ELECTRIC DRIVES

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ELECTRIC DRIVES

• INTRODUCTION TO ELECTRIC DRIVES
• MODULE 1
• Dr. Nik Rumzi Nik Idris
• Dept. of Energy Conversion, UTM
• 2013

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• 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
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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 1523 will be covering variable speed drives

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
Power In
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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 (PE, control system, machine
  design, sensors)
• Several research area
• Expanding

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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
• 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

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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
• 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

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• 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.

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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
(lt1950)

• 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

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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Overview of AC and DC drives
After semiconductor devices were introduced
(1950s)
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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
v
x
Fm
M
Ff
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
Rotational motion
- Normally is the case for electrical drives
J
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
For constant J,
Torque dynamic present during speed transient
Angular acceleration
Larger net torque and smaller J gives faster
acceleration
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics

• A step change in speed requires an infinite
  driving power
• Therefore ? is a continuous variable

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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
Integrating the equation with time and setting
the initial speed ?(0) 0, we obtain the
following
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• 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 !
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
Combination of rotational and translational
motions
Te r(Fe), Tl r(Fl), v r?
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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics effect of
gearing
Tlequ Tl1 a2Tl2
a2 n1/n2?2/?1
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque-speed quadrant of operation
1
2

• T -ve
• ve
• Pm -ve

• T ve
• 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

3
4

• T -ve
• -ve
• Pm ve

• T ve
• -ve
• Pm -ve

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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque-speed quadrant of operation

• 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 2 Forward braking
Quadrant 1 Forward motoring
Quadrant 3 Reverse motoring
Quadrant 4 Reverse braking
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Motor steady state torque-speed characteristic
(natural characteristic)
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
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

SPEED
TORQUE
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Load steady state torque-speed characteristic
Constant torque, e.g. gravitational torque
(active load)
TL rFL r g M sin ?
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Load steady state torque-speed characteristic
Hoist drive
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• 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
Torque
Speed
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque and speed profile
Speed profile
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) ?
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque and speed profile
0 lt t lt10 ms Te 0.01(0) 0.01(0) 5 Nm 5
Nm 10ms lt t lt25 ms Te 0.01(100/0.015)
0.01(-66.67 6666.67t) 5 (71
66.67t) Nm 25ms lt tlt 45ms Te 0.01(0)
0.01(100) 5 6 Nm 45ms lt t lt 60ms Te
0.01(-100/0.015) 0.01(400 -6666.67t) 5
-57.67 66.67t
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque and speed profile
speed (rad/s)
100
Speed profile
25
60
t (ms)
10
45
Torque (Nm)
72.67
torque profile
71.67
6
5
45
25
10
60
t (ms)
-60.67
-61.67
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• 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
45
25
10
60
t (ms)
-65
For the same system and with the motor torque
profile given above, what would be the speed
profile?
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Ratings of converters and motors
Torque
Continuous torque limit
Power limit for continuous torque
Maximum speed limit
Speed
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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 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
Thermal capacity, C (Ws/oC) Surface A,
(m2) Surface temperature, T (oC)
p2
Emitted heat power (convection)
Input heat power (losses)
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
Power balance
Heat transfer by convection
, where ? is the coefficient of heat transfer
Which gives
With ?T(0) 0 and p1 ph constant ,
, where
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
t
Cooling transient
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
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• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
Short time intermittent duty
p1
t
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
Short time intermittent duty
t
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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
t
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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
• 0.92, nominal efficiency
• ?T? 50oC, steady state temperature rise due to pn

Also,
If we assume motor is solid iron of specific heat
cFE0.48 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
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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,