A single-phase transformer rated 7.2 kVA, 1.2 kV/120 V has a primary winding
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A single-phase transformer rated 7.2 kVA, 1.2 kV/120 V has a primary winding

3.1. A single-phase transformer rated 7.2 kVA, 1.2 kV/120 V has a primary winding of 800 turns. Determine (a) the turns ratio and the number of turns in the secondary winding and (b) the currents carried by the two windings when the transformer delivers its rated kVA at rated voltages. Hence, verify Equation (3.6).


Below is Fig 3.2


3.5. For the pair of mutually coupled coils shown in Figure 3.4, develop an equivalent-T network in the form of Figure 3.5. Use the parameter values given in Prob. 3.4 and assume that the turns ratio a equals 2. What are the values of the leakage reactances of the windings and the magnetizing susceptance of the coupled coils?

Below is Fig 3.4


3.7. A single-phase transformer is rated 440/220 V, 5.0 kVA. When the low-voltage side is short circuited and 35 V is applied to the high-voltage side, rated current flows in the windings and the power input is 100 W. Find the resistance and reactance of the high- and low-voltage windings if the power loss and ratio of reactance to resistance is the same in both windings.

3.10. A single-phase transformer rated 30 kVA, 1200/120 V is connected as an auto-transformer to supply 1320 V from a 1200 V bus.

a. Draw a diagram of the transformer connections showing the polarity marks on the windings and directions chosen as positive for current in each winding so that the currents will be in phase.

b. Mark on the diagram the values of rated current in the windings and at the input and output.

c. Determine the rated kilovoltamperes of the unit as an autotransformer

d. If the efficiency of the transformer connected for 1200/120 V operation at rated load unity power factor is 97%, determine its efficiency as an autotransformer with rated current in the windings and operating at rated voltage to supply a load at unity power factor.

3.11. Solve Prob. 3.10 if the transformer is to supply 1080 V from a 1200 V bus.

Hint
EngineeringA transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. Transformers are employed for widely varying purposes. For example, a transformer is often used to reduce the voltage of conventional power circuits to operate low-voltage devices and to raise the voltage from electric generators so that electric po...

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