Download Electromagnetism Notes-3 - Physics-S3

Electromagnetism Notes-3
Electromagnetic Induction
The Transformer and its use in
the National Grid
Model National Grid-1
2
Model National Grid-1
Voltage IN /V
Power of bulb /W
Voltage OUT /V
2.5
0.75
0.9
3
Model National Grid-1
Voltage IN /V
Power of bulb /W
Voltage OUT /V
2.5
0.75
0.9
Voltage IN /V
Power of bulb /W
Voltage OUT /V
12
24
1.4
4
Model National Grid-2
5
Model National Grid-2
Voltage Power
IN /V
/W
2.5
0.75
Turns
ratio
Step-up
V OUT /V
Step-down
V IN /V
Turns
ratio
Final
Voltage
OUT /V
1 : 10
22
18
10 : 1
1.7
6
Model National Grid-2
Voltage Power
IN /V
/W
2.5
0.75
Voltage Power
IN /V
/W
12
24
Turns
ratio
Step-up
V OUT /V
Step-down
V IN /V
Turns
ratio
Final
Voltage
OUT /V
1 : 10
22
18
10 : 1
1.7
Turns
ratio
Step-up
V OUT /V
Step-down
V IN /V
Turns
ratio
Final
Voltage
OUT /V
1 : 20
220
180
20 : 1
9.1
7
Inducing an emf without physical movement
8
Inducing an emf without physical movement-2
9
Magnetic circuit links electric circuits
10
Inducing an emf without physical movement-3
11
Faraday’s transformer 1831
12
Transformer principle-1
13
Transformer principle-2
Primary Turns
Secondary Turns
Input Voltage /V
Output Voltage /V
30
10
2.16
0.71
30
20
2.16
1.45
30
30
2.16
2.18
30
40
2.16
2.93
30
50
2.16
3.50
30
60
2.16
4.38
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Questions
•
•
•
•
Independent variable ? Kind of variable ?
Dependent variable ?
Control variables ?
How does output voltage depend on Number
of turns ?
• Type of input voltage ?
15
Graph of Output Voltage /V against No. of turns
From graph, how does output
voltage depend number of turns on
secondary coil ?
16
The Transformer Equation
17
Energy Losses in a transformer
• In practice, if the voltage is doubled the current is more than
halved due to small energy losses in the transformer:1. The windings of copper wire do have resistance and heat is
produced in them
2. The alternating magnetic flux induces currents in the core,
Eddy currents, which cause heating in the iron core
3. The field lines in the core are repeatedly changed from one
direction to the other and back again. This requires energy
and also causes the core to get hot.
4. Leakage of field lines in the iron core reduces the output
voltage and current
18
Step down transformer
19
Alternative National Grid model
20
Low voltage transmission
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Efficiency of low voltage transmission
Voltage IN
= 11.9 V
Voltage OUT = 1.4 V
Current IN
= 2.79 A
Current OUT = 0.68 A
Power IN
= 11.9 x 2.79
Power OUT
= 15.6 W
Efficiency
= Power IN / Power OUT
= 1.4 x 0.68
= 1.1 W
Efficiency
= 1.1 / 15.6 = 7%
22
High voltage transmission
23
Efficiency of high voltage transmission
Voltage IN
= 12.5 V
Voltage OUT = 10.0 V
Current IN
= 2.32 A
Current OUT = 1.60 A
Power IN
= 12.5 x 2.32
Power OUT
= 29.0 W
Efficiency
= Power IN / Power OUT
= 10.0 x 1.60
= 16.0 W
Efficiency
= 16 / 29 = 55%
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Currents in an ideal transformer
For a transformer, with no energy losses, i.e. 100% efficient
Power IN = Power OUT
If the output voltage is doubled, the output current is halved.
A small current at high voltage delivers same power as large current at low
voltage.
A power of 1000W can be delivered by Current of 10A at voltage 100V or
Current of 1A at 1000V
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Heating effect of a current
• Much Greater heating effect with large
current than with small current.
• Power (W) = (Current in A)2 x Resistance (Ω)
• 10 fold increase in voltage leads to
• 10 fold decrease in current and a
• 100 times decrease in heat produced
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