Transformers READING Text, Sections 14-1 through 14-9 OBJECTIVES After performin

Question

Transformers READING Text, Sections 14-1 through 14-9 OBJECTIVES After performing this experiment, you will be able to: 2. Show the phase relationships between the primary and secondary of a center-tapped transformer 3. Compute the turns ratio required for matching a signal generator to a speaker 4. Demonstrate how an impedance-matching transformer can increase the power transferred to a load. MATERIALS NEEDEID One 12.6 V center-tapped transformer One small impedance-matching transformer (approximately 600 to 800 ) One small speaker (4 or 8 ) For Further Investigation One 100 resistor SUMMARY OF THEORY A transformer consists of two (or more) closely coupled coils that share a common magnetic field. When an ac voltage is applied to the first coil, caled the primary, a voltage is induced in the second coil, called the secondary. The voltage that appears across the secondary is proportional to the transformer tuns ratio. The turns ratio is found by dividing the number of turns in the secondary winding by the number of tuns in the primary winding. The turns ratio, n, is directly proportional to the primary and secondary voltages. That is N, V For most work, we can assume that a transformer has no internal power dissipation and that all the magnetic flux lines in the primary also cut through the secondary- that is, we can assume the transformer is ideal The ideal transformer delivers to the load 100% of the applied power. Actual transformers have losses due to magnetizing current, eddy currents, coil resistance, and so forth. In typical power applications, transformers are used to change the ac line voltage from one voltage to another or to isolate ac grounds. For the ideal transformer, the secondary voltage is found by multiplying the turns ratio by the applied primary voltage. That is, Since the ideal transformer has no internal losses, we can equate the power delivered to the primary to the power delivered by the secondary. Since P IV we can write 237

Explanation / Answer

1.(a)Resistance ratio=measured secondary resistance/measured primary resistance

Therefore Resistance ratio=0.9/23.67=0.038

(b)The winding resistance is a function of the conductor crossectional area, length, material and temperature. The resistance will increase with the number of turns simply because the length increases with each additional turn.So for the Resistance ratio to be equal to turns ratio ,the conductors in primary and secondary sides should be identical and area of crosssection of transformer should be same .Even when same conductors are used turns ratio will not be same due to the reason that magnetic losses also exists which are being neglected .

2.The difference in ratios can be due to shorted turns or high resistance connections in the lead circuitry

and also as Accuracy of the measuring instruments<0.1% .

3.Marked Skipped

4.Ideal transformer with no load is like a pure inductor . Since power lost is due to resistive components ,therefore power supplied to an ideal transformer is zero

5.

(a) Since in an ideal transformer power loss=0

power supplied = power consumed by load

Given Primary voltage Vp=115V and primary current Ip=200mA

so power supplied=Vp*Ip=115*0.2=23W=power consumed by load

(b) since power consumed by load is 23W in part (a)

Given secondary voltage Vs=24V

Therefore secondary current Is = P/Vs = 23/24 =958.34mA

(c) Turns ratio= Vp/Vs = 115/25 = 4.6

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