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ELECTRIC DRIVES
INTRODUCTION TO ELECTRIC DRIVES
MODULE 1
Dr. Nik Rumzi Nik Idris
Dept. of Energy Conversion, UTM
2006
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Electrical Drives
Drives are systems employed formotion control
Require prime movers
Drives that employ electric motors as
prime movers are known as Electrical Drives
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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 1522 will be covering variable speed drives
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Example on VSD application
motor pump
valve
Supply
Constant speed Variable Speed Drives
Power
In
Power lossMainly in valve
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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Example on VSD application
motor pump
valve
Supply
motorPEC pump
Supply
Constant speed Variable Speed Drives
Power
In
Power loss
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Power lossMainly in valve
Power outPower
In
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Power lossMainly in valve
Power out
motor pump
valve
Supply
motorPEC pump
Supply
Constant speed Variable Speed Drives
Example on VSD application
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Power
In
Power loss
Power
In
Power out
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Conventional electric drives (variable speed)
Bulky
Inefficient
inflexible
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Small
Efficient
Flexible
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives
Inter-disciplinary
Several research area
Expanding
Machine designSpeed sensorlessMachine Theory
Non-linear controlReal-time controlDSP application
PFCSpeed sensorlessPower electronic converters
Utility interface
Renewable energy
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
e.g. Single drive - sensorless vector control from Hitachi
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
e.g. Multidrives system from ABB
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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
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
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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 controllers
DSP/microprocessor flexible, lower bandwidth - DSPs perform
faster operation than microprocessors (multiplication in single
cycle), can perform complex estimations
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
Extracted from Boldea & Nasar
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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
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
Before semiconductor devices were introduced (
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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
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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.
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
M
v
Fm
Ff
dt
MvdFF fm !
Newtons law
Linear motion, constant M
First order differential equation for speed
Second order differential equation for displacement
Ma
dt
xdM
dt
vdMFF
2
2
fm !!!
x
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
First order differential equation for angular frequency (or velocity)
Second order differential equation for angle (or position)
2
2
m
le dt
dJ
dt
dJTT
U
!
[
!
With constant J,
Rotational motion
- Normally is the case for electrical drives
dt
JdTT mle
[!
U
Te , [m
Tl
J
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
dtdJTT m
le[!For constant J,
dt
dJ
m[
Torque dynamic present during speed transient
dt
dm
[Angular acceleration (speed)
The larger the net torque, the faster the acceleration is.
0.19 0.2 0. 21 0. 22 0. 23 0. 24 0. 25-200
-10 0
0
10 0
200
s
peed
(rad/s
)
0.19 0.2 0. 21 0. 22 0. 23 0. 24 0. 25
0
5
10
15
20
torque
(Nm)
Elementary principles of mechanics
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
dt
vdMFF
le!
Combination of rotational and translational motions
r r
[Te, [
Tl
Fl Fe
v
M
Te = r(Fe), Tl = r(Fl), v =r[
dt
dMrTT 2
le
[!
r2M - Equivalent moment inertia of the
linearly moving mass
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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
MotorTe
Load 1,
Tl1
Load 2,
Tl2
J1
J2
[m
[m1
[m2
n1
n2
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Motor
Te
Load 1,
Tl1
Load 2,
Tl2
J1
J2
[m
[m1
[m2
n1
n2
Motor
Te
Jequ
Equivalent
Load , Tlequ
[m2
2
21equ JaJJ !
Tlequ = Tl1 + a2Tl2
a2 = n1/n2
Elementary principles of mechanics effect of gearing
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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
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
SPEED
TORQUE
Frictional torque (passive load) Exist in all motor-load drivesystem simultaneously
In most cases, only one or two
are dominating
Exists when there is motion
T~ C
Coulomb friction
T~ [
Viscous friction
T~ [2
Friction due to turbulent flow
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E
TL
Te
Vehicle drive
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Constant torque, e.g. gravitational torque (active load)
SPEED
TORQUE
Gravitational torque
gM
FL
TL = rFL = r g M sin E
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Hoist drive
Speed
Torque
Gravitational torque
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load and motor steady state torque
At constant speed, Te= TlSteady state speed is at point of intersection between Te and Tl of the
steady state torque characteristics
TlTe
Steady state
speed
[r
Torque
Speed[r2[r3 [r1
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
10 25 45 60 t (ms)
speed
(rad/s)
100
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) ?
Speed profile
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
10 25 45 60 t (ms)
speed
(rad/s)
100
0 < t
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
10 25 45 60
speed(rad/s)
100
10 25 45 60
Torque(Nm)
72.67
71.67
-60.67
-61.67
56
t (ms)
t (ms)
Speed profile
torque profile
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
10 25 45 60
Torque
(Nm)
70
-65
6
t (ms)
For the same system and with the motor torque profilegiven above, what would be the speed profile?
J = 0.001 kg-m2, B = 0.1 Nm/rads-1and Tload = 5 Nm.
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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
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Electrical machines can be overloaded as long their temperaturedoes 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
p2p
1 Thermal capacity, C (Ws/oC)Surface A, (m2)
Surface temperature, T (oC)Input heat power
(losses)
Emitted heat power
(convection)
Ambient temperature, To
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Power balance:
21 ppdt
dTC !
Heat transfer by convection:
)TT(Ap o2 E!
C
pT
C
A
dt
Td 1!(E
(
Which gives:
XE
!( /th e1A
pT
A
C
E!X, where
With (T(0) = 0 and p1 = ph = constant ,
, whereE
is the coefficient of heat transfer
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
tX
T(
tX
X(!( /te)0(TT
T(
XE!(
/th e1A
pT
Heating transient
Cooling transient
A
ph
E
)0(T(
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
The duration of overloading depends on the modes of operation:
Continuous duty
Short time intermittent duty
Periodic intermittent duty
Continuous duty
Load torque is constant over extended period multiple
Steady state temperature reached
Nominal output power chosen equals or exceeds continuous load
T(
t
A
p n1E
X
p1n
Losses due to continuous load
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
Operation considerably less than time constant, X
Motor allowed to cool before next cycle
Motor can be overloaded until maximum temperature reached
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t1X
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
Ap s1E
maxT( A
p n1
E
t
T(
p1
p1n
p1s
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t1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
X t
T(
XE
!( /ts1 e1A
pT
maxT(A
p n1
E
X
E!E/ts1n1 1
e1A
p
A
p
X
u
/t
s1n1
1
e1pp1
/tn1
s1
te1
1
p
p
1
X}
e
X
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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
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
p1
t
heating coollingcoolling
coolling
heating
heating
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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
L= 0.92, nominal efficiency
(Tg= 50oC, steady state temperature rise due to pn
kW911
pp n1 !
L! Also, C/W180
50
9000
T
pA o1 !!
(!E
g
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 X, thermal time constant = 384000/180 = 35 minutes
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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,
0 0. 5 1 1. 5 2 2 .5
x 104
0
5
10
15
20
25
30
35
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque-speed quadrant of operation
[
T12
3 4
T +ve
[ +vePm +ve
T -ve
[ +vePm -ve
T -ve
[ -vePm +ve
T +ve
[ -vePm -ve
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Ratings of converters and motors
Torque
Speed
Powerlimit for
continuoustorque
Continuous
torquelimit
Maximum
speed limit
Powerlimit for
transienttorque
Transient
torquelimit
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Steady-state stability
Recommended