1. Transformer DKT 123 1

2. 1.1 Introduction to Transformer.  Transformer is a device that changes ac electrical power at one voltage level to ac electric power at another voltage level through the action of magnetic field. Figure 1.1: Block Diagrams of Transformer. 2

3. 1.2 Applications of Transformer. Why do we need transformer? The Power Grid 3

4. 1.2 Applications of Transformer. Why do we need transformer? (a) Step Up. In modern power system, electrical power is generated at voltage of 12kV to 25kV. Transformer will step up the voltage to between 110kV to 1000kV for transmission over long distance at very low lost. The Power Grid The Transmission Tower 4

5. 1.2 Applications of Transformer. Why do we need transformer? (b) Step Down. The transformer will stepped down the voltage to the 12kV to 34.5kV range for local Distribution. In homes, offices and factories stepped down to 240V. The Power Grid The Substation The Utility poles 5

6. 1.3 Types and Constructions of Transformer.  Power transformers are constructed on two types of cores; (i) Core form. (ii) Shell form. Figure 1.2: Core Form and Shell Form. 6

7. Core form.  The core form construction consists of a simple rectangular laminated piece of steel with the transform winding wrapped around the two sides of the rectangle. Shell form.  The shell form construction consists of a three-legged laminated core with the winding wrapped around the center leg. Cont’d… 7

8. Construction.  Transformer consists of two or more coils of wire wrapped around a common ferromagnetic core. The coils are usually not directly connected.  The common magnetic flux present within the coils connects the coils.  There are two windings; (i) Primary winding (input winding); the winding that is connected to the power source. (ii) Secondary winding (output winding); the winding connected to the loads. Cont’d… Figure 1.3: A Simple Transformer. 8

9. Operation.  When AC voltage is applied to the primary winding of the transformer, an AC current will result i L or i 2 (current at load).  The AC primary current i 2 set up time varying magnetic flux  in the core. The flux links the secondary winding of the transformer. Cont’d… 9

10. Operation.  From the Faraday law, the emf will be induced in the secondary winding. This is known as transformer action.  The current i 2 will flow in the secondary winding and electric power will be transfer to the load.  The direction of the current in the secondary winding is determined by Len’z law. The secondary current’s direction is such that the flux produced by this current opposes the change in the original flux with respect to time. Cont’d… 10

11. FARADAY’S LAW In 1831 two people, Michael Faraday in the UK and Joseph Henry in the US performed experiments that clearly demonstrated that a changing magnetic field produces an induced EMF (voltage) that would produce a current if the circuit was complete. If current produces a magnetic field, why can't a magnetic field produce a current ? Michael Faraday 1.4 General Theory of Transformer Operation. 11

12. When the switch was closed, a momentary deflection was noticed in the galvanometer after which the current returned to zero. When the switch was opened, the galvanometer deflected again momentarily, in the other direction. Current was not detected in the secondary circuit when the switch was left closed. 12

13. Faraday found that the induced e.m.f. increases if (i) the speed of motion of the magnet or coil increases. (ii) the number of turns on the coil is made larger. (iii) the strength of the magnet is increased. An e.m.f. is made to happen (or induced) in a conductor (like a piece of metal) whenever it 'cuts' magnetic field lines by moving across them. This does not work when it is stationary. If the conductor is part of a complete circuit a current is also produced. 13

14. Faraday’s Law E = Electromotive force (emf) Φ = Flux N = Number of turn t = time Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. 14

15. Inserting a magnet into a coil also produces an induced voltage or current. The faster speed of insertion/ retraction, the higher the induced voltage. 15

16.  According to the Faraday’s law of electromagnetic induction, electromagnetic force (emf’s) are induced in N 1 and N 2 due to a time rate of change of  M, Where, (1.1) e = instantaneous voltage induced by magnetic field (emf), = number of flux linkages between the magnetic field and the electric circuit.  = effective flux Figure 1.4: Basic Transformer Components. 16

17.  Lenz’s Law states that the direction of e 1 is such to produce a current that opposes the flux changes.  If the winding resistance is neglected, then equation (1.1) become; (1.2)  Taking the voltage ratio in equation (1.2) results in, (1.3) Cont’d… 17

18.  Neglecting losses means that the instantaneous power is the same on both sides of the transformer; (1.4)  Combining all the above equation we get the equation (1.5) where a is the turn ratio of the transformer. (1.5) Cont’d… a > 1  Step down transformer a < 1  Step up transformer a = 1  Isolation Transformer 18

19.  According to Lenz’a Law, the direction of e is oppose the flux changes, and the flux varies sinusoidally such that (1.6)  Substitute eqn(1.6) into eqn(1.2) (1.7)  The rms value of the induce voltage is; (1.8)  =  max sin  t Cont’d…  max 19

20.  Losses are composed of two parts; (a) The Eddy-Current lost. Eddy current lost is basically loss due to the induced current in the magnetic material. To reduce this lost, the magnetic circuit is usually made of a stack of thin laminations. (b) The Hysteresis loss. Hysteresis lost is caused by the energy used in orienting the magnetic domains of the material along the field. The lost depends on the material used. Cont’d… 20

21. 1.5 The Ideal Transformer.  An Ideal transformer is a lossless device with an input winding and an output winding.  Zero resistance result in zero voltage drops between the terminal voltages and induced voltages  Figure below shows the relationship of input voltage and output voltage of the ideal transformer. An Ideal Transformer and the Schematic Symbols. 21

22.  The relationship between voltage and the number of turns. N p, number of turns of wire on its primary side. N s, number of turns of wire on its secondary side. V p (t), voltage applied to the primary side. V s (t), voltage applied to the secondary side. where a is defined to be the turns ratio of the transformer. Cont’d… 22

23.  The relationship between current into the primary side, I p (t), of transformer versus the secondary side, I s (t), of the transformer;  In term of phasor quantities; -Note that V p and V s are in the same phase angle. I p and I s are in the same phase angle too. - the turn ratio, a, of the ideal transformer affects the magnitude only but not the their angle. Cont’d… 23

24.  The dot convention appearing at one end of each winding tell the polarity of the voltage and current on the secondary side of the transformer.  If the primary voltage is positive at the dotted end of the winding with respect to the undotted end, then the secondary voltage will be positive at the dotted end also. Voltage polarities are the same with respect to the doted on each side of the core.  If the primary current of the transformer flow into the dotted end of the primary winding, the secondary current will flow out of the dotted end of the secondary winding. Cont’d… 24

25. Example 1: Transformer. How many turns must the primary and the secondary windings of a 220 V-110 V, 60 Hz ideal transformer have if the core flux is not allowed to exceed 5mWb? Solution: For an ideal transformer with no losses, From the emf equation, we have 25

26. 1.5.1 Power in an Ideal Transformer.  Power supplied to the transformer by the primary circuit is given by ; where,  p is the angle between the primary voltage and the primary current.  The power supplied by the transformer secondary circuit to its loads is given by the equation; where,  s is the angle between the secondary voltage and the secondary current.  Voltage and current angles are unaffected by an ideal transformer,  p –  s  he primary and secondary windings of an ideal transformer have the same power factor. 26

27.  The power out of a transformer; - apply V s = V p / a and I s = a I p into the above equation gives, - The output power of an ideal transformer is equal to the input power. Cont’d… 27

28.  The reactive power, Q, and the apparent power, S;  In term of phasor quantities; -Note that V p and V s are in the same phase angle. I p and I s are in the same phase angle too. - the turn ratio, a, of the ideal transformer affects the magnitude only but not the their angle. Cont’d… 28

29. Example 2: Ideal Transformer. Consider an ideal, single-phase 2400V-240V transformer. The primary is connected to a 2200V source and the secondary is connected to an impedance of 2    find, (a) The secondary output current and voltage. (b) The primary input current. (c) The load impedance as seen from the primary side. (d) The input and output apparent power. (e) The output power factor. 29

30. Example 2: Ideal Transformer. Consider an ideal, single-phase 2400V-240V transformer. The primary is connected to a 2200V source and the secondary is connected to an impedance of 2    find, Solution: 30

31. .. Cont’d…Example 2 31

32. 1.7 The Exact Equivalent Circuit of a Real Transformer.  Figure below is an exact model of a transformer.  To analyze the transformer it is necessary to convert the entire circuit to an equivalent circuit at a single voltage level as in Figure 1.8. Model of a Real Transformer 32

33. 1.7 The Exact Equivalent Circuit of a Real Transformer.  To analyze the transformer it is necessary to convert the entire circuit to an equivalent circuit at a single voltage level. (b) The Transformer Model Referred to its Secondary Windings. (a) The Transformer Model Referred to its Primary Windings. 33

34.  Symbols used for the Exact Equivalent Circuit above; Cont’d… 34

37. Refer to primary side 37 Refer to Secondary side

42. The Approximate Equivalent Circuit of a Transformer Approximate Transformer Model Referred to the Primary Side. 42  The equivalent impedance for the circuit is;

43. Approximate Circuit Model of a Transformer Referred to the Secondary.  The equivalent impedance for the circuit is; 43

44. 1.9 Transformer Voltage Regulation and Efficiency.  Voltage regulation is a measure of the change in the terminal voltage of the transformer with respect to loading. Therefore the voltage regulation is defined as:  At no load, V s = V p / a and the voltage regulation can also be express as; 44

45. 1.9 Transformer Voltage Regulation and Efficiency.  In the per-unit system;  For ideal transformer VR=0. It is a good practice to have as small voltage regulator as possible. 45

46. Example of Transformer Voltage Regulation. Cont’d… 46

47.  Transformer Efficiency, efficiency of a transformer is defined as follows;  For Non-Ideal transformer, the output power is less than the input power because of losses. These losses are the winding or I 2 R loss (copper losses) and the core loss (hysteresis and eddy-current losses). Cont’d… 47

48.  Thus, in terms of the total losses, P losses, the above equation may be expressed as;  The winding or copper loss is load dependent, whereas the core loss is constant and almost independent of the load on the transformer. Cont’d… 48

49. Example 3: Transformer Voltage Regulation. 49

50. Cont’d…Example 3 1.1 1 50

51. 1.10 Open Circuit and Short Circuit. Open Circuit Test.  The open circuit test is conducted by applying rated voltage at rated frequency to one of the windings, with the other windings open circuited. The input power and current are measured.  For reasons of safety and convenience, the measurements are made on the low-voltage (LV) side of the transformer. 51

52. Short Circuit Test.  The short-circuit test is used to determine the equivalent series resistance and reactance.  One winding is shorted at its terminals, and the other winding is connected through proper meters to a variable, low-voltage, high-current source of rated frequency.  The source voltage is increased until the current into the transformer reaches rated value. To avoid unnecessary high currents, the short-circuit measurements are made on the high-voltage side of the transformer. Cont’d… 52