Post on 23-Oct-2014
ELECTRIC DRIVES
INTRODUCTION TO ELECTRIC DRIVESMODULE 1
Dr. Nik Rumzi Nik IdrisDept. of Energy Conversion, UTM
2006
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Electrical Drives
Drives are systems employed for motion control
Require prime movers
Drives that employ electric motors as prime movers are known as Electrical Drives
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
Example on VSD application
motor pump
valve
Supply
Constant speed Variable Speed Drives
PowerIn
Power lossMainly in valve
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
motor pump
valve
SupplymotorPEC pump
Supply
Constant speed Variable Speed Drives
PowerIn
Power loss
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Power lossMainly in valve
Power outPowerIn
Power lossMainly in valve
Power out
motor pump
valve
SupplymotorPEC pump
Supply
Constant speed Variable Speed Drives
Example on VSD application
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
PowerIn
Power loss
PowerIn
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
• Small
• Efficient
• Flexible
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives
• Inter-disciplinary
• Several research area
• Expanding
Machine designSpeed sensorlessMachine Theory
Non-linear controlReal-time controlDSP applicationPFCSpeed sensorless Power electronic converters
Utility interfaceRenewable energy
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
e.g. Single drive - sensorless vector control from Hitachi
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
e.g. Multidrives system from ABB
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
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
Extracted from Boldea & Nasar
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
Before semiconductor devices were introduced (<1950)• AC motors for fixed speed applications• DC motors for variable speed applications
After semiconductor devices were introduced (1950s)
• Variable frequency sources available – AC motors in variable speed applications
• Coupling between flux and torque control• Application limited to medium performance applications –
fans, blowers, compressors – scalar control
• High performance applications dominated by DC motors – tractions, elevators, servos, etc
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
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.
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
M
v
Fm
Ff
dtMvd
FF fm
Newton’s law
Linear motion, constant M
• First order differential equation for speed• Second order differential equation for displacement
Ma
dtxd
Mdtvd
MFF 2
2
fm
x
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
2m
le dtd
Jdt
dJTT
With constant J,
Rotational motion
- Normally is the case for electrical drives
dtJd
TT mle
Te , m
Tl
J
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
dtd
JTT mle
For constant J,
dt
dJ m
Torque dynamic – present during speed transient
dt
d m 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
-100
0
100
200
spee
d (r
ad/s
)
0.19 0.2 0.21 0.22 0.23 0.24 0.250
5
10
15
20
torq
ue (
Nm
)
Elementary principles of mechanics
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
dtvd
MFF le
Combination of rotational and translational motions
r r
Te,
Tl
Fl Fe
v
M
Te = r(Fe), Tl = r(Fl), v =r
dtd
MrTT 2le
r2M - Equivalent moment inertia of the linearly moving mass
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
mm1
m2
n1
n2
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
MotorTe
Load 1, Tl1
Load 2, Tl2
J1
J2
mm1
m2
n1
n2
MotorTe
Jequ
Equivalent Load , Tlequ
m2
221equ JaJJ
Tlequ = Tl1 + a2Tl2
a2 = n1/n2
Elementary principles of mechanics – effect of gearing
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
SPEED
TORQUE
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
T~ C
Coulomb friction
T~
Viscous friction
T~ 2
Friction due to turbulent flow
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Hoist drive
Speed
Torque
Gravitational torque
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
TlTe
Steady state speed
r
Torque
Speedr2r3r1
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
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
le TBdtd
JT
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
10 25 45 60
speed (rad/s)
100
10 25 45 60
Torque (Nm)
72.6771.67
-60.67
-61.67
56
t (ms)
t (ms)
Speed profile
torque profile
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 profile given above, what would be the speed profile?
J = 0.001 kg-m2, B = 0.1 Nm/rads-1 and Tload = 5 Nm.
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
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
p2p1 Thermal capacity, C (Ws/oC)
Surface A, (m2)Surface temperature, T (oC)Input heat power
(losses)
Emitted heat power(convection)
Ambient temperature, To
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Power balance:
21 ppdtdT
C
Heat transfer by convection:
)TT(Ap o2
Cp
TC
Adt
Td 1
Which gives:
/th e1A
pT
AC
, where
With T(0) = 0 and p1 = ph = constant ,
, where is the coefficient of heat transfer
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
t
T
t
/te)0(TT
T
/th e1A
pT
Heating transient
Cooling transient
Aph
)0(T
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
Ap n1
p1n
Losses due to continuous load
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
t1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
Ap s1
maxT Ap n1
t
T
p1
p1n
p1s
t1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
t
T
/ts1 e1A
pT
maxT Ap n1
/ts1n1 1e1A
pA
p /ts1n1
1e1pp1
/tn1
s1
te11
pp
1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
p1
t
heating coollingcoolling
coolling
heating
heating
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
Example of a simple case – p1 rectangular periodic pattern
pn = 100kW, nominal powerM = 800kg= 0.92, nominal efficiencyT= 50oC, steady state temperature rise due to pn
kW911
pp n1
Also, C/W180
509000
Tp
A o1
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
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque-speed quadrant of operation
T
12
3 4
T +ve +vePm +ve
T -ve +vePm -ve
T -ve -vePm +ve
T +ve -vePm -ve
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
4-quadrant 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 1Forward motoring
Quadrant 2Forward braking
Quadrant 3Reverse motoring
Quadrant 4Reverse braking
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Ratings of converters and motors
Torque
Speed
Power limit for continuous torque
Continuous torque limit
Maximum speed limit
Power limit for transient torque
Transient torque limit
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Steady-state stability