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ELECTRIC DRIVES
INTRODUCTION TO ELECTRIC DRIVES
MODULE 1
Dr. Nik Rumzi Nik Idris
Dept. of Energy Conversion, UTM
2013
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 1523 will be covering variable speed drives
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
valve
Supply
Power
In
motor
pump
Power out
Power loss
Mainly in valve
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
valve
Supply
Power
In
motor
Supply
pump
PEC
Power out
Power loss
Mainly in valve
Power
In
motor
pump
Power out
Power loss
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
valve
Supply
Power
In
motor
Supply
pump
PEC
Power out
Power loss
Mainly in valve
Power
In
motor
pump
Power out
Power loss
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
Machine design
Speed sensorless
Machine Theory
Utility interface
Renewable energy
Non-linear control
Real-time control
DSP application
PFC
Speed sensorless
Power electronic converters
•
Inter-disciplinary
•
Several research area
•
Expanding
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
• Natural speed-torque characteristic is not compatible with load
requirements
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
• Electrical isolation between control circuit and power circuit is
needed:
• Malfuction in power circuit may damage control circuit
• Safety for the operator
• Avoid conduction of harmonic to control circuit
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
Sensors
• Sensors (voltage, current, speed or torque) is normally
required for closed-loop operation or protection
• Electrical isolation between sensors and control circuit is
needed for the reasons previously explained
• The term ‘sensorless drives’ is normally referred to the
drive system where the speed is estimated rather than
measured.
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 semiconductor devices were introduced (1950s)
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
IEEE Transactions on Industrial Electronics, 2004.
(Buja, Kamierkowski, “Direct torque control of PWM inverter-fed AC motors - a survey”,
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
v
x
Newton’s law
M
Fm  Ff 
Fm
Ff
dMv 
dt
Linear motion, constant M
dv 
d2 x
Fm  Ff  M
 M 2  Ma
dt
dt
•
•
First order differential equation for speed
Second order differential equation for displacement
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics

Rotational motion
- Normally is the case for electrical drives
Tl
Te  Tl 
Te , m
dJm 
dt
J
With constant J,
dm 
d 2
Te  Tl  J
J 2
dt
dt
•
•
First order differential equation for angular frequency (or velocity)
Second order differential equation for angle (or position)
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
For constant J,
dm 
dt
dm 
dt
dm
dt
Torque dynamic – present during speed transient
Angular acceleration
Larger net torque and smaller J gives faster acceleration
speed (rad/s)
200
100
0
-100
-200
0.19
0.2
0.21
0.22
0.23
0.24
0.25
0.2
0.21
0.22
0.23
0.24
0.25
20
torque (Nm)
J
Te  Tl  J
15
10
5
0
0.19
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
A drive system that require fast acceleration must have
• large motor torque capability
• small overall moment of inertia
As the motor speed increases, the kinetic energy also increases.
During deceleration, the dynamic torque changes its sign and thus
helps motor to maintain the speed. This energy is extracted from the
stored kinetic energy:
J is purposely increased to do this job !
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
Combination of rotational and translational motions
Fe
Fl

M
r
Te, 
r
Tl
v
Fe  Fl  M
dv 
dt
Te  Tl  r 2M
Te = r(Fe),
d
dt
r2M - Equivalent moment inertia of the
linearly moving mass
Tl = r(Fl),
v =r
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
Motor
Te
m1
m
n1
Load 1,
Tl1
J2
m2
J1
n2
Load 2,
Tl2
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics – effect of gearing
m
Motor
Te
m1
Load 1,
Tl1
n1
m2
J1
Motor
Te
m
n2
J2
Load 2,
Tl2
J equ  J1  a 22 J 2
Equivalent
Load , Tlequ
Tlequ = Tl1 + a2Tl2
Jequ
a2 = n1/n2=2/1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Motor steady state torque-speed characteristic (natural
characteristic)
SPEED
Synchronous mch
Induction mch
Separately / shunt DC mch
Series DC
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
Frictional torque (passive load)
SPEED
T~ C
T~ 2
T~ 
• Exist in all motor-load drive
system simultaneously
• In most cases, only one or two
are dominating
• Exists when there is motion
TORQUE
Coulomb friction
Viscous friction
Friction due to turbulent flow
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Constant torque, e.g. gravitational torque (active load)
SPEED
Gravitational torque
Vehicle drive
Te
TORQUE
TL

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
Te
Torque
Tl
Steady state
speed
r3
r1r
r2
Speed
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
speed
(rad/s)
Speed profile
100
10
25
The system is described by:
J = 0.01 kg-m2,
45
60
t (ms)
Te – Tload = J(d/dt) + B
B = 0.01 Nm/rads-1 and
Tload = 5 Nm.
What is the torque profile (torque needed to be produced) ?
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
speed
(rad/s)
d
Te  J  B  Tl
dt
100
10
25
45
60
t (ms)
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
speed
(rad/s)
100
Speed profile
10
25
45
60
t (ms)
Torque
(Nm)
72.67
71.67
torque profile
6
5
10
-60.67
-61.67
25
45
60
t (ms)
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
Torque
(Nm)
70
J = 0.001 kg-m2, B = 0.1 Nm/rads-1
and Tload = 5 Nm.
6
10
25
45
60
t (ms)
-65
For the same system and with the motor torque profile
given above, what would be the speed profile?
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
Ambient temperature, To
p1
Input heat power
(losses)
Thermal capacity, C (Ws/oC)
Surface A, (m2)
Surface temperature, T (oC)
p2
Emitted heat power
(convection)
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Power balance:
C
dT
 p1  p 2
dt
Heat transfer by convection:
, where  is the coefficient of heat transfer
p 2  A(T  To )
Which gives:
dT A
p

T  1
dt
C
C
With T(0) = 0 and p1 = ph = constant ,
T 

ph
1  e t / 
A

, where  
C
A
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
ph
A
T
T 

ph
1  e t / 
A

Heating transient
T

t
T  T(0)  e  t / 
 T ( 0)
Cooling transient

t
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
The duration of overloading depends on the modes of operation:
Continuous duty
Load torque is constant
over extended
Continuous
duty period multiple
Short time intermittent duty
Steady state temperature reached
Periodic intermittent duty
Nominal output power chosen equals or exceeds continuous load
p1n
A
T
Losses due to continuous load
p1n

t
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
p1s
p1
p1n
p 1s
A
T
p1n
A
Tmax
t1

t
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations

T
T 
p1n
A
Tmax
t1


pp11nn p1ps 1s1 1eet1 / t1 / 
A A
Short time intermittent duty

p1s
1  e t / 
A

p1s
1



 t1 / 
p1n 1  e
t1
t
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
heating
coolling
heating coolling
heating coolling
t
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
= 0.92, nominal efficiency
T= 50oC, steady state temperature rise due to pn
1 
p1  pn   1  9kW
 
Also,
A 
p1
9000

 180 W / o C
T
50
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,
35
30
25
20
15
10
5
0
0
0.5
1
1.5
2
2.5
4
x 10
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque-speed quadrant of operation

T -ve
 +ve
Pm -ve
2
1
T +ve
 +ve
Pm +ve
T
3
T -ve
 -ve
Pm +ve
4
T +ve
 -ve
Pm -ve
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
4-quadrant operation

m
Te
m
• Direction of positive torque will
produce positive (forward) speed
Quadrant 2
Forward braking
Quadrant 1
Forward motoring
Quadrant 3
Reverse motoring
Quadrant 4
Reverse braking
Te
m
• Direction of positive (forward)
speed is arbitrary chosen
Te
T
Te
m
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Ratings of converters and motors
Torque
Transient
torque limit
Continuous
torque limit
Power limit for
transient torque
Power limit for
continuous torque
Maximum
speed limit
Speed
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Steady-state stability