Power Electronics and Motor Drives
Dr. Ahmad Shukri Bin Abu Hasim
Universiti Pertahanan Nasional Malaysia (UPNM)
Past of electrical machines and variablespeed Drive
• First variable DC drives introduced by Ward Leonhard
• using mercury arch rectifier
• silicon-based devices
• Squirrel cage IM was introduced in 1889
• Slip-ring was introduced in 1890 with rotor resistance control
• Kramer drive was introduced in 1904
• Until early 1970, DC drives used in many applications
• based on frequency & voltage conversion
• Field-oriented control technique was introduced by German researchers
in 1985
• Others vector control were introduced in 1990 by Peter Vas
Present of electrical machines and variablespeed Drive
DC Drives
• Application of DC motor drives slowly reduces
• introduce improved DC drives
• reduced their cost
• Increase reliability
AC Drives
• PWM-VSI and IGBT widely used in AC drives in the power range to 200kW
• ease of application
• good power factor
• provides good dynamic performance
• also applied in brushless DC drives
• PWM-CSI using thyristors
• IM drives up to 3MW
• SM for higher-power range > 4MW and high speed > 5000 rpm
Present ……..continue
AC Drives
• Cycloconverter-fed AC drives
• suitable for driving low-speed & large-horse power AC motors
• e.g. shaft-winder, rolling mill drives
• Cycloconverter-fed synchronous machine
• e.g. ship propulsion drives, large compressors, large conveyor systems
• Static kramer drives where the torque is proportional to the square of the speed
• e.g. fans, pumps and blowers
• Vector control- allows to control separately the flux and torque producing
components of the supply currents
• Speed or position sensorless vector control by utilising the voltage (e.g. DC link)
and currents
• PM Brushless DC drives suitable for better dynamic performance because
the motor has very low inertia
Present ……..continue
AC Drives
• Vector control drives manufacturers
• Control Technique, ABB, Eurotherm Drives, Siemens, Mitsubishi, Hitachi, etc.
• Implementation of Vector control
• direct and indirect methods
• stator-flux oriented control
• Rotor-flux oriented control
• Magnetising-flux-oriented control
• First Direct Torque Control drives introduced by ABB
• DTC features
• direct control of flux and torque
• indirect control of currents and voltages
• reduced torque oscillation
• absence of coordinate transformation, voltage modulation block and voltage
decoupling circuits
Present ……..continue
AC Drives
• Vector control drives manufacturers
• Control Technique, ABB, Eurotherm Drives, Siemens, Mitsubishi, Hitachi, etc.
• Implementation of Vector control
• direct and indirect methods
• stator-flux oriented control
• Rotor-flux oriented control
• Magnetising-flux-oriented control
• First Direct Torque Control drives introduced by ABB
• DTC features
• direct control of flux and torque
• indirect control of currents and voltages
• reduced torque oscillation
• absence of coordinate transformation, voltage modulation block and voltage
decoupling circuits
Present ……..continue
Disadvantages of DTC
• during starting and low speed operation
• during changes in torque command
• requires flux and torque estimator
• Motor drives incorporated with artificial intelligence capabilities
• Hitachi introduced fuzzy-logic control
• Yaskawa introduced Neural-network based control
• Switched reluctance motor
• highly non-linear
• consist of a salient-pole stator and salient pole rotor
• does not require any rotor conductors or PM
• Attractive features:
• require only unidirectional stator winding currents
• results in an economical and robust controller
• high output power
• high starting torques combined with wide speed range
• fast dynamic response, rugged and robust construction
Future trends electrical machines and
variable-speed Drive
• Market share of drives will significantly increase
• Universal drives will incorporate vector and direct torque control
• DC motor – continued but slowly declining share, replace by improve design
• Cage induction motor dominant in long-term economy
• PMSM – increase role( cheaper PM required)
• Open loop drives – further drives required with improved performance
• Mechanical sensorless drives – increased role
• Intelligent control, non-linear observers, self-tuning controllers
Motor Drives
Introduction To Motor Drives
Basic function of electric motor:
“To drive mechanical loads by converting electrical energy”
POWER INPUT
SOURCE
Vi, ii
PROCESSOR + CONTROL
POWER
PROCESSOR
POWER OUTPUT
Vo, io
Motor
Actuator
Equipment
Computer
Utility line
CONTROLLER
(PI, PID, FL, NN)
measurement
Reference
LOAD
Introduction To Motor Drives
Power Source
High-level
Command
Motor
Host
Computer
Controller
Power
Electronic Converter
Command
signal
Feedback
Devices
Load
Introduction To Motor Drives
motor drive controls the speed, torque, direction and
resulting horsepower of a motor.
Traditional method:
•
Uncontrolled operation- motors running at constant speed
• i.e. when the compressor-motors in a refrigerator turns ON,
it runs at a constant speed
Adjustable method:
•
Controlled operation, motors can running at different speed
• To full fill industry requirements for different applications
»
Electric vehicle
Introduction To Motor Drives
History
Process industry (i.e. Oil refineries and chemical factories)
Controlled parameters are flow rates of gases and fluids
A pump operated at a constant speed
A throttling valve controls the flow rates
Constant flow rates
Flow rates
•
•
•
•
Motor speed (rad/s)
Introduction To Motor Drives
Trend: Adjustable speed drives (ASDs)
Process industry (i.e. Oil refineries and chemical factories)
Controlled parameters are flow rates of gases and fluids
A pump operated at variable speed
Different flow rates
Much easier to automate the process plant
Offer much higher energy efficiency
Low maintenance
Flow rates
•
•
•
•
•
•
Motor speed (rad/s)
Adjustable Frequency Drives Concept
The principle of speed control for adjustable frequency drives is based on the
following fundamental formula for AC motors
•
•
P is based on its design & manufacturer
The drives control the frequency applied to the motor
o
o
•
frequency is controlled by potentiometer (external signal-Analog) or
microprocessor (ADC-digital)
Automatically maintain the required Volts/cycle (V/f) ratio to the motor at any
speed
Ns (motor speed) is then proportional to this applied frequency
Direct Drive Motor
Standard Motor Drive System
Consist of 4 main elements
2
1
3
4
What is an Electric-Motor Drive
Controller
Hardware
Basic
• Discrete
• Op-Amp
High Performance
• DSP
• Microprocessor
• ASIC
Operation
Control Technique
• Position
• Speed
• Current/torque
• V/F
• Vector control
• Direct torque control
Control Algorithm
• PI
• PID
• PD
• Fuzzy Logic
• Neural Network
• Fuzzy-Neural
Processor-based Controller
What is an Electric-Motor Drive
Power Processing Unit
Hardware
Rectifier
Switch-mode Converter
DC-DC Converter
Uncontrolled
• Diode rectifier
bridge
Controlled
• Phase controlled
Thyristors (SCR)
• Step-Down (Buck) Converter
• Step-Up (Boost) Converter
• Buck-Boost Converter
• Cuk DC-DC converter
• Full Bridge Converter
DC-AC Inverter
• Voltage-Source
Inverter (VSI)
• Current-Source
Inverter (CSI)
• Load Commutated (LCI)
What is an Electric-Motor Drive
Voltage Source Inverter (VSI)
DC-AC
Voltage-Source
Inverter (VSI)
Square-Wave Inverter
(Voltage Controlled)
SW Implementation
Sinusoidal PWM Inverter
(Voltage Controlled)
SW Implementation
Space Vector
(Six-step modulation)
Current-Source
Inverter (CSI)
Sinusoidal PWM Inverter
(Current Controlled)
PWM Implementation
Hysteresis
Current Controlled
Ramp-comparison
Current Controlled
What is an Electric-Motor Drive
Electric Motors
DC Motors
Induction Asynchronous
Motors
Synchronous
Motors
Brushed
DC Motor
Brushed-less
DC Motor
Squirrel-Cage
IM
Reluctance
Motors
Square
Wave
PM Syn.
(sinusoidal)
Motor
Wound-field
Motor
Stepper
Motor
Switch_reluctance
Motor
Slip-ring
(Wound)
What is an Electric-Motor Drive
Feedback Devices
Speed
Position
Resolver
Tachogenerator
Encoder
Current
Current shunt
Current
Transformer
Voltage
Potentiometer
Differential amplifier
Absolute
Encoder
What is an Electric-Motor Drive
Tachogenerator: DC generator which gives voltage proportional to speed
Motor speed
•
Tacho. Voltage
•
Encoder: a slotted disk and light source giving a pulsetrain which has a
frequency proportional to speed e.g. 1000 pulses per revolution (PPR)
What is an Electric-Motor Drive
A wide
variety of measuring devices for collecting data from the manufacturing process for use in
feedback control.
Two types of measuring devices;
(1) Sensor – detects the physical variable (such as temperature, force, or pressure).
(2) Transducer – converts the physical variable into an alternative form (commonly electrical
voltage). Some cases, the sensor and transducer are the same device; e.g. a limit switch that
converts the mechanical movement of a lever to close an electrical contact.
Measuring devices can be classified into two basic categories;
(1) Analog – produces a continuous analog signal such as electrical voltage, e.g. thermocouple,
strain gages, and potentiometer. The signal must be convertered to digital data by an analog-todigital converter.
(2) Discrete – devided into two categories: binary and digital.

A binary measuring device produces on/OFF signal i.e. Limit switch, photoelectric sensors
and proximity switches

A digital measuring device produces a digital output signal
1. a set of parallel status bits (photoelectric sensor array)
2. A series of pulses that can be counted (optical encoder.
Types of motor drives
Motor
Drives
Direct Torque
Control (DTC)
Traditional
Motor Drive
High performance
Motor drives
Vector Control
Scalar control
Constant V/f
Principle
Separately-excited DC
motor
Losses in motor drives
Power losses and energy efficiency
Power losses and energy efficiency
Motor speed
Motor speed
Motor speed
Motor speed
Motor speed
Motor speed
Common Speed Reference Profiles
Time
Time
Time
Time
Time
Time
Common Speed responses
Overview- The Analog Drive
• Traditional Analogue Drive
 Velocity represented by analogue input
voltage range ±10V
 Zero volts represents the stationary condition
and intermediate voltage represent speeds
proportional to the voltage
• Various adjustment needed to tune an analogue
drive are usually made with potentiometers.
 Disadvantages;
Difficult when dealing with complicated
system
Repeating the adjustment on subsequent
units (time consume)
Overview- The Digital Drive
• Tuning can be performed by sending data from
terminal or computer.
 Advantages;
 Easy repetition between units since it
facilitates fully-automatic self tuning
 Able to convert analogue signal to digital
signal using ADC
• Generally, digital drive used more in conjunction
with brushless servo motors than DC brush
motor.
• The velocity feedback is derived either from an
encoder or resolver and again is processed as
digital information.
Digital Speed Control
• Sampling interval for digital speed control in the range of 50μs to 600 μ s
• Shorter sampling times, produce better the dynamic performance of the
drives
• The required speed is 0.01 rpm to 12,000 rpm (e.g. high speed cutting)
Analogue DC Drive Operation
• The function of the system is to control motor speed in response
to an analog input voltage.
• Motor velocity is measured by a tach generator attached to the motor
shaft. This produce a voltage proportional to speed that is compared
with the incoming velocity demand signal.
• The result of this comparison is torque demand.
• Speed is too low, drive deliver more current (torque to accelerate the
load)
• Speed is too high or velocity demand is reduce, current flow in the
motor will be reversed (braking torque).
•
This type of amplifier is often referred to as a four-quadrant drive. This
means that it can produce both acceleration and braking torque in either
direction of rotation.
• A torque demand from the velocity amplifier amounts to a request for
more current in the motor.
Analogue DC Drive Operation
• Control current is used to compares between the torque
demand with the current in the motor. This current is
measured by a sense resistor R, which produces a voltage
proportional to motor current.
•
The inner feedback loop referred as torque amplifier since
its purpose is to create torque in response to a demand
from the velocity amplifier.
Analogue drive system
Digital Drive Operation
• All the main control function are carried out by the microprocessor
• The feedback information is derived from the encoder attached to
the motor shaft. The encoder generates pulse stream from which
the processor can determine the distance travelled, thus by
calculating the pulse frequency it is possible to measure velocity.
• Digital drive perform the same operation as analogue counterpart,
but does so by solving a series equations. The microprocessor is
programmed with a mathematical model of the equivalent
analogue system. This model predicts the behaviour of the
system.
Digital drive system
Digital Drive Operation
•
To solve the equations
takes a finite amount of
time (typical between
100μs and 2ms)
•
During this time, torque
demand remain constant,
no response to change at
the input or output.
•
This “update time” is
critical factor which must
be kept to minimum
values for performance
of a digital servo and
high performance
system.

Tuning the digital drive is
performed either by push buttons
or sending numerical data from a
computer (no adjustment
involve).

Tuning data is used to set various
coefficients in drive algorithm
hence determines the system
behaviour.

In some application such as an
arm robot, the change of inertia
may well be 20 or more, therefore
it requires the drive to be retuned to maintain the stability
performance. This can be
achieved by sending the new
tuning values in the appropriate
point the operating cycle.
Brushless Motor Drives
• Previous figure show the layout of the drive for three-phase
trapezoidal motor. The switch set is based on H-bridge, but uses
three bridge leg instead of two.
• Motor winding are connected between the three bridge legs with
no connection to the star point at the junction windings.
• By turning two appropriate transistor, the current can be flow in
either direction through any two motor windings..
• The current path depends on rotor position and direction of
rotation, so the transistor are selected based on logic driven from
the commutation encoder. The required current feedback is
provided by sense resistors connected in series with two of the
motor leads.
• The voltage signal derived from these resistors must be decoded
and combine to provide a useful current reference, while the
communication encoder used to interpret the information
Brushless Motor Drives
Three-phase trapezoidal motor
Advantages of Permanent Magnet Motor
Drives over the Induction Motor Drives
1. The neodymium boron PM machine has a lower inertia when compared with
a rotor cage of IM. This make for a faster response for a given electric torque
or the torque to inertia ratio of these PM machines is higher.
2. Higher efficiency – because negligible rotor losses in permanent magnet.
The rotor losses in the IM is depending on the operating slip. The heat of the
IM is transferred to the machine tools and work pieces, thus affecting the
machining operation.
3. The IM requires a source of magnetizing current for excitation. The PM
machine already has the excitation in the form of the rotor magnet
4. The need for magnetizing current – requires a larger rated rectifier and
inverter for IM than for a PM of the same output capacity
5. The PM is smaller in size than an IM based on the same capacity. The power
density of PM is higher. ( space & weight)
Advantages of Induction Motor Drives over
the Permanent Magnet Motor Drives
1. Larger field weakening range and simple of control in that region
2. Lower cogging torque
3. Less expensive feedback transducers such as an incremental
rotor position encoder for IM instead of an absolute position
encoder or resolver that is required by the PM motor drives.
4. Lower cost
5. Much higher rotor operating temperatures that are allowed in IM
than PM
Similarities between the PMSM and BDCM
1. PMSM - replaces a field coil, brushes, and slip rings of the wound rotor
synchronous machines (WRSM) with permanent magnet
2. PMSM – generates a sinusoidal back emf just like an IM or WRSM
4. BDCM – similar to brush DC motor but the commutator and brush gear are
replaced by a permanent magnet.
• Commutator in the brush DC motor converts the input DC current into
approximately rectangular shaped currents of variable frequency
• For BDCM, the permanent magnet provides field excitation and the
rectangular-shaped current directly applied to the stator of the BDCM
5. The magnet in the PMSM or BDCM can be either buried or surface mounted
• An expoxy glue is used to fix the magnets to the rotor of the
Surface-mount PM motor
• Buried PM machines are more difficult to construct – more robust and
tend to be used for high-speed applications.
6. Rotor position feedback is needed in both drives to convert the stator
current reference into phase current references
Differences between the PMSM and BDCM
1. PMSM
• a sinusoidal back emf
• Winding arrangement of the stator and shaping of the magnet
• Sinusoidal stator currents are to produce a steady torque
EMF
Current
PMSM back emf and current waveforms
Differences between the PMSM and BDCM
2. BDCM
• a trapezoidal back emf
• trapezoidal stator currents are to produce a steady torque
EMF
Current
Emf and current waveforms of the BDCM
Selection criteria of the PMSM and BDCM
1. Cost
2. Power density
• Applications like robotics and aerospace actuators required as low a weight
as possible for a given output power.
• Power density is limited by heat dissipation capability of the machine
• For PM motor – most of the losses are developed in the stator in
terms of copper, eddy currents, and hysteresis losses.
Rotor losses are assumed negligible
Therefore, for a given frame size, the motor that develops lower losses
Will be capable of higher power density
• The BDCM is capable of supplying 15% more power than the PMSM from
the same frame size, that is, the power density can be 15% larger, provided
the core losses are equal.
3. Torque to inertia ratio
• It is also possible to obtain 15% more electric torque if they have the same
rated speed. If the rotor inertias are equal, then the torque-to-inertia ratio
of the BDCM can be as much 15% higher than PMSM.
Selection criteria of the PMSM and BDCM
4. Speed range
• The speed rage of a PMSM would be higher than that of a BDCM of the
same parameters
• Speed range of the PMSM depends on the motor parameters, its current
rating, the back emf waveform, and the maximum output voltage from the
inverter.
5. Torque Per Unit Current
• In servo applications, the drive should operated at maximum torque
per unit current (to minimised the required input current for a given torque).
as results the copper, inverter, and rectifier losses are also minimised.
• The total motor torque consists of electric and reluctsnce torque components
• The electric torque is produced as a results of the interaction of the
stator current with the airgap flux
• The reluctance torque is produced as a result of reluctance variation
due to rotor saliency
• The torque/(unit peak current) is higher in the BDCM by a factor of 1.33
Selection criteria of the PMSM and BDCM
4. The ripple torque of the BDCM is higher than that of the PMSM.
• The ripple torque in the PMSM is due only to the ripple in currents.
These ripple torques are of high frequency and are easily damped out
by the rotor
• The BDCM has a commutation ripple that depends of the speed of the machine
This makes the motor less suitable for high-performance position applications
• Buried PM motor are capable of higher torque per unit current than surface
Mount PM motor. It can increase up to 40% with proper design (due to
contribution of the reluctance torque)
5. Continuous rotor position feedback is needed by the PMSM for proper operation,
whereas the BDCM requires rotor position feedback information only every 60º.
6. Buried PM motors are more sensitive to parameters changes than surface-mounted
PMSM because of the absence of the reluctance torque in surface-mount motor
Selection criteria of the PMSM and BDCM
Characteristics
Overall size/power density
Acceleration
Shaft speed limits
Thermal overload
Service life
Wear
Robustness
Moment of inertia
Control complexity
DC
Drive
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Synchronous
AC Drive
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Asynchronous
AC Drive
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Conveyor System
Winding Machine
Engraving Machine
Winding Machine
Speed
reference
Hysteresis
Controller
Speed
Controller
_
_
Feedback
Devices
DC
Chopper
DC Motor