Lecture Noted - Electrical Technology

ELECTRICAL TECHNOLOGY

2.10 POWER FACTOR

2.11 POWER IN AC CIRCUIT

45

ELECTRICAL TECHNOLOGY

2.12 THREE PHASE SYSTEM

• In electrical engineering, three-phase electric power systems have at least three
conductors carrying alternating current voltages that are offset in time by one-third
of the period.

• A three-phase system may be arranged in delta (∆) or star (Y) (also denoted as wye
in some areas).

• There are two types of system available in electric circuit, single phase and three
phase system.

• In single phase circuit, there will be only one phase, i.e. the current will flow through
only one wire and there will be one return path called neutral line to complete the
circuit.

• So in single phase minimum amount of power can be transported. The generating
station and load station will also be single phase. This is an old system using from
previous time .

The three line in three phase R,
Y, B corresponding to Red, Yellow
and Blue, are equal inmagnitude
and differ in phase angle by 120°.

46

ELECTRICAL TECHNOLOGY

2.12.1 Differences between three phase system and single phase system

47

ELECTRICAL TECHNOLOGY

TUTORIAL CHAPTER 2

1. Calculate the total inductance (LT) where there are connected in series:

i . 0.02H, 44mH and 400pH
ii. 0.05H, 30mH and 750pH
2. Calculate the total inductance (LT) where there are connected in parallel:

i . 0.08H, 400mH and 400pH
ii. 15mH, 50mH

3. Based on figure below, calculate the total inductance (LT)

4. A coil with value of 0.2H is connected with AC circuit 200V, 50Hz. Calculate the
Inductive reactance (XL)

5. A coil with 6H connected to AC 12V 50 Hz. Calculate the current flow.
. A capacitor with 50µF connected to AC 115 V 60 Hz. Calculate capacitance reactance

and current flow.

7. A capacitor with 120µF connected with 500 V 50 Hz. Calculate capacitance reactance
and current flow.

8. A capacitor 1000µF connected to AC 20 V 50 Hz.
i. Calculate current flow
ii. What is the effect of the current if the frequency change to 1000 Hz.

48

ELECTRICAL TECHNOLOGY

9. Capacitor with 50pF connected with 240V power supply. Calculate the charge
and energy stored in the capacitor.

10. Capacitor with 8pF connected with 240V, 50Hz power supply. Calculate
the Capacitance Reactance (XC)

11. Calculate the total capacitance (CT ) which connected in series and parallel
for combination of three capacitors below:
i. 120 µF, 240 µF and 360 µF

12 . Calculate the value of capacitor which is connected in series with other
60pF capacitor, where the total capacitance is 15pF.

13. Based on figure below, calculate total capacitance, CT and total charge, QT

14. A circuit with 3 capacitors C1, C2 and C3 are connected in series with values 3µF,
4pF and 1µF respectively. If the voltage supply is 100V, calculate:

i . Total capacitance ,CT
ii . TotalCharge, QT
iii. Voltagedropat C1
15. Series RL with 150Ω and 25mH connected to AC 60 V, 100Hz. Calculate the current

and phase angle refer to supplied voltage.
16. Series RLC with R=33Ω, L=50mH and C=10µF. Voltage supply 75 V, 200Hz.

Calculate I, VR , VC ,VL and phase angle refer to the supplied voltage.
17. A circuit with 250Ω resistance, 0.2 H inductance and 1µF capacitance

connected in series to AC 100 V, 50 Hz. Calculate:
i. Impedance, Z
ii. Current, I.

49

ELECTRICAL TECHNOLOGY

18. Series RLC circuit with 20Ω, 0.1H and 40µF respectively was connected to AC
230V, 50 Hz. Calculate:

i. Impedance, Z
ii. Current, I
iii. Voltage drop at each components, VR, VL,VC
iv. Power factor
v. Draw the vector diagram
19. A circuit with resistance 50Ω, inductance 0.15 H and capacitance 100µF connected

inseries with AC 100 V, 50 Hz. Calculate:
i. Inductive reactance, capacitive reactance and impedance
ii. Current
iii. Voltage drop at each components V R , VC and VL

20. AC circuit 200V, 50 Hz connected in series with resistance 40Ω, inductance
reactance 20Ω and capacitance reactance 12Ω. Calculate:

i . Impedance, Z .
ii. Current , I
iii. Voltage drop at each components V R , VC and VL
iv. Phaseangle
v. Powerfactor
vi. Draw the Vector diagram

21. Series RLC circuit with 50Ω, 10mH and 100µF. Supplied with AC 240 <30˚ V 50 Hz.
Calculate:

i . Impedance ,Z .
ii . Current, I
iii . Voltage drop at each components V R , VL and VC
iv . Powerfactor

50

ELECTRICAL TECHNOLOGY

3 BASIC PRINCIPLES OF

ELECTROMAGNETISM

3.1 INTRODUCTION

Word magnet comes from the word “magnetic” (lodestones / black stone) found in
magnesia, turkey.
• a piece of iron (or an ore, alloy, or other material) that has its component atoms so

ordered that the material exhibits properties of magnetism, such as attracting other
iron-containing objects or aligning itself in an external magnetic field.

• Magnets have two poles. These poles decide whether the ferromagnetic material
is going to repelor attract. The attraction of two magnets will happen if the poles
are opposite. For example, if themagnets are positive to positive they will repel, if
they are negative to positive they attract andstick together

3.2 MAGNET

There are various types of magnets:
a)Self-magnetic
Is a type of iron which is found in minerals and has a natural magnetism.
b ) A r t i f i cia l m a g n e t i c
Made of the assistance of other magnetic or electrical assistance. Two type of artificial
magnetic which is:
i. Permanent magnet -hold onto their magnetic capabilities for long durations.
Characteristic : formed from hardened steel,
• Magnet has been magnetized first by electromagnetic and strong magnetism

after magnetization resources released.
• used in the moving coil, the speaker, motor construction electricity, telephone,

current measuring device and

51

ELECTRICAL TECHNOLOGY

ii. Temporary magnet -their magnetic capabilities is temporary.
• Characteristic : formed by electric currents
• Temporarymagnets take on the properties of a magnet if they are touching

something magnetic. This happens when a paperclip becomes magnetic as
it touches a permanent magnet. Things like wood or glass does not have the
properties to become temporary magnets.
• Used in electric bells, relays and e.t.c

3.3 MAGNETS CHARACTERISTIC

• Flux Pattern In Magnetic Field - Magnetic Field in Magnetic Bar

52

ELECTRICAL TECHNOLOGY

• The magnetic field from a bar magnet
• Flux direction is always moving from north to south
• Attraction - Force by which two bodies are pulled toward each other,
• Opposite poles attract each other. Different poles repels each other
• Magnetic field - Area around the magnet where magnetic forces represented by

lines of force are exerted resulting in electron movement.

3.4 ELECTROMAGNETISM

Definition: electromagnetic is coil of wire windings on a magnetic iron with has
magnetism when current flows through the coil wire and lose their magnetism when
the flow stops. It is also known as magnetic electromagnetic transient is under
the influence of magnetization and magnetization disappears after the source is
removed or stopped.
• A simple example is to wind a coil of wire (solenoid) with a flow of current in it

Figure 3.4: Electromagnet using a nail

53

ELECTRICAL TECHNOLOGY

• In this experiment, when the supply voltage from the battery is on, the current
will flow through the coil and the nail will become a temporary magnet, it will lost
the magnetic character once the current is off.

• The result is solenoid coil containing an electric current produces a magnetic
field. The magnetic field strength depends on:-

i . the number of coils
ii. total current flowing
iii . typeofthematerial
iv. length of the conductor

3.5 MAGNETIC QUANTITIES

3.5.1 MAGNETIC FLUX

• Definition: magnetic flux is the amount of lines or dots around a magnet. Magnetic
flux formed in the region or zone of a magnetic field.

• It can be seen visually by using iron fillings sprinkled onto a sheet of paper or by
using a small compass to trace them out.

• Symbol of the Flux Unit = Weber (Wb)
• Formula:

• the flux in a magnetic field can be determined using three methods:
i. Compass - the direction of the magnetic field can determine by placing
one or more compasses on a card and observing their direction. Note that

the current must be DC (direct current), such as from a battery. Otherwise
with AC, the direction of the current and a second.

54

ELECTRICAL TECHNOLOGY

ii. Right hand rule – Grip the wire in right hand and the thumb shows the
current direction and fingers shows the direction of magnetic field around
the wire.

Figure 3.5.2: The right hand rule (source: http://
physicsdetective.com)

iii. Maxwell Screw Rule
According to this rule, if a right-handed screw is turned then the
movement direction shows the direction current while the rotation
direction is the direction of the magnetic field

55

ELECTRICAL TECHNOLOGY

Figure 3.5.3: Maxwell screw rule (source: http://
physicsdetective.com)

• Then the physical action of screwing into and out of the paper indicates the
direction of the current in the conductor and therefore, the direction of rotation
of the electromagnetic field around it as shown below.

• Magnetic Field around a current-carrying conductor

• The needle deflections show that a magnetic field exists in a circular form
around a conductor. When the current flows upward the direction of the field
is clockwise as viewed from the top.However, if the polarity of the battery is
reversed so that the current flows downward, the direction of the field is counter
clockwise.

56

ELECTRICAL TECHNOLOGY

b) Two Conductor
One of the more interesting and strange things that happens with current-
carrying conductors and their magnetic fields is between two parallel
current carrying wires. Since the magnetic fields are three dimensional and
have vectors i.e., have both magnitude

3.5.2 FLUX DENSITY

• Definition: total flux through the conductor cross-sectional area of a magnet or
magnetic field

• Magnetic flux is the amount of magnetic field produced by a magnetic source.
• Symbol : B
• Formula :
• Unit : Weber/m2 (Wb/m2) or Tesla (T)

Figure 3.5.2.1: Magnetic Flux Density

57

ELECTRICAL TECHNOLOGY

3.5.3 ELECTROMAGNETIC MOVE FORCE
3.5.4 MAGNETIC FIELD STRENGTH

58

ELECTRICAL TECHNOLOGY

59

ELECTRICAL TECHNOLOGY

3.5.5 PERMEABILITY

3.5.6 RELUCTANCE

60

ELECTRICAL TECHNOLOGY

3.6 ELECTROMAGNETIC INDUCTION

Definition: When a conductor is moved across a magnetic field and cut through
the flux, anelectromagnetic force (e.m.f.) is produced in the conductor. This effect
is known as electromagnetic induction. The effect of electromagnetic induction will
cause induced current.

3.6.1 RELUCTANCE

a. Flux cuts conductor
When the magnet is moved back and forth, the galvanometer needle will moves. It shows
that the presence of a current in the magnetic field. The needle immediately returns to
zero when the magnet is not moving. Faraday confirmed that a moving magnetic field
is necessary in order for electromagnetic induction to occur.

Figure 3.6.1.1: Flux cuts conductor (source: www.electronics-
tutorials.ws)

61

ELECTRICAL TECHNOLOGY

Figure 3.6.2 : Conductor cuts flux

62

ELECTRICAL TECHNOLOGY

63

ELECTRICAL TECHNOLOGY

64

ELECTRICAL TECHNOLOGY

4 TRANSFORMER

4.1 INTRODUCTION TO ELECTRIC TRANSFORMER

• Electricity and Energy has become an important part of everyday life. Most of the
applianceswork with electricity. One of the main reasons that we use alternating
AC voltages andcurrents in our homes and workplace’s is that it can be easily
generated at a convenientvoltage, transformed into a much higher voltage and
then distributed around the country.

• Atransformer converts alternating current (AC) from one voltage to another voltage.
It has nomoving parts and works on a magnetic induction principle and it can be
designed to “step-up”or “step-down” voltage. In other words Electrical transformer
is a device that is used toincrease or decrease the voltage of alternating current
or voltage.V

Figure 4.1: The major components of today’s
power grids

65

ELECTRICAL TECHNOLOGY

4.2 PRINCIPLES OF TRANSFORMER

When an alternating voltage ( VP ) is applied to the primary coil, current flows through
the coil which in turn sets up a magnetic field around itself, called mutual inductance,
by this current flow according to Faraday’s Law of electromagnetic induction. The
strength of the magnetic field builds up as the current flow rises from zero to its
maximum value which is given as dΦ/dt.
• When the magnetic lines of flux flow around the core, they pass through the turns

of thesecondary winding, causing a voltage to be induced into the secondary coil.
The amount of voltage induced will be determined. Also this induced voltage has
the same frequency as the primary winding voltage.
• Then we can see that the same voltage is induced in each coil turn of both windings
because the same magnetic flux links the turns of both the windings together.
As a result, the total induced voltage in each winding is directly proportional to
the number of turns in that winding. However, the peak amplitude of the output
voltage available on the secondary winding will be reduced if the magnetic losses
of the core are high.
• In other words, Transformers DO NOT Operate on DC Voltages.

66

ELECTRICAL TECHNOLOGY

4.3 RANSFORMER STRUCTURE

A single phase voltage transformer basically consists of two electrical coils of wire,
one called the “Primary Winding” and another called the “Secondary Winding” that
are wrapped together around a closed magnetic iron circuit called a “core”. This soft
iron core is not solid but made up of individual laminations connected together to
help reduce the core’s losses. These two windings are electrically isolated from each
other but are magnetically linked through the common core allowing electrical power
to be transferred from one coil to the other.

Generally, the primary winding of a transformer is connected to the input voltage
supply and converts or transforms the electrical power into a magnetic field. While
the secondary winding converts this magnetic field into electrical power producing
the required output voltage as shown.

67

ELECTRICAL TECHNOLOGY

• When a transformer is used to “increase” the voltage on its secondary winding
with respect tothe primary, it is called a Step-up transformer. When it is used to
“decrease” the voltage on thesecondary winding with respect to the primary it is
called a Step-down transformer.

• However, a third condition exists in which a transformer produces the same
voltage on itssecondary as is applied to its primary winding. In other words, its
output is identical with respect to voltage, current and power transferred. This
type of transformer is called an”Impedance Transformer” and is mainly used for
impedance matching or the isolation ofadjoining electrical circuits.

• The difference in voltage between the primary and the secondary windings is
achieved bychanging the number of coil turns in the primary winding ( NP )
compared to the number of coilturns on the secondary winding ( NS ). As the
transformer is a linear device, a ratio now existsbetween the number of turns of
the primary coil divided by the number of turns of thesecondary coil. This ratio,
called the ratio of transformation, more commonly known as atransformers “turns
ratio”, ( TR ). This turns ratio value dictates the operation of thetransformer and
the corresponding voltage available on the secondary winding.

4.4 STEP-DOWN AND STEP-UP TRANSFORMER

A step-up transformer is one whose secondary voltage is greater than its primary
voltage. This kind of transformer “steps up” the voltage applied to it. For instance, a
step up transformer is needed to use a 220V product in a country with a 110V mains
supply.

A step-down transformer is one whose primary voltage is greater than its secondary
voltage. This kind of transformer “steps down” the voltage applied to it. For instance,
a step down transformer needed to uses an 110V product in a country with a 220V
supply. So a step up transformer increases the voltage and a step down transformer
decreases the voltage.

68

ELECTRICAL TECHNOLOGY

A transformer is made from two or more coils of insulated wire wound around a core
made of iron. When voltage is applied to one coil (frequently called the primary or
input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently
called the secondary or output). The turns ratio of the two sets of windings determines
the amount of voltage transformation. Transformers “step up” or “step down” voltage
according to the ratios of primary to secondary wire turns.

̵̵ If k>1 : Step up transformer
̵̵ If k < 1 : Step down transformer

A transformer designed to increase voltage from primary to secondary is called a step-
up transformer. A transformer designed to reduce voltage from primary to secondary
is called a step-down transformer.

4.5 TRANSFORMER RATIO

69

ELECTRICAL TECHNOLOGY

70

ELECTRICAL TECHNOLOGY

4.6 LOSS AND EFFICIENCY IN TRANSFORMERS

The losses which occur in a transformer on load can be divided into two groups:

71

ELECTRICAL TECHNOLOGY

v. The efficiency of the transformer when
the transformer is on a full load stage.

72

ELECTRICAL TECHNOLOGY

4.7 IDEAL TRANSFORMERS

An ideal transformer is an imaginary transformer which has
-No copper losses (no winding resistance)
-No iron loss in core
-No flux leakage
In other words, an ideal transformer gives output power exactly equal to the input
power.

4.8 TRANSFORMER TYPES AND APPLICATION

A variety of types of electrical transformer are made for different purposes. Despite
their design differences, the various types employ the same basic principle as
discovered in 1831 by Michael Faraday, and share several key functional parts.

4.8.1 RELUCTANCE

Autotransformers are different from traditional transformers because autotransformers
share a common winding. On each end of the transformer core is an end terminal for
the winding, but there is also a second winding that connects at a key intermediary
point, forming a third terminal. The first and second terminals conduct the primary
voltage, while the third terminal works alongside either the first or second terminal
to provide a secondary form of voltage. The first and second terminals have many
matching turns in the winding. Voltage is the same for each turn in the first and
second terminal. An adaptable autotransformer is another option for this process.
By uncovering part of the second winding and using a sliding brush as the second
terminal, the number of turns can be varied, thus altering voltage.

The advantages of an autotransformers are less cost (because using less copper),
less volume, less weight, low copper lost, high efficiency, output voltage is achievable
if a sliding contact is used and use sliding brush.

73

ELECTRICAL TECHNOLOGY

4.8.2 POLYPHASE TRANSFORMERS

This type of transformer is commonly associated with three phase electric power,
which is a common method of transmitting large amounts of high voltage power,
such as the national power grid. In this system, three separate wires carry alternating
currents of the same frequency, but they reach their peak at different times, thus
resulting in a continuous power flow. Occasionally these “three-phase” systems have
a neutral wire, depending on the application. Other times, all three phases can be
incorporated into one, multiphase transformer. This would require the unification and
connection of magnetic circuits so as to encompass the three-phase transmission.
Winding patterns can vary and so can the phases of a Polyphase transformer.

As a type of leakage transformer, resonant transformers depend on the loose
pairing of the primary and secondary winding, and on external capacitors to work in
combination with the secondary winding. They can effectively transmit high voltages,
and are useful in recovering data from certain radio wave frequency levels.

74

ELECTRICAL TECHNOLOGY

4.8.4 AUDIO TRANSFORMER

Originally found in early telephone systems, audio transformers help isolate potential
interference and send one signal through multiple electrical circuits. Modern
telephone systems still use audio transformers, but they are also found in audio
systems where they transmit analogue signals between systems. Because these
transformers can serve multiple functions, such as preventing interference, splitting
a signal, or combining signals, they are found in numerous applications. Amplifiers,
loudspeakers, and microphones all depend on audio transformers in order to properly
perform

4.8.5 SUBSTATION TRANSFORMER

The transformer is the heart of the substation. The transformer changes the relationship
between the incoming voltage and current and the outgoing voltage and current.
Substation transformers are rated by their primary and secondary voltage relationship
and their power carrying capability. For example, a typical substation transformer
would be rated 69-13 kV and 20 MVA, meaning the primary or high voltage is 69kV,
the secondary or low voltage is 13kV and the transformer has a power rating of
20MVA or 20,000 kVA. Substation transformers like most utility transformers, consist
of a core and coils immersed in oil in a steel tank. The oil serves both as an insulator
and as a coolant to keep the core at reliable operating temperatures.

75

ELECTRICAL TECHNOLOGY

TUTORIAL

1. State the definition of transformer.
2. Define transformer with the aid of diagram
3. Sketch the step up and step down transformers
4. Explain the uses of step up and step down transformer with the aid of diagrams.
5. List the types of transformer in terms of application.
6. Define autotransformers
7. Explain advantages of auto transformers
8. An ideal transformer is connected to a supply of 10kVA, 60Hz. A 1000 turns has wound

to a coiled at a 230V primary winding and a 150 turns at a secondary winding. Calculate:
i. The ratio of a transformer, k
ii. The secondary voltage, Vs
iii. The primary current current, Ip
iv.The secondary current, Is
v.Identify the type of transformer being used
9. State TWO (2) basic structure of transformer
10.A 30kV/1200V, transformer is to have approximate 15V per turn and operate with flux

density of 1.2 5T. Calculate:
i.The number of primary and secondary turns
ii. The cross sectional area of the core
11. A transformers with 2000V/200V and 20kVA has 66 turns of secondary coil. Calculate the :
i.Primary Turns
ii. Primary and secondary currents
iii. Ratio of the transformers
iv.Determine the type of transformers
12. With the aid of diagram, explain the principle operation of transformer.
13. The number of windings for a transformer are Np = 1500 turns and Ns = 5000 turns.
i. Calculate the ratio of K.
ii. Determine the type of transformer
iii.Draw the symbol of transformer to differentiate the number of windings
iv.If the ratio k=1.5 and total primary windings is 2000 turns, find the number of turns in
the secondary windings.

76

ELECTRICAL TECHNOLOGY

5 AC ELECTRICAL

MACHINE

5.1 INTRODUCTION

• As well as DC, ac also has two types of machines, i.e. generators and motors
• Generator is a device used to convert mechanical energy into electrical energy.
• Motor is a device used to convert electrical energy to mechanical energy.
• Both machines are based on the principle of “electromagnetic induction” discovered

in 1831 by Michael Faraday, a British scientist.

Figure 5.1: AC Motor

77

ELECTRICAL TECHNOLOGY

5.2 AC GENERATOR

• The AC generator (alternator) is a device that converts mechanical energy into
electrical energy using the principle of electromagnetic induction.

• The amount of voltage generated depends on the following:
i. The strength of the magnetic field.
ii. The angle at which the conductor cuts the magnetic field.
iii. The speed at which the conductor is moved
iv. The length of the conductor within the magnetic field.
• Voltage generated is the voltage obtained from the action of electromagnetic

induction law.
• Terminal voltage is the voltage supply to the terminal.

5.2.1 PRINCIPLE OPERATION OF AC GENERATOR (ALTERNATOR)

• A basic generator consists of a magnetic field, an armature, slip rings, brushes
and a resistive load.

• The magnetic field is usually an electromagnet. An armature is any number of
conductive wires wound in loops which rotates through the magnetic field.

• For simplicity, one loop is shown. When a conductor is moved through a magnetic
field, a voltage is induced in the conductor.

• As the armature rotates through the magnetic field, a voltage is generated in the
armature which causes current to flow.

• Slip rings are attached to the armature and rotate with it.
• Carbon brushes ride against the slip rings to conduct current from the armature

78

ELECTRICAL TECHNOLOGY

5.2.2 THE DIFFERENCES BETWEEN AC GENERATOR AND DC GENERATOR

5.3 AC MOTOR

• AC motors are used worldwide in many applications to transform electrical energy
into mechanical energy.

• There are many types of AC motors, but three phase AC induction motors, is the
most common type of motor used in industrial applications.

• AC motors provide the motive power to lift, shift, pump, drive, blow, drill, and perform
a variety of other tasks in industrial, domestic, and commercial applications.

• The induction motor, the most versatile of the AC motors, has truly emerged as
the prime mover in industry, powering machine tools, pumps, fans, compressors,
and a variety of industrial equipment.

• Asynchronous and synchronous electric motors are the two main categories of ac
motors.

79

ELECTRICAL TECHNOLOGY

5.4 OPERATING PRINCIPLES OF AC MOTOR

AC Motors Convert Electric Energy into Mechanical Energy
• When a conductor is moving across a magnetic field a voltage is induced
• If the conductor is part of a closed circuit there will be a current induced
• In a motor, the induction principle is utilized in reverse
• A live conductor is placed in a magnetic field
• The conductor is influenced by a force which tries to move it through the magnetic

field.

5.4.1 AC MOTOR STRUCTURE.

The AC motor is made up of two basic parts
ᴼᴼ Stator -The stationary section that contain the windings (magnetic field)

80

ELECTRICAL TECHNOLOGY

ᴼᴼ Rotor - The rotating section that contains the conductors

The other parts in the AC motor structure are:
ᴼᴼ Slip ring- is a rotating metallic device that allows the transmission of power

andelectrical signals from a stationary to a rotating structure

ᴼᴼ Carbon brush - is a fixed contact used to transmit electrical current from a static
to arotating part, in a motor or generator, and as regards DC machines, ensure
a spark-free commutation.

81

ELECTRICAL TECHNOLOGY

5.4.2 TYPES OF AC MOTORS

• AC electric motors are further subdivided into single phase and three phase
motors. Singlephase AC electrical supply is what is typically supplied in a home.
Three phase electricalpower is commonly only available in a factory setting. DC
electric motors are also split intotypes. These include brush motors, brushless
motors, and stepper motors.

• Of these types, brush electric motors are by far the most common. They are easy to
buildand very cost effective. Their major drawback is that they use carbon brushes
to transferelectrical current to the rotating part, and these brushes wear over time
and eventuallyresult in the failure of the electric motor. The DC brushless motor
eliminates the brushes,but is more costly and requires much more complicated
drive electronics to operate.

• A stepper motor is a special type of brushless motor that is used primarily in
automationsystems. A stepper motor uses a special type of construction that
allows a computerizedcontrol system to “step” the rotation of the motor. This is
very important whencontrolling a robotic arm.

82

ELECTRICAL TECHNOLOGY

5.4.3 DIFFERENCES BETWEEN DC AND AC MOTORS

ᴼᴼ The main differences between DC Motor and AC motor

83

ELECTRICAL TECHNOLOGY

5.5 AC MACHINE CALCULATION

84

ELECTRICAL TECHNOLOGY

TUTORIAL

1. Define AC motor
2. With the aid of diagram, explain two basic structures of a motor.
3. List THREE (3) differences between AC motor and DC motor.
4. A three-phase induction motor 4poles rotates at 1440 rpm when supplied with 240 V, 50 Hz.

Calculate the following:
i.Synchronous speed
ii.Percent slip
iii.Rotor frequency
iv. Stator frequency
5. The frequency of the supply to the stator of an 8-pole induction motor is 50Hz and the rotor

frequency is 3 Hz. Determine:
i. The slip
ii. The rotor speed Nr
6. A three phase induction motor is supplied with 415V, and 12 poles from a 50Hz system.

Calculate the:
i. The synchronous speed
ii. The speed of the rotor when the slip is 4%
iii. The rotor frequency when speed of the rotor is 60 0rpm
7. State the definition of slip, synchronous speed and rotor speed.
8. A three phase induction motor, 4 poles 60Hz rotating at 5% slip. Calculate:
9. An alternating current machine three-phase induction motor constructed with eight poles,

andsupplied with 415V, 150 Hz. If the percentage is 7% slip, determine these values:
i.Synchronous speed
ii.rotor speed
iii.Rotor frequency if the new value of the rotor speed is 1500 rpm.
iv.Stator frequency

85





ELECTRICAL TECHNOLOGY

1