1. The document discusses the principles of rotating machines and electromagnetism. It explains how a current passing through a wire in a magnetic field experiences a force due to the interaction between the current and the magnetic field. 2. Fleming's left hand rule is introduced to determine the direction of force on a current-carrying conductor in a magnetic field. The thumb represents the direction of force, the first finger the magnetic field direction, and the second finger the current direction. 3. The basic principles of how electric motors work are explained. An electric current in a loop experiences forces in a magnetic field, creating torque to rotate the loop and armature. This principle is used to transform electrical energy to mechanical motion

1. 027 Electrical & Electronics Principles
© Seychelles Institutes of Technology Prepared by Mr. Dinesh
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6.0B ROTATING MACHINES
6.B.1 Understanding motor effect
When a current is passed through a magnetic field a force is produced which acts on the wire.
Fig 7
Fig 7 shows part of a wire carrying a current placed in the
magnetic field produced by two strong magnets The magnets
have opposite poles facing and the field lines point from N to S.
You need to be able to use Fleming’s Left Hand Rule to
workout. The direction of the force that acts on the wire. This is
also called the Motor Rule.
First
Finger
= Field (magnetic from North to South)
seCond
Finger
=
Current (conventional current from
+ive to –ive)
thuMb = Movement of the wire
Fleming’s right Hand Rule
The direction of induced current in a straight wire.
First
Finger
= Field (magnetic from North to South)
seCond
Finger
=
Current (conventional current from
+ive to –ive)
thuMb = Movement of the wire
Wire carrying current
Direction of force

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6.B.2 Understanding electromagnetism
Electromagnetism describes the relationship between electricity and magnetism. An electric current
creates a magnetic field and a moving magnetic field will create a flow of charge.
6.B.3 A bar magnet produces magnetic fields the around it
6.B.4 Magnetic fields also can be produce by an electric current in a wire

3. 027 Electrical & Electronics Principles
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6.B.5 Electromagnet
Electromagnet is a temporary magnet. It is made by winding a coil of insulated wire round a soft iron
core. Fig 8 shows a simple electromagnet.
Fig 8: a simple electromagnet
6.B.6 Right hand grip rule
Right-hand grip rule is used to determine the direction of a magnetic field from the current direction in a
conducting wire. The thumb indicates north. The magnetic field direction of a solenoid or a coil can be
determined by the right-hand grip rule for solenoid as follows:
Fig 9: Right hand grip rule

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6.B.7 Symbols used to indicate direction of magnetic fields
6.B.8 Understanding electric motors
Electrical motors uses the concept of motor rule principle, fleming’s right hand rule, electromagnetism
and right hand grip rule in order to function. There are various types of electric motors are to be
explained in the part of electrical machines.

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6.B.9 Electrical machines
There are 3 main categories of electrical machines
 Direct current
 Alternating current
 Alternating current synchronous
6.B.10 Classification of electrical motors
ELECTRIC MOTORS
Shunt
Permanent
magnet
CompoundSeries
DC MOTORS
Selfexcited Induction
Three phaseSingle phase
SynchronousSeparately excited
AC MOTORS

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6.0B ROTATING MACHINES
6.B.11 Basic principle of how electrical motor works
Electric motors involve rotating coils of wire which are driven by the magnetic force exerted by
a magnetic field on an electric current. They transform electrical energy into mechanical energy.
An electric current in a magnetic field will experience a force
If a current carrying conductor bent into a loop, then the two sides
of the loops which are at right angles to the magnetic fields will
experiences forces in opposite direction.
The pair of forces will create a turning influence or torque to rotate
the coil.
Practical motors have several loops on an armature to provide a
more uniform torque and the magnetic field is produced by an
electromagnet called the field coils.
Fig 10 : Basic principle of how electrical motor works

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6.B.12 Solved questions on application of Fleming’s rule
Work example 1
Diagram 1 shows a thin strip of aluminium foil held between the poles of a strong magnet
Question (a)
When the switch is closed the aluminium foil moves. Explain why.
Solution
When the switch is closed the circuit is complete and a current flows through the aluminium foil.
The current produces a magnetic field around the aluminium foil.
This magnetic field interacts with the field of the magnet, causing a force to act on the aluminium foil.
Question (b)
The direction in which the wire moves can be predicted using Fleming’s left-hand rule.

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Question (b)i
When Fleming’s left-hand rule is used, what does each of the following represent?
1 the thumb
2 the first finger
3 the second finger
Solution
thumb = direction of the force
first finger = direction of the magnetic field from north to south
second finger = direction of the current
Question (b)ii
In which direction will the aluminium strip in diagram 1 move when the switch is closed?
Solution
Upwards
Question (c)
Diagram 3 shows the cross-section through a loudspeaker.

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Question c(i)
Explain how the alternating current input from the amplifier causes the loudspeaker cone to vibrate.
Solution
The current passing through the coil is always at right angles to the magnetic field lines. So depending
on the direction of the current there will be a force either forwards or backwards on the coil. With an
alternating input the direction of the force will reverse every time the current changes direction. This
will cause the coil to vibrate. Since the coil is joined to the loudspeaker cone, when the coil vibrates so
will the cone.
Work example 2
Show the direction of force on the diagram below
Solution

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Work example 4
How is the direction of the Catapult Effect (or Motor Effect) predicted by Fleming's Left Hand Rule?
Solution
Explanation
The fields have the same direction and repel the wire. On the other side of the wire the fields have
opposite directions and attract the wire. The wire is pushed at 90° to the direction of the magnetic field
from the permanent magnets. This is called the catapult effect (or motor effect) and is used to make a
simple electric motor spin round.

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Work example 4
Define a solenoid. Explain how does the solenoid behave like a magnet?
Solution
Fig (a) Solenoid Fig (b) A bar of magnet
Fig (c) Electromagnet
Explanation
A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is
called a solenoid.
The pattern of the magnetic field lines around a current-carrying solenoid is shown above. Compare the
pattern of the field with the magnetic field around a bar magnet, they are similar.
One end of the solenoid behaves as a magnetic north pole, while the other behaves as the south pole.
The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the
magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the
solenoid.
A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic
material, like soft iron, when placed inside the coil (Fig c). The magnet so formed is called an
electromagnet.