Control of Electric Machine (EPM431)

“Electrical Drive Systems ”

Lecture No.1
28/9/2024
by:
Dr. Naema Mansour
 Course plan

Course plan

5th level Academic year (2023/2024) 1st semester

Course Title: Control of Electrical machine Course Code: EPM 431

Prerequisites Power electronics (EPM325) 4 CH

Number of weekly contact hours:

Lecture Tutorial Practical

3 2 -
 1. Course Contents
No. Topics Week
1 • Electric Drive: Concept, classification, parts and advantages of electrical dives.
1
• Types of Loads, Components of load toques, Fundamental torque equations
2 DC & AC motor drives:
Course plan • Comparison AC and DC drive
2
• Structure and modeling of DC machine.
Basic Characteristics of DC Motors.
3 DC motor drives:
• Speed control methods of DC motor. 3
• Modeling of DC motors, block diagram & Transfer function.
4 Quiz No. 1 3
4 DC motor drives:
• Operating Modes of DC motor. 4
• Single phase fully controlled and half controlled DC drives.
5 DC motor drives:
• Three phases fully controlled and half controlled DC drives. 5
• Dual converter control of DC drives.
6 DC motor drives:
• Chopper controlled DC motor drives 6

Mid-term
8 Induction motor drives:
• Stator voltage variation by three phase controllers. 8
• Speed control using chopper resistance in the rotor circuit.
9 Induction motor drives: -
• Slip power recovery scheme.
9
• Pulse width modulated inverter fed and current source inverter fed induction
motor drive.
10 Induction motor drives: -
• Volts/Hertz Control. 10
• Vector or Field oriented control.
11 Quiz No. 2 10
Synchronous motor drives:
11 • Variable frequency control. 11
• Voltage source inverter fed synchronous motor drive.
Synchronous Reluctance Motor drive.
12 12
Permanent-Magnet Motors drive.
13 Laboratory exam. 13
Drive system:

❖Drives can be defined as systems employed for motion control,

for example transportation, fans, pumps, robots…..

❖Prime movers are required in drive system to provide the

movement- can be diesel engine, petrol engines, hydraulic
motors, electric motors…

❖Drives that use electrical motors as a prime movers are known as

electric drives.
 Historical Review
❖In the past, horse-power replaces the hand power drive by using animals.

❖ These animals were replaced by mechanical drive powered by wind-mills, water wheels and
turbines, steam engine, internal combustion engine (ICE) and electrical machines (Electric
Drives).

Wind-Mills Water Wheels Steam Engine

 Drive system

❖It may have prime mover as diesel engine or petrol engine,

steam engines, gas or steam turbine, electric motors for
providing mechanical energy to have the motion control.

❖Many industries require electric motors whose speed can be

varied.
 Drive system are generally classified as:-

Internal combustion
engine (ICE)
Drive system

Mechanical drives
Steam Engine

Electrical drives Electric motor.

Electrical drives concerned with:

1) Starting operation.

2) Speed control operation.

3) Braking operation.
 system.
Drive
A system which has ability to provide a motion control.

❖Motion control is required in large number of industrial and domestic

applications like:- ❖Robotics required high precision for
1) Transportations. controlling the position. So, high performance
2) Rolling mills. position controlled drives are employed in such
3) Textile machines. systems
4) Paper machines.
5) Washing machines. ❖In many applications a reversal in direction
of rotation is also required in addition to
6) Fans. speed variation such as washing machine.
7) Pumps
8) Robots.
 Electric Drive

An electromechanical system that employs an

electric motor as the prime mover instead of a
diesel engine, steam or gas turbines, to control the
motion and processes of different machines and
mechanisms.
 A form of machine equipment designed to convert electrical
energy into mechanical energy and provide electrical
control of the processes.
Electric Drive

An electromechanical device designed to convert electrical

energy into mechanical energy to impart motion to different
machines and provides electrical control for various kinds of
processes by means of “controller”. The aim of the controller
is to adjust or stabilize the speed of the motor to suit a given
industrial task.
Applications of Electrical Drives
➢Electric drives are used in several industrial applications
such as: Machine Tool:-
1) Electrical drives are extensively used in the huge ❖ Any stationary power-
number of domestic (washing machine) as well as driven machine that is
industrial applications which includes motors, used to shape or form
transportation systems, factories ( textile mills, parts made of metal or
paper mill, cement mill, sugar mill, steel mill, other materials such as
machine tool, petrochemical industry). turning machines (lathes
2) Electric traction:- (electric traction means and boring mills), shapers
transportation of materials from one location to
another location). The different types of electric and planers, drilling
tractions mainly include electric trains, buses, machines, milling
trolleys, trams, and solar-powered vehicles inbuilt machines, grinding
with battery. machines, power saws,
3) Cranes and Hoist Motor. and presses.
4) Elevators (Lifts).
5) Robotic actuators.
Advantages of Electrical Drives
➢Electrical drives are widely used because they have:-
1) Flexible control characteristics particularly when power electronic converters
are employed.
2) Starting and braking is easy and simple.
3) Provides a wide range of torques over a wide range of speeds (both AC
and DC motor).
4) Availability of wide range of electric power.
5) Works to almost any type of environmental conditions.
6) No exhaust gases emitted (cleaner operation).
7) Capable of operating in all 4-quadrants of torque –speed plane.
8) Can be started and accelerated at very short time.
9) High efficiency.
10) Long life.
11) Low maintenance requirements, and low noise.
Choice of Electrical Drives:

The choice of an electrical drive depends on a number of factors. Some important factors
are:

1) Steady state operation requirements: (nature of speed-torque characteristics, speed

regulation, speed range, efficiency, duty cycle, quadrants of operation, speed
fluctuations, rating).
2) Transient operation requirement:- (values of acceleration and deceleration,
starting, braking, speed reversing).
3) Requirement of sources: (types of source, its capacity, magnitude of voltage, power
factor, harmonics).
4) Capital and running cost, maintenance needs, life periods. Duty cycle is the ratio of time a
load or circuit is ON compared to
5) Space and weight restrictions. the time the load or circuit is OFF.
Duty cycle, sometimes called "duty
6) Environment and location. factor," is expressed as a percentage
7) Reliability. of ON time.
 Block diagram of electrical drive system.
A modern electric drive system has five main functional blocks a mechanical
load, a motor, a power modulator, a power source and a controller.

The basic structural diagram of electrical drive system

Source
Power Provides the energy to the drive system. It may be DC or AC (single phase or three-
phase)
Converter

Interfaces the motor with the power source and provides the motor with adjustable
voltage, current and/or frequency.
Motor Mechanical Controller

Supervises the operation of the whole system to ensure stability and enhance the
overall performance. Its basic function is comparing the system variables with
desire values, and then adjust the converter output until the system achieves the
desired performance
load:

Depends on the customer needs and the industrial process.

Selected according to the power level, environmental factors and performance

required by the load. Ex: if load requires high starting torque so DC series motor is
better than induction motor.
1) Electrical Sources (Input Power)
❖Power Source:-
❖The power source offers the necessary energy for the system. Both
the converter and the motor interfaces by the power source to provide
changeable voltage, frequency and current to the motor.
1) Some drives are powered from batteries or DC motors.
2) Very low power drives are generally fed from single-phase
sources.
3) Rest of the drives is powered from a three-phase source.
❖ Low and medium power motors are fed from a 400V supply.
❖ For higher ratings, motors may be rated at 3.3 kV, 6.6 kV and
11 kV.
❖The power controlling of the motor can be described as:-
❖The way by which the electrical motor will be forced to
track the speed-torque characteristics that are suitable to
the load application.

❖This task can be achieved with:-

✓Power modulator.
✓Controller.
2) Power Modulators
❖This modulator can be used to regulate the output power of the supply.
➢The functions of the power modulator are:-
1) Modulates the flow of power from the source to the motor in such a manner that motor is imparted
speed-torque characteristics required by the load.
2) It converts electrical energy of the source in the form suitable to the motor (if the source is DC and an
induction motor is used then the power modulator convert the DC into AC).
3) Selects the mode of operation of the motor, i.e. Motoring or Braking.
4) During transients like starting, braking and speed reversal, it keeps the motor current within
the acceptable limits. During the transient operations like starting, breaking and speed reversing, the
excessive current drawn by the source. This excessive current drawn from the source may overload it
or may cause a voltage drop. Hence power modulator restrict the source and motor current.
➢Types of Power Modulators
❖In the electrical drive system, the power modulators can be any one of the following:
1) Controlled rectifiers (AC-to-DC converters) provide a variable DC output voltage from a fixed AC
voltage.
2) Inverter (DC-to-AC converters).
3) AC voltage controllers (AC-to-AC-converter) converts a fixed voltage, fixed frequency AC electrical
input supply to obtain variable voltage in output delivered.
4) DC chopper (DC-to-DC-converter) provide a variable DC voltage from a fixed DC voltage..
5) Frequency changers (PWM inverter). These inverters are able to produce ac voltages of variable
magnitude and frequency.
3) Controller (control unit)
❖Control unit takes two inputs:-
1) Reference signal.
2) Feedback signal.
❖It does a certain operation on these signals (for example, a comparison). Based
on the error signal generated by the comparison process, the control unit initiates
the command to the power modulator.

❖Basically, power modulator is a converter consisting controlled semiconductor devices.

❖Control unit produces the firing pulses as a command to achieve the required speed –
torque characteristic of the load.
❖Control unit consisting firing circuits, it may be implemented using linear & digital
integrated circuits, transistors and microprocessor or DSP when sophisticated control is
required.
❖The complexity of the control unit depends on the desired drive performance and the
type of motors used
3) Mechanical Load:
❖The mechanical load usually defined as a machine such as pump, fan, robot, machine tool, trains and
drills that are coupled with motor shaft.
❖It is very important that you should consider the type of load that is being put on each electric motor, as
this can affect the opportunity for energy saving.
Torque-Speed Characteristics of Mechanical Load:
Fundamental Torque Equations
❖Mechanical load exhibit wide variations of speed-torque characteristics. Load torques are generally
speed dependent and can be represented by an empirical formula such as:-
𝒌
𝒏 𝒏
𝑻 = 𝑪𝑻𝒓 𝑷 = 𝑻𝝎 𝝎 = 𝟐𝝅
𝒏𝒓 𝟔𝟎
Where 𝐶 is a proportionally constant.
𝑻𝒓 is the load torque at the rated speed 𝒏𝒓 . 𝒌 = 𝟎,
𝑪=𝟏
𝒏 is the operating speed.
𝒌 is an exponential coefficient representing the torque dependency speed.
𝑷 is the mechanical power. Variable-
𝝎 is the angular speed. Constant
torque
𝒌 = 𝟐,
power
Figure shows the typical mechanical loads. 𝒌 = −𝟏,
Most common motor load types
❖Certain load types are characteristic in the industrial world. Knowing the load
profile (speed range, torque and power) is essential when selecting a suitable
motor and frequency converter for the application.

❖The common load types are three types or may also a combination of theses
types:

1) Constant torque load.

2) Quadratic torque load (variable torque load).
3) Constant power load.
 Typical torque speed
𝒌 = 𝟎, characteristics of 𝒌 = 𝟎,
𝑪=𝟏 𝑪=𝟏
mechanical loads
𝒌
𝒏
𝑻 = 𝑪𝑻𝒓
𝒏𝒓 Variable-
Variable- torque
torque Typical Power speed 𝒌 = 𝟐,
Constant 𝒌 = 𝟐,
power characteristics of Constant
𝒌 = −𝟏, power
mechanical loads. 𝒌 = −𝟏,

According to the torque speed characteristics, most of the industrial loads can be classified into the following
categories:
1) Constant torque type load:- such as conveyors, screw, reciprocating compressors, and crushers. With these
loads, the amount of power consumed is in direct proportion to the useful work done. So halving the speed will
halve the energy consumed. The characteristics of this type of mechanical load are represented by setting 𝑘 is
equal to 𝑧𝑒𝑟𝑜 and 𝐶 equals to 𝟏.While torque is independent of speed.
2) Variable-torque loads (Torque proportional to square of the speed ). The torque-speed characteristic is
parabolic, 𝒌 = 𝟐 such as washing machine, fans and pumps. With this type of load, reducing the speed of the
motor by even a small amount can save a lot of energy.
3) Constant-power loads (Torque inversely proportional to speed ) such as a milling, boring machines, road
vehicle and traction. With these loads, the power is constant so there will rarely be energy savings when the
speed is reduced.
1) Constant torque type load.
➢ A constant torque load implies that the torque required to keep the load running
is the same at all speeds.
➢ Torque is constant and power is linearly proportional to the speed. With
constant torque loads, the torque is not a function of speed. As speed is changed,
the load torque will remain fairly constant and the horsepower will change
linearly with the speed.
➢ Constant torque loads require the same amount of torque at low speeds as at
high speeds. Torque remains constant throughout the speed range, and the
horsepower increases and decreases in direct proportion to the speed.
➢ For example, if the speed increases by 50%, then the power required to drive
the operation will increase 50% while the torque remains constant.
➢ Typical constant torque applications controlled by variable frequency drives
include:-
❖Conveyors- Agitators- Crushers- Surface winders- Displacement pumps -
Air compressors.

A good example is
a drum-type hoist,
where the torque
required varies
with the load on
the hook, but not
with the speed of
Surface winders Agitators Conveyors A drum-type hoist, hoisting.
2) Variable torque load.
❖Torque proportional to square of the speed 𝑻 ∝ 𝝎𝟐 .
❖Quadratic torque 𝑻 ∝ 𝝎𝟐 is the most common load type.
❖Typical applications are centrifugal pumps and fans.
❖The torque is quadratically, and the power 𝑷 ∝ 𝝎𝟑 is cubically
proportional to the speed.
𝑷 = 𝑻𝝎
➢Have the largest energy saving potential controlled by variable ❖ Variable torque loads require
frequency drives. The power – speed relationship is also referred much lower torque at low speeds
to as the 'Cube Law’. than at high speeds. Variable
➢When controlling the flow by reducing the speed of the fan or torque loads include most
pump, a relatively small speed change will result in a large centrifugal and axial pumps, fans
reduction in power absorbed, as fans and pumps where the and blowers and many mixers.
speed varies, reducing the speed of the motor by even a small
amount can save a lot of energy.
❖As an example, when the speed of a variable torque load is
reduced by 𝟓𝟎% or one half, the torque required to drive the load
is reduced to one-quarter or 𝟐𝟓%. The horsepower is reduced to
the speed cubed, which is 𝟏/𝟖, 𝒐𝒓 𝟏𝟐. 𝟓% of that required to
drive the load at full speed.
1) Centrifugal fans:-
❖A centrifugal fan or centrifugal blower is a motor that moves air. It pulls the
air inside the blower and then pushes it out at a 90º angle. A centrifugal fan is
typically used for residential applications to move air through a duct in your
home [air conditioning system, furnace, or range hood duct].
2) Centrifugal pumps.
❖A centrifugal pump is a mechanical device designed to move a fluid by
means of the transfer of rotational energy from one or more driven rotors,
called impellers. Fluid enters the rapidly rotating impeller along its axis and
is cast out by centrifugal force along its circumference through the impeller’s
vane tips. The action of the impeller increases the fluid’s velocity and
pressure and also directs it towards the pump outlet. Centrifugal pumps are
commonly used for pumping water, solvents, organics, oils, acids, bases and
any ‘thin’ liquids in both industrial, agricultural and domestic applications.
3) Constant power load.
❖A constant power load is normal when material 𝒌 = 𝟎,
𝑪=𝟏
is being rolled and the diameter changes during
rolling. The power is constant and the torque is
inversely proportional to the speed. Variable-

❖Constant horsepower loads require high torque Constant

torque
𝒌 = 𝟐,
power
at low speeds and low torque at high speeds, 𝒌 = −𝟏,

which means constant horsepower at any speed.

❖Constant horsepower loads include grinders,
winding machines and lathes.
𝒌 = 𝟎,
❖ For constant horsepower loads, the torque 𝑪=𝟏

loading is a function of speed up to 100%

operating speed. As the speed of the operation is
decreased, the torque increases so that the Variable-
torque
horsepower required remains essentially 𝒌 = 𝟐,
constant. Constant
power
𝒌 = −𝟏,
5) Electrical Motors
➢To drive the mechanical load electrical motors are required. Basically there are
two types of motors: A.C. motors and D.C. motors.
➢Most commonly used electrical motor for speed control applications are the
following:
1. DC Machines
a) Shunt motors.
b) Series motors.
c) Compound motors
2. AC Machines
a) Induction motors (wound rotor, squirrel-cage )
b) Synchronous motors (wound field & permanent magnet).
3. Special Machines
a) Brush less DC motors.
b) Stepper motors.
c) Switched reluctance motors.
Classification of the electric machine according to power
source and operating principles.
 The basic criterion in selecting an electric motor for a given drive
application is:

❖ It meets power level and performance required by the load during

steady state and dynamic operation.

❖Environmental factors: In industry such as in food processing,

chemical industries and aviation where the environment must be clean
and free from arc. Induction motors are used instead of DC motor
6) Sensing Unit:-
➢ In order to provide command to the power modulator to control the flow of power
from source to load, sensing unit provides input to the control unit.

1) Speed Sensing (From Motor) Speed can be sensed by using a tachometer.

Wind speed can be sensed by anemometer similarly both speed and velocity
can be measured by the speed meter.
2) Torque Sensing.
3) Position Sensing.
4) Current sensing and voltage sensing from lines or from motor terminals or
from Load.
5) Temperature Sensing. Thermistor is a device which is used for temperature
measurement.
Classification of Electrical Drives
➢Electrical Drives are classified into two types based on supply:-
1) AC drives
2) DC drives.
➢Electrical Drives are classified into two types based on running speed:-
1) Constant speed drives
2) Changeable speed drives.
➢Electrical Drives are classified into two types based on a number of motors:-
1) Single motor drives (A single motor is dedicated to a single load.
Applications include electric saw, drill, disk drive, fan, washer, dryer,
blender etc).
2) Multi-motor drives (Each operation of the mechanism is taken care of
by a separate drive motor, for example, robot)
➢Electrical Drives are classified into two types based on control parameter:-
1) Stable torque drives
2) Stable power drives.