Delmar's Standard Textbook of Electricity 5th. Edition by Stephen L. Herman | 9781111539153 | 1111539154

Basic Electrical

Delmar's Standard Textbook of Electricity

78 SOLVED PROBLEMS

15 SOLVED PROBLEMS

Question: 25.1

Assume that the RLC parallel circuit shown in Figure 25–2 is connected to a 240-V, 60-Hz line. The resistor has a resistance of 12 Ω, the inductor has an inductive reactance of 8 Ω, and the capacitor has a capacitive reactance of 16 Ω. Complete the following unknown values: Z—impedance of the ...

Verified Answer:

Impedance The impedance of the circuit is the reci...

Question: 28.2

A single-phase transformer is rated at 50 kVA and has a secondary voltage of 240 V. The nameplate reveals that the transformer has an internal impedance (%Z) of 2.5%. What is the short-circuit current for this transformer? ...

Verified Answer:

I_{secondary} = \frac{50, 000 VA}{240 V}[...

Question: 11.8

An electric annealing oven is located in an area with an ambient temperature of 125°F. The oven contains a 50-kW electric heating element and is connected to 480 volts. The conductors are to be copper with type THHN insulation. The termination temperature is not known. The furnace is expected to ...

Verified Answer:

Determine the amount of current needed to operate ...

Question: 19.1

In this circuit, one resistor is connected in parallel with two inductors (Figure 19–11). The frequency is 60 Hz. The circuit has an apparent power of 6120 VA, the resistor has a resistance of 45 Ω, the first inductor has an inductive reactance of 40 Ω, and the second inductor has an inductive ...

Verified Answer:

Impedance Before it is possible to calculate the i...

Question: 11.7

Assume that a bank of heating resistors is rated at 28 kW and is connected to 240 V. The resistors operate for more than three hours at a time and are located in an area with an ambient temperature of 86ºF. Aluminum conductors with Type THWN-2 insulation are to be used to connect the heaters. What ...

Verified Answer:

Determine the amperage of the load using Ohm’s law...

Question: 23.2

In this circuit, a resistor and a capacitor are connected in parallel to a 400-Hz line. The power factor is 47.1%, the apparent power is 4086.13 VA, and the capacitance of the capacitor is 33.15 µF (Figure 23–8). Find the following unknown values: ∠θ—angle theta P—true power VARsC—capacitive VARs ...

Verified Answer:

Angle Theta The power factor is the cosine of angl...

Question: 22.2

A small indicating lamp has a rating of 2 W when connected to 120 V. The lamp must be connected to a voltage of 480 V at 60 Hz. A capacitor will be connected in series with the lamp to reduce the circuit current to the proper value. What value of capacitor will be needed to perform this job? ...

Verified Answer:

The first step is to determine the amount of curre...

Question: 22.1

One resistor and two capacitors are connected in series to a 42.5-V, 60-Hz line (Figure 22–8). Capacitor 1 has a reactive power of 3.75 VARs, the resistor has a true power of 5 W, and the second capacitor has a reactive power of 5.625 VARs. Find the following values: VA—apparent power IT—total ...

Verified Answer:

Because the reactive power of the two capacitors i...

Question: 24.3

The RLC series circuit shown in Figure 24–8 contains an inductor with an inductive reactance of 62 Ω and a capacitor with a capacitive reactance of 38 Ω. The circuit is connected to a 208-V, 60-Hz line. How much resistance should be connected in the circuit to limit the circuit current to a value ...

Verified Answer:

The first step is to determine the total impedance...

Question: 25.2

In the circuit shown in Figure 25–6, a resistor, an inductor, and a capacitor are connected to a 1200-Hz power source. The resistor has a resistance of 18 Ω, the inductor has an inductance of 9.76 mH (0.00976 H), and the capacitor has a capacitance of 5.5 μF. What is the impedance of this circuit? ...

Verified Answer:

The first step in finding the impedance of this ci...

Question: 21.2

A small indicating lamp has a rating of 2 W when connected to 120 V. The lamp must be connected to a voltage of 480 V at 60 Hz. A capacitor will be connected in series with the lamp to reduce the circuit current to the proper value. What value of capacitor will be needed to perform this job? ...

Verified Answer:

The first step is to determine the amount of curre...

Question: 16.3

An inductor with negligible resistance is connected to a 480-V, 60-Hz line. An ammeter indicates a current flow of 24 A. How much current will flow in this circuit if the frequency is increased to 400 Hz? ...

Verified Answer:

The first step in solving this problem is to deter...

Question: 15.9

A half-wave rectifier is connected to 277 VAC. What is the average DC voltage? ...

Verified Answer:

average = RMS ×

\frac{0.9}{2}

avera...

Question: 15.6

A sine wave has a peak value of 354 V. What is the RMS value? ...

Verified Answer:

RMS = peak × 0.707 RMS = 354 V × 0.707 RMS = 250.2...

Question: 13.1

A coil has an inductance of 1.5 H and a wire resistance of 6 Ω. If the coil is connected to a battery of 3 V, how long will it take the current to reach its full Ohm’s law value of 0.5 A (3 V/6 Ω = 0.5 A)? ...

Verified Answer:

To find the time of one time constant, use the for...

Question: 2.1

An elevator must lift a load of 4000 lb to a height of 50 ft in 20 s. How much horsepower is required to operate the elevator? ...

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Find the amount of work that must be performed, an...

Question: 2.2

A water heater contains 40 gallons of water. Water weighs 8.34 lb per gallon. The present temperature of the water is 68ºF. The water must be raised to a temperature of 160ºF in one hour. How much power will be required to raise the water to the desired temperature? ...

Verified Answer:

First determine the weight of the water in the tan...

Question: 15.3

A sine wave has a maximum voltage of 350 V. At what angle of rotation will the voltage reach 53 V? ...

Verified Answer:

\sin \angle \theta=\frac{E_{\text {(INST) }...

Question: 16.2

Assume an inductor with a negligible resistance is connected to a 36-V, 400-Hz line. If the circuit has a current flow of 0.2 A, what is the inductance of the inductor? ...

Verified Answer:

The first step is to determine the inductive react...

Question: 10.10

A manufacturing plant has a cooling pond located 4000 ft from the plant. Six pumps are used to circulate water between the pond and the plant. In cold weather, however, the pumps can freeze and fail to supply water. The plant owner decides to connect electric-resistance heaters to each pump to ...

Verified Answer:

The first step in the solution of this problem is ...