459112057-1-Introduction-to-Electrical-Drives-ppt.ppt

Introduction to Electrical Drives
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering
Dr. Ungku Anisa, December 2009 1
EEEB443 - Control & Drives

Definition of Electrical Drives
 Drives – system employed
for motion control
 Motion control requires
prime movers
 Electrical Drives – Drives
that employ Electric
Motors as prime movers
Electrical Drives -> Electric
Motor as Prime Mover
Prime Mover
Drives -> Motion Control

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
Dr. Ungku Anisa, December 2009 EEEB443 - Control & Drives 3

Conventional Electric Drives
 Ward-Leonard system –
introduced in 1890s
 Disadvantage :
 Bulky
 Expensive
 Inefficient
 Complex

Modern Electric Drives
 Small (compact)
 Efficient
 Flexible
 Interdisciplinary
Power Source Power Processing Unit Motor Load
Control
Reference
Control
Unit
feedback

Electric Drives Application
 Line Shaft Drives
 Oldest form
 Single motor,
multiple loads
 Common line
shaft or belt
 Inflexible
 Inefficient
 Rarely used

 Single-Motor, Single-
Load Drives
 Most common
 Eg: electric saws,
drills, fans, washers,
blenders, disk-
drives, electric cars.

 Multimotor Drives
 Several motors,
single mechanical
load
 Complex drive
functions
 Eg: assembly
lines, robotics,
military airplane
actuation.

Basic Components of Electric
Drives
 Power Source
 Motor
 Power Processing Unit (Electronic Converter)
 Control Unit
 Mechanical Load
Power Source
Power
Processing Unit
Motor Load
Control
Reference
Control
Unit
feedback

Drives - Motor
• 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.:
Electrical
energy
Mechanical
energy
Motor
• application
• cost
• efficiency
• environment
• type of source available

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

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

 DC to AC:

 DC to DC:

 AC to DC:

 AC to AC:

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

Drives – Component Selection
• Several factors affecting drive selection:
• Steady-state operation requirements
• nature of torque-speed profile, speed regulation, speed range,
efficiency, quadrants of operations, converter ratings
• Transient operation requirements
• 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

DC or AC Drives?
DC Drives
AC Drives
(particularly Induction Motor)
Motor • requires maintenance
• heavy, expensive
• limited speed (due to
mechanical construction)
• less maintenance
• light, cheaper
• high speeds achievable (squirrel-
cage IM)
• robust
Control Unit Simple & cheap control even
for high performance drives
• decoupled torque and flux control
• Possible implementation using
single analog circuit
Depends on required drive
performance
• complexity & costs increase with
performance
• DSPs or fast processors required in high
performance drives
Performance Fast torque and flux control Scalar control – satisfactory in some
applications
Vector control – similar to DC drives

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
Motor Load
Te , m TL
Represent motor-load
system as equivalent
rotational system

Systems
• First order differential equation for angular frequency (or velocity)
• Second order differential equation for angle (or position)
 
2
2
dt
d
J
dt
d
J
T
T m
L
e





With constant inertia J,
 
dt
J
d
T
T m
L
e




Te , m
TL
Torque equation for equivalent motor-load system:
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
(1)
(2)

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 motor-
load system
Motor
Te
Load 1,
TL0
Load 2,
TL1
J0
J1
m
m
m1
n
n1
TL0
TL1
Motor
Te
J
Equivalent
Load , TL
m
TL

Systems with Gears
 Gear ratio a1 =
 Neglecting losses in the transmission:
 Hence, equivalent motor-load inertia J is:
Kinetic energy due
to equivalent inertia
=  kinetic energy of moving parts
1
2
1
0 J
a
J
J 

(3)
(4)

Systems with Gears
 If 1 = transmission efficiency of the gears:
 Hence, equivalent load torque TL is:
Power of the equivalent
motor-load system
=  power at the loads
1
1
1
0

L
L
L
T
a
T
T 
 (5)

Systems – Example 1
 Figure below shows a motor driving three loads. Assuming there are no
losses in the system, calculate the:
 total moment of inertia of the system referred to
the motor shaft
 amount of torque the motor must produce
to drive the loads
 output power of the motor
Motor
T e
Load 1,
T L1 = 10 Nm
Load 2,
T L2 = 10 Nm
Jm=1.5kgm-2
m 1500 rpm
N1
N2
N3
Load 3,
T L3 = 6 Nm
J1 = 2kgm-2
J2= 7kgm-2
J3= 5kgm-2
m3
500 rpm
m2
300 rpm

Systems with Belt Drives
 By neglecting slippage,
equations (4) and (5) can
still be used.
 However,
where:
Dm = diameter of wheel driven by motor
DL = diameter of wheel mounted on load shaft
L
m
D
D
a 
1 (6)

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 motor-
load system
Motor
Te
J
Equivalent
Load , TL
m
TL

 Neglecting losses in the transmission:
 Hence, equivalent motor-load inertia J is:
Kinetic energy due
to equivalent inertia
=  kinetic energy of moving parts
2
1
1
0 









m
v
M
J
J

(7)

 If 1 = transmission efficiency of the transmission system:
 Hence, equivalent load torque TL is:
Power of the equivalent
motor-load system
=  power at the loads and motor










m
L
L
v
F
T
T


1
1
1
0
(8)

Relation between Translational and
Rotational Motions
 The relationship between the torques and linear forces are:
 Relationship between linear and angular velocity:
 Hence, assuming the mass M is constant:
1
1 rF
T  m
m rF
T 

r
v 
dt
dv
M
F
Fm 
 1
dt
d
Mr
T
Tm

2
1 


 An example of a hoist drive employing gears is shown below.
The system can be represented by an equivalent system shown
on the right. Write down the equation for the:
 Equivalent system moment of inertia
 Equivalent system load torque
Hoist drive
Equivalent system

 If the motor is rated at 19kW, is the motor sufficient to drive
the two loads?
 The translational motion load now has to lift a weight of 1200
kg at the same speed of 1.5m/s. Is the motor still capable to
drive both loads at the same motor speed of 1420 rpm?

Components of Load Torque
• Load torque can be divided into:
• Friction torque – 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  m
2).
• Mechanical Load Torque TL - torque to do useful
mechanical work.

Mechanical Load Torque
• Torque to do useful mechanical work TL – depends on
application.
• Load torque is function of speed
• where k = integer or fraction
• Mechanical power of load:
• and
k
m
L
T 

m
L
T
P 
 m
m n
60
2
 
Angular speed
in rad/s
Speed
in rpm

 Torque independent
of speed , k = 0
 Hoist
 Elevator
 Pumping of water
or gas against
constant pressure

 Torque proportional
to square of speed ,
k = 2
 Fans
 Centrifugal pumps
 Propellers

 Torque inversely
proportional to
speed , k = -1
 Milling machines
 Electric drill
 Electric saw

Motor T- characteristic – variation of motor torque with speed
with all other variables (voltage and frequency) kept constant.
Loads will have their own T- characteristics.
Steady State Operating Speed
Synchronous motor
Induction motor
Separately excited
/ shunt DC motor
Series DC motor
SPEED
TORQUE

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
TL
Te
Steady state
Speed, r
Torque
Speed
r2
r3
r1
By using power
electronic
converters, the
motor characteristic
can be varied

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.
 Condition for stable equilibrium:
m
e
m
L
d
dT
d
dT


 (9)

 Evaluated using steady-state T- characteristic of motor and
load.
 Assume a disturbance causes speed drop to r’
 At the new speed r’,
Te TL
Steady-state point A
at speed = r
r
r’
Te’
TL’
Te’ > TL’
motor accelerates
operation restored to steady-
state point
m
T
Steady-state speed is of
stable equilibrium
m
e
m
L
d
dT
d
dT



 
dt
d
J
T
T m
L
e




 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
r
r’
TL’
Te’
Te’ < TL’
motor decelerates
operation point moves away
from steady-state point
m
T
Point B is at UNSTABLE
equilibrium
m
e
m
L
d
dT
d
dT



 
dt
d
J
T
T m
L
e




Torque-Speed Quadrant of
Operation
m
Te
Te
m
Te
m
Te
m

T
•Direction of positive
(forward) speed is
arbitrary chosen
•Direction of positive
torque will produce
positive (forward) speed
Quadrant 1
Forward motoring
Quadrant 2
Forward braking
Quadrant 3
Reverse motoring
Quadrant 4
Reverse braking
P = +ve
P = -ve
P = -ve
P = +ve
m
e
T
P 

Electrical energy
Mechanical energy
MOTOR
P = + ve

References
 El-Sharkawi, M. A., Fundamentals of Electric Drives,
Brooks/Cole Publishing Company, California, 2000.
 Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha
Science Int. Ltd., UK, 2001.
 Krishnan, R., Electric Motor Drives: Modelling, Analysis and
Control, Prentice-Hall, New Jersey, 2001.
 Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
 Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

459112057-1-Introduction-to-Electrical-Drives-ppt.ppt

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