Questions and Answers of Introduction To Electrical Power

Identify the advantage and disadvantage of using a Design D motor where high starting torque is required.
A synchronous motor is running at 70% rated load torque when the torque angle is 20°. Determine its pull-out torque Tmax (steady-state stability limit torque) in percentage of the rated torque. If
A 1000-hp synchronous motor running at 80% rated load has a torque angle of 25°. Determine its pull-out torque Tmax in percentage of the rated torque. If the motor load is increased to 100% in one
Identify the adverse effects of motor current during starting.
A 250-hp, three-phase, 460-V, 60-Hz, ac motor has power factor of 0.90 lagging and efficiency of 92%. Determine the line current it will draw under 10% overload and the percent change in stator
Explain the difference between the electrical degrees and mechanical degrees in four-pole ac machines.
A water pumping system in a ship is designed to pump 90 m3/h of fresh water against 80 m of total pressure head. Determine the motor horsepower rating and the motor control center kilovolt-ampere
A water pumping system in a ship is designed to pump 250 m3/h fresh water against 70 m of total pressure head. Determine the motor horsepower rating and the motor control center kilovolt-ampere
What would happen if a 460-V, 60-Hz motor is connected to 460-V dc supply lines?
A three-phase, 460-V, 60-Hz induction motor with a Δ-connected stator is designed to have the magnetic flux density in the core at the saturation limit of 1.6 T. If this machine is connected to
A three-phase, 125-hp, 460-V, 50-Hz induction motor with a Δ-connected stator is designed to have a magnetic flux density in the core at the saturation limit of 1.65 T. If this machine is connected
What would happen if a 240-V dc shunt motor is connected to 240-V, 50-Hz supply lines?
A 150-hp, three-phase, 460-V induction motor has the design life of 20 years at 110°C rise in conductor temperature above the ambient air. If it is continuously operated under unbalanced line
A 200-hp, three-phase, 460-V induction motor has a design life of 25 years at 110°C rise in conductor temperature above the ambient air. If it is continuously operated under unbalanced line voltages
Explain the difference between dual-frequency and dual-voltage motors in construction and operation.
A three-phase induction motor in a 690-V distribution system under an abnormal condition is working on highly unbalanced three-phase voltages Ṽa= 450∠ 0°, Ṽb= 450∠−90°, and Ṽc=
A three-phase induction motor in a 440-V distribution system under an abnormal condition receives highly unbalanced three-phase voltages Ṽa = 360∠0°, Ṽb= 360∠−90°, and Ṽc =
Identify the effect of unbalanced voltage on motor operation and the remedy to maintain its normal service life.
A 100-hp, 240-V dc shunt motor runs at 3450 rpm at rated conditions. If its field current is decreased by 15% while keeping the same load current, determine its new speed.
Identify the major advantage of a dc motor.
A three-phase induction motor running from 460-V supply lines is drawing 60-A rated current at 0.90 power factor lagging. The power losses estimated from the manufacturer’s data sheet at this load
Why have dc motors fallen out of favor now?
A three-phase, 50-hp, 460-V, letter code-G induction motor is connected to 480 VLL lines via a transformer with 0.034 + j 0.097 Ω/ph impedance and cable with 0.006 + j 0.003 Ω/ph in series.
What is required to change the direction of rotation of the induction motor and the dc shunt motor?
Determine the kilovolt-ampere load on a generator for powering a crude oil pump if (1) The pumping rate at discharge manifold is 5000 m3/h against 100-m pressure head, (2) The pump pressure head to
List a few candidate applications where high-power series motors can be beneficial and the reason thereof.
Determine the running speed of a 100-hp, 440-V, four-pole, 60-Hz synchronous motor at full load. If the voltage and frequency both were reduced to 80% and the load is reduced to 64%, determine its
For a three-phase, 60-Hz induction motor with phase values of stator voltage V = 120 V, Rrotor = 1 Ω, and Lrotor = 1 mH, compute and plot using EXCEL the rotor current and motor torque for slip
For the motor of Problem 17, compute and plot using EXCEL the rotor current and motor torque from slip s = 1 (starting) to 0.05 (full load) with rotor resistance varying from 0.5 to 2 Ω/phase. Then,
Determine the kilovolt-ampere and voltage ratings of a Δ–Y connected transformer to power a three-phase Y-connected load drawing 600-A line current at 480 V from a three-phase, 4160-V source
Explain the difference between power transformer, voltage (potential) transformer, and current transformer.
Determine the voltage and kilovolt-ampere ratings of Y–Δ connected transformer to power a three-phase Y-connected load drawing 800-A line current at 480 V from a three-phase, 4160-V source.
A 3000-kVA, three-phase transformer bank steps down line voltage from 12.5 kV to 480 V. The HV side is connected in Δ, and the LV side in grounded Y. Determine (1) The HV side–line and phase
Why does the three-phase transformer not need the fourth core leg for the return flux?
A 5000-kVA, three-phase transformer bank steps down line voltage from 12.5 kV to 480 V. The HV side is connected in Δ and the LV side in grounded Y. Determine (1) The HV side - line and phase
Three single-phase transformers, each rated 100 kVA, are connected in Δ–Δ to power a three-phase 300-kVA load. If one of them is removed for repairs, determine: (1) The line voltage on the load
Three single-phase transformers, each rated 75 kVA, are connected in Δ–Δ to power a three-phase 225-kVA load. If one of them is removed for repairs, determine:(1) The line voltage and line
Which type of transformers are used in indoor power systems and why? How do they compare in cost, volume, and weight with other types commonly used outdoors?
Determine the base current and base impedance of a single-phase system based on 50 kVA and 120 V.
What are the instrument transformers, and how do they differ from the power transformer?
Determine the base current and base impedance of a single-phase system based on 500 kVA and 277 V.
A single-phase, 36-kVA, 480/120-V transformer has series impedance Z = 5% stated on its nameplate. Determine Z in ohms looking: (1) From the HV side source when connected as a step-down transformer
Why is electrical equipment using pressurized gas (or vacuum) as the insulating mediums more compact than the dry-type equipment?
A single-phase, 24-kVA, 480/120-V transformer has Z = 4% stated on its nameplate. Determine Z in ohms looking from(1) The HV side source when connected as a stepdown transformer. (2) The LV side
A single-phase, 225-kVA, 460/120-V transformer has 6% impedance and 96% efficiency. Find its series R and X in percent and in ohms looking from the HV side source.
If you place a dry-type transformer in an oil tank, it can deliver greater kilovolt-ampere load and withstand higher lightening voltages. Discuss why.
Determine the rms fault currents on both sides of a 100-kVA, single-phase, 460/120-V transformer with 6% impedance as shown in the following figure. Assume the generator bus infinite and all cable
A single-phase, 150-kVA, 460/120-V transformer has 5% impedance and 97% efficiency. Find its series R and X in percent and in ohms looking from the HV side source.
Determine the rms fault currents on both sides of a 50-kVA, single-phase, 480/120-V transformer with 5% impedance as shown in the figure below. Assume that all generators behind the bus are large
The three-phase transformer costs less than three single-phase transformers in a three-phase system. Even then, three single-phase transformers are used in Δ–Δ connected to form the three-phase
A single-phase, 100-kVA, 480/120-V step-down transformer powers a load impedance of 12 Ω on the LV side. Determine (1) The current on the HV side from conventional transformer calculations and(2)
A single-phase, 100-kVA, 480/120-V step-down transformer powers a load impedance of 8 Ω on the LV side. Determine (1) The current on the HV side from conventional transformer calculations and (2)
A 25-kVA, single-phase, 460/120-V transformer powers 6-Ω load as shown in the figure below. Determine the HV and LV side currents in amperes and also in percent based on the rated voltage on two
Consider a transformer rated 15 kVA, single-phase, 480/120 V supplying 9.6-Ω load as shown in the following figure (top). In calculations specific to a given transformer, it is customary to take the
Which part of the system would get overloaded if the kilovolt-ampere loading is not reduced after one of the three single-phase transformers connected in Δ–Δ is removed for service? What will be
Why is the magnetizing current at the instant of connecting the transformer primary to the supply lines very high even when there is no short or no load connected on the secondary side?
A 5000-kVA transformer has series impedance of R = 3% and X = 7%. Determine the percentage of rise in the output voltage on unloading the transformer from full load at 0.8 pf lagging to zero load,
A three-phase, 500-kVA, 4160/480-V transformer bank has R = 1.0% and X = 5%. Using Equation 2.15 or 2.18 with pu values, determine the % voltage rise in the secondary voltage on unloading from the
A 2-MVA, 4160/480-V transformer has series impedance of R = 2% and X = 6%. Determine the (1) Full-load voltage regulation of the transformer at 0.85 pf lagging and (2) Secondary output voltage on
Explain in your own words the concept of reflected impedance or the impedance retransformation.
Determine a three-phase 480V/208Y–120V transformer kilovolt-ampere rating if the daily variations in the secondary load current are 150 A for 5 h, 170 A for 1 h, and 130 A for 18 h, taking into
A cable with neoprene insulation has nominal ampacity of 250 A in 30°C ambient air. Determine its ampacity for use in a boiler room where the ambient air is 50°C.
Between AWG 30 and AWG 10 conductors, which is a heavier conductor, and how many of each can be placed in a 1-in. width?
A cable made with polypropylene insulation has nominal ampacity of 200 A in standard 40°C ambient air. Determine its ampacity for use in the ship’s engine room uptake where the ambient air is
A 1-kV, 750-kcmil copper cable in a metallic tray has dc resistance of 0.016 Ω per 1000 ft. at 25°C. Using the tables provided in this chapter, determine its 60-Hz resistance at operating
In specifying the conductor size, what do terms MCM and kcmil stand for?
A 1-kV, AWG 4 copper conductor has dc resistance of 0.30 Ω per 1000 ft. at 25°C. Using the tables provided in this chapter, determine its 60-Hz resistance at an operating temperature of 110°C.
A three-phase, Y-connected, 4.16 kVLL feeder has R = 200 mΩ and X = 35 mΩ, both per phase per 1000-ft. length. Determine the Vdrop per 1000 ft. per MVA load so that it can be easily reused for
List two major factors that limit the cable ampacity.
Select a three-phase 6.6-kV cable to carry 525 A/phase in a single-banked tray and determine its (1) Voltage drops per 100 ft. at operating temperature of 90°C, (2) Outer sheath diameter, and (3)
Give two reasons why heavy conductors are made of thin strands as opposed to solid copper.
Select a three-phase, 13.8-kV cable to power a 24,000-hp propulsion motor that has 96% efficiency and 90% pf lagging. Determine the % voltage drop if the switchboard is 250 ft. away from the motor.
Explain the difference between the skin effect and the proximity effect and how they increase the conductor ac resistance over the dc resistance.
A special three-phase cable is assembled as shown in Figure 6.4 using three one-conductor cables, each with a conductor diameter of 1 in. and an insulated outer diameter of 1.5 in. The net copper
Select a three-phase, 460-V power distribution cable for a 100-hp induction motor that has 95% efficiency and 0.85 pf lagging. The motor is located 20 m from the switchboard. Determine the
The term effective is used for the rms value of ac voltage or current and also for the cable impedance. Explain the term effective impedance and its use.
A three-phase, 460-V, 60-Hz, 200-hp induction motor has design efficiency of 90% and pf of 0.85 lagging. The cable reactance from the control center to the motor is 0.015 Ω/ph. If the motor pf were
How are two voltages, 120 and 240 V, obtained in residential power distribution from a single secondary coil of a single-phase step-down transformer?
A three-phase, 100-hp, 460-V, letter code G induction motor is started directly from 480-V lines. The internal (Thevenin) source impedance and the cable impedance add up to a total of 0.01 + j0.02
How many voltages can be obtained from a three-phase Y-connected transformer?
Explain the advantage of an ungrounded system that is usually preferred in navy and commercial ships.
Two rectangular copper bus bars, each 1/4 in. thick × 4 in. wide with facing flats, are separated by a 1/2-in. space between the flats. Determine per foot run of bus bars:(1) The resistance and
Explain the function of neutral ground and chassis ground.
On detecting a ground fault on an ungrounded system, how would you make sure that it is indeed a ground fault and not an anomaly?
Which bus bar configuration—facing flats or facing edges—results in lower voltage drop per ampere per meter length?
How do capacitors improve the voltage regulation (i.e., reduce the voltage drop)?
Sketch the magnetic flux pattern of two parallel bus bars and virtually concentric bus bars for a single-phase distribution system.
In dealing with high power at high frequency (e.g., 10 MW at 10 kHz), what type of the distribution line (round hollow tubes or rectangular bus bars) would you consider to keep the voltage drop low,
Identify the difference between the circuit breaker and the disconnect switch.
Explain how the most frequent unsymmetrical L-G fault soon leads to a three-phase symmetrical L-L-L-G fault.
Determine the base current and base impedance in a three-phase power system on the base of 10 MVA3-ph and 6.6 kVLL, first using single-phase formulas and then using three-phase formulas, recognizing
What system parameters do you need in order to calculate the fault current magnitude?
A three-phase 13.8-kV distribution center has a source (Thevenin) impedance of 0.5 + j2.5 Ω/phase. Determine the symmetrical rms and the worst-case asymmetrical first peak value of the fault current
A three-phase, 480-V generator powers a load via single-phase feeders and 25-kVA, single-phase, 480/120-V step-down transformer with series impedances of 1 + j5% as shown below. The single-phase
By hydraulic analogy, explain how the first peak of the transient fault current gets much greater than the symmetrical peak.
A three-phase, 10-MVA, 4.16-kV generator with sub transient reactance Xdʺ = 20% feeds a three-phase, 1-MVA, 4.16 kV/480 V transformer with Xtrfr = 5% via a cable that has the leakage reactance
Explain the R-L-C circuit physics leading to the asymmetrical fault current peak almost twice in the worst case than the symmetrical peak.
For a three-phase fault at the spark location shown in the figure below with all equipment three-phase and generator of finite size, determine the symmetrical rms fault current. Ignore the series
Explain the terms d-axis and q-axis in the synchronous generator and motor.
A three-phase, 6600-V main bus feeds a three-phase, 2-MVA, 6600/480-V transformer with 5% impedance. The transformer powers multiple loads, including a three-phase, 500-hp motor as shown in the

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