Questions and Answers of Introduction To Electrical Power

A buck converter has 220-V dc input voltage, RL = 18 Ω, switching frequency of 5 kHz, and on-time of 80 μs. If the average source current is 10 A, determine: (1) The average output voltage,
For a low-voltage buck converter with Vout = 3.5 V, fs = 50 kHz, L = 200 μH, C = 500 μF, Vin = 12 V, and Iout = 1.5 A, determine the peak-to-peak ripple ΔVout in the output voltage.
A ship was designed to travel at 15 knots with a dedicated 4.6-kV, 30-MWe propulsion power plant consisting of the generators, transformers, VFDs, and motors rated accordingly. A new line of ships is
A buck converter is operating at 2-kHz switching frequency from a 220-V dc source. The inductance is 30 mH. If the output voltage is 110 V and the load resistance is 15 Ω, determine:(1) The switch
Why can the triplen harmonics currents not flow in the Δ-connected system?
A transistor junction has a total power loss of 25 W, and the junction to case thermal resistance is 0.9°C/W. The case is mounted on heat sink #9 with a 50-μm-thick mica insulation with thermal
A transistor junction has a power loss of 45 W and a junction to case thermal resistance of 1.1°C/W. The case is mounted on the heat sink #6 with 35-μm-thick mica insulation with thermal grease
The power train devices in a power electronics converter are mounted on a metal base plate 10 cm high × 20 cm wide. The total switching power loss in the devices is 40 W. Determine the temperature
The power electronics devices in a converter assembly are mounted on a metal base plate 15 cm high × 25 cm wide. The total switching power loss in the devices is 95 W. Determine the temperature rise
How does the variable-speed drive (VSD) for a dc motor differ for the VFD for an ac motor in construction and in operation?
Explain how the VFD can also be used for soft starting a large motor and the benefits of doing so.
Why might metal bands on the permanent magnet synchronous motor rotor pose overheating risk? What is the remedy?
Explain the construction and working of the special transformer used to build a 12-pulse VFD.
A three-phase 1-MW bus has a Thevenin source voltage of 520 V and a Thevenin source impedance of 5 + j35 mΩ/phase. Determine the bus voltage when delivering the rated current at 0.85 pf lagging.
A 50-Hz 800-kW bus has a Thevenin source voltage of 480 V and a Thevenin source impedance of 3 + j25 mΩ/phase. Determine the bus voltage when delivering rated current at 0.90 pf lagging.
In a power distribution system, the line voltage often drops by 50% for 10 ms. Does it meet the ANSI standard on quality of power?
The actual values of the harmonic currents in a practical 6-pulse converter are given in the Ih/I1 ratios below, which are less than the theoretical values of I1/Ih.Determine the THDi with the
The values of the harmonic currents measured in a six-pulse converter are given in the Ih/I1 ratios below, which are less than the theoretical values of I1/Ih.Determine the THDi with the actual and
What is the major cause of harmonic currents in the electrical power system?
A three-phase, Y-connected load draws current containing the following Fourier harmonics (all rms amperes) in each line:Determine (1) The total (true) rms value of the current,(2) The THD based on
A three-phase, Y-connected load draws current containing the following Fourier harmonics (all rms amperes) in each line:Determine (1) The total (true) rms value of the current, (2) The THD based on
A power electronics converter draws the fundamental and harmonic currents from a transformer as listed below.Determine (1) The total rms current in the transformer output and (2) The K-rating of
Explain the difference between the static source impedance and the dynamic source impedance.
A power electronics converter draws the fundamental and harmonic currents from a transformer as listed below.Determine: (1) The total rms current in the transformer output and(2) The K-rating of the
A 480-V bus in an industrial plant supplying power to several feeders has a Thevenin source impedance behind the bus equal to 0.003 + j0.015 Ω/phase. A power factor correction capacitor rated 600
Explain the term ringing in the bus voltage, and when does it occur?
A 460-V bus in a factory supplying power to several feeders has a Thevenin source impedance behind the bus equal to 0.002 + j 0.01 Ω/phase. A power factor–correction capacitor rated 500 kVAR3ph is
The bus in Example 16.5 supplies a 200-kVA, 480-V power electronics load that has the harmonic spectrum shown in Table E5.1 below, which also shows the harmonic impedances behind the bus.
Where and how do we use the superposition theorem when the source voltage has many harmonics?
It is desired to filter out the 5th harmonic from the 60-Hz supply lines those powers a six-pulse converter in an industrial power distribution system. Design the harmonic filter capacitor using a
What is the transformer K-rating, and why does the higher K-rated transformer cost more?
Determine the value of filter inductance required to filter out the 11th harmonic from a 60-Hz bus voltage that supplies a 12-pulse converter with a 100-kVAR, 4160-V bus capacitor.
For a 60-Hz 4160-V bus that supplies power to 12-pulse converter, determine two separate harmonic filter inductance values: (1) To filter out the 11th harmonic with a 100-kVAR capacitor and (2) To
Identify equipment around your work area that may severely suffer in performance due to poor quality of power.
The product of the fundamental voltage and the fundamental current results in a net average power delivered to the loads. On the other hand, the product of the fundamental voltage and a harmonic
Identify major ill effects of poor quality of power on the performance of a large induction motor.
A three-phase 460-V transformer is to be procured to power a motor drive that will draw a line current containing the following harmonics as per the VFD supplier:Determine the three-phase
How does one identify poor quality of a power anywhere (at home, office, industry, etc.)?
A 120-V power source with no feedback voltage regulator has the static source impedance of 0.2 + j 0.6 Ω. Determine its steady-state voltage drop at its terminals under an increase of 8 A in the
What are the triplen harmonic currents, and where are they able and unable to flow?
The utility line voltage displayed on a harmonic spectrum analyzer shows the harmonic content of 10%, 7%, 5%, 3%, and 1% of the fundamental— percentages based on the fundamental magnitude—for the
Explain how a very clean harmonic-free power is obtained on navy ships.
Explain how cable shielding and twisting significantly reduce the electromagnetic interference (EMI).
A power electronics device has an on-state power loss of 40 W and switching loss in watts equal to 1.1 × switching frequency in kilohertz. The junction to case thermal resistance is 1.85°C/W, and
A power electronics device has an on-state power loss of 30 W and switching loss in watts equal to 1.5 × switching frequency in kilohertz. The junction to case thermal resistance is 2.0°C/W, and
Briefly summarize the mathematical analogy between the heat transfer and the flow of electrical current.
How would you determine the thermal time constant of a piece of equipment merely by using two readings of a thermometer that are 1 min apart?
A power converter assembly with 800-W power loss is mounted on a 20-cm square base plate cooled by water. The water tubes are of 2-mm internal diameter placed on a 15-cm square with four 90° bends
Why is passing the required quantity of cooling water not sufficient to limit the equipment temperature rise?
A power converter assembly with 1.2-kW power loss is mounted on a 30-cm square base plate cooled by water. The water tubes are of 2-mm internal diameter placed on a 25-cm square frame with four 90°
A piece of electrical equipment is cooled by a labyrinth (Figure 17.6) of 1.0-cm ID water tubes made of 15 straight sections, each 0.6 m long joined at the ends by 90° connectors and 0.1-m straight
Passing less water through a thinner tube can remove the same heat with a given length of tube. What is the downside of it?
A piece of electrical equipment is cooled by a labyrinth of 1.0-cm ID water tubes made of 12 straight sections, each 80 cm long joined at ends by 90° connectors and 12-cm straight bridging pieces.
How can you reduce the pressure drop in the water-cooling path into one-half?
A vertical heat exchanger dissipates 1 MJ/h per pair of cooling plates that are 25 cm wide, 50 cm high, and separated by a 1.5-mm water gap. Determine the water flow rate and pressure drop to limit
A large vertical heat exchanger dissipates 10 kW per pair of cooling plates that are 30 cm wide, 95 cm high, and separated by 10-mm water gap. Determine the water flow rate and pressure drop to limit
Discuss your experience (if any) in water cooling of the electrical or power electronics equipment.
A 10-MW water-cooled power electronics converter is 97% efficient. Determine the water requirement in gallons per minute if the available cooling water inlet temperature is 30°C and the outlet
A three-phase, three-cm outer diameter cable in air has a resistance of 2 mΩ/phase per meter and carries 100-A/phase load. Determine its outside surface temperature (rise + ambient) if the ambient
A 300-hp, 480-V, three-phase, 50-Hz, four-pole, 1446-rpm induction motor is driving a large ventilating fan. When it is running from a VFD output of 25 Hz with a constant V/f ratio, determine: (1)
A three-phase, 60-Hz, 100-hp, 1750-rpm induction motor is driven by a variable-frequency drive. Determine(1) The power, torque and full load speed when operating at 20 Hz and (2) The power, torque,
Identify at least two electrical products (other than the transformer, motor, generator, and those identified in Section 2.1) that work on Faraday’s law of electromagnetic induction. 2.1 FARADAY'S
With VFD, explain how the motor torque varies below and above the rated speed.
A VFD drives a three-phase, 60-Hz, 100-hp, 1760-rpm induction motor. Determine (1) The power, torque, and full load speed when operating at 40 Hz and (2) The power, torque, and full load speed when
A small ship designed to travel at 10 knots needs 2000-kWe propulsion power and 500 kWe for housekeeping loads. If it is desired to redesign the power plant for a higher speed of 15 knots, determine
With VFD, explain how the motor horsepower varies below and above the rated speed.
Determine 2 / j1.5 in θ-form.
A 10,000-hp, 60-Hz, 1200-rpm, 4.1-kV, three-phase, Y-connected synchronous motor draws the armature current of 1500 A at 0.80 leading power factor under rated load. It has synchronous reactance of
A three-phase, Y-connected, 4.16 kVLL feeder has R = 50 mΩ and X = 30 mΩ, both per phase per 1000-ft. length. Determine the voltage drop in the feeder at (1) 0.8 power factor lagging and (2)
Select a three-phase, 11-kV cable to power a 20,000-hp propulsion motor that has 97% efficiency and 95% pf lagging. Determine the % voltage drop if the distance from the switchboard to the motor is
Select a three-phase 4.6-kV cable to carry 240 A/phase in single-banked tray and determine its outer sheath diameter and weight per 1000 ft. The cable must limit the voltage drop below 30 V/phase per
In the distribution systems shown in the figure below, the generator impedance is not known. Determine the symmetrical rms fault current for a three-phase fault at the spark location. The cable
Determine the base current and base impedance in a three-phase power system on the base of 50 MVA3-ph and 11 kVLL, first using single-phase formulas and then using three-phase formulas, recognizing
A daily power consumption profile of a customer is shown in the table below, where the kilowatt loads are averaged over the period listed and kilovolt-ampere demand meter readings are peak values
A buck converter operating at 60-Hz switching frequency has Ton = 7 ms. Determine the average source current if the load current is 12 A.
A boost converter has an input voltage of 115 V and an output voltage of 230 V dc connected to a 10-Ω resistor and 2-mH inductor load. If the on-time is 3 ms, determine :(1) The switching
A boost converter has an input of 60 V dc, output of 100 V dc, inductance of 3 mH, and load resistance 4 Ω. If its switching frequency is 2 kHz, determine:(1) The output and input currents and (2)
A water pump driven by a 1000-hp induction motor pumps 100,000 m3/day at rated speed. If the motor were run at slower speed using VFD to reduce the flow rate to 80% over a longer time to pump the
A water pump driven by a 1800-hp induction motor pumps 200,000 m3/day at rated speed. If the motor were run at slower speed using VFD to reduce the flow rate to 60% over a longer time to pump the
In an unthrottled fluid pipeline, explain why you should expect a higher percentage of reduction in the motor input power than in the flow output rate.
An existing cruise ship New Horizon was designed to travel at 20 knots with a dedicated 6.6-kV, 46-MWe propulsion power plant consisting of generators, transformers, VFDs, and motors rated
List key types of inverters that are used in VFDs.
A cargo ship travels 5000 nautical miles transatlantic at 25 knots speed, taking 200 h and using 520,000 gal. of fuel oil. If the speed were reduced by 10%, determine the percent change in total
A ship travels 3000 nautical mi. at 20 knots speed using 400,000 gal. of fuel oil. If the speed was reduced by 15%, determine the percent change in total gallons of fuel consumption.
What is the primary reason for maintaining a constant V/f ratio at the motor terminal at all speeds?
A 100-hp, three-phase, 60-Hz, four-pole, 1750-rpm, 0.85-pf induction motor operates with VFD connected to 460-V lines. The VFD design has CSI fed from a dc link. If the motor needs to run at 980 rpm,
A 200-hp, three-phase, 60-Hz, six-pole, 1150-rpm, 0.90-pf induction motor operates with VFD connected to 480 V lines. The VFD design has a current source inverter (CSI) fed from a dc link. If the
Why is a voltage boost needed in the VFD output voltage when starting and at low motor speeds?
A 500-hp, 460-V, three-phase, 60-Hz, four-pole, 1728-rpm induction motor is driving a large ventilating fan. When it is running from a variable-frequency drive (VFD) output of 230 V, 30 Hz with a
A three-phase, 600-V generator feeds a load via single-phase feeder and 50-kVA, single-phase 600/120-V step-down transformer with series impedances of 2 + j6% as shown in the figure below. The
A three-phase 6.6-kV distribution center has a source impedance of 0.1 + j0.5 Ω/phase. Determine the symmetrical rms value and the worst-case asymmetrical first peak of the fault current in a
A 15-MVA, three-phase, 6.6-kV generator with subtransient reactance X′′d = 15% feeds a three-phase, 1-MVA, 6.6-kV/480 V transformer with Xtrfr = 7% via cable that has a leakage reactance Xcable
For a three-phase fault at the spark location in the figure below with all equipment three-phase and the generator of finite size, determine the symmetrical rms fault current: (1) In a 4.2-kV line
A three-phase, 4.16-kV main bus feeds a three-phase, 5-MVA, 4.16-kV/480-V transformer with 6% impedance. The transformer powers multiple loads, including a three-phase, 1000-hp motor as shown in the
A three-phase, 1000-kVA, 4.16-kV Δ/480-V grounded-Y transformer with 5% impedance feeds a three-phase, 100-hp, 460-V motor. If a three-phase symmetrical fault occurs in the feeder, determine the
A 1500-kVA, three-phase, 460-V, Y-connected generator with neutral solidly grounded has R = 2% and subtransient Xd = 14%. It supplies two 750-kVA, three-phase, 208-V Y-connected feeders, one of
Estimate the peak asymmetrical and the average asymmetrical rms fault currents coming out of a 20-MVA, three-phase, 4160-V, GrY generator terminal if a symmetrical fault develops somewhere in the
Determine the symmetrical rms fault current for a three-phase fault at the spark location shown in the following figure. Ignore all cable impedances. 11-KV 60-MVA 3-ph Gen 1 + j12% Propulsion loads 2
A three-phase distribution cable gets three-phase fault 150 ft. away from the 4160-V, Y-connected generator with neutral grounded. The subtransient impedance of the generator is 0.1 + j0.4 Ω/ph, and
How do the basic circuit laws and theorems in dc and ac differ?

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