AC alternating current and power analysis
- 2. Introduction • Power is the most important quantity in electric utilities, electronic, and communication systems, because such systems involve transmission of power from one point to another. Also, every industrial and household electrical device—every fan, motor, lamp, pressing iron, TV, personal computer— has a power rating that indicates how much power the equipment requires; exceeding the power rating can do permanent damage to an appliance. The most common form of electric power is 50- or 60-Hz ac power. The choice of ac over dc allowed high-voltage power transmission from the power generating plant to the consumer.
- 3. Instantaneous and Average Power The instantaneous power p(t) absorbed by an element is the product of the instantaneous voltage v(t) across the element and the instantaneous current i(t) through it. Assuming the passive sign convention, • The instantaneous power (in watts) is the power at any instant of time. We can also think of the instantaneous power as the power absorbed by the element at a specific instant of time. Instantaneous quantities are denoted by lowercase letters.
- 4. Let the voltage and current at the terminals of the circuit be Where Vm and Im are the amplitudes (or peak values), and ϴv and ϴi are the phase angles of the voltage and current, respectively. The instantaneous power absorbed by the circuit is We apply the trigonometric identity
- 5. This shows us that the instantaneous power has two parts. The first part is constant or time independent. Its value depends on the phase difference between the voltage and the current. The second part is a sinusoidal function whose frequency is 2ω which is twice the angular frequency of the voltage or current. A sketch of p(t) in is shown in the figure, where is the period of voltage or current. We observe that p(t) is periodic, and has a period of since its frequency is twice that of voltage or current. We also observe that p(t) is positive for some part of each cycle and negative for the rest of the cycle. When p(t) is positive, power is absorbed by the circuit. When p(t) is negative, power is absorbed by the source; that is, power is transferred from the circuit to the source. This is possible because of the storage elements (capacitors and inductors) in the circuit.
- 6. Average Power • The instantaneous power changes with time and is therefore difficult to measure. The average power is more convenient to measure. In fact, the wattmeter, the instrument for measuring power, responds to average power. • The average power, in watts, is the average of the instantaneous power over one period. Thus, the average power is given by
- 7. Average Power Consider two special cases of the above equation. When the voltage and current are in phase. This implies a purely resistive circuit or resistive load R, and Where
- 8. Case 2: Average Power • When we have a purely reactive circuit, and showing that a purely reactive circuit absorbs no average power. In summary, A resistive load (R) absorbs power at all times, while a reactive load (L or C ) absorbs zero average power.
- 12. For the circuit shown in the figure, find the average power supplied by the source and the average power absorbed by the resistor.
- 13. Solution: We apply mesh analysis as shown in Figure(b). For mesh 1,
- 17. Questions?
Editor's Notes
- #5: The instantaneous power p(t) entering the circuit. The instantaneous power changes with time and is therefore difficult to measure.