Introduction of Electrical Drives By H. S. Darji Department of Electrical Engineering U. V. Patel College of Engineering
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Reference Books
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Definition of Electrical Drives “An electrical drive is defined as a form of machine equipment designed to convert electrical energy into mechanical energy & provide electrical control of this process.”
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Block diagram of an Electric Drives Power Source
Power Processing Unit
Motor
Load
feedback Control
Control
Reference
Unit
Small (compact)
Flexible
Efficient
Interdisciplinary 4
Basic Components of Electric Drives Power Source
Control Reference
Power Processing Unit
Control
Motor
Load
feedback
Unit
Power Source Motor Power Processing Unit (Electronic Converter) Control Unit Mechanical Load 5
Basic Components of Electric Drives – Power Source • Provides energy to electric motors • Regulated (e.g: utility) or Unregulated (e.g. : renewable
energy) • Unregulated power sources must be regulated for high efficiency – use power electronic converters • DC source • batteries • fuel cell • photovoltaic
• AC source • single- or three- phase utility • wind generator 6
Basic Components of Electric Drives - Motor Electrical Motor energy
Mechanical energy
• Obtain power from electrical sources
• DC motors - Permanent Magnet or wound-field (shunt,
separately excited, compound, series) • AC motors – Induction, Synchronous (wound –rotor, IPMSM, SPMSM), brushless DC • Selection of machines depends on many factors, e.g.: • application • cost • efficiency
• environment • type of source available 7
Basic Components of Electric Drives – Power Processing Unit • Provides a regulated power supply to motor • Enables motor operation in reverse, braking and variable
speeds • Combination of power electronic converters Controlled rectifiers, inverters –treated as ‘black boxes’ with certain transfer function More efficient – ideally no losses occur Flexible - voltage and current easily shaped through switching control Compact Several conversions possible: AC-DC , DC-DC, DC-AC, AC-AC 8
Basic Components of Electric Drives – Power Processing Unit DC to AC:
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Basic Components of Electric Drives – Power Processing Unit DC to DC:
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Basic Components of Electric Drives – Power Processing Unit AC to DC:
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Basic Components of Electric Drives – Power Processing Unit AC to AC:
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Basic Components of Electric Drives – Control Unit Supervise operation Enhance overall performance and stability Complexity depends on performance requirement Analog Control – noisy, inflexible, ideally infinite bandwidth • Digital Control – immune to noise, configurable, smaller bandwidth (depends on sampling frequency) • DSP/microprocessor – flexible, lower bandwidth, real-time • DSPs perform faster operation than microprocessors (multiplication in single cycle), complex estimations and observers easily implemented • • • •
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Advantages of Electrical Drives Flexible control characteristic particularly when power electronic converters are employed Wide range of speed, torque and power High efficiency – low no load losses Low noise Low maintenance requirements, cleaner operation Electric energy easily transported Adaptable to most operating conditions Available operation in all four torque-speed quadrants 14
Choice of Electrical Drives • Several factors affecting drive selection: • Steady-state operation requirements •
•
Transient operation requirements •
•
nature of torque-speed profile, speed regulation, speed range, efficiency, quadrants of operations, converter ratings
values of acceleration and deceleration, starting, braking and reversing performance
Power source requirements •
Type, capacity, voltage magnitude, voltage fluctuations, power factor, harmonics and its effect on loads, ability to accept regenerated power
Capital & running costs • Space and weight restrictions • Environment and location • Efficiency and reliability •
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Electric Drives Application Line Shaft Drives Oldest form Single motor, multiple loads Common line shaft or belt Inflexible Inefficient Rarely used 16
Electric Drives Application Single-Motor,
Single-Load Drives Most common Eg: electric saws,
drills, fans, washers, blenders, disk-drives, electric cars.
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Electric Drives Application Multimotor Drives Several motors, single mechanical load Complex drive functions Eg: assembly lines, robotics, military airplane actuation. 18
DC or AC Drives? DC Drives Motor
• requires maintenance • heavy, expensive • limited speed (due to mechanical construction)
Control Unit Simple & cheap control even for high performance drives • decoupled torque and flux control • Possible implementation using single analog circuit
Performance Fast torque and flux control
AC Drives (particularly Induction Motor) • less maintenance • light, cheaper • high speeds achievable (squirrelcage IM) • robust Depends on required drive performance • complexity & costs increase with performance • DSPs or fast processors required in high performance drives
Scalar control – satisfactory in some applications Vector control – similar to DC drives
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Torque Equation for Rotating Systems Motor drives a load through a transmission system (eg.
gears, V-belts, crankshaft and pulleys) Load may rotate or undergo translational motion Load speed may be different from motor speed Can also have multiple loads each having different speeds, some may rotate and some have translational motion Represent motorload system as equivalent rotational system
Te , m Motor
TL Load
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Torque Equation for Rotating Systems Torque equation for equivalent motor-load system: TL Te , m
With constant inertia J,
• •
d Jm Te TL dt
(1)
where: J = inertia of equivalent motor-load system, kgm2 m = angular velocity of motor shaft, rads-1 Te = motor torque, Nm TL = load torque referred to motor shaft, Nm
d m d 2 Te TL J J 2 dt dt
(2)
First order differential equation for angular frequency (or velocity) Second order differential equation for angle (or position) 21
Torque Equation for Rotating Systems with Gears Low speed
applications use gears to utilize high speed motors Motor drives two loads: Load 1 coupled
directly to motor shaft Load 2 coupled via gear with n and n1 teeth
Need to obtain
equivalent motorload system
m
m
Motor Te
Load 1, TL0
n TL0
J1 TL1
n1
J0
m1
Motor Te
m TL
J
Load 2, TL1
Equivalent Load , TL
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Torque Equation for Rotating Systems with Gears Gear ratio a1 =
(3)
Neglecting losses in the transmission:
Kinetic energy due to equivalent inertia
=
kinetic energy of moving parts
Hence, equivalent motor-load inertia J is:
J J0 a J
2 1 1
(4)
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Torque Equation for Rotating Systems with Gears If 1 = transmission efficiency of the gears:
Power of the equivalent motor-load system
=
power at the loads
Hence, equivalent load torque TL is:
TL TL 0
a1 TL1
1
(5)
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Torque Equation for Rotating Systems with Translational Motion Motor drives two
loads: Load 1 coupled
directly to motor shaft Load 2 coupled via transmission system converting rotational to linear motion
Need to obtain
equivalent motorload system
Motor Te
m TL
J
Equivalent Load , TL
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Torque Equation for Rotating Systems with Translational Motion Neglecting losses in the transmission:
Kinetic energy due to equivalent inertia
=
kinetic energy of moving parts
Hence, equivalent motor-load inertia J is:
v1 J J 0 M 1 m
2 (7)
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Torque Equation for Rotating Systems with Translational Motion If 1 = transmission efficiency of the transmission system:
Power of the equivalent motor-load system
=
power at the loads and motor
Hence, equivalent load torque TL is:
F1 v1 TL TL 0 1 m
(8)
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Torque Equation for Rotating Systems – Example
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Components of Load Torque(Tl) • Load torque can be divided into: • Friction torque (TF) -present at motor shaft and in various parts of load. •
•
Viscous friction torque Tv – varies linearly with speed (Tv m). Exists in lubricated bearings due to laminar flow of lubricant Coulomb friction torque TC – independent of speed. Exists in bearings, gears coupling and brakes.
• Windage torque (Tw)-exists due to turbulent flow of air or
liquid. •
Varies proportional to speed squared (Tw m2).
• Mechanical Load Torque (TL ) - torque required to do the useful mechanical work. 29
Mechanical Load Torque • Torque to do useful mechanical work TL – depends
on application. • Load torque is function of speed •
TL
k m
where k = integer or fraction
• Mechanical power of load: • and P T L
m
m
2 nm 60
Angular speed in rad/s
Speed in rpm 30
Torque-Speed Characteristics of Load 1) 2) 3) 4)
Torque independent of speed Linear rising Torque-Speed Non-Linear rising Torque-Speed Non-Linear falling Torque-Speed
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Torque-Speed Characteristics of Load Torque
independent of speed , k = 0 Hoist Elevator Pumping of water or gas against constant pressure 32
Torque-Speed Characteristics of Load Torque
proportional to square of speed , k =2 Fans Centrifugal pumps 33
Torque-Speed Characteristics of Load Torque inversely
proportional to speed , k = -1 Milling machines Electric drill
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Classification of Electrical Drives Group Drive(Shaft Drive) Individual Drive Multi-Motor Drive
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Classification of Electrical Drives Group Drive(Shaft Drive) “If Several groups of Mechanisms or Machines are organized on one shaft & driven by one motor, the system is called a group drive (Shaft Drive)”
Disadvantages There is no flexibility, Addition of an extra machine to the main
shaft is difficult. The efficiency of the drive is low, because of the losses occurring in several transmitting mechanisms. The complete drive system requires shutdown if the motor, requires servicing or repair. The system is not very safe to operate The noise level at the work spot is very high. 36
Classification of Electrical Drives Individual Drive “If a single motor is used to drive a given mechanism & it does all the jobs connected with load, the drive is called an individual drive”
Examples • Single Spindle drilling machine • Lathe machines
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Classification of Electrical Drives Multi-Motor Drive “In a Multi-Motor drive, each operation of the mechanism is taken care of by a separate drive motor. The system contains several individual drives, each of which is used to operate its own mechanism”
Examples • Metal cutting machine tool • Rolling mills • Travelling cranes 38
Dynamic Conditions of a drive system • Dynamic conditions occur in a electric drive system
when operating point changes from one steady state condition to another, following a change introduced in the system variables. This variables may be mechanical such as speed, torque etc. or electrical such as voltage, current etc. • These conditions generally exist during starting, braking and speed reversal of the drive. • The dynamic conditions arise in a variable speed drive when transition from one speed to another is required. 39
Dynamic Conditions of a drive system • The drive may also have transient behavior if there are sudden changes of load, supply, voltage or frequency. • The dynamic behavior of a drive has a close relation to its stability. A drive is said to be stable if it can go from one state of equilibrium to another following a disturbance in one of the parameters of the system. • Stability can be identified as either steady-state or transient. 40
Dynamic Conditions of a drive system • The condition of stability depend on the operating point.
The dynamics of the drive can be investigated using the Torque balance equation given by
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system The load torque occurring in mechanical system may be Passive or active. Passive torque If the torque always opposes the direction of motion of drive motor it is called a passive torque. Active torque Load torque which have the potential to drive the motor under equilibrium condition are called active load torque. 46
Steady State Operating Speed Motor T- characteristic – variation of motor torque with speed with all other variables (voltage and frequency) kept constant. SPEED Synchronous motor Induction motor
Series DC motor
Separately excited / shunt DC motor
TORQUE
Loads will have their own T- characteristics. 47
Steady State Operating Speed • At constant
speed, Te= TL • Steady state speed is at point of intersection between Te and TL of the steady state torque characteristics
By using power TL electronic converters, the motor characteristic can be varied
Torque Te
Steady state Speed, r
r3
r1
r2
Speed 48
Steady State Stability Drives operate at steady-state speed (when Te = TL) only
if the speed is of stable equilibrium. A disturbance in any part of drive causes system speed to depart from steady-state point. Steady-state speed is of stable equilibrium if: system will return to stable equilibrium speed when subjected to a disturbance Steady-state stability evaluated using steady-state T- characteristic of motor and load. dTe dTL Condition for stable equilibrium: (9) dm dm 49
d m e L dt Evaluated using steady-state T- characteristic of motor and load.
Steady State Stability T T
J
Assume a disturbance causes speed drop to r’ At the new speed r’,
Te’ > TL’
Steady-state point A at speed = r
m Te
TL
motor accelerates
operation restored to steadystate point Steady-state speed is of stable equilibrium
dTe dTL dm dm
r r’
TL’
Te’
T 50
Steady State Stability
d m Te TL J dt
Let’s look at a different condition!
Assume a disturbance causes speed drop to r’ At the new speed r’,
Te’ < TL’
Steady-state point B at speed = r
m
Te
TL
motor decelerates
operation point moves away from steady-state point Point B is at UNSTABLE equilibrium
dTe dTL dm dm
r r’
Te’
TL’
T 51
Torque-Speed Quadrant of Operation •Direction of positive m
Te
Te
(forward) speed is arbitrary chosen •Direction of positive torque will produce positive (forward) speed
m
P = -ve P = +ve Quadrant 2 Quadrant 1 Forward braking Forward motoring
T
Quadrant 3 Quadrant 4 Reverse motoring Reverse braking Te P = +ve P = -ve Te
m
P Tem Electrical energy
m
MOTO R P = + ve
Mechanical energy 52
