4The operating principle of electromagnetic devices
4The operating principle of electromagnetic devices.docx
Theoretical material for the lesson,
definitions for concepts
Our lesson today is…….
How is wind power converted into
Most of the electricity we use is
generated by electromagnetic induction. This process goes on in the generators
at work in power stations, in wind turbines (Figure) and, on a much
smaller scale, in bicycle dynamos. It is the process whereby a conductor and a
magnetic field are moved relative to each other to induce, or generate, a
current or electromotive force(e.m.f.).
generator is an electric generator that converts mechanical energy into
electrical energy in form of alternative emf or alternating current. AC generator works on the principle of
Parts of an
generator consists of two poles i.e is the north pole and south pole of a
magnet so that we can have a uniform magnetic field. There is also a coil which
is rectangular in shape that is the armature. These coils are connected to the
slip rings and attached to them are carbon brushes.
rings are made of metal and are insulated from each other. The brushes are
carbon brushes and one end of each brush connects to the ring and other
connects to the circuit. The rectangular coils rotate about an axis which
is perpendicular to the magnetic field. There is also a shaft which
Working of an AC Generator
armature rotates between the poles of the magnet upon an axis perpendicular to
the magnetic field, the flux which links with the armature changes
continuously. Due to this, an emf is induced in the armature. This produces an
electric current through the galvanometer and the slip rings and brushes.
galvanometer swings between the positive and negative values. This indicates
that there is an alternating current flowing through the galvanometer.
Emf induced in an
coil of N turn and area A
is rotated at v revolutions per second in a uniform magnetic field
B, then the motional emf produced is e = NBA(2πv)sin(2πv)t, where we assume that at time t =
0 s, the coil is perpendicular to the field. The direction of the induced
emf is given by Fleming’s right-hand rule or the Lenz’s law.
right-hand rule states that, stretch the forefinger, the middle finger and the
thumb of the right hand such that they are manually perpendicular to each
other. If the forefinger indicates the direction of the magnetic field, rhumb
indicates the direction of the motion of the conductor. The middle finger
indicates the direction of the induced current in the conductor.
symmetric three-phase power supply system, three conductors each carry an
alternating current of the same frequency and voltage amplitude relative to a
common reference but with a phase difference of one third the period. The
common reference usually connects to ground and often to a current-carrying
conductor that is neutral.
Due to the
phase difference, the voltage on any conductor reaches its peak at one-third of
a cycle after one of the other conductors and one-third of a cycle before the
remaining conductor. This phase delay gives constant power transfer to a
balanced linear load. It also makes it possible to produce a rotating magnetic
field in an electric motor and generate other phase arrangements using
Relay Working Principle
does an electromagnetic relay work?
As the below figure showed, electromagnetic relay consists of electromagnet,
armature, spring, movable contact and stationary contact.
Usually an electromagnetic relay has two circuits, low-voltage control circuit
and high-voltage working circuit.The low-voltage control circuit includes an
electromagnetic relay coil, a low-voltage power supply and a switch. The
high-voltage working circuit includes a high-voltage power supply, a motor and
the contacts of the electromagnetic relay.
The working principle of electromagnetic relays is not complicated, and it
operates mainly according to the principle of electromagnetic induction.
Switching on the power in the low-voltage control circuit, the current goes
through the coil of the electromagnet to generate a magnetic field. Then the
armature generates a suction force to making the movable contact and stationary
contact touching. Thus the working circuit is powered on and the motor begins
to work. When switching off the power in the low-voltage control circuit, the
current in the coil will disappear and the armature under the action of the
spring will separate the movable contact and stationary contact. The working
circuit is disconnected and the motor stops working.
speaking, an electromagnetic relay uses electromagnet to control “on” or “off” status of
the operating circuit. When placing voltage to both ends of a coil, the coil
will be flowed with current and generate electromagnetic effect. The
electromagnet will attract armature to the iron core against tension of spring,
so as to pull the movable contact of the armature to the stationary contact
(normally open contact, or NO). When cutting off power, attraction of the
electromagnet will disappear and the armature will restore its position under
tension of the spring to release the movable contract from the stationary contact
(normally closed contract or NC). The pulling and releasing are used to control
opening and closing of the circuit. Normally open and closed contacts
respectively refer to the stationary contract is in the state of “on” when the
coil is cut off from power and the stationery contract is in the state of “off”
when the coil is connected to power.
One of the main reasons that we use
alternating AC voltages and currents in our homes and workplace’s is that AC
supplies can be easily generated at a convenient voltage, transformed (hence
the name transformer) into much higher voltages and then distributed around the
country using a national grid of pylons and cables over very long distances.
The reason for transforming the voltage to a
much higher level is that higher distribution voltages implies lower currents
for the same power and therefore lower I2*R losses along the
networked grid of cables. These higher AC transmission voltages and currents
can then be reduced to a much lower, safer and usable voltage level where it
can be used to supply electrical equipment in our homes and workplaces, and all
this is possible thanks to the basic Voltage Transformer.
A Typical Voltage Transformer
The Voltage Transformer can
be thought of as an electrical component rather than an electronic component. A
transformer basically is very simple static (or stationary) electro-magnetic
passive electrical device that works on the principle of Faraday’s law of
induction by converting electrical energy from one value to another.
The transformer does this by linking together
two or more electrical circuits using a common oscillating magnetic circuit
which is produced by the transformer itself. A transformer operates on the
principals of “electromagnetic induction”, in the form of Mutual
Mutual induction is the process by which a
coil of wire magnetically induces a voltage into another coil located in close
proximity to it. Then we can say that transformers work in the “magnetic
domain”, and transformers get their name from the fact that they “transform”
one voltage or current level into another.
Transformers are capable of either increasing
or decreasing the voltage and current levels of their supply, without modifying
its frequency, or the amount of electrical power being transferred from one
winding to another via the magnetic circuit.
A single phase voltage transformer basically
consists of two electrical coils of wire, one called the “Primary Winding” and
another called the “Secondary Winding”. For this tutorial we will define the
“primary” side of the transformer as the side that usually takes power, and the
“secondary” as the side that usually delivers power. In a single-phase voltage
transformer the primary is usually the side with the higher voltage.
These two coils are not in electrical contact
with each other but are instead wrapped together around a common closed
magnetic iron circuit called the “core”. This soft iron core is not solid but
made up of individual laminations connected together to help reduce the core’s losses.
The two coil 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. When an
electric current passed through the primary winding, a magnetic field is
developed which induces a voltage into the secondary winding as shown.
Single Phase Voltage Transformer
In other words, for a transformer there is no
direct electrical connection between the two coil windings, thereby giving it the
name also of an Isolation Transformer. 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
job of the secondary winding is to convert this alternating magnetic field into
electrical power producing the required output voltage as shown.
Transformer Construction (single-phase)
·VP - is
the Primary Voltage
·VS - is
the Secondary Voltage
·NP - is the Number of Primary Windings
·NS - is the Number of Secondary Windings
·Φ (phi) - is the Flux Linkage
Notice that the two coil windings are not
electrically connected but are only linked magnetically. A single-phase
transformer can operate to either increase or decrease the voltage applied to
the primary winding. When a transformer is used to “increase” the voltage on
its secondary winding with respect to the primary, it is called a Step-up
transformer. When it is used to “decrease” the voltage on the secondary
winding with respect to the primary it is called a Step-down
However, a third condition exists in which a
transformer produces the same voltage on its secondary 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 of adjoining electrical circuits.
The difference in voltage between the primary
and the secondary windings is achieved by changing the number of coil turns in
the primary winding ( NP ) compared to the number of coil turns on the
secondary winding ( NS ).
As the transformer is basically a linear
device, a ratio now exists between the number of turns of the primary coil
divided by the number of turns of the secondary coil. This ratio, called the
ratio of transformation, more commonly known as a transformers “turns ratio”,
( TR ). This turns ratio value dictates the
operation of the transformer and the corresponding voltage available on the
It is necessary to know the ratio of the
number of turns of wire on the primary winding compared to the secondary
winding. The turns ratio, which has no units, compares the two windings in
order and is written with a colon, such as 3:1 (3-to-1). This means in this example, that if there
are 3 volts on the primary winding there will be 1 volt on the secondary
winding, 3 volts-to-1 volt. Then we can see that if the ratio between the
number of turns changes the resulting voltages must also change by the same
ratio, and this is true.
Transformers are all about “ratios”. The ratio
of the primary to the secondary, the ratio of the input to the output, and the
turns ratio of any given transformer will be the same as its voltage ratio. In
other words for a transformer: “turns ratio = voltage ratio”. The actual number
of turns of wire on any winding is generally not important, just the turns
ratio and this relationship is given as:
A Transformers Turns Ratio
Assuming an ideal transformer and the phase
angles: ΦP ≡ ΦS
Note that the order of the numbers when
expressing a transformers turns ratio value is very
important as the turns ratio 3:1 expresses
a very different transformer relationship and output voltage than one in which
the turns ratio is given as: 1:3.
Instructions for demonstrations and safety
Warning: experiments should be performed under the supervision of teachers
or students followthe instructions of safety procedures.
Additional guidelines for organizing a
moment. Establishing emotional state. Checking for absent students.
provides a class discussion for:How is wind power
converted into electricity?
students are called on to respond to questions and share their own
opinions/thoughts. Then she explains that electricity is
generated by electromagnetic induction. This process goes on in the generators
at work in power stations, in wind turbines.
introduces the topic and objectives of the lesson, assess criteria.
asks learners to divide into 3 groups and study the operational principle of
electromagnetic relay, generator and transformer based on a given model.
4.Groups assess each other’s presentations and provide fair and
helpful feedback by using a assessment criteria.
asks Learners individually answer the MisConceptual
can use A, B, C, D and E cards to show their answers. The questions are shown
at the interactive board.
Laminate a set of cards so
every member of the class has five, with
A,B,C, D and E written
on them. Ask questions with five answers,
you their answer.
Encourage them not to look
at other people’s responses so as to copy.
6.The Teacher checks and assesses each student’s answer and provides
fair and helpful feedback.
the end of the lesson students are encouraged to reflect on what they have
learned and what they need to improve.
Recommendations for formative assessment
discuss learning objectives and assess criteria.
Activity 2.A class discussion
about:How is wind power converted into electricity?
are called on to respond to questions and share their own
are divided into 3 groups and study the operational principle of
generator and transformer based on a given model. The
results of group works
should be given in the form of presentations to be defended
by learners. Assessment
criteria should be agreed in advance.
Activity 4. Groups assess each other’s presentations and provide fair and
helpful feedback by
Activity 5. Learners individually answer the MisConceptual
can use A, B, C,
and E cards to show their answers. The questions are shown at the interactive
a set of cards so every member of the class has five, with
and E written
on them. Ask questions with five answers,
them not to look at other people’s responses so as to copy.
Activity 6. The Teacher checks and assesses each student’s answer and provide
fair and helpful
Activity 7. At the end of the lesson students are encouraged to reflect on what
they have learned
what they need to improve.
Answers, criteria for
assignments, additional materials for the lesson
Assessment criteria for group work
Use Faraday’s law of electromagnetic induction to
explain the operating principle of electromagnetic relay, generator and
Explain the working principle of electromagnetic
Explain the working principle generator;
Explain the working principle of transformer;
useful links and literature
Giancoli, Physics Principles with Applications, Seventh edition 2014.
Physics for You IGCSE Updated Edition 2011