EE 2

100問 • 1年前

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問題一覧

If load on a dc shunt motor is increased, its speed is decreased due primarily to:

decrease in back emf

If field resistance of a dc shunt generator is increased beyond its critical value, the generator:

will not build up

The intrinsic magnetic dipole moments of protons and neutrons are much less than that of an electron because:

their masses are greater

A hollow conductor is positively charged. A small uncharged metal ball is lowered by a silk thread through a small opening in the top of the conductor and allowed to touch surface. After the ball is removed, it will have:

no appreciable charge

In a simple high-pass RC filter, if the value of capacitance is doubled, the cutoff frequency is:

halved

As the load is increased, the speed of a dc shunt motor:

reduces slightly

The all-day efficiency of a transformer depends primarily on:

both the amount of load and the duration of load

An electron is on the z axis moving toward the xy plane but it has not reached that plane yet, At that instant:

there is only a displacement current through the xy plane

Radio receivers are usually tuned by adjusting the capacitor of an LC circuit. If C = C₁ for a frequency of 600 kHz, then for a frequency of 1200 kHz one must adjust C to:

C/4

You push a permanent magnet with its north pole away from you toward a loop of conducting wire in front of you. Before the north pole enters the loop the current in the loop is:

counterclockwise

In an RLC series circuit, which is connected to a source of emf em cos(wt), the current lags the voltage by 45° if:In an RLC series circuit, which is connected to a source of emf em cos(wt), the current lags the voltage by 45° if:

R = ωL – 1/ωC

Suppose you are looking into one end of a long cylindrical tube in which there is a uniform electric field, pointing away from you. If the magnitude of the field is decreasing with time the field lines of the induced magnetic field are:

circles

The core of a transformer is made in a laminated form to:

prevent eddy currents

A series RL circuit is connected to an emf source of angular frequency ω. The current:

lags the applied emf by tan¯¹ (ωL/R)

The resistance of the primary coil of a well-designed, 1: 10 step-down transformer is 1W. With the secondary circuit open, the primary is connected to a 12 V ac generator. The primary current is:

essentially zero

Displacement current exists in the region between the plates of a parallel plate capacitor if:

the capacitor is being discharged

An inductance L and resistance R are connected in series to an ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. The rate of increase of the energy stored in the inductor is a maximum:

at the time t = (L/R) In 2 after the switch is closed

Two of Maxwell’s equations contain a path integral on the left side and an area integral on the right. For them:

the path must be the boundary of the area

The primary of a 3:1 step-up transformer is connected to a source and the secondary is connected to a resistor R. The power dissipated by R in this situation is P. If R is connected directly to the source it will dissipate a power of:

P/9

The statement that magnetic field lines form closed loops is a direct consequence of:

Gauss’ law for magnetism

The electrical analog of a spring constant k is:

1/C

The magnetic field of Earth is roughly the same as that of a magnetic dipole with a dipole moment of about:

10 23J/T

counterclockwise

A charged capacitor and an inductor are connected in series. At time = 0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time after t = 0 that the energy stored in the electric field of the capacitor is a maximum is:

T/2

An RLC series circuit is driven by a sinusoidal emf with angular frequency ω. If ω is increased without changing the amplitude of the emf the current amplitude increases. If L is the inductance, c is the capacitance, and R is the resistance, this means that:

ωL < 1/ωC

At any instant of time the total magnetic flux through a stationary conducting loop is less in magnitude than the flux associated with an externally applied field. This might occur because:

the applied field is normal to the loop and increasing in magnitude

A magnetic field parallel to the x axis with a magnitude that decreases with increasing x but does not change with y and z is impossible according to:

Gauss` law for magnetism

When the amount of back emf of a shunt motor will increase?

When the field is strengthened

Why is skewing done in rotor slots?

To suppress unwanted harmonics

Oil for transformer cooling should have:

Low viscosity

When two electricity neutral materials are rubbed together, one acquires a net positive charge and the other

Gain electrons

How the voltage of the bus-bar to which several alternators that are paralleled may be raised?

By simultaneously increasing field excitation of all alternators

The number of commutator segments in a dc machine is equal to the number of:

Coils

A DC voltage source is connected across a series RLC circuit. Under steady state conditions the applied DC voltage drops entirely across the:

C only

A power distribution system consists of a 4800-volts, 3-phase bus supplied from an electric utility system having a 3-phase short-circuit capacity of 20MVA through a transformer whose reactance is 12% on a 5MVA base. Calculate the percent voltage rise obtained by switching in a 3-phase bank of 1000 kVAR shunt capacitors.

7.4%

What is the value of air gap flux density in case of induction motor?

0.35 to 0.6 Wb/m²

An autotransformer is used to connect a 12.6-kV distribution line to a 13.8-kV distribution line. It must be capable of handling 2000 kVA. There are three phases, connected Y-Y with their neutrals solidly grounded. How much apparent power must the windings of each autotransformer handle?

63.5 kVA

Given a cast steel core of toroid with an air gap of 1mm cut into it, determine the current necessary in a 600 turn coil wound around the core to produce a flux across the air gap of 0.6 mWb. Assume it is known from a magnetization curve that cast steel requires an H of 400 A-t/m for a flux density of 0.6 Tesla. Refer to the figure below and neglect any fringing in the air gap.

1 A

A circular lifting mgnet for a crane is to be designed so that with a flux density of 30,000 lines per square inch in each air gap, the length of each air gap is 0.5 inch. Leakage and saturation effects are such that the magnetomotive force for the air gaps is 0.85 of the magnetomotive force for the complete magnetic circuit. The mean length per turn of winding is 24 inches and the magnet is to operate with an applied terminal voltage of 70 volts with the winding at a temperature of 60°C. What is the pull or tractive force in pounds per square inch for the magnet?

12.5

What type of winding is the armature of a dc machine?

A closed-circuit winding

Which of the following law is the magnetic equivalent of Coulomb’s law?

Biot Savart’s Law

For a coil of 100 turns wound around a toroidal core of iron with a relative permeability of 1,000, find the current necessary to produce a magnetic flux density of 0.5 Tesla in the core. The dimensions of the core are given in the figure shown below.

1.2 A

The number of parallel paths in an integral slot winding with P poles is:

P/2

A system response is given as c(t) = 12 – 3e^-7t. What is the forced response?

10¹³ ergs/sec is equal to how many kilowatts?

1000

What is the maximum output power of a cellular transmitter?

3 W

Which of the following positions will lead to critical damping?

x(t) = 8te^-13t

What is the maximum distance between two receptacles in a single-family dwelling unit?

1,800 mm

Which of these is NOT a simplex type of electronic communications?

Telephone

Which of the following principles will the operation of fiber-optic cable is based?

Reflection

What is the reason why thermal powerplant decrease capacity during summer?

Temperature rises of the lake

In steam power plant which component needs maximum maintenance attention?

Boiler

The thickness of insulation provided on the conductor in the cable depends on which among the following factors?

Operating voltage

A floodlight emitting 25,000 candelas in the center of its beam is aimed at 600 to a point on the ground 20 meters away. The illumination of the point in lux is:

62.5

Two-wattmeter method of measuring three phase power is applied to a three-phase balanced load. Two wattmeter reads 6717 watts and 2658 watts, respectively. What is the pf of the load?

0.8

The impedance coil absorbs 250 watts when connected across 220 V, 60 Hz mains. It is then connected across 110 V, 25 Hz mains and also absorbs 250 watts. What is the inductance of the coil?

0.149 H

In EPIRA law, it is an energy loss due to tampering of meters and erroneous meter reading:

non-technical loss

A generator rated 600 kVA, 2400 V, 60 cycles, 3-phase, 6 poles and wye-connected has 10% synchronous reactance. If a three-phase fault occurs, what will be the short circuit current?

1443 A

Which of the following limits the number of coils that may be placed on the armature of a DC motor or generator?

Coil to coil arcing due to the breakdown of insulation

Given that a transfer function is T(s) = 2/(s – 3), find the system response y(t) to an input x(t) = u(t), a unit step, assuming zero initial conditions.

y(t) = -2/3 + (2/3)^e3t

When you place a measuring meter in parallel to the load, the instrument is a/an _____.

Voltmeter

You are facing a loop of wire which carries a clockwise current of 3.0 A and which surrounds an area of 5.8 x 10ˉ² m². The magnetic dipole moment of the loop is:

0.17 A · m², away from you

A control system consists of subsystems and __________ assembled for the purpose of obtaining a desired output with desired performance, given a specified input.

processes

At a certain point of the system network the positive, negative and zero sequence impedances are 0.25 pu, 0.25 pu and 0.3 pu respectively. The base MVA is 100. The level at that point is 34.5 kV. Determine the zero-sequence current for a line to ground fault.

2091 A

The color reserved for use in equipment grounding is:

Green

The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, or to both, at the service, at each building or structure where supplied by feeder(s) or branch circuit(s), or at the source of separately derived system.

Grounding electrode conductor

In Philippine Distribution Code, the utility requires the consumer to operate ant a power factor not less than percent.

85%

What is the meaning of SCADA?

Supervisory Control and Data Acquisition

The emf induced in a coil by the charging of current in the neighboring coils is known as __ emf

mutually induced

The rate of passage of radiant energy evaluated by reference to the luminous sensation produced by it is called

Luminous Flux

Regarding Thevenin's equivalent, which of the following is not correct?

The Thevenin`s equivalent resistance is calculated when all voltage sources are open circuited.

In air blast circuit breakers, the pressure of air is of the order of ___.

20 to 30 kg/sq.cm

Which of the following types of fault that used only the equivalent positive impedance of the circuit in the short-circuit calculation?

Three phase fault

The position of a spring is defined as x(t) = 4 cos 8t – 0.25 sin 8t. Simplify x(t) by expressing into one function only.

4.0078 sin (8t – 266.42°)

A three-phase, 60 Hz, transmission line has its conductors arranged in a triangular configuration so that the two distances between conductors are 5 m and the third is 8 m. The conductors have an outside diameter of 0.25 inch. Find the inductive reactance in ohm per km per phase of the transmission line.

0.586

Two identical transformers bank on open-delta serve a balanced 3 phase load of 26kVA at 240V, 60Hz. What is the minimum size of each in kVA needed to serve this load?

Bundled conductors are used in transmission lines, the effective capacitance and inductance will respectively:

Increase and decrease

It is the total number of sustained customer power interruptions within a given period divided by the total number of customers served within the same period.

SAIFI

An electric heater carries 12 A at 110 V, is submerged in 22.5 lbs of water for 30 minutes. What will be the final temperature of the water if its initial temperature is 35°F?

135.43 F

Step potential and touch potential is associated with:

Earthing of th substation

A small wind generator is designed to generate 50kW of power at a wind velocity of 25mi/h. What is the approximate blade diameter?

36 ft

For the circuit shown below, given V₁ = 10 V, R₁ = 16 kΩ, R₂ = 4 kΩ, R₃ = 30 Ω, R₄ = 60 Ω. Ix is:

-2/60 A

A transformer is so designed that primary and secondary windings have what coupling?

Tight magnetic coupling

A 3-phase, alternator has generated emf per phase of 230 V with 10% 3rd harmonic and 6% 5th harmonic content. Calculate rms line voltage for star connection.

230.2 V

All the resistances in figure are 1 Ω each. The value of current ‘I’ is:

8/15 A

An 8-pole duplex lap winding will have how many parallel paths?

Assertion (A): Kirchhoff’s voltage law states that a closed path in a network, the algebraic sum of all voltages in a single direction is zero. Reason (R): Law of conservation of charge is the basis of this law.

A is true but R is false

In one 132 / 33 kV transformer the LV resistance is 0.02 per unit. What is the resistance referred to HV side?

0.02 pu

For the circuit shown in the figure is Vx = 10 V and ix = 5 A. The value of Vs is:

-2.5 V

What can you describe the maximum flux produced in the core of a transformer?

It is inversely proportional to supply frequency

The rotor electromotive force of the IM is observed to make 100 complete alterations per minute. What is the calculated slip?

2.78%

A DC shunt generator builds up to a voltage of 220 V at no load while running at its rated speed. If the speed of the generator is raised by 25% keeping the circuit conditions unaltered, then the voltage to which the machine would build up will ______.

increase to 1.25 times 220 V

Assertion (A): Two wires of same length with different cross-sectional areas are connected in series. The heat produced by the current is more in the thicker wire. Reason (R): The thicker wire has low resistance.

A is false, but R is true

When all the resistances in the circuit are of one ohm each, the equivalent resistance across the points A and B will be:

0.5 ohms

Calculate the loss of energy caused by hysteresis in 1 hour in 50 kg of iron when subjected to cyclic magnetic changes. The frequency is 25 Hz, the area of hysteresis loop is equivalent in area to 240 J/m³/cycle and the density of iron is 7.8 g/cm³.

138456 J

When the efficiency of a transformer will be maximum?

When copper loss is equal to constant loss.

Which motor never use belt-connected loads?

Series

Given the following circuit, which of the elements of the circuit have their current direction and voltage polarity in the proper relationship for positive power absorbed by the element?

C, D, E

The armature of a 4-pole d.c. generator has a volume of 12 × 10¯³ m³. During rotation, the armature is taken through a hysteresis loop whose area is 20 cm² when plotted to a scale of 1 cm = 100 AT/m, 1 cm = 0·1 Wb/m². Determine the hysteresis loss in watts when the armature rotates at a speed of 900 r.p.m.

72 W

100

In what application commutatively compounded motors are used?

Where sudden heavy loads for short duration is required.

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問題一覧

If load on a dc shunt motor is increased, its speed is decreased due primarily to:

decrease in back emf

If field resistance of a dc shunt generator is increased beyond its critical value, the generator:

will not build up

The intrinsic magnetic dipole moments of protons and neutrons are much less than that of an electron because:

their masses are greater

no appreciable charge

In a simple high-pass RC filter, if the value of capacitance is doubled, the cutoff frequency is:

halved

As the load is increased, the speed of a dc shunt motor:

reduces slightly

The all-day efficiency of a transformer depends primarily on:

both the amount of load and the duration of load

An electron is on the z axis moving toward the xy plane but it has not reached that plane yet, At that instant:

there is only a displacement current through the xy plane

Radio receivers are usually tuned by adjusting the capacitor of an LC circuit. If C = C₁ for a frequency of 600 kHz, then for a frequency of 1200 kHz one must adjust C to:

C/4

You push a permanent magnet with its north pole away from you toward a loop of conducting wire in front of you. Before the north pole enters the loop the current in the loop is:

counterclockwise

R = ωL – 1/ωC

circles

The core of a transformer is made in a laminated form to:

prevent eddy currents

A series RL circuit is connected to an emf source of angular frequency ω. The current:

lags the applied emf by tan¯¹ (ωL/R)

The resistance of the primary coil of a well-designed, 1: 10 step-down transformer is 1W. With the secondary circuit open, the primary is connected to a 12 V ac generator. The primary current is:

essentially zero

Displacement current exists in the region between the plates of a parallel plate capacitor if:

the capacitor is being discharged

at the time t = (L/R) In 2 after the switch is closed

Two of Maxwell’s equations contain a path integral on the left side and an area integral on the right. For them:

the path must be the boundary of the area

P/9

The statement that magnetic field lines form closed loops is a direct consequence of:

Gauss’ law for magnetism

The electrical analog of a spring constant k is:

1/C

The magnetic field of Earth is roughly the same as that of a magnetic dipole with a dipole moment of about:

10 23J/T

counterclockwise

T/2

ωL < 1/ωC

At any instant of time the total magnetic flux through a stationary conducting loop is less in magnitude than the flux associated with an externally applied field. This might occur because:

the applied field is normal to the loop and increasing in magnitude

A magnetic field parallel to the x axis with a magnitude that decreases with increasing x but does not change with y and z is impossible according to:

Gauss` law for magnetism

When the amount of back emf of a shunt motor will increase?

When the field is strengthened

Why is skewing done in rotor slots?

To suppress unwanted harmonics

Oil for transformer cooling should have:

Low viscosity

When two electricity neutral materials are rubbed together, one acquires a net positive charge and the other

Gain electrons

How the voltage of the bus-bar to which several alternators that are paralleled may be raised?

By simultaneously increasing field excitation of all alternators

The number of commutator segments in a dc machine is equal to the number of:

Coils

A DC voltage source is connected across a series RLC circuit. Under steady state conditions the applied DC voltage drops entirely across the:

C only

7.4%

What is the value of air gap flux density in case of induction motor?

0.35 to 0.6 Wb/m²

63.5 kVA

1 A

12.5

What type of winding is the armature of a dc machine?

A closed-circuit winding

Which of the following law is the magnetic equivalent of Coulomb’s law?

Biot Savart’s Law

1.2 A

The number of parallel paths in an integral slot winding with P poles is:

P/2

A system response is given as c(t) = 12 – 3e^-7t. What is the forced response?

10¹³ ergs/sec is equal to how many kilowatts?

1000

What is the maximum output power of a cellular transmitter?

3 W

Which of the following positions will lead to critical damping?

x(t) = 8te^-13t

What is the maximum distance between two receptacles in a single-family dwelling unit?

1,800 mm

Which of these is NOT a simplex type of electronic communications?

Telephone

Which of the following principles will the operation of fiber-optic cable is based?

Reflection

What is the reason why thermal powerplant decrease capacity during summer?

Temperature rises of the lake

In steam power plant which component needs maximum maintenance attention?

Boiler

The thickness of insulation provided on the conductor in the cable depends on which among the following factors?

Operating voltage

A floodlight emitting 25,000 candelas in the center of its beam is aimed at 600 to a point on the ground 20 meters away. The illumination of the point in lux is:

62.5

Two-wattmeter method of measuring three phase power is applied to a three-phase balanced load. Two wattmeter reads 6717 watts and 2658 watts, respectively. What is the pf of the load?

0.8

The impedance coil absorbs 250 watts when connected across 220 V, 60 Hz mains. It is then connected across 110 V, 25 Hz mains and also absorbs 250 watts. What is the inductance of the coil?

0.149 H

In EPIRA law, it is an energy loss due to tampering of meters and erroneous meter reading:

non-technical loss

A generator rated 600 kVA, 2400 V, 60 cycles, 3-phase, 6 poles and wye-connected has 10% synchronous reactance. If a three-phase fault occurs, what will be the short circuit current?

1443 A

Which of the following limits the number of coils that may be placed on the armature of a DC motor or generator?

Coil to coil arcing due to the breakdown of insulation

Given that a transfer function is T(s) = 2/(s – 3), find the system response y(t) to an input x(t) = u(t), a unit step, assuming zero initial conditions.

y(t) = -2/3 + (2/3)^e3t

When you place a measuring meter in parallel to the load, the instrument is a/an _____.

Voltmeter

You are facing a loop of wire which carries a clockwise current of 3.0 A and which surrounds an area of 5.8 x 10ˉ² m². The magnetic dipole moment of the loop is:

0.17 A · m², away from you

A control system consists of subsystems and __________ assembled for the purpose of obtaining a desired output with desired performance, given a specified input.

processes

2091 A

The color reserved for use in equipment grounding is:

Green

Grounding electrode conductor

In Philippine Distribution Code, the utility requires the consumer to operate ant a power factor not less than percent.

85%

What is the meaning of SCADA?

Supervisory Control and Data Acquisition

The emf induced in a coil by the charging of current in the neighboring coils is known as __ emf

mutually induced

The rate of passage of radiant energy evaluated by reference to the luminous sensation produced by it is called

Luminous Flux

Regarding Thevenin's equivalent, which of the following is not correct?

The Thevenin`s equivalent resistance is calculated when all voltage sources are open circuited.

In air blast circuit breakers, the pressure of air is of the order of ___.

20 to 30 kg/sq.cm

Which of the following types of fault that used only the equivalent positive impedance of the circuit in the short-circuit calculation?

Three phase fault

The position of a spring is defined as x(t) = 4 cos 8t – 0.25 sin 8t. Simplify x(t) by expressing into one function only.

4.0078 sin (8t – 266.42°)

0.586

Two identical transformers bank on open-delta serve a balanced 3 phase load of 26kVA at 240V, 60Hz. What is the minimum size of each in kVA needed to serve this load?

Bundled conductors are used in transmission lines, the effective capacitance and inductance will respectively:

Increase and decrease

It is the total number of sustained customer power interruptions within a given period divided by the total number of customers served within the same period.

SAIFI

An electric heater carries 12 A at 110 V, is submerged in 22.5 lbs of water for 30 minutes. What will be the final temperature of the water if its initial temperature is 35°F?

135.43 F

Step potential and touch potential is associated with:

Earthing of th substation

A small wind generator is designed to generate 50kW of power at a wind velocity of 25mi/h. What is the approximate blade diameter?

36 ft

For the circuit shown below, given V₁ = 10 V, R₁ = 16 kΩ, R₂ = 4 kΩ, R₃ = 30 Ω, R₄ = 60 Ω. Ix is:

-2/60 A

A transformer is so designed that primary and secondary windings have what coupling?

Tight magnetic coupling

A 3-phase, alternator has generated emf per phase of 230 V with 10% 3rd harmonic and 6% 5th harmonic content. Calculate rms line voltage for star connection.

230.2 V

All the resistances in figure are 1 Ω each. The value of current ‘I’ is:

8/15 A

An 8-pole duplex lap winding will have how many parallel paths?

A is true but R is false

In one 132 / 33 kV transformer the LV resistance is 0.02 per unit. What is the resistance referred to HV side?

0.02 pu

For the circuit shown in the figure is Vx = 10 V and ix = 5 A. The value of Vs is:

-2.5 V

What can you describe the maximum flux produced in the core of a transformer?

It is inversely proportional to supply frequency

The rotor electromotive force of the IM is observed to make 100 complete alterations per minute. What is the calculated slip?

2.78%

increase to 1.25 times 220 V

A is false, but R is true

When all the resistances in the circuit are of one ohm each, the equivalent resistance across the points A and B will be:

0.5 ohms

138456 J

When the efficiency of a transformer will be maximum?

When copper loss is equal to constant loss.

Which motor never use belt-connected loads?

Series

Given the following circuit, which of the elements of the circuit have their current direction and voltage polarity in the proper relationship for positive power absorbed by the element?

C, D, E

72 W

100

In what application commutatively compounded motors are used?

Where sudden heavy loads for short duration is required.