmission is neglected , then the kinetic energy due to equivalent inertia must be the same as the kinetic energy of various moving parts. Thus, ω 𝒎 𝟐 𝟎 ω 𝒎 𝟐 𝟏 ω 𝒎𝟏 𝟐 𝟏 𝟎 ω 𝒎𝟏 ω 𝒎 𝟐 If in addition to load directly coupled to the motor with inertia 𝟎 there are m other loads with moment of inertias 𝟏 , 𝟐 ,……, 𝒎 and gear teeth ratios of 𝟏 , 𝟐 ,……., 𝒎 then ELECTRICAL ENGINEERING DEPARTMENT

16. 16 Equivalent values of drive parameters: Power at the loads and motor must be same. If the transmission efficiency of gears be η 𝟏 then, 𝒍 ω 𝒎 𝒍𝟎 ω 𝒎 η 𝟏 𝒍𝟏 ω 𝒎𝟏 W h e r e 𝒍 𝒍 𝟎 𝒍 𝟏 𝒍 is the total equivalent torque referred to motor shaft ω 𝒎𝟏 η 𝟏 ω 𝒎 If m loads with torques 𝒍𝟏 , 𝒍𝟐 , ……, η 𝒍𝒎 are coupled through gears with teeth ratios 𝟏 , 𝟐 ,……., 𝒎 and transmission efficiencies η 𝟏 , η 𝟐 , ….., η 𝒎 in addition to one directly coupled , then, η η η ELECTRICAL ENGINEERING DEPARTMENT

17. 17 Equivalent values of drive parameters: 1. Loads With Translational Motion: ELECTRICAL ENGINEERING DEPARTMENT

18. 18 Equivalent values of drive parameters: 2. Loads With Translational Motion: Let us consider a motor driving two loads, one coupled directly to its shaft and other through a transmission system converting rotational motion to linear motion. Here, -- Moment of inertia of motor and load directly coupled to its shaft -- Load Torque directly coupled to motor. -- Mass of load with translational system -- Velocity of load with translational system -- Force of load with translational system ELECTRICAL ENGINEERING DEPARTMENT

19. 19 Equivalent values of drive parameters: If the losses in the transmission is neglected , then the kinetic energy due to equivalent inertia must be the same as the kinetic energy of various moving parts. Thus, 𝟏 v 𝟏 𝟐 ω 𝒎 𝟐 𝟎 ω 𝒎 𝟐 v ω If in addition to one load directly coupled to the motor shaft, there are m other loads with translational velocities v , v , …, v v ω and masses , ,…, v ω m o t i o n w i t h . v ω ELECTRICAL ENGINEERING DEPARTMENT

20. 20 Equivalent values of drive parameters: P o w e r a t th e l o a d s a n d m o t o r m u s t b e s a m e . I f t h e t r a n s m i s s i on efficiency is η 𝟏 then, 𝒍 ω 𝒎 𝒍𝟎 ω 𝒎 η 𝟏 𝟏 𝟏 η ω If in addition to one load directly coupled to the motor shaft, there are m other loads with translational motion with velocities v 𝟏 , v 𝟐 , …, v 𝒎 and masses 𝟏 , 𝟐 ,…, 𝒎 . η ω η ω η ω ELECTRICAL ENGINEERING DEPARTMENT

21. 21 Load Torque Components: Different Components of Load Torque are as follow: a) Friction Torque ( ): Friction torque will be present at the motor shaft and also in various parts of the load. Friction torque has following three components: 1. Viscous Friction Torque: Friction torque which varies linearly with speed is called 𝑽 Viscous Friction Torque 𝑽 . It is given by, ω 𝒎 …Here B is the Viscous Friction Coefficient. Coulomb Friction Torque: Friction torque which is independent of speed, is known as Coulomb Friction 𝑪 . Standstill Friction Torque: Friction which is present only at standstill. It is not taken into account in dynamic analysis 𝑺 .

22. 22 Load Torque Components: Different Components of Load Torque are as follow: b) Windage Torque 𝑾 : When motor runs, wind generates a torque opposing the motion, this is known as windage torque. This torque is proportional to speed squared, is given by, 𝑾 ω 𝒎 𝟐 c) Torque Required To Do Useful Mechanical Work: Nature of this torque depends on particular application; It may be constant and independent of speed It may be some function of speed It may depend on the position or path followed by load It may be time invariant or Time-variant It may vary cyclically Its nature may also change with loads mode of operation

23. 23 Load Torque Components: Hence for finite speed, the load torque is given by ω 𝒎 𝑪 ω 𝒎 𝟐 𝒍 𝑳 𝑪 ω 𝒎 𝟐 ) is very small compared to ω 𝒎 and I n m a n y a p p l i c a t i o n s ( negligible compared to 𝑳 . Hence neglected. If there is torsional elasticity in shaft coupling the load to motor, coupling torque is present and it is given by, Rotational stiffness of shaft (N-m/rad) torsion angle of coupling (rad)

24. 24 Nature and classification of Load Total torque required to do useful mechanical work is called load torque. The nature of load torque depends on particular application 1. Torque Independent of Speed: In low speed hoist torque is constant and independent of speed. At low speeds windage torque is negligible hence torque is mainly due to gravity which is constant and independent of speed. In paper mill drive , coulomb friction dominates over other torque. Hence torque is independent of speed. Here Scharge motor (Inverted Polyphase induction Motor) is used.

25. 25 Nature and classification of Load 2. Load Torque Function of Speed: In Fans, Compressors, and airplanes windage torque dominate, here load torque is proportional to speed squared . Similar nature of load torque can be observed when motion is opposed by any other fluid. For e.g. Water in Centrifugal Pumps Ship Propellers Load Torque Proportional to speed squared ω

26. 26 Nature and classification of Load In high speed hoist, viscous friction and windage have high magnitude in addition to gravity. Thus speed torque curve will be High Speed Hoist Speed Torque Curve In traction load, moving on levelled ground , the speed curve is as shown. Due to large stiction and need for accelerating heavy mass, the motor torque required for starting is much larger than run it at full speed

27. 27 Nature and classification of Load In coiler (used in coil making) drive, torque is function of speed . It is hyperbolic in nature. Here developed power is nearly constant at all speeds. Coiler drive speed torque curve, constant power loads.

28. 28 Classification of Load Load torques are broadly classified into: 1. Active Load Torque L o a d t o r qu e s w h i ch h av e th e p o t e n t i a l t o d r i v e th e m o t or und e r equilibrium condition are called active load torques. Such load torques are due to Gravitational Force Tension Compression and torsion undergone by an elastic body. 2. Passive Load Torque The load torque which always oppose the motion and change their sign on reversal of motion are called passive load torque. These torques are due to Friction Windage cutting

29. 29 Steady state stability a m o t o r l o a d s y s t e m i s t o r q u e e q u a l s t o l o a d Equilibrium speed of o bt a i n e d w h e n m o t o r torque. Drive will operate in steady state at equilibrium s p ee d o f s t a b l e s p ee d , p r o v i d e d i t i s t h e equilibrium. Case I: Stable Equilibrium Point: The equilibrium point will be termed as stable when the operation will be restored to it after a small departure from it due to a disturbance in motor or load . Examine the steady state stability of equilibrium point A.

30. 3/17/2021 ELECTRICAL ENGINEERING DEPARTMENT 30 Steady state stability Consider the disturbance causes increase in speed. At new speed the motor torque is greater than load torque, hence motor will accelerate and operation will be restored to A Similarly an increase in ω in speed caused by a disturbance will make load torque greater than motor torque, r e s u l ti n g int o deceleration and restoration of operation to point A.

31. 31 Steady state stability Let us examine equilibrium point B which is obtained when the same motor drives another load. A decrease in speed causes the load torque to become greater than the motor torque, drive decelerates and operating point will move away from B.

32. 32 Steady state stability Similarly when working at B an increase in speed will make motor torque greater than the load torque which will move the operating point away from B. Thus B is unstable point of equilibrium.

33. 33 Steady state stability Stable Equilibrium Point Unstable Equilibrium Point

34. 34 Load Equalization Need: In some drive applications, load torque fluctuates widely within short intervals of time, for example in pressing machines a large torque of short duration is required during pressing operation, otherwise the torque is nearly zero. Other examples are rolling mills and reciprocating pumps In such drives, if motor is required to supply peak torque demanded by load, first motor rating has to be high also the motor will draw a pulsed current from the supply When amplitude of pulsed current forms an appreciable proportion of supply line capacity, it gives rise to line voltage fluctuations which adversely affect other loads connected to line. In some applications like blooming mills, load fluctuation may also adversely affect the stability of source Above mentioned problems of fluctuating loads are overcome by mounting a flywheel on motor shaft in non reversible drives.

35. 35 Load Equalization T h e m o t or s p ee d t o r qu e characteristics is made sagging Case-I: During High Load Period: Here load torque will be much larger compared to motor torque, deceleration occurs producing a large dynamic torque Case-II: During Light load period Here the motor torque exceeds the load torque causing acceleration, speed is brought back to its original value before next high load period. Variation of motor and load torques is shown below B C ω 𝒎 ω 𝒎𝒐 A ω 𝒎𝒓 𝒓 𝒎 𝒂 𝒙 𝒍 𝒉 T 𝒎 𝒂 𝒙 T 𝒎𝒊𝒏 T 𝒍 T 𝒍 T 𝒍 ω 𝒎 ω 𝒎 T 𝒎 T 𝒎 t 𝒍 t 𝒉 t

36. 36 Load Equalization It shows the peak motor torque is less than peak load torque, hence a motor with much smaller rating than peak load can be used and peak current drawn by motor from the source is reduced by a large amount. Fluctuations in motor torque and speed is reduced. Hence the power drawn from source fluctuates very little, this is called load equalization. In variable speed flywheel can not be mounted as it will increase transient time of the drive by large amount

37. 37 Important Equation: For Steady State Speed: 𝒎 Motor Load Dynamic Equation: 𝒎 𝒍 𝒍 ω 𝒎 𝟎 Total Moment of Inertia referred to Motor Shaft 𝟐 𝟏 𝟏 ……. Total Moment of Inertia of load having Rotational Motion 𝟎 𝟏 v 𝟏 ω 𝒎 𝟐 ……. Total Moment of Inertia of load having Translational Motion 𝟎 𝟐 𝟏 𝟏 𝟏 v 𝟏 ω 𝒎 𝟐 …… Total Moment of Inertia of load having combine Motion

38. 38 Important Equation: 𝒍 𝒍 𝟎 Total Load Torque referred to Motor Shaft: 𝟏 𝒍𝟏 ……. Total Load Torque of load having Rotational Motion 𝒍 𝒍 𝟎 ……. Total Load Torque of load having Translational Motion 𝒍 η 𝟏 𝟏 𝟏 η 𝟏 ω 𝒎 𝟏 η 𝟏 𝒍 𝟏 𝟏 𝟏 η 𝟏 ω 𝒎 …… Total Load Torque of load having combine Motion 𝒍𝟎 P o w e r 𝒍 ω 𝒎

39. 3 9 Analogy With Newton’s Second Law: The Acceleration ‘a’ of an object as produced by a net force F is directly proportional to the magnitude of Net Force, in the same direction as the net force and inversely proportional to the mass m of the object. …….. If m is constant. Here we can relate F, m, v with ( m l , J and ω m in case of motors.