Motors and Generators

Motors and Generators

Motors and Generators: 1. Motors and magnetic fields x

Perform a first hand investigation to investigate the motor effect

Investigation: Building a simple motor Aim: To perform a simple first-hand investigation to investigate the motor effect Equipment: -Small permanent bar magnet (2 x 5cm) -2 large paper clips -dowel/marker (1.9cm diameter) -connecting wire -sandpaper -packing foam (5 x 5cm square, 2cm deep) -D-ell battery -24 gauge insulated wire Method: 1/ Wrap the wire around the dowel 10-20 times, leaving 5cm free wire at each end. Remove dowel so that you have a spring of wire

5 cm

2/

3/

Squeeze the coil together and wrap one end to secure coil:

Wind the free end as shown below. Ensure the leads are well-centred and straight.

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Motors and Generators

4/

Sand only the top half of each lead, ensuring the paint is completely removed from the top half.

5/

Unfold each paperclip and assemble as shown below: Plastic straw Paperclip

Magnet

Coil of copper wire

Foam block Tape/elastic band Battery (D-cell)

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Motors and Generators

6/ When the battery is connected and the bare wires of the copper coil are in contact with the paperclips, the coil should rotate freely. 7/ Reverse the polarity of the battery and reconnect. You should observe the coil rotating in the opposite direction. What is actually happening? S

‘Bare’ wire Wire insulation

N

Paperclip ‘brush’

No current, therefore no magnetic field in the coil. The coil experiences no torque I

S

S

N

N

If coil is given a push, contact is made, current flows and the coil acts as an electromagnet. The North Pole of the magnet attracts the South Pole of the coil, repelling the north pole and causing the coil to spin. I

S N

N

S

Attraction between opposite poles and the inertia keeps it rotating while current flows. I

S

N

N

S

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Motors and Generators

On completing ½ a turn, the insulation turns the current off. The inertia of the coil carries it through another turn past the insulation. When current flows again, the torque is in the same direction so the coil continues to rotate in the same direction. Describe qualitatively and quantitatively the force between long parallel currentII F k 1 2 carrying conductors: l d In 1820, physicist Andre-Marie Ampere investigated the force exerted between two long parallel wires. He found that when the wires carry current in the same direction, the force is attractive. If the currents flow in opposite directions, the force exerted repels the wires. x

This shows that electrical currents create magnetic fields and forces, which interact with other magnetic fields and electrical currents. The direction of the force between two parallel current carrying wires can be explained using the right hand palm rule.

Currents in same direction, so the resulting magnetic fluxes are also in the same direction. Hence, the wires attract.

Currents in opposite directions, so the resulting magnetic fluxes are in opposite directions. Hence the wires repel. The force between the two wires depends on the product of the current in the wires, the perpendicular distance between the wires and the length of the wires. Increasing the current and the length of the wires increases the force. Increasing the separation of the wires decreases the force.

Mathematically,

F l

k

I1 I 2 where d

F= Force, in Newtons L= length of conductors, in metres Robert Lee Chin

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Motors and Generators I1 and I2 are the respective currents in the conductors, in amps k = magnetic force constant = 2.0 x 10-7 TmA-1 F v I1I 2 for constant l and d F v l for constant I1I 2 and d Fv

1 for constant I1I 2 and l d

The unit of current, the ampere, is defined in terms of the force between two parallel current carrying wires. When one ampere of current flows between two infinitely long wires separated by the distance of 1 metre in a vacuum, they experience a force of 2.0 x 10-7 newtons. x

Solve problems using:

F l

k

I1 I 2 d

Examples: 1) Calculate the force per unit length between two long, parallel wires currying 15.3A and 12.7A and separated by 1.00cm. State the direction of the force, given that the currents are in opposite directions. I1 I 2 15.3 u 12.7 194.31A d 0.01m k 2.0 u x10 -7 TmA 1 II F 194.31A k 1 2 2.0 u x10 -7 3.8862 u 10 3 Nm 1 # 3.88Nm -1 (3 s.f.) l d 0.01 Since the currents in the wires are in opposite directions the force/unit length is 3.88Nm -1 repelling the wires

I1

2) Two long, parallel wires are carrying equal currents. The wires are 10.0cm apart. The force between them is found to be 8.25x10-5Nm-1, attracting each other. Find the magnitude, and relative direction, of the currents in the wires. F I2 8.25 u 10 5 Nm 1 d 0.1m k 2.0 u x10 -7 TmA 1 l

I1 I 2

Fd kl

Since I1

8.25 u 10 5 u 0.1 41.25 2.0 u x10 -7 12 , the current flowing in each is given by I

41.25

6.42261... # 6.42A (3 s.f.)

Because the force attracts the wires, the currents are flowing parallel to each other. 3) Two wires run parallel for a length of 1.48m. The total force acting between them over this length is 6.44x10-4N when they are carrying currents of 8.90A and 14.5A. How far apart are they?

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Motors and Generators

F

6.44 u 10 4 N l 1.48m I1 I 2

F l

k

?d

I1 I 2 d k

l

FI1 I 2

2.0 u x10 -7

8.90 u 14.5 129.05A k

1.48 6.44 u 10  4 u 129.05

2.0 u x10 -7 TmA 1

3.561 u 10 6 # 3.56 u 10 6 m (3 s.f.)

Two power cables, both carrying 30.0A of current in the same direction, are separated by a distance of 8.00cm. The cables run parallel over a distance of 25.0m. What is the total force acting between them?

30 2

I1

12

F

k

x

Identify that the motor effect is due to the force acting on a current carrying conductor in a magnetic field

lI1 I 2 d

900 A d 2.0 u x10 -7

0.08m l 25 u 900 0.08

25.0m 5.625 u 10  2 # 5.63 u 10  2 (3 s.f.) attracting the wires

The force between two current carrying conductors is relatively weak. However, this effect can be made much more powerful by increasing number of wires and the strength of the magnetic fields involved. The effect of the force forms the basis of the electric motor. This is called the motor effect. To determine the direction of the force on a wire in a magnetic field, use the right-hand palm rule:

Conventional Current, I Magnetic field, B

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Force out of page

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Motors and Generators

Magnetic field due to current

Current into page

External magnetic field

Force on wire due to interaction of magnetic fields

Discuss the effect, on the magnitude of the force on a current-carrying conductor, of variations in: -the strength of the magnetic field in which it is located -the magnitude of the current in the conductor -the length of the conductor in the external magnetic field -The angle between the direction of the external magnetic field and the direction of the length of the conductor x

The force equation for a conductor in an external magnetic field is: F BIl sin T where, F

force in newtons

B magnetic flux strength, in teslas I current, in amps l length of conductor, in metres T the angle between the wire and the external magnetic field

I θ B

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Motors and Generators

From the force equation:

FvB FvI Fvl Hence, an increase or decrease in the magnetic field strength, current or length of the wire in the field will increase or decrease the force proportionally. Effect on magnitude of force when the angle, is the only variable Angle, θ (°) 0 10 20 30 40 45 50 60 70 80 90 Force (N), 0 0.17 0.34 0.50 0.64 0.71 0.77 0.87 0.94 0.98 1.0 to 2 s.f. From above, we can see that the force is zero when the current is parallel to the external magnetic field. As the angle increases, approaching 90°, the force increases in proportion to the sine of the angle. When the current is perpendicular to the field, the force is a maximum i.e. F = BIl Solve problems and analyse information about the force on current carrying conductors in magnetic fields using: F BIl sin T Examples:

1) A DC power line 250m long is orientated NE-SW in the Earth’s magnetic field that has strength of 1.5 x 10-5 T in the north-south direction. If the current carried by the power line is 150A flowing from the NE calculate the magnitude and direction of the force exerted on the power line due to the interaction of the current carrying wire and the magnetic field.

S B θ

I

N Robert Lee Chin

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Motors and Generators B 1.5 u 10 5 T l

250m I 150A T

45q

(1.5 u 10 5 ) u 150 u 250 u sin 45q

F

BIl sin T

0.39774... # 0.40 N (3 s.f .) directed downwards

x

Define torque as the turning force of using: W

Fd

The diagram shows how one force can cause an object to rotate (clockwise in this example):

Force

d = perpendicular distance between forces

Pivot point or axi s Force

The diagram shows how two forces can cause an object to rotate (anticlockwise in this example):

Force d

Torque is defined as the turning effect of a force around an axis, pivot or fulcrum. We say the force exerts a torque around the axis. Mathematically, W

Fd where,

W the torque, in Newton metres F the applied force perpendicular to the axis of rotation, in Newtons d the perpendicular distance from : the line of action to the force or the two forces (if two forces are involved), in metres

x

Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces

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Motors and Generators Torque on a current-carrying loop in a magnetic field Consider a single rectangular coil or loop of wire ABCD placed in a plane parallel to a magnetic field B. Let AB CD l and BC AD a . a B

C

Axis of rotation

b

B A

D

_

+

Sides BC and AD experience no force because they are aligned parallel to the field. Sides AB and CD which lie perpendicular to the field each experience a force of: F= BIl. The forces on sides AB and CD are always opposite so the net force on the loop is always zero. The resulting torque is in the same direction, causing the loop to rotate. Using the right hand palm rule, it can be shown that the force on side AB is directed into the page while the force on side CD is directed out of the page. Hence, the loop rotates in a clockwise direction when viewed front on. Since Torque = Force × distance, the torque acting on the sides AB and CD is W BIla BIA where A is the area of the coil. When there are n coils: W nBIA However, as the coil rotates, there are positions where no rotation occurs due to the forces on the wires. When the coil is inclined at an angle, θ, to the magnetic field, then:

W nBIA cos T

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Motors and Generators

D

B

θ A

When the plane of the coil is parallel to the magnetic field, the force AB and CD is equal in magnitude but opposite in direction- force on AB is downwards force and upwards on side CD. The torque is at maximum and in clockwise direction as the perpendicular distance to the line of action is at maximum. Sides BC and AD experience no force as they are parallel to the field. B

C

Torque

B A

D

_

+ Axis of Rotation

B

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A

B

11

Motors and Generators Sides BC and AD experience force outwards. As the force on BC and AD is always equal in magnitude but opposite in sign, their net effect on the loop is zero. The force on sides AB and CD remain in the same direction throughout the rotation and their effect is to create a torque around the axis.

C B

Torque

B

D A

_

+

D Axis of Rotation

B

θ A

As the plane of the loop rotates perpendicular to the field, the torque drops to zero as the perpendicular distance to the line of force is zero. The force on ends AB and CD is at maximum as they are now perpendicular to the magnetic field.

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Motors and Generators C

D

B

_ B

+ A

D

Axis of Rotation

B

A

As the loop completes a full rotation, the direction of the torque alternates: the torque will always rotate the loop to be perpendicular to the field. The force on sides AD and BC returns to zero.

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Motors and Generators C

B

Torque

B A

D

+

_

Axis of Rotation A

D

B

Torque vs. rotation of coil relative to magnetic field in a DC motor: Torque

-90°

0°

90°

180°

270°

360°

Rotation (°)

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Motors and Generators x

Solve problems and analyse information using: W

nBIA cos T

Examples: 1) A rectangular coil of wire 2cm by 4cm, consisting of 200 turns is laced in a magnetic field of 0.02T. If a current of 4A is flowing in the coil, calculate: a) The angle between the plane of the coil and the plane of the field when maximum torque is experienced From the equation, W v cos T . Since the maximum value of cos T is 1 and cos 0q 1 , the maximum torque is attained when the coil is parallel to the field i.e. the angle between coil and field is zero. b) The maximum torque experienced by the coil

A

0.02 u 0.04

W

nBIA

0.0008m 2 B 0.02T I

200 u 0.02 u 4 u 0.02

4A n

200

0.32Nm -1

2) In the coil above, calculate the torque when the coil is at 45° to the magnetic field

nBIA

0.32Nm -1 T

45 q

W

nBIA cos T

0.32 u cos 45q

0.22627... # 0.226 Nm 1 (3s.f .)

x x

Describe the main features of a DC electric motor and the role of each feature Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets

Electric motors can be classed as AC or DC motors. These can be further divided into: -DC Commutator motors -Induction motors (DC) -Synchronous AC motors

Armature laminations

Coil contacts

Axle

Rotor coils

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Commutator

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Motors and Generators

DC commutator motor

Part Description Role For the external magnets (Stator) one of the following arrangements is used: a) a pair of permanent Two permanent magnets on The magnets supply the magnetic magnets (field opposite sides of the motor. field that interacts with the current magnets) in a simple They are curved to surround in the coil to produce torque. The motor the armature. curved shape of the magnets produces a radial magnetic field such that the coil is always parallel to the field throughout rotation. This N S means torque stays at a constant maximum, providing smooth motion. b) pair of electromagnetic Each static coil is wound The stator coils produce a magnetic coils in more complex around a soft iron core field similar to permanent magnets motors attached to the motor casing. i.e. with North and South poles facing each other. However, the coils and the iron core greatly increase the strength of the magnetic field

Armature

Rotor coils

Split-ring commutator

Commutator for a motor with 2 rotors coils

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A laminated soft iron cylinder mounted on an axle. Laminated conductors (electromagnets) are imbedded in slots into the metal core to reduce the air gap between conductors and stator. The coils of copper wire that wrap around the armature. Simple motors will only have one but more complex motors may have several rotor coils. They are insulated with a clear lacquer and are connected by their ends to the commutator A broad ring of metal mounted on one end of the axle, separated into a number of even sections, depending on the number of rotor coils. Each opposing set of coils connects to one rotor.

The armature holds the rotor coils. The iron core concentrates the magnetic field, increasing the torque. The laminations reduce heating effects due to eddy currents. The coils provide the torque as the current passing through them interacts with the external magnetic field. They transfer torque to the armature and thereby the axle.

Provides electrical connection between the rotor coils and the external circuit. It reverses the polarity of the current every half rotation, ensuring that the torque remains in the same direction.

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Motors and Generators Brushes

Axle

Thin, flexible copper brushes or compressed carbon blocks, connected to the external circuit. They are mounted on opposite sides and are held by springs to maintain contact with the commutator A cylindrical bar of hardened steel passing through the centre of the armature

The brushes maintain electrical contact between the commutator and external circuit as it rotates. The springs reduce sparking. Provides a centre of rotation for the other components. Allows useful work to be taken from the motor using pulleys or gears

DC motor Advantages -large starting torque is able to move loads quickly -constant torque even if load varies i.e. self-regulating (back AMF decreases when load is applied) -able to control motor speed by controlling electromagnet strength -able to reverse rotation by reversing field current direction DC Motor

Advantages large starting torque is able to move loads quickly constant torque even if load varies i.e. selfregulating (back AMF decreases when load is applied)

Disadvantages Sparking at commutator is potentially dangerous Noise due to contact of brushes with commutator

able to control motor speed by controlling electromagnet strength -able to reverse rotation by reversing field current direction

Requires a starting resistor to prevent ‘burning out’

DC motors are most commonly used in industrial applications, especially where variable rotation speeds are needed e.g. power tools x

Identify data sources, gather and process information to qualitatively describe the application of the motor effect in: -the galvanometer -the loudspeaker

The Galvanometer The galvanometer forms the basis of ammeters and voltmeters. It is designed mainly to detect current rather than measure it. It consists of a coil of fine wire wrapped around an iron core which is suspended in a permanent U-shaped magnet. A coiled spring is attached to the coil which is attached to a pointer. The spring serves to bring the pointer back to the zero position. When DC current passes through it coil, it experiences magnetic deflecting torque. As the coil rotates, so does the pointer until the magnetic torque is balanced by the restoring torque of the spring. The scale is designed to measure current in both directions and the concave Robert Lee Chin

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Motors and Generators surface of the magnet poles produces a radial magnetic field which ensures constant torque. Hence, the angular deflection of the pointer is proportional to the magnitude of the current. The scale can be calibrated to read either voltage or current. Construction of a moving coil galvanometer:

Scale

Pointer

N

Magnetic coil seen end-on

S

Iron core

Coiled spring

The loudspeaker The loudspeaker consists of a coil wrapped around a central pole piece. The central pole piece forms part of the permanent magnet, which is shaped like an all-round horseshoe magnet. The central pole is surrounded by a circular North Pole. The central core is attached to a cone of stiff paper or plastic. When an alternating current is passed through the coil, it interacts with the permanent magnet, causing the cone to vibrate back and forth according to the modulated signal. As the cone vibrates, it amplifies the desired sound via sound waves. Construction of a moving coil loudspeaker:

Vibrating cone

N S

N

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Central pole piece

Coil

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Motors and Generators

2. The Generator x

Outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet

In 1831, Faraday demonstrated that relative motion between a magnetic field and a conductor will induce a current in the conductor. This is known as electromagnetic induction and forms the basis for electrical power generation in modern society. One such apparatus consisted of two wires electrically connected to a copper disc via copper brushes. The copper disc was placed in between the poles of a horseshoe magnet. When the disc was rotated, the relative motion with the conductor induced an electrical current in the wire which was detected by a galvanometer which was connected to the wiring. Faraday’s apparatus for electromagnetic induction: Copper brushes Horseshoe magnet

N

Copper disc

Galvanometer

Handle to turn disc

x

Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet

Investigation: Generating an electric current Aim: To perform an investigation to generate an electric current by the relative motion of a coil near a magnet Equipment: -digital multimeter -1m length of single strand copper wire - cardboard tube -alligator clips -A bar magnet Method: 1/Make the wire into a tight coil by wrapping around the cardboard tube. Tape the ends of the coil to the tube. Do not overlap the coils. This coil will act as a solenoid

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Motors and Generators

2/ Connect wires to multimeter using alligator clips 3/ Set the multimeter to read micro amps (μA).

N

S

4/ Observe what happens when the magnet: -is pushed to the centre of the solenoid -Is held stationary in the solenoid -Is removed -Is held stationary and the solenoid is moved Results:

Motion of magnet pushed to centre of solenoid Stationery in solenoid Removed from solenoid Kept stationery and coil is moved

Observation Current detected on multimeter No current detected Current detected in opposite direction Current is detected on multimeter

Conclusion: An electrical current is produced when there is relative motion between a magnet and a coil. x

Plan, choose equipment and resources for, and perform a first-hand investigation to predict and verify the effect of a generated current when:

-the distance between the coil and the magnet is varied -the strength of the magnet is varied -the relative motion between the coil and magnet is varied Investigation: Factors affecting the strength of an induced current Aim: To verify the factors affecting the strength of the current induced by the relative motion between a coil and a magnet Theory: An electrical current is generated by the relative motion between a coil and a magnet. The factors affecting the size of the current are: -Distance between the coil and the magnet -Strength of the magnet

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Motors and Generators -Relative motion of the coil and magnet Equipment: -Solenoid from previous experiment -Alligator clips -Bar magnet -multimeter Method: 1/ Connect the solenoid to the multimeter using alligator clips. Set multimeter to read micro amps. 2/ Place the magnet on its end with the North Pole facing up. Move the coil over the magnet rapidly, noting the direction of the current. Move the coil more slowly and note what happens to the reading. 3/ Move the coil rapidly in the opposite direction Take note of the direction of the current. 4/ Keep the coil still and move the magnet in and out of the coil at different speeds with the North Pole moving into the coil. Note what happens to the reading. 5/ Tape two bar magnets together so the north poles are together and repeat step 4. 7/ Move the magnet back and forth at a constant speed towards the coil, stopping at a distance of 1cm from the coil. Then, move the magnet back and forth to a distance of 5cm from the coil at the same speed. Results:

Variable/s Magnet is stationery and North pole faces up. Coil moved rapidly and slowly. Coil is stationary and magnet moves Stronger magnet Magnet moved closer and further away from coil

Observation The current changes direction as the coil moves up and down. The faster the current moves, the larger the current detected. A current is detected Larger current detected The closer the coil and magnet, the larger the current detected

-The direction of the current is dependant on the polarity of the magnetic field with respect to the coil. -The faster the relative motion of the coil and the magnet, the larger the current generated -The stronger the magnet, the larger the induced current -The closer the magnet and the coil move, the larger the current Conclusion: The size of the induced current produced when there is relative motion between a coil and a magnet increases when: the distance decreases and increases; the speed of the motion is increased; a stronger magnet is used.

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Motors and Generators x x

Define magnetic field strength B as magnetic flux density Describe the concept of magnetic flux in terms of magnetic flux density and surface area

The number of magnetic lines emerging from an imaginary surface in a magnetic field is called the magnetic flux. The density of these lines i.e. the magnetic flux per area (B) is a measure of the intensity of the magnetic field. The greater the flux density, the more intense the magnetic field within a given area. The symbol used in equations to define magnetic flux is B and the unit of measurement is the tesla. Mathematically, I B where, A B magnetic flux density, in teslas I the magnetic flux, in A the area perpendicular to the magnetic flux

A

x

B

Describe generated potential difference as the rate of change of magnetic flux through a circuit

When there is relative motion between a conductor and a magnetic field, a voltage or electromotive force (EMF) is produced. Faraday’s law states that the induced EMF is proportional to the rate of change of flux through the circuit. What causes the induced EMF? Consider a wire with the current flowing into the page. The wire moves to the right angles to a magnetic field, B. The wire contains equal numbers of positive and negative nuclei but only the electrons can move. Using the right hand plan rule (for negative charges), it can be shown they experience a force into the page. This leaves a deficiency of electrons at one end i.e. a potential difference or EMF is produced. The charges continue to separate until the electric force of repulsion between like charges balances the magnetic force.

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Motors and Generators

B

B Flow of electrons into page

Wire moves to the right Opposing force on wire

x

Movement of wire

Account for Lenz’s Law in terms of the conservation of energy

Consider the situation below. The North Pole of a bar magnet is moved towards a coil carrying current. The current induced in the coil due to this relative motion creates a magnetic field opposing the change that produced this EMF. Using the right hand grip rule, it can be shown that the direction of the magnetic flux produced by the coil opposes the flux from the magnet. Thus, the induced current is simply a result of the work done to overcome the magnetic repulsion. North Pole is moved towards coil. The induced current opposes the motion:

N

S

Movement of magnet

_

+

When the magnet is removed from the coil, the induced current opposes the withdrawal of the North Pole i.e. energy is required to remove the magnet. Because the direction of the current alternates, so does the magnetic flux. North Pole removed from coil. The induced current and hence, the induced magnetic field changes direction to oppose the motion:

N

S

Movement of magnet

_ Robert Lee Chin

+

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Motors and Generators

Lenz’s law states that the magnetic field of an induced current opposes the change that caused it. Lenz’s law is simply a consequence of the law of conservation of energy. If the induced current aided the motion of the magnet, kinetic energy would be produced indefinitely and energy would be created. x

Relate Lenz’s law to the generation of back emf in motors

A working motor has the two things required to generate an EMF: a moving coil and a magnetic field. The applied EMF induces another EMF in the coil, whose direction opposes the motion of the coil (from Lenz’s law). Because this motion opposes the applied EMF it is called the back EMF. x

Explain that in electric motors, back emf opposes the supply emf

When a motor is working, a back EMF is produced. This limits the current in the coil and hence limits the speed of the motor. When a motor begins working, an applied EMF produces a voltage to cause the motor to rotate. As the applied EMF increases, the current and speed of the motor increases. Hence, the back EMF also increases such that: net EMF = applied EMF – back EMF. Eventually, the motor reaches a steady speed when applied EMF equals back EMF. When a motor is started, the back EMF is small and so the current may burn out the motor. To prevent this, a starting resistance is placed in the coil. As the motor speeds up, the back EMF increases and the starting resistance can be removed. x

Explain the production of eddy currents in terms of Lenz’s law

An EMF is induced in a conductor whenever it is placed in a region of changing magnetic flux. In the case of a solid conductor, the back EMF induces its own currents which travel in persistent vortices, hence the name ‘eddy currents’. Eddy currents can result from moving a conductor within a magnetic field or having the conductor within a changing magnetic flux. Situation #1: Conductor moving within a magnetic field: Consider a rectangular metal plate ABCD moving to the right due to an applied force, perpendicular to a magnetic field directed into the page:

A

B

F D

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C

24

Motors and Generators

From Lenz’s law, the induced current inside the magnetic field must produce a magnetic field which opposes the applied force. Applying the right-hand palm rule, the induced current flowing in the side BC within the magnetic field must flow up the page from C to B. The current flowing in the part of the plate not yet in the field travels in the opposite direction such that a circular eddy current flowing anticlockwise is induced. Using the right hand-grip rule, we can confirm that the magnetic field produced by the eddy current opposes the initial magnetic field, and hence the force produced by the eddy current opposes the applied force. The current flows down the page from B to C for the part of the plate not yet in the field. The result is a circular eddy current flowing anticlockwise. When the plate is entirely within the magnetic field, no eddy currents are induced. When the plate leaves the field, the eddy current flow clockwise to resist this change. Situation #2: Conductor in the presence of changing magnetic flux Consider a conducting plate placed perpendicular, in a uniform magnetic field.

If the field strength is increased, then the induced current in the plate will be such that it produces a magnetic field directed out of the page. Applying the right-hand grip rule, this can only occur when the current travels in a circular anticlockwise motion. x

Gather, analyse and present information to explain how induction is used in cook tops in electric ranges

Induction stovetops use coils of copper wire (electromagnets) placed under a glass-ceramic cook top to generate heat for cooking. AC current is passed through the coil which produces oscillating magnetic fields. When a ferromagnetic-based metal pan is placed on the heating plate, the oscillating magnetic fields induce eddy currents in the pan. The eddy currents produce joule heating (I2R) due to the resistivity of the metal. The heated metal cooks the contents of the pot.

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Motors and Generators Advantages 83-90% efficiency (compared to 40% for gas) Does not heat surrounding air Heating is almost instant Ceramic-glass cook top is easy to clean No risk of burning hand on stove top

x

Disadvantages Only functions with ferromagnetic pans Expensive to install

Gather secondary information to identify how eddy currents have been utilised in electromagnetic braking

Consider a solid aluminium or copper disk spinning on its axle. Arranging magnets perpendicular to the spinning disk creates eddy currents in the disc. The magnetic fields produced by the eddy currents oppose the motion of the disc, quickly bringing it to rest. Eddy current breaking has been utilised by industry and transportation.

Motion of disc Eddy current

S N

Eddy currents are used for electromagnetic braking in some Japanese ‘bullet’ trains. During breaking, electromagnets induce eddy currents in the metal railings or the moving metal wheels of the train. The eddy currents induced in the rails or wheels oppose the forward motion of the train. Because the strength of the eddy currents is proportional to the speed of the train, the braking effect decreases as the speed of the train decreases, resulting in a smooth stop. Eddy current electromagnetic braking is also used in some amusement park rides. There are two arrangements commonly used. The first involves placing copper plates to the passenger seat which passes through strong, fixed magnets near the bottom of the ride. As the copper plate passes through the magnetic fields, eddy currents are induced. Because the size of the eddy currents is proportional to the speed of the ride, the braking force is gradually reduced and the rise stops smoothly. The other approach is to place the magnets in the passengers’ seats. As the passenger seat falls during the last half of the descent, they pass between copper sheets. Eddy currents are induced, bringing the ride to a smooth stop.

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Motors and Generators

3. Generators for large-scale power production x

Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

Aim: to demonstrate the production of an alternating current Equipment: -Solenoid (insulated wire coil of many turns) -bar magnet -ammeter (micro amps) -alligator clips -dynamo set to AC setting Method: 1/ Connect the solenoid to the zero-centred ammeter, using alligator clips. 2/ move the bar magnet in and out of the coil. Observe the direction of the current as the solenoid and the magnet are in relative motion 3/ Connect the dynamo to the ammeter, using alligator clips 4/ Rotate the handle of an AC dynamo and observe the ammeter needle as the coil rotates within the magnetic field. Results: As the north pole of the bar magnet was moved inside the coil, the ammeter gave a positive reading. When the magnet was removed, the direction of the ammeter needle gave a negative current reading. When this motion was repeated continuously, the needle moved back and forth i.e. an alternating current was produced. The AC dynamo had a coil of wire that was continuously rotated within opposite poles of two bar magnets. As the coil was rotated, the needle of the ammeter moved back and forth. The faster the coil was rotated, the faster the current alternated and the grater the size of the Ac current. Conclusion: An AC current can be generated when a coil and a magnet are in relative motion and when a coil of wire is continuously rotated within a pair of magnets.

x

Describe the main components of a generator

Generators convert mechanical energy into electrical energy. In theory, a motor can function as a generator and vice versa. Generator component Rotor coils

Robert Lee Chin

Description A single loop, or more commonly a loop of many copper wires wound around the armature which rotates within a magnetic field. The coils connect to the external circuit via slip rings (AC) or split ring commutator (DC) 27

Motors and Generators Armature Stator (permanent magnet/electromagnet) Slip rings (AC) or split ring commutator (DC) Brushes

A cylinder of laminated iron mounted onto a hardened steel axle. Torque applied to the axle makes the armature rotate Two fixed permanent magnets or electromagnets which supply the magnetic field. Metal rings attached to the axle which transfer generated current to brushes Carbon blocks which maintain electrical conduction from the coils to external circuit

Real Generators Real generators are slightly more complex than just described. Real generators commonly use an auxiliary DC generator called the exciter to provide DC current for the electromagnets. The armature of real generators also carries hundreds of copper coils. In most AC generators, the armature is the stator and the field structure is the rotor. This arrangement means the slip rings carry current from the exciter to the field structure, which reduces sparking because the current o the exciter is smaller. Most generators have three or more sets of electromagnets. This produces multiple voltages each rotation, increasing efficiency. x

Compare the structure and function of a generator to the electric motor

The motor and generator have the same structure. The main difference is that a motor converts electrical into mechanical energy while a generator converts mechanical energy into electrical energy. Structure of motor compared to generator Component Motor function Generator function Where current induces a Where a current is induced Rotor magnetic field by cutting magnetic field O p p o s e s t h e c o i l i n d u c e d Induces the electrical current Stator (permanent magnetic field to produce in the moving coil magnet/electromagnet) motion Ensures the induced current Slip rings (AC) or split ring Ensures torque is in one direction only is in one direction only commutator (DC) Connects the external circuit Connects the rotor coils to Brushes to the rotor coils the external circuit

Robert Lee Chin

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Motors and Generators x

Describe the differences between AC and DC generators

The main difference is the way the rotor coils are connected to the external circuit. AC generators are connected via a pair of slip rings which maintain constant electrical connection. As the induced EMF changes polarity with every half rotation, the voltage in the external circuit varies like a sine wave and current alternates direction: Voltage vs. Time graph for AC generator is sinusoidal:

Voltage Time

DC generators are connected via a split-ring commutator. The commutator reverses the polarity of the voltage every half rotation, so the voltage and current are always in one direction. Voltage vs. Time graph for DC generator:

Voltage Time

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Motors and Generators

x

Gather secondary information to discuss advantages/disadvantages of AC and DC generators and relate these to their use

The advantages and disadvantages of AC and DC generators relate to: -The use of a commutator in DC and slip rings in AC -Where the current is induced -The voltage output Commutators (DC) and Split rings (AC) The split ring commutator used in DC is prone to wearing due to friction and is more difficult to construct than the slip rings used in AC generators. The commutator is not always connected to the circuit, so it is less efficient in producing electricity than slip rings, which are permanently connected. As a result, slip rings require less maintenance and are more reliable than commutators. Induced Currents In DC generators, current is induced in the moving rotor coils and drawn to the external circuit via the commutator and brushes. The larger the current, the larger the rotor coils and supporting armature needs to be. Larger currents may also induce sparking when the commutator is not in contact with the brushes. In AC generators, current is induced in the stator windings and the rotor produces the magnetic field. The current is much easier to draw from the stationary stator compared with the moving rotor for DC. Thus, AC generators are more practical for use in power stations to supply electricity to the grid. Voltage Output The voltage output for DC generators can be made ‘smoother’ by using several coils at angles to each other- the more coils, the smoother the voltage output. This is advantageous for equipment that requires a steady voltage. AC generators produce a sinusoidal voltage output and require rectifiers to convert to DC voltage. The voltage from AC generators is easily stepped up and down. This means high voltage can be used for electricity transmission, then stepped down for domestic use. However, due to the oscillating nature of AC, it is more likely to cause fatalities due to heart fibrillations in domestic use. x

Analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities

In the early days of electricity, famous inventor, Thomas Edison pioneered the electricity supply in the US. He favoured DC electricity which already worked well with incandescent bulbs, motor and storage batteries which could provide useful back-up energy during blackouts. Edison had also invented a DC electricity meter which allowed consumers to be billed in proportion to their power consumption. In addition, no AC system was available at the time. His main competitor, George Westinghouse developed an alternative AC electricity system after buying the patents from AC pioneer, Nikola Tesla. This rivalry was due to several reasons. Firstly, Edison was an experimenter but lacked the mathematical mind required to understand AC electricity, which Tesla possessed. Tesla has previously worked for Edison, but was undervalued. Robert Lee Chin

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Motors and Generators Edison’s DC system was disadvantageous because the voltage drop due to the resistance of the system made electricity transmission prohibitive over long distances. DC generating plants had to be within 1-2km of the load. Higher voltages could not be used because there were no devices which could transform a high transmission voltage to a lower voltage for domestic use. Westinghouse’s AC system utilised a transformer between high voltage electricity transmission and domestic use. This meant that AC long-distance electricity transmission was much more efficient than DC, so fewer, larger power stations could supply a given area. The AC system was also more reliable for factories, elevators and other consumer machines. In retaliation, Edison began a smear campaign including spreading disinformation on fatal AC accidents, lobbying against AC and publicly electrocuting animals using an AC generator. When the New York government made electrocution the form of execution, Edison, in fact, lobbied for AC to demonstrate just how dangerous it was and secretly funded the invention of the electric chair! Westinghouse opposed this, saying it was “cruel and unusual”. Despite Edison’s unreasonable logic, AC eventually became the dominant form of electricity transmission worldwide. x

Discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer

There are several reasons for energy loss during electricity transmission: - Resistance of wiring -eddy currents -magnetic hysteresis -Friction in rotor bearings Resistive energy losses Heat is generated in the wire due to the resistance of the metal. Although this is small, the long distances involved in transmission causes major energy losses. The power lost in transmission is given by P = V I or P = I 2 R . Since the power lost, is proportional to the square of the current, electricity is transmitted using high voltage and low current. This is why AC voltage is predominately used, because it can be easily stepped up or down. However, DC is becoming used more due to advances in solid state technology which have made stepping voltages up and down easier. Conductors such as aluminium and copper are used for the wiring, because they have low resistance. The resistance is inversely proportional to the cross section of the conductor, so the thicker the wire, the lower the energy loss. However, heavier conductors such as copper require heavier structural supports. The high voltages used for electricity transmission require high poles and large insulator, which are expensive to build, maintain and can have adverse effects on the environment. Inductive energy losses Energy is also lost in the form of eddy currents induced in the iron core of transformers. This applies to both step-up transformers at the power station and step-down transformers at the substation and power poles. Robert Lee Chin

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Motors and Generators

Eddy current energy losses can be minimised through the use of laminated iron cores in transformers. Using granular ferrites for transformer cores also reduces incidence of eddy currents whilst allowing magnetic flux to change freely. However, excess heat is still produced and as overheating can cause damage to transformers, cooling techniques are used. This includes cooling fins, radiator pipes filled with cooling oil and electric fans. x

Gather and analyse information to identify how transmission lines are: -insulated from supporting structures -protected from lightning strikes

Insulation from supporting structures Transmission lines can carry voltages as high as 500kV. To reduce the likelihood of discharge between the conductor and the support towers (pylons), transmission lines are separated from their support structures by chain or stack insulators. The insulator stacks consist of ceramic or glass discs joined by metal links or rubber discs with a fibreglass core. The metal discs in ceramic insulators are separated by the non-conductive discs and fibreglass is also a non-conductor. The disc shape of the insulators also helps to prevent the build up of moisture or dust which could provide a conductive path across the insulator surface. Insulating chains can be as long as 2m- generally, the higher the voltage, the longer the chain.

Ceramic/glass or rubber discs

Metal links or fibreglass core

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Motors and Generators

Protection from lightning strikes The metal pylons themselves can act as a conductor to take the charge to the ground. The pylons are well-earthed with a large surface area of metal buried beneath the ground. The pylons are also well-spaced to ensure that if one tower is truck, adjacent towers will not be damaged. The uppermost wires do not transmit electricity and are called shield wires. They are connected directly to the pylons without any insulators. If struck, they conduct charge harmlessly to the ground. x

Assess the effects of the development of AC and DC generators on society and the environment

Effects on Society The development of AC generators has led to the widespread availability of low-cost energy. AC generators are simpler and cheaper to construct than DC generators. Because AC electricity can be easily transformed, power can be transmitted at high voltages, resulting in lower energy losses over long distances. This has resulted in the development of reliable AC electricity networks for domestic and industrial used throughout the world, which has had both positive and negative environmental effects. DC generators are still widely used for applications where power does not need to be distributed large distances and for battery systems. It has led to many lifestyle improvements in the form of “labour saving” devices such as washers, refrigerators and air conditioning. Other tasks can now be achieved that were impossible in the past such as modern communication systems and computer networks for finance, business and entertainment. The DC generator has many useful applications including vehicle starting, wind and solar power, electronics and other battery systems. The dependence on electricity has it downfalls. It has led to a reduction in unskilled labour, leading to higher rates of unemployment. Essential serves such as hospitals are forced to have back up a back-up generators and disruptions or blackouts can cause major disruption, loss of productivity and can even precipitate an economic crisis. Effects on the environment AC power plants can be located far away from urban areas, shifting pollution away from people and improving urban environmental health. Power transmission lines which criss-cross the country require power poles and pylons to be constructed, often cutting through environmentally sensitive areas. Air pollution from the burning of fossil fuels in thermal power stations contributes to acid rain and carbon dioxide emissions. Nuclear power stations produce radioactive waste which can remain dangerous to the environment for millennia. Hydro-electricity results in the damming of major rivers and flooding of valleys to provide energy for power stations.

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Motors and Generators The effects of AC generators on society are almost all positive. People now enjoy a much higher standard of living, increased convenience and leisure and many technologies have been made possible by AC electricity. However, the dependence on electricity can have detrimental effects when power is disrupted or when people do not have access to electricity. Many of the environmental effects are negative including long-term environmental degradation. Society favours the social benefits of electricity over the environmental, impacts and we have not yet learned how to use AC electricity in a sustainable way.

4. Transformers and their uses Describe the purpose and principles of transformers in electrical circuits A transformer can be defined as a device which in which an input AC current is transformed into an output AC current of a different voltage. It works based on the principle of electromagnetic induction. The transformer serves to “step-up” electricity to high voltages for efficient electricity transmission, then “step-down” the voltage for convenient and safe domestic use. Australia’s domestic voltage is 240V single phase AC (50Hz). Industry and commercial supply is 415V 3-phase AC. Most appliances, such as light and motors are designed to run on these voltages. Some devices such as MP3 players require much lower voltages while some CRT televisions require 1500V. Transformers are placed in the circuit between the AC supply and the device to step-up or step-down the voltage. Many transformers can supply a range of secondary voltages. Transformers consist of two coils of wire in close proximity, wrapped around a solid iron core. The coil providing the input AC voltage is called the primary coil and the coil receiving the output voltage is called the secondary coil. The primary and secondary coils have a different number of turns. Oscillating AC voltages in the primary coil induces oscillating magnetic fields in the iron core. The magnetic fields in the core connect with those in the secondary coil, hence inducing a different voltage. The coils are said to be inductively coupled.

+

Soft iron core

+

Primary AC supply voltage

Secondary AC output voltage _

_

B Primary coil

Robert Lee Chin

Secondary coil

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Motors and Generators x

Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced

Investigation: Modelling the structure of a simple transformer Aim: To perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced Equipment: -insulated wire -low input AC power supply (of known voltage) -multimeter set to measure voltage -iron ring Method: 1/ Construct the transformer by winding two coils as shown. Ensure one coil has significantly more windings than the other. Iron ring

Insulated copper windings

2/

Connect one coil to the power supply as the primary coil and low input AC voltage. Check with a multimeter whether a voltage is induced in the second coil with the power supply switched on and with it switched off.

3/

Compare the output voltage from the secondary coil with the input voltage to the primary coil for several settings of the power supply voltage.

4/

Reverse the connections to the two coils, so that the secondary coil becomes the primary coil, and repeat the above investigation. Determine which arrangement models a step-up transformer, with secondary voltage higher than primary voltage, and which a step-down transformer.

Results: No. of turns Voltage (V) No. of turns Voltage (V)

Robert Lee Chin

Primary (input) coil np= 100 Vp=12 np= 5.0 Vp=0.6

Secondary (output) Coil ns=5.0 Vs=0.6 ns=100 Vs= 12

Transformer type Step-down Step-up

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Motors and Generators

x

Compare step-up and step-down transformers

Step-up Transformer Step-down transformer Consists of two inductively coupled wires wound around a laminated iron core More turns in secondary than primary More turns in primary than secondary Higher voltage in secondary i.e. higher Lower voltage in secondary i.e. lower output output voltage voltage Lower current in secondary i.e. lower current Higher current in secondary i.e. higher output current output Used to increase voltage in power stations for Used at substations and towns to reduce efficient long-distance transmission voltage for domestic and industrial use Use in high-voltage appliance such as TV Solid state appliances such as radios, screens computers, MP3s, mobile phones… x

Identify the relationship between the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage

In an ideal transformer, no energy losses occur: voltage in primary number of turns in primary voltage in secondary number of turns in secondary

Mathematically,

Vp

np

Vs

ns

In a step up transformer, Vp < Vs and np < ns

Primary coil-fewer turns

Secondary coil- many turns

In a step-down transformer, Vp < Vs and np < ns

Primary coil-many turns

Robert Lee Chin

Secondary coil- fewer turns

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Motors and Generators

Explain why voltage transformations are related to the conservation of energy

x

The law of conservation of energy states that energy can neither be created nor destroyed merely transformed into other forms. It follows that for an ideal transformer, power in = power out. That is,

power in primary power in secondary Since power voltage u current ? Vp I p Vs I s Vp

Is Ip

Vs x

np ns

Solve problems and analyse information about transformers using:

Vp

np

Vs n s 1) A small step-down transformer rectifier unit has an output of 8.00 V from 240 V mains input. Its secondary coil contains 60 turns of wire. a) How many turns are in the primary coil? 8.0 V Vp 240 V n s 60 turns

Vs np

Vp

ns

Vs

?np

n s u Vp Vs

60 u 240 8

180 turns

The primary coil has180 turns b) What is the purpose of the unit being a rectifier? The mains supply is 240V AC. The unit converts the AC current into DC current and the transformer steps down the voltage so that it is suitable to use for small electronic devices such as laptops, mobile phones and MP3’s. x

Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome

Transformers use the principle of induction to transform voltages and as such, eddy currents are induced, circulating in the plane perpendicular to the magnetic field produced by the iron core. They produce heat due to the resistivity of the iron which represents an energy loss and can damage the transformer components. To reduce eddy currents, the iron core is made of laminated iron separated by thin sheets of insulating lacquer. This reduces the circulation of eddy currents to the thickness of one lamina rather the entire core, thus reducing the heating effect. Robert Lee Chin

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Motors and Generators

The core still becomes hot so other strategies include: -Heat-sink fins are added to the transformer casing to provide a large surface area to dissipate heat -The transformer casing may be coloured black to help radiate heat more efficiently -Non-conducting oil around the transformer to transport heat to the outside of the transformer. Cooling pipes and radiator slats may also be used -Transformers mounted above ground with circulating fans. -Placement in well-ventilated area. x

Gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use

Electricity used for domestic and industrial use is typically 240V or 415V AC. If there were no transformers, electricity would have to be transmitted at these low voltages. For a given power output, the current required would be very large and hence, there would be significant energy losses and possible damage to conductors. Large cities would require separate power stations for different voltages. This would be costly, unreliable and inefficient. At power stations, the steam/water-powered turbines drive alternators, producing three phase voltages of 25kV. Step-up transformers increase the voltage to 500kV for distribution via transmission lines. The currents at these voltages are relatively small, so energy losses are minimised. Transformers are used to progressively step-down voltages as they reach consumers. Regional sub-stations reduce voltages to 110kV for regional distribution. Local substations step these voltages down to 66kV, then 1kV. Pole or underground transformers step this down for domestic use (240V) or industry (415V).

Power station 3-phase 25kV AC

x

Transmission lines 500 kV AC

Regional Substation 110 kV AC

Local substation 66 kV AC

Neighbourhood distribution 1 kV

Pole/underground transformer 240 V/415V

Explain the role of transformers in electricity sub-stations

Electricity from power stations is transmitted through the national grid at voltages of up to 500kV. The high voltages are necessary to reduce energy losses due to resistance in the wiring as electricity is carried over long distances. The role of transformers in substations is to progressively lower the voltages to safe, useful levels as they reach consumers for domestic or industrial use. The voltage output is chosen to match the power demands and the distances the electricity is transmitted. x

Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer

The mains electricity supply to homes in Australia is 240V AC. Most appliances, such as lights and fans are designed to operate on these voltages. Some appliances contain components which require different voltages. For example, a microwave turntable and Robert Lee Chin

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Motors and Generators transductor may be connected directly to the mains, while the display panel uses lower voltages supplied by a step-down transformer. Many electronic devices in the home such as mobile phones, laptops and modems are designed to run on batteries. These are designed to operate on low DC voltages. A step-down transformer-rectifier may be built into the connection plug to lower the voltage and convert the AC current to DC. Other appliances such as CRT televisions require voltages as high as 1500V in order to accelerate the electrons towards the screen. A step-up transformer provides the required voltages. x

Discuss the impact of the development of the transformer on society

The development of the transformer has resulted in efficient, long-distance electricity transmission. Remote areas have access to grid-supplied electricity. This has raised living standards in many areas by providing electricity for lighting, refrigeration, air conditioning, electronics and rural industries. Large cities have been allowed to spread, because transformers have made electricity readily available. This has led to... Power stations and industry can be built away from cities, closer to the fuel source such as coal, hydroelectricity or natural gas. This has relocated pollution away from residential and urban areas. However, this means many people now have to travel further to their workplace. Electricity has become an affordable, essential commodity thanks to the transformer

5. AC Motors x

Describe the main features of an AC electric motor

There are two main types of electric AC motors: universal motors and induction motors. Universal AC motors Universal motors are similar in design to the DC motor, except they do not have a commutator, because the current alternates at 50Hz. Instead, they have a pair of permanent slip rings connected to the brushes. Universal AC motor disadvantages -Not self-starting -Torque varies with load -Can only operate at 50Hz (3000rpm), although gear boxes allow speed to be varied

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Motors and Generators Induction Motors The induction motor is the most commonly used electric motor. Its invention followed Tesla’s discovery that magnetic fields can be rotated if two coils placed perpendicular to each other are supplied with AC current 90◦ out-of-phase. Phase shift for two-phase induction motor: Phase a

Current

Phase(◦) Phase b 0◦

90◦

180 ◦

270◦

360◦

All induction motors work on Faraday’s principle of induction: a magnetic field produced in the stator by the AC currents field winding current (a solenoid) induces AC ‘eddy currents’ in the field windings of the rotor. The eddy currents in the rotor induce their own magnetic fields which causes the rotor to rotate. DC current will NOT work with induction motors. There are several types of induction motors: -Squirrel cage induction motor -single phase “ “ -split phase “ “ The induction motor consists of a rotor housed inside a stator. The rotor windings consist of solid aluminium or copper bars joined at the ends by a ring of metal. This shape allows eddy currents to circulate in loops through adjacent bars. The rotor windings are imbedded in a laminated iron armature. The rotor is mounted onto an axle. There are no slip rings or commutators and it is not connected to a power supply ‘Squirrel cage’ rotor windings:

Eddy currents circulate in cage

Robert Lee Chin

Aluminium or copper bars

End rings

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Motors and Generators The rotor turns because of the rotation of the magnetic fields in the stator. The stator consists of a series of electromagnets i.e. wire coils wound around a laminated soft iron core. For each current phase, a pair of electromagnets is used, with opposite poles facing each other. This is connected to a power supply so that the applied magnetic field and hence, the induced eddy currents in the rotor are maximised. Schematic of three-phase squirrel cage motor:

Stator poles

Squirrel cage rotor

Opposite stator poles are connected to a circuit receiving one phase of the three-phase current. The AC current constantly changes the polarity of the electromagnets but ensures that opposite poles face each other. The effect of using three pairs of electromagnets out of phase is to create an apparent rotating magnetic field in the stator. The induced eddy currents in the rotor interact with the changing applied magnetic fields. The rotor literally ‘chases’ the magnetic fields in the stator, producing torque. Induction motor rotor: Laminated Stator pole (electromagnet) End disc Axle

‘Squirrel cage’ imbedded in laminated iron armature

Robert Lee Chin

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Motors and Generators Advantages of AC induction motor over DC motors -Simpler to construct -Have no mechanical contacts and thus require less maintenance -Lighter than DC motors of equivalent power output -Able to be controlled efficiently using switching devices and microprocessors -Electricity supply is AC x

Perform an investigation to demonstrate the principle of an AC induction motor

Investigation: Modelling an induction AC motor Aim: To construct a working model to demonstrate how induction is used in an AC electric motor. Equipment: -A sharpened lead pencil -A hand drill -A bar magnet -Sticky tape -Blutak -Empty aluminium can -A pair of scissors -20cm of light cotton thread Method: 1/ Cut the bottom of the soft drink can, taking care not to cut yourself on any sharp edges so that you end up with a round disc 2/ Attach the cotton thread to the centre of the disc using Blutak so that it is balanced and can hang horizontally Cotton thread

Aluminium can base

N

S

Bar magnet

Pencil in drill chuck

3/

Attach a bar magnet perpendicular to a pencil so it forms a ‘T’ shape. Mount the pencil in the chick of a hand drill so the magnet is close to the hanging aluminium disc.

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Motors and Generators

4/

Rotate the hand drill to make the magnet spin in one direction. Spin in the other direction. Observe.

5/

Replace the magnet or the aluminium disc with a non-magnetic material. Repeat steps24 and observe.

Results: The rotating magnet caused the disc to rotate in the same direction. A rotating magnetic field placed close to a conductor disc induces eddy currents in the disc. The eddy currents induce magnetic fields which, by Lenz’s law, oppose the change which caused it. i.e. the applied magnetic field and the induced magnetic field have opposite polarities. This magnetic attraction causes the disc to rotate as the disc is literally ‘dragged’ along. Conclusion: A magnetic field that is rotating relative to a conductor induces eddy currents in the conductor which cause the conductor to rotate. x

Gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry.

Home Electrical energy → radiant heat: -kettles -stovetops -ovens -toaster Electrical energy → light energy: -incandescent bulbs -fluorescent tubes -television screens -computer monitors -mobile phones Electrical energy → sound energy: -hi-fi speakers -headphones -MP3 players and IPods Electrical energy → microwaves: -microwave ovens Electrical energy → kinetic energy: -food processors -fans -electric drill Electrical energy → infrared energy: -radiant heater -television remote Electrical → chemical energy: -battery recharger

Robert Lee Chin

Industry Electrical energy → kinetic energy: -industrial ,machinery -conveyors -elevators Electrical energy → x-rays: -x-ray machines

Electrical energy → light energy: -laser beams (communication) Electrical energy → chemical energy: -car batteries -electroplating Electrical energy → radio waves: -television transmission -radio

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