1. Converts AC voltage to another
Transformers
Converts AC voltage to another

2. Introduction
A transformer is a device which uses the phenomenon of mutual induction to change the values of alternating voltages and currents. In fact, one of the main advantages of a.c. transmission and distribution is the ease with which an alternating voltage can be increased or decreased by transformers.
Losses in transformers are generally low and thus efficiency is high.
Being static they have a long life and are very stable.

3. Transformers
An AC transformer consists of two coils of wire wound around a core of iron.
The side connected to the input AC voltage source is called the primary and has N1 turns.
The other side, called the secondary, is connected to a resistor and has N2 turns.
The core is used to increase the magnetic flux and to provide a medium for the flux to pass from one coil to the other.
Section 33.8

4. Transformer
An A.C. device used to change high voltage low current A.C. into low voltage high current A.C. and vice-versa without changing the frequency In brief, a transformer: 1. Transfers electric power from one circuit to another 2. It does so without a change of frequency 3. It accomplishes this by electromagnetic induction 4. Where the two electric circuits are in mutual inductive influence of each other.

5. Transformers range in size from the miniature units used in electronic applications to the large power transformers used in power stations. The principle of operation is the same for each.
A transformer is shown on the next slide as consisting of two electrical circuits linked by a common ferromagnetic core. One coil is termed the primary winding which is connected to the supply of electricity, and the other the secondary winding, which may be connected to a load.

6. Principle of operation
It is based on principle of MUTUAL INDUCTION. According to which an e.m.f. is induced in a coil when current in the neighbouring coil changes.

8. Constructional detail : Shell type
Windings are wrapped around the center leg of a laminated core.

9. Core type
Windings are wrapped around two sides of a laminated square core.

10. Sectional view of transformers
Note:
High voltage conductors are smaller cross section conductors than the low voltage coils

11. Working of a transformer
1. When current in the primary coil changes being alternating in nature, a changing magnetic field is produced 2. This changing magnetic field gets associated with the secondary through the soft iron core 3. Hence magnetic flux linked with the secondary coil changes. 4. Which induces e.m.f. in the secondary.

13. Ideal Transformers Zero leakage flux:
-Fluxes produced by the primary and secondary currents
are confined within the core
The windings have no resistance:
- Induced voltages equal applied voltages
The core has infinite permeability
- Reluctance of the core is zero
- Negligible current is required to establish magnetic flux
Loss-less magnetic core
- No hysteresis or eddy currents

14. Ideal transformer V1 – supply voltage ; I1- noload input current ;
V2- output voltgae; I2- output current
Im- magnetising current;
E1-self induced emf ; E2- mutually induced emf

15. Transformers, cont. Assume an ideal transformer In the primary,
Eddy-current losses are minimized by using a laminated core.
Assume an ideal transformer
One in which the energy losses in the windings and the core are zero.
Typical transformers have power efficiencies of 90% to 99%.
In the primary,
The rate of change of the flux is the same for both coils.
The voltage across the secondary is
Section 33.8

16. Transformers – Step-up and Step-down
The voltages are related by
When N2 > N1, the transformer is referred to as a step-up transformer.
When N2 < N1, the transformer is referred to as a step-down transformer.
The power input into the primary equals the power output at the secondary.
I1ΔV1 = I2ΔV2
The equivalent resistance of the load resistance when viewed from the primary is
Section 33.8

17. Transformer principle of operation
When the secondary is an open-circuit and an alternating voltage V1 is applied to the primary winding, a small current—called the no-load current I₀—flows, which sets up a magnetic flux in the core. This alternating flux links with both primary and secondary coils and induces in them e.m.f.’s of E1 and E2 respectively by mutual induction.
The induced e.m.f. E in a coil of N turns is given by
E = -N 𝑑∅ 𝑑𝑡 volts,
where 𝑑∅ 𝑑𝑡 is the rate of change of flux.

18. In an ideal transformer, the rate of change of flux is the same for both primary and secondary and thus E₁/N₁ = E₂/N₂, i.e. the induced e.m.f. per turn is constant.
Assuming no losses, E₁ = V₁ and E₂ = V₂. Hence
V₁ N₁ = V₂ N₂ or V₁ V₂ = N₁ N₂

19. V₁/V₂ is called the voltage ratio and N₁/N₂ the turns ratio, or the ‘transformation ratio’ of the transformer. If N₂ is less than N₁ then V₂ is less than V₁ and the device is termed a step-down transformer. If N₂ is greater then N₁ then V₂ is greater than V₁ and the device is termed a step-up transformer.
When a load is connected across the secondary winding, a current I₂ flows. In an ideal transformer losses are neglected and a transformer is considered to be 100% efficient.

20. Hence input power = output power, or V₁I₁ = V₂𝐼₂, i. e
Hence input power = output power, or V₁I₁ = V₂𝐼₂, i.e., in an ideal transformer, the primary and secondary volt-amperes are equal.
V₁ V₂ = I₂ I₁
Thus in general, V₁ V₂ = 𝑁₁ 𝑁₂ = I₂ I₁
The rating of a transformer is stated in terms of the volt-amperes that it can transform without overheating. Thus, the transformer rating is either V₁I₁ or V₂I₂, where I₂ is the full-load secondary current.

21. Transformer Efficiency
Transformer efficiency is defined as (applies to motors and generators ):
Types of losses incurred in a transformer:
Copper I2R losses winding losses
Iron or Core losses Hysteresis losses
Eddy current losses
Therefore, for a transformer, efficiency may be calculated using the following:
Electrical Machines

22. Losses in a transformer
Core or Iron loss:
Copper loss:

23. Condition for maximum efficiency

24. Contd.,
The load at which the two losses are equal =

25. AUTO TRANSFORMER
The autotransformer is by definition, a transfor-mer consisting of only one winding with a part of its turns being common to both primary and secondary circuits i.e. it is a transformer in which part of the winding is common to both the primary and secondary circuits. Part of the load in the receiver circuit is supplied directly from the supply circuit through the primary winding, the remainder being supplied indirectly through the secondary winding by electro-magnetic induction.
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26. As such, there is no primary or secondary
As such, there is no primary or secondary. Any two points on the winding can be connected to the supply and likewise a load may be connected across any two points. A minimum of two voltage taps are required for an auto-transformer to perform a useful task. An auto-transformer does not provide electrical isolation between the input and output so must not be used in safety critical applications such as portable tool transformers, arc welders or car battery chargers. Suitable applications are in supply voltage matching where only a small difference exists between input and output voltages.
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28. Though the advantages of auto-transformers have been known, for a long time they were not used earlier as power units, only much later interest in the auto-transformers had increased because of the vast development of electrical networks and the necessity to produce transfor-mer units of larger capacities. The auto-transfor-mer does not differ from the ordinary transformers in its fundamental principles. The same laws that govern the ampere-turn relations in the ordinary transformer hold good for auto-transformers also.
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29. But it differs essentially in the manner of conne-ction to the circuits in the primary and secondary systems.
In the ordinary transformers the primary and secondary windings are magnetically interconne-cted but electrically separate.
In the auto-transformers, the windings are both magnetically and electrically interconnected. The auto-transformers may have one single continu-ous winding with one or more taps brought-out or it may have two or more distinct coils electrically connected.
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30. Autotransformers
Autotransformers are transformers in which the primary and secondary windings are coupled magnetically and electrically.
This results in lower cost, and smaller size and weight.
The key disadvantage is loss of electrical isolation between the voltage levels. This can be an important safety consideration when the turns ratio is large. For example in stepping down 7160/240 V we do not ever want 7160 on the low side!

31. Smaller size and weight,
Some of the advantages of the auto-transformer over the ordinary transformer having the same output are:
Lower cost,
Smaller size and weight,
Greater efficiency since the losses are lower,
Smaller exciting current and better regulation.
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JAY B. THAKAR

32. DISADVANTAGES
a) The secondary winding of the auto-transformer will experience high voltage in step down operation when an open circuit occurs on the common winding. This occurrence is very rare.
b) The impedance of the auto-transformer due to the common winding is less than the two winding transformer resulting in a higher fault current being available. This can be overcome by specifying the correct transformer impedance to limit the available fault current to within the equipment duty ratings.
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JAY B. THAKAR

33. c) The auto-transformer provides less of a barrier to electrical noise than does a comparable two winding transformer.
d) Auto-transformers share a common ground return between secondary and primary windings for earth fault currents which could lead to increased operating times to allow other devices to operate first. Directional ground fault relays will be required to distinguish between primary and secondary transformer earth faults.
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JAY B. THAKAR

34. e) Auto-transformers have the limitation of not suppressing harmonic currents and acting as another source of ground fault currents. An auxiliary Delta winding not connected to the outside of the tank may be required to absorb some of the harmonic currents.
These problems are same for auto transfor-mers and star-star connected two winding transformers.
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35. An auto-transformer (circuit diagram)

36. example
A transformer for home use of a portable radio reduces 120-V ac to 9.0-V ac. (Such a device also contains diodes to change the 9.0-V ac to dc.). The secondary contains 30 turns and the radio draws 400 mA. Calculate:
(i) the number of turns in the primary;
(ii) the current in the primary; and
(iii) the power transformed.
NB: DC voltages do not work in a transformer because there would be no changing magnetic flux.

37. Why power is transmitted at very high voltages
Transformers play an important role in the transmission of electricity. Power plants are often situated some distance from metropolitan areas, so electricity must often be transmitted over long distances. There is always some power loss in the transmission lines, and this loss can be minimized if the power is transmitted at high voltage, using transformers.

38. Transmission lines. An average of 120 kW of electric power is sent to a small town from a power plant 10 km away. The transmission lines have a total resistance of 0.40 Ω. Calculate the power loss if the power is transmitted at a) 240 V and b) 24,000 V.
SOLUTION We cannot use P = V²/R because if R is the resistance of the transmission lines, we do not know the voltage drop along them; the given voltages are applied across the lines plus the load (the town). But for each case we can determine the current I in the lines, and find the power loss from P = I²R.

39. The power loss in the lines, PL,is then
(a) If 120 kW is sent at 240 V, the total current will be
I = P/V = 120,000/240
= 500 A
The power loss in the lines, PL,is then
PL= I²R
= (500)²(0.40) = 100 kW.
Thus over 80 % of all the power would be wasted as heat in the power lines.

40. (b) If 120 kW is sent at 24,000 V, the total current will be
I = P/V = 120,000/24000
= 5.0 A
The power loss in the lines is then
PL= I²R = (5.0)²(0.40) = 10 W,
which is less than 1/100 of 1 %. We see that the greater the voltage, the less the current and thus the less power is wasted in the transmission lines. It is for this reason that power is usually transmitted at very high voltages, as high as 700 kV.

41. More practice problems
A transformer has 500 primary turns and 3000 secondary turns. If the primary voltage is 240 V, determine the secondary voltage, assuming an ideal transformer.
An ideal transformer, connected to a 240 V mains, supplies a 12 V, 150 W lamp. Calculate the transformer turns ratio and the current taken from the supply.
A transformer for home use of a portable radio reduces 120-V ac to 9.0-V ac. (Such a device also contains diodes to change the 9.0-V ac to dc.). The secondary contains 30 turns and the radio draws 400 mA. Calculate:
(i) the number of turns in the primary;
(ii) the current in the primary; and
(iii) the power transformed.