Electric drives

ELECTRIC DRIVES
INTRODUCTION TO ELECTRIC DRIVES
MODULE 1
S. Samsudeen
Electrical Energy Conversion

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

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

Example on VSD application
motor pump
valve
Supply
Constant speed Variable Speed Drives
Power
In
Power loss
Mainly in valve
Power out

motor pump
valve
Supply
motorPEC pump
Supply
Power
In
Power loss
Power out
Power loss
Mainly in valve
Power outPower
In

Power loss
Mainly in valve
Power out
motor pump
valve
Supply
motorPEC pump
Supply
Power
In
Power loss
Power
In
Power out

Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible

Modern electric drives (With power electronic converters)
• Small
• Efficient
• Flexible

Modern electric drives
• Inter-disciplinary (PE, control system, machine
design, sensors)
• Several research area
• Expanding
Machine design
Speed sensorless
Machine Theory
Non-linear control
Real-time control
DSP application
PFC
Speed sensorless
Power electronic converters
Utility interface
Renewable energy

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

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

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.

Overview of AC and DC drives
Extracted from Boldea & Nasar

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

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

After semiconductor devices were introduced (1950s)

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

Classification of IM drives (Buja, Kamierkowski, “Direct torque control of PWM inverter-fed AC motors - a survey”,
IEEE Transactions on Industrial Electronics, 2004.

Elementary principles of mechanics
M
v
Fm
Ff
( )
dt
Mvd
FF fm =−
Newton’s law
Linear motion, constant M
• First order differential equation for speed
• Second order differential equation for displacement
( ) Ma
dt
xd
M
dt
vd
MFF 2
2
fm ===−
x

• First order differential equation for angular frequency (or velocity)
• Second order differential equation for angle (or position)
( )
2
2
m
le
dt
d
J
dt
d
JTT
θ
=
ω
=−
With constant J,
Rotational motion
- Normally is the case for electrical drives
( )
dt
Jd
TT m
le
ω
=−
θ
Te , ωm
Tl
J

dt
d
JTT m
le
ω
+=
For constant J,
( )
dt
d
J mω
Torque dynamic – present during speed transient
( )
dt
d mω Angular acceleration
Larger net torque and smaller J gives faster acceleration
0.19 0.2 0.21 0.22 0.23 0.24 0.25
-200
-100
0
100
200
speed(rad/s)
0.19 0.2 0.21 0.22 0.23 0.24 0.25
0
5
10
15
20
torque(Nm)

dt
d
JTT m
le
ω
+=
dt
d
JTT m
mlmem
ω
ω+ω=ω
dt
d
Jpp m
mLD
ω
ω+=⇒
Driving
power
Load
power
Change
in KE
• A step change in speed requires an infinite driving power
• Therefore ω is a continuous variable

dt
d
JTT m
le
ω
+=
dt
d
JTT m
mlmem
ω
ω+ω=ω
dt
d
Jpp m
mLD
ω
ω+=⇒
Integrating the equation with time and setting the initial speed ω(0) =
0, we obtain the following:
wD = pD dτ
0
t
∫ = pL dτ
0
t
∫ + ωm J
dωm
dτ0
t
∫ dτ
wD =wL +J ωm
0
ω
∫ dωm
wD = wL +
1
2
Jωm
2

A drive system that require fast acceleration must have
• small overall moment of inertia
• large motor torque capability
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 !

( )
dt
vd
MFF le =−
Combination of rotational and translational motions
r r
ω
Te, ω
Tl
Fl Fe
v
M
Te = r(Fe), Tl = r(Fl), v =rω
dt
d
MrTT 2
le
ω
=−
r2
M - Equivalent moment inertia of the
linearly moving mass

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
Load 1,
Tl1
Load 2,
Tl2
J1
J2
ωm
ωm1
ωm2
n1
n2

Motor
Te
Load 1,
Tl1
Load 2,
Tl2
J1
J2
ωm
ωm1
ωm2
n1
n2
Motor
Te
Jequ
Equivalent
Load , Tlequ
ωm
2
2
21equ JaJJ +=
Tlequ = Tl1 + a2Tl2
a2 = n1/n2=ω2/ω1
Elementary principles of mechanics – effect of gearing

Torque-speed quadrant of operation
ω
T
12
3 4
T +ve
ω +ve
Pm +ve
T -ve
ω +ve
Pm -ve
T -ve
ω -ve
Pm +ve
T +ve
ω -ve
Pm -ve
• Quadrant of operation is
defined by the speed and
torque of the motor
• Most rotating electrical
machines can operate in 4
quadrants
• Not all converters can
operate in 4 quadrants

Torque-speed quadrant of operation
ω
T
Te
ωm
Te
Te
Te
ωm
ωm
ωm
• Quadrant of operation is
defined by the speed and
torque of the motor
• Most rotating electrical
machines can operate in 4
quadrants
• Not all converters can
operate in 4 quadrants
Quadrant 1
Forward motoring
Quadrant 2
Forward braking
Quadrant 3
Reverse motoring
Quadrant 4
Reverse braking

Motor steady state torque-speed characteristic (natural
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

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~ ω
Viscous friction
T~ ω2
Friction due to turbulent flow

α
TL
Te
Vehicle drive
Constant torque, e.g. gravitational torque (active load)
SPEED
TORQUE
Gravitational torque
gM
FL
TL = rFL = r g M sin α

Hoist drive
Speed
Torque
Gravitational torque

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
ωr
Torque
Speedωr2ωr3
ωr1

Torque and speed profile
10 25 45 60 t (ms)
speed
(rad/s)
100
The system is described by: Te – Tload = J(dω/dt) + Bω
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

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 TB
dt
d
JT +ω+
ω
=

10 25 45 60
speed
(rad/s)
100
10 25 45 60
Torque
(Nm)
72.67
71.67
-60.67
-61.67
5
6
t (ms)
t (ms)
Speed profile
torque 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.

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

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 (i2
R)
- Core losses – hysteresis and eddy current
- Friction losses – bearings, brush windage

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
p2
p1
Thermal capacity, C (Ws/o
C)
Surface A, (m2
)
Surface temperature, T (o
C)Input heat power
(losses)
Emitted heat power
(convection)
Ambient temperature, To

Power balance:
21 pp
dt
dT
C −=
Heat transfer by convection:
)TT(Ap o2 −α=
C
p
T
C
A
dt
Td 1
=∆
α
+
∆
Which gives:
( )τ−
−
α
=∆ /th
e1
A
p
T
A
C
α
=τ, where
With ∆T(0) = 0 and p1 = ph = constant ,
, where α is the coefficient of heat transfer

tτ
T∆
t
τ
τ−
⋅∆=∆ /t
e)0(TT
T∆
( )τ−
−
α
=∆ /th
e1
A
p
T
Heating transient
Cooling transient
A
ph
α
)0(T∆

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 n1
α
τ
p1n
Losses due to continuous load

Operation considerably less than time constant, τ
Motor allowed to cool before next cycle
Motor can be overloaded until maximum temperature reached

t1
τ
A
p s1
α
maxT∆
A
p n1
α
t
T∆
p1
p1n
p1s

t1
τ t
T∆
( )τ−
−
α
=∆ /ts1
e1
A
p
T
maxT∆
A
p n1
α
( )τ−
−
α
=
α
/ts1n1 1
e1
A
p
A
p ( )τ−
−≥ /t
s1n1
1
e1pp
1
/t
n1
s1
te1
1
p
p
1
τ
≈
−
≤ τ−

Load cycles are repeated periodically
Motors are not allowed to completely cooled
Fluctuations in temperature until steady state temperature is reached

p1
t
heating coolling
coolling
coolling
heating
heating

Example of a simple case – p1 rectangular periodic pattern
pn = 100kW, nominal power
M = 800kg
η= 0.92, nominal efficiency
∆T∞= 50o
C, steady state temperature rise due to pn
kW91
1
pp n1 =





−
η
= Also, C/W180
50
9000
T
p
A o1
==
∆
=α
∞
If we assume motor is solid iron of specific heat cFE=0.48 kWs/kgo
C,
thermal capacity C is given by
C = cFE M = 0.48 (800) = 384 kWs/o
C
Finally τ, thermal time constant = 384000/180 = 35 minutes

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 10
4
0
5
10
15
20
25
30
35

Electric drives

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Electric drives

Similar to Electric drives (20)

Recently uploaded

Recently uploaded (20)

Electric drives