Comprehensive Problems
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Comprehensive Problems
- 11.86 A transmitter delivers maximum power to an antenna when the antenna is adjusted to represent a load of 75-Ω resistance in series with an inductance of 4 μH. If the transmitter operates at 4.12 MHz, find its internal impedance.
- 11.87 In a TV transmitter, a series circuit has an impedance of 3 kΩ and a total current of 50 mA. If the voltage across the resistor is 80 V, what is the power factor of the circuit?
- 11.88 A certain electronic circuit is connected to a 110-V ac line. The root-mean-square value of the current drawn is 2 A, with a phase angle of 55°.
- (a) Find the true power drawn by the circuit.
- (b) Calculate the apparent power.
11.89 An industrial heater has a nameplate that reads:
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210 V 60 Hz 12 kVA 0.78 pf lagging Determine:
- (a) the apparent and the complex power
- (b) the impedance of the heater
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11.90 A 2000-kW turbine-generator of 0.85 power factor operates at the rated load. An additional load of 300 kW at 0.8 power factor is added.What kVAR *
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(b) Calculate the charge per kWh with a flat-rate tariff if the revenue to the utility company is to remain the same as for the two-part tariff.
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11.85 A regular household system of a single-phase threewire circuit allows the operation of both 120-V and 240-V, 60-Hz appliances. The household circuit is modeled as shown in Fig. 11.96. Calculate:
- (a) the currents I1, I2, and I*n*
- (b) the total complex power supplied
- (c) the overall power factor of the circuit
Figure 11.96
For Prob. 11.85.
of capacitors is required to operate the turbinegenerator but keep it from being overloaded?
11.91 The nameplate of an electric motor has the following information:
Line voltage: 220 V rms Line current: 15 A rms Line frequency: 60 Hz Power: 2700 W
Determine the power factor (lagging) of the motor. Find the value of the capacitance C that must be connected across the motor to raise the pf to unity.
- 11.92 As shown in Fig. 11.97, a 550-V feeder line supplies an industrial plant consisting of a motor drawing 90 kW at 0.8 pf (inductive), a capacitor with a rating of 20 kVAR, and lighting drawing 10 kW.
- (a) Calculate the total reactive power and apparent power absorbed by the plant.
- (b) Determine the overall pf.
- (c) Find the magnitude of the current in the feeder line.
* An asterisk indicates a challenging problem.
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11.93 A factory has the following four major loads:
- A motor rated at 5 hp, 0.8 pf lagging (1hp = 0.7457 kW).
- A heater rated at 1.2 kW, 1.0 pf.
- Ten 120-W lightbulbs.
- A synchronous motor rated at 1.6 kVAR, 0.6 pf leading.
- (a) Calculate the total real and reactive power.
- (b) Find the overall power factor.
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(a) Calculate the cost of capacitors needed.
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(b) Find the savings in substation capacity released.
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(c) Are capacitors economical for releasing the amount of substation capacity?
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(a) At what frequency is maximum power transferred to the speaker?
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(b) If Vs = 4.6 V rms, how much power is delivered to the speaker at that frequency?
Figure 11.98
For Prob. 11.95.
- 1.96 A power amplifier has an output impedance of 40 + j8 Ω. It produces a no-load output voltage of 146 V at 300 Hz.
- (a) Determine the impedance of the load that achieves maximum power transfer.
- (b) Calculate the load power under this matching condition.
- 1.97 A power transmission system is modeled as shown in Fig. 11.99. If Vs = 440⧸0*°* rms, find the average power absorbed by the load.
Figure 11.99 For Prob. 11.97.
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