Transformers

1. Chapter 14 Transformers

2. Objectives Explain mutual inductance Describe how a transformer is constructed and how it works Explain how a step-up transformer works Explain how a step-down transformer works Discuss the effect of a resistive load across the secondary winding

3. Objectives Discuss the concept of a reflected load in a transformer Discuss impedance matching with transformers Explain how the transformer acts as an isolation device Describe a practical transformer

4. Mutual Inductance When two coils are placed close to each other, a changing electromagnetic field produced by the current in one coil will cause an induced voltage in the second coil because of the mutual inductance Mutual inductance is established by the inductance of each coil and by the amount of coupling between the two coils. To maximize coupling, the two coils are wound on a common core

5. Coefficient of Coupling The coefficient of coupling (k) between two coils is the ratio of the lines of force (flux) produced by one coil linking the second coil (?1-2) to the total flux produced by the first coil (?1) The coefficient of coupling depends on the physical closeness of the coils and the type of core material on which they are wound Construction and core shape are also factors influencing coefficient of coupling

6. The Basic Transformer Source voltage is applied to the primary winding The load is connected to the secondary winding The core provides a physical structure for placement of windings and a magnetic path so that the magnetic flux lines are concentrated close to the coils Typical core materials are: air, ferrite, and iron Air and ferrite cores are used at high frequencies Iron cores are used for AF and power applications

7. Turns Ratio Turns ratio (n) is defined as the ratio of the number of turns in the secondary winding (Nsec) to the number of turns in the primary winding (Npri) n = Nsec/Npri

8. Direction of Windings The direction of the windings determines the polarity of the voltage across the secondary winding with respect to the voltage across the primary Phase dots are used to indicate polarities

9. Step-up Transformers A transformer in which the secondary voltage is greater than the primary voltage is called a step-up transformer The ratio of secondary voltage (Vsec) to primary voltage (Vpri) is equal to the ratio of the number of turns in the secondary winding (Nsec) to the number of turns in the primary winding (Npri) Vsec/Vpri = Nsec/Npri

10. Step-Down Transformer A transformer in which the secondary voltage is less than the primary voltage is called a step-down transformer The amount by which the voltage is stepped down depends on the turns ratio The turns ratio of a step-down transformer is always less than 1

11. Loading the Secondary Winding When a load resistor is connected to the secondary winding, there is a current through the resulting secondary circuit because of the voltage induced in the secondary coil This secondary current results in a primary current Isec = (Npri/Nsec)Ipri

12. Primary Power Equals Load Power For an ideal transformer, the power delivered in the secondary equals the power in the primary In a real transformer, some power is dissipated in the transformer, so primary power is always greater than secondary power In an ideal transformer, power transfer is not related to the turns ratio Ppri = VpriIpri = VsecIsec = Psec

13. Reflected Load From the viewpoint of the primary circuit, a load connected across the secondary winding of a transformer appears to have a resistance that is not necessarily equal to the actual resistance of the load The actual load is essentially �reflected� into the primary circuit altered by the turns ratio This reflected load is what the primary source effectively see, and it determines the amount of primary current

14. Reflected Load The effective resistance that the primary sees is: Rpri = (Npri/Nsec)2RL

15. Impedance Matching When a source is connected to a load, maximum power is delivered to the load when the load impedance is equal to the fixed internal source impedance One application of transformers is in the matching of a load resistance to a source resistance in order to achieve maximum transfer of power This is termed impedance matching Transformers designed specifically for impedance matching usually show the input and output impedance they are designed to match

16. Matching Transformer The reflected-resistance characteristics provided by a transformer are used to make the load resistance appear to have the same value as the source resistance

17. The Transformer as an Isolation Device Transformers are useful in providing electrical isolation between the primary circuit and the secondary circuit because there is no electrical connection between the two windings In a transformer, energy is transferred entirely by magnetic coupling

18. DC Isolation A transformer does not pass dc, therefore a transformer can be used to keep the dc voltage on the output of an amplifier stage from affecting the bias of the next amplifier The ac signal is coupled through the transformer between amplifier stages

19. Power Line Isolation Transformers are often used to electrically isolate electronic equipment from the ac power line

20. Practical Transformers Real transformers have winding resistance, resistance in series with each winding, resulting in less than idea secondary voltage Hysteresis losses are core losses due to the continuous reversal of the magnetic field due to the changing direction of the primary current Eddy currents result in more heat losses in the core material. Eddy currents are produced when voltage is induced in the core material itself

21. Practical Transformers Magnetic flux leakage is the result of some of the magnetic flux lines breaking out of the core and passing through the surrounding air back to the other end of the winding Magnetic flux leakage results in less secondary voltage Winding capacitance results in a bypassing effect across the primary winding and the secondary load Winding capacitance increases with frequency

22. Transformer Power Rating and Efficiency Transformers are typically rated in volt-amperes (VA), primary/secondary voltage, and operating frequency The efficiency of a transformer is a measure of the percentage of input power that is delivered to the output ? = (Pout/Pin)100%

23. Tapped Transformers The center tap (CT) transformer is equivalent to two secondary windings with half the voltage across each Center tap windings are used for rectifier supplies and impedance-matching transformers

24. Multiple-Winding Transformers Multiple-winding transformers have more than one winding on a common core. They are used to operate on, or provide, different operating voltages

25. Auto Transformers In an autotransformer, one winding serves as both the primary and the secondary. The winding is tapped at the proper points to achieve the desired turns ratio for stepping up or down the voltage

26. Summary A normal transformer consists of two or more coils that are magnetically coupled on a common core There is mutual inductance between two magnetically coupled coils When current in one coil changes, voltage is induced in the other coil The primary is the winding connected to the source, and the secondary is the winding connected to the load

27. Summary The number of turns in the primary and the number of turns in the secondary determine the turns ratio The relative polarities of the primary and secondary voltages are determined by the direction of the windings around the core A step-up transformer has a turns ratio greater than 1

28. Summary A step-down transformer has a turns ratio less than 1 A transformer cannot increase power In an ideal transformer, the power from the source (input power) is equal to the power delivered to the load (output power) If the voltage is stepped up, the current is stepped down, and vice versa

29. Summary A load across the secondary winding of a transformer appears to the source as a reflected load having a value dependent on the reciprocal of the turns ratio squared An impedance-matching transformer can match a load resistance to an internal source resistance to achieve maximum power transfer to the load by selecting the proper turns ratio A typical transformer does not respond to dc

30. Summary Conversion of electrical energy to heat in an actual transformer results from winding resistances, hysteresis loss in the core, eddy currents in the core, and flux leakage