2. 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
2
3. 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
3
4. Conventional Electric Drives
Ward-Leonard system –
introduced in 1890s
Disadvantage :
Bulky
Expensive
Inefficient
Complex
4
6. Electric Drives Application
Line Shaft Drives
Oldest form
Single motor,
multiple loads
Common line
shaft or belt
Inflexible
Inefficient
Rarely used
6
7. Electric Drives Application
Single-Motor, Single-
Load Drives
Most common
Eg: electric saws, drills,
fans, washers, blenders,
disk-drives, electric cars.
7
8. Electric Drives Application
Multi-motor
Drives
Several motors,
single
mechanical
load
Complex drive
functions
Eg: assembly
lines, robotics,
military
airplane
actuation.
8
9. Basic Components of Electric Drives
Power Source
Motor
Power Processing Unit (Electronic Converter)
Control Unit
Mechanical Load
9
feedback
10. Basic Components of Electric 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.:
10
Electrical
energy
Mechanical
energy
Motor
• application
• cost
• efficiency
• environment
• type of source available
11. Basic Components of Electric 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
11
12. Basic Components of Electric 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
12
17. Basic Components of Electric 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
17
18. Basic Components of Electric 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
18
19. 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
19
20. 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
20
Motor Load
Te , m TL
Represent motor-
load system as
equivalent
rotational system
21. Torque Equation for Rotating Systems
21
• 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)
22. Torque Equation for Rotating 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
22
Motor
Te
Load 1,
TL0
Load 2,
TL1
J0
J1
m
m
m1
n
n1
TL
0 TL1
Motor
Te
J
Equivalent
Load , TL
m
TL
23. Torque Equation for Rotating Systems
with Gears
Gear ratio a1 =
Neglecting losses in the transmission:
Hence, equivalent motor-load inertia J is:
23
Kinetic energy due
to equivalent inertia
= kinetic energy of moving parts
1
2
1
0 J
a
J
J
(3)
(4)
24. Torque Equation for Rotating Systems
with Gears
If 1 = transmission efficiency of the gears:
Hence, equivalent load torque TL is:
24
Power of the equivalent
motor-load system
= power at the loads
1
1
1
0
L
L
L
T
a
T
T
(5)
25. Torque Equation for Rotating 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
25
L
m
D
D
a
1 (6)
26. Torque Equation for Rotating 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
26
Motor
Te
J
Equivalent
Load , TL
m
TL
27. Torque Equation for Rotating Systems
with Translational Motion
Neglecting losses in the transmission:
Hence, equivalent motor-load inertia J is:
27
Kinetic energy due
to equivalent inertia
= kinetic energy of moving parts
2
1
1
0
m
v
M
J
J
(7)
28. Torque Equation for Rotating Systems
with Translational Motion
If 1 = transmission efficiency of the transmission
system:
Hence, equivalent load torque TL is:
28
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)
29. 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:
29
1
1 rF
T m
m rF
T
r
v
dt
dv
M
F
Fm
1
dt
d
Mr
T
Tm
2
1
30. 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.
30
31. 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
31
k
m
L
T
m
L
T
P
m
m n
60
2
Angular speed
in rad/s
Speed
in rpm
32. Torque-Speed Characteristics of
Load
32
1) Torque independent of speed
2) Linear rising Torque-Speed
3) Non-Linear rising Torque-Speed
4) Non-Linear falling Torque-Speed
33. Mechanical Load Torque
Torque independent
of speed , k = 0
Hoist
Elevator
Pumping of water
or gas against
constant pressure
33
34. Mechanical Load Torque
Torque
proportional to
square of speed ,
k = 2
Fans
Centrifugal
pumps
Propellers
34
37. Classification of Electrical Drives
37
Group Drive(Shaft Drive)
“If Several groups of Mechanisms or Machines are organized on one
shaft & driven by one motor, the system is called a group drive (Shaft
Drive)”
Disadvantages
• There is no flexibility, Addition of an extra machine to the main shaft is
difficult.
• The efficiency of the drive is low, because of the losses occurring in
several transmitting mechanisms.
• The complete drive system requires shutdown if the motor, requires
servicing or repair.
• The system is not very safe to operate
• The noise level at the work spot is very high.
38. Classification of Electrical Drives
38
Individual Drive
“If a single motor is used to drive a given mechanism &
it does all the jobs connected with load, the drive is
called an individual drive”
Examples
• Single Spindle drilling machine
• Lathe machines
39. Classification of Electrical Drives
39
Multi-Motor Drive
“In a Multi-Motor drive, each operation of the mechanism is
taken care of by a separate drive motor. The system contains
several individual drives, each of which is used to operate its
own mechanism”
Examples
• Metal cutting machine tool
• Rolling mills
• Travelling cranes
40. Dynamic Conditions of a
drive system
40
• Dynamic conditions occur in a electric drive system
when operating point changes from one steady state
condition to another, following a change introduced in
the system variables. This variables may be mechanical
such as speed, torque etc. or electrical such as
voltage, current etc.
• These conditions generally exist during starting,
braking and speed reversal of the drive.
• The dynamic conditions arise in a variable speed drive
when transition from one speed to another is required.
41. Dynamic Conditions of a
drive system
41
• The drive may also have transient behavior if there are
sudden changes of load, supply, voltage or frequency.
• The dynamic behavior of a drive has a close relation
to its stability. A drive is said to be stable if it can go
from one state of equilibrium to another following a
disturbance in one of the parameters of the system.
• Stability can be identified as either steady-state or
transient.
42. Dynamic Conditions of a drive
system
42
• The condition of stability depend on the operating
point.
The dynamics of the drive can be investigated using
the Torque balanceequation given by
47. Dynamic Conditions of a drive system
47
The load torque occurring in mechanical system may be
Passiveoractive.
Passive torque
If the torque always opposes the direction of motion of
drive motor it is called a passive torque.
Active torque
Load torque which have the potential to drive the motor
under equilibrium condition are called active load
torque.
48. 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
48
Synchronous motor
Induction motor
Separately excited
/ shunt DC motor
Series DC motor
SPEED
TORQUE
49. 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
49
TL
Te
Steady state
Speed, r
Torque
Speed
r2
r3
r1
By using power electronic converters, the
motor characteristic can be varied
50. 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:
50
m
e
m
L
d
dT
d
dT
(9)
51. Steady State Stability
Assume a disturbance causes speed drop to r’
At the new speed r’,
51
Te’ > TL’
motor accelerates
operation restored to steady-state
point
Steady-state speed is of
stable equilibrium
Te TL
Steady-state point A
at speed = r
r
r’
Te’
TL’
m
T
m
e
m
L
d
dT
d
dT
dt
d
J
T
T m
L
e
Evaluated using steady-state T-
characteristic of motor and load.
52. Steady State Stability
Let’s look at a different condition!
Assume a disturbance causes speed drop to r’
At the new speed r’,
52
Te’ < TL’
motor decelerates
operation point moves away
from steady-state point
Point B is at UNSTABLE
equilibrium
Te
TL
Steady-state point B
at speed = r
r
r’
TL’
Te’
m
T
m
e
m
L
d
dT
d
dT
dt
d
J
T
T m
L
e
53. Torque-Speed Quadrant of Operation
53
•Direction of positive
(forward) speed is
arbitrary chosen
•Direction of positive
torque will produce
positive (forward)
speed
m
Te
Te
m
Te
m
Te
m
T
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
