rotating machines part 2

027 Electrical & Electronics Principles

Pg 146.0B 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

© Seychelles Institutes of Technology Prepared by Mr. Dinesh

Wire carrying current

Direction of force



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




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


http://sciencecity.oupchina.com.hk/npaw/student/glossary/solenoid.htm

http://sciencecity.oupchina.com.hk/npaw/student/glossary/current.htm

http://sciencecity.oupchina.com.hk/npaw/student/glossary/magnetic_field.htm



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.




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

Self excited Induction

Three phaseSingle phase

SynchronousSeparately excited

AC MOTORS



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


http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elecur.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forwir.html#c1



6.B.12 Solved questions on application of Fleming’s rule

Work example 1Diagram 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.




Question (b)iWhen Fleming’s left-hand rule is used, what does each of the following represent?1 the thumb 2 the first finger3 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)iiIn which direction will the aluminium strip in diagram 1 move when the switch is closed?

SolutionUpwards

Question (c)Diagram 3 shows the cross-section through a loudspeaker.




Question c(i)Explain how the alternating current input from the amplifier causes the loudspeaker cone to vibrate.

SolutionThe 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 2Show the direction of force on the diagram below

Solution




Work example 4How 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.


http://www.gcsescience.com/pme12.htm

http://www.gcsescience.com/pme10.htm



Work example 4Define 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.


rotating machines part 2

Engineering