Download EE 123 HO final exam 2s 2011

EE 123 – ELEMENTARY ELECTRICAL ENGINEERING AND INSTRUMENTATION
ALTERNATING CURRENT CIRCUITS
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HO3
For multiplication or division of phasors, we use ______ form?
a. polar
b. rectangular
c. exponential
d. trigonometrical
For addition or subtraction of phasors , we use ________ form?
a. rectangular
b. polar
c. exponential
d. trigonometrical
In the complex number 4 + j7, 7 is called _________ component.
a. imaginary
b. real
c. in-phase
d. quadrature
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When a phasor is multiplied by j , it is rotated through ______ in counterclockwise direction.
a. 540 deg
b. 360 deg
c. 90 deg
d. 270 deg.
A current of 3 + j4 amperes is flowing through a circuit. The magnitude of current is _____?
a. 5 A
b. 1 A
c. 7 A
d. 1.33 A
A phasor 2 angle 180 degrees can be expressed as ______?
a. –2
b. j2
c. –j2
c. 2
The sum of two conjugate numbers results in _________?
a. complex number
b. in phase component only
c. quadrature component only
d. none of the above
The kva drawn by an ac circuit is given by (3 + j4) kva. The true power drawn by the circuit is _?
a. 3 kW
b. 4 kW
c. 5 kW
d. 1 kW
The kva drawn by an ac circuit is given by (3 + j4) kva. The apparent power drawn by the circuit is?
a. 3 kW
b. 4 kVar
c. 5 kva
d. 1 kW
The kva drawn by an ac circuit is given by (3 + j4) kva. The reactive power drawn by the circuit is ?
a. 3 kW
b. 5 kw
c. 4 kvar
d. 1 kW
An alternating voltage or current is a _____ ?
a. scalar quantity
b. phasor
c. vector quantity
d. none of the above
An alternating voltage is given by v = 20 sin 157 t. The frequency of the alternating voltage is __?
a. 50 Hz
b. 25 Hz
c. 100 Hz
d. 75 Hz
The reciprocal of impedance is _______?
a. resistance
b. reactance
c. admittance
d. conductance
The cosine of the angle between the voltage and the current is___
a. reactive factor
b. power factor
c. Form Factor
d. peak factor
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The phenomenon whereby a circuit stores electrical energy is called _____
a. inductance
b. capacitance
c. resistance
d. susceptance
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A voltage or current that is reversed at regular intervals is called ____ voltage or current.
a. Direct b. Steady State c. Sinusoidal
d. none of these
The instrument used to indicate phase relation between current and voltage is the _?
a. megger
b. power factor meter
c. voltmeter
d. galvanometer
To calculate the V.A. one needs to know the _______
a. voltage and current
b/ inpedance and conductance
resistance and impedance
d. ohms and resistance
Reactance will cause the current in a circuit to vary only when ______
a. current flows
b. DC current flows
c. There is no resistance in the circuit
d. There is resistance in the circuit
Frequency is measured in _____
a. hertz
b. voltage
c. rpm
d. foot pounds
Which of the following would cause the most power to be dissipated in the form of heat?
a. XL
b. resonance
c. Xc
d. resistance
_____ is the combined opposition to current by resistance and reactance.
a. Q
b. Z
c. Xc
d. I2R
An electrician in the industry would first check the ______ to correct a low power factor.
resistance
b. Hysteresis c. inductive load
d. reluctance
As the power factor of a circuit is increased _______?
a. reactive power is decreased
b. active power is decreased
c. reactive power is increased
d. both active and reactive power are increased
The AC system is preferred to the DC system because ___?
a. DC voltage cannot be used for domestic appliances
b. DC motors do not have speed control
c. AC voltages can be easily changed in magnitude
d. high – voltage AC transmission is less efficient
A capacitor opposes ____________.
a. both a change in voltage and current b. change in current
c. change in voltage
d. none of these
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Rheostat and potentiometers are types of _______ resistor?
a. film
b. variable
c. fixed
d. wirewound
What percentage of the maximum (peak) voltage is the effective (R.M.S.) voltage?
a. 100 %
b. 70.7 %
c. 63.7 %
d. 57.7 %
Permeability is ____________.
a. the opposite of conductance
b. a measure of the ease with which magnetism passes through any substance
c. the total resistance to current flow
d. the liquid substance in a battery
An AC ammeter or voltmeter is calibrated to read RMS values; this means the meter is reading the __ value.
a. maximum
b. peak
c. average
d. effective
Rheostat and potentiometers are types of _______ resistor?
a. film
b. variable
c. fixed
d. wirewound
An electrical equipment that converts electrical energy into mechanical energy is:
a. converter
b. generator
c. motor
d. transformer
An electrical equipment which converts mechanical energy into electrical energy is__?
a. converter
b. motor
c. generator
d. transformer
An electrical device which converts electrical energy in another form by magnetic induction is __?
a. converter
b. motor
c.. transformer
d. generator
An electrical device that converts ac signal into dc signal is _____?
a. converter
b. inverter
c. rectifier
d. diode
An electrical device that converts DC signal into AC signal is _____?
a. converter
b. inverter
c. rectifier
d. diode
A transformer will work on _______________?
a. a.c only
b. d.c only
c. ac as well as dc
d. none of the above
The transformer is an efficient device because it ________?
a. is a static device
b. uses inductive coupling
c. uses electric coupling
d. uses capacitive coupling
A transformer transfers electrical energy from primary to secondary usually with a change in __?
a. frequency
b. voltage
c. power
d. time period
The speed of a __________ motor is practically constant?
a. series
b. shunt
c. commulatively compounded
d.differentially compounded
___________ motor has the best speed regulation.
a. series
b. commulatively compounded
c. shunt
d.differentially compounded
_________ is most suitable for punch presses?
a. shunt
b. series
c. differentially compounded
d. commulatively compounded
In a vacuum cleaner, we generally use ___________ motor?
a. shunt
b. commulatively compounded
c. series
d.differentially compounded
The most suitable motor for elevators is the __________ motor?
a. shunt
b. commulatively compounded
c. series
d.differentially compounded
The greatest percentage of power loss in a dc motor is due to ___________?
a. windage loss
b. core loss
c. copper loss
d. friction loss
The cause of excessive sparking at the brushes is due to _________?
a. dirt in the commutator
b. loose coupling
c. misalignment of machine
d. worn bearings
Excessive motor vibration is caused by ________________?
a. too much brush tension
b. worn bearings
c. open armature coil
d. bent shaft
Overheating of a dc motor is often due to _____________?
a. insufficient end play
b. loose parts
c. overloads
d. rough commutators
Hot bearings of a dc motor may be caused by _________?
a. poor ventilation
b. incorrect voltage
c. loose coupling
d. lack of or dirty lubricants
Intermittent sparking at the brushes of a dc motor may be caused due to _______?
a. an open armature coil b. intermittent load
c. loose coupling
d. incorrect voltage
DC series motors are used in those applications where _____ is required?
a. high starting torque
b. constant speed
c. low no-load speed
d. none of the above
By looking at which part of the motor, it can be easily confirmed that a particular motor is DC motor?
a. Frame
b. Shaft
c. Commutator
d. Stator
The armature of a dc machine is made of ___________?
a. silicon steel
b. cast steel
c. wrought iron
d. soft iron
ILLUMINATION
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The study of illumination involves:
a. design of lighting system
c. study of lighting & wiring system
b. calculation of wire sizes
d. study of photometry and its application
The luminous intensity of a lamp is expressed in
a. candela
b. lumens
c. watts
d. lux
Energy radiated continuously in the form of lighting waves is called
a. luminous flux
b. luminous intensity c. lumen
d. illumination
Defined as the ratio of the lumens actually received by a particular surface to the total lumens
emitted by the luminous source
a. coefficient of utilization
b. maintenance factor
c. depreciation factor
d. lux
A lighting system which partially all the lights on the illuminated area is essentially downward and
comes directly from the lighting units.
a. direct lighting
b. semi-direct lighting
c. general diffuse
d. indirect
The difference in brightness between an object and its background is
a. contrast
b. glare
c. Hue
d. Tint
A strong steady dazzling light or reflection is
a. glare
b. contrast
c. hue
d. brightness
A combination of the arc discharge characteristics of a fluorescent lamp and the compact
focusable shape of the incandescent lamp is
a. metal halide lamp b. mercury lamp
c. sodium halide
d. neon
The heart of the lighting system is the
a. light source
b. light
c. environment
d. fixture
The room dimensions, wall and ceiling colors, lighting system, spacing of lamps, are the items that
determines the
a. maintenance factor
b. coefficient of utilization
c. room factor
d. depreciation factor
The maximum operating temperature of TW conductors is
a. 60oC
b. 75oC
c. 75oC
d. 90oC
Minimum internal depth of outlet box
a. 24 mm
b. 12 mm
c. 20 mm
d. 6 mm
Standard illumination level for indoor lighting in dwellings
a. 150 lux
b. 1500 lux
c. 500 lux
d. 1000 lux
Continuous load of over-current device in a panelboard shall mean an operation of:
a. less than 3 hours
b. more than three hours
c. exactly three hours
d. 3 hours or more
The purpose of insulation around a conductor of a cable is to provide a degree of mechanical
protection and also to
a. improve manual handling
b. prevent internal heat loss
c. improve wiring flexibility
d. reduce leakage current
A good conductor is one that has:
a. high resistivity
b. low conductivity
c. low permeability
d. high conductivity
What is the minimum nominal cross sectional area of insulated copper cables that can be used for
power and lighting circuits?
a. 4.0 mm2
b. 2.5 mm2
c. 1.5 mm2
d. 1.0 mm2
The purpose of filling an underground cable joint with epoxy resin is to prevent
a. entry of moisture
b. contact between conductor
c. corrosion of the conductor
d. mechanical movement
The filament of an incandescent lamp is made of:
a. copper
b. tungsten
c. nichrome
d. aluminum
A common plastic insulating material for secondary wires is
a. rubber
b. polyethylene
c. cotton braid
d. cambric
ENERGY CONVERSION AND POWER PLANT
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ELECTRIC GENERATOR
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ELECTRIC MOTOR
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TRANSFORMER
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HYDRO ELECTRIC PLANT
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BATTERY
SOLAR POWER STATION
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GEOTHERMAL POWER
STATION
NUCLEAR POWER STATION
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WIND POWER PLANT
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DIESEL POWER PLANT
An electrical equipment that converts mechanical energy to
electrical energy
An electrical equipment that converts electrical energy to
mechanical energy
An electrical equipment that converts electrical energy of one level
to electrical energy of another level
An electric generating power station that converts the energy of
water to electrical energy
A device that converts chemical energy to electrical energy
An electric generating plant that converts the heat or light energy of
the sun into electrical energy
An electric generating plants that converts the heat from the
earth’s crust into electrical energy
An electric generating plant that converts the heat produced by
chain reaction into electrical energy
An electric generating plant that converts the energy of moving air
into electrical energy
An Electric generating plant that produces electricity through
thermodynamic process.
FORMULAS FOR POWER PLANT
1. Demand Factor =
Maximum demand
----------------------------Connected load
Sum of individual maximum demand
2. Group Diversity Factor = ---------------------------------------------------------------Actual maximum demand of group
Maximum demand of consumer group
3. Peak Diversity Factor = ------------------------------------------------------------------------------Demand of consumer group at time of system peak demand
Average load for period
4. Load Factor = ---------------------------------------------------------------------------------------Peak load for period, either instantaneous or average
Average Load for period
5. Capacity Factor = -----------------------------------------------------------------Rated Capacity of the plant
Maximum Load
= -------------------------------------------------- X LOAD Factor
Rated Capacity of the plant
( Also termed as Plant Factor or Use Factor )
Maximum Load
6. Utilization factor = ---------------------------------------------------Rated Capacity of the plant
7.
Capacity factor = Utilization Factor x load Factor
NOTES ON Machines
PARTS OF A DC MACHINE:
a. Two Main Parts:
1. Stationary part designed mainly for producing magnetic flux
2. Rotating part called armature, where mechanical energy is converted into
electrical energy (generator) or electrical energy converted into mechanical
energy (motor).
b. Various parts
1. Frame
4. Field poles
7. Commutator poles
2. Armature
5. Brush Gear
8. Armature windings
3. Commutator
6. Armature shaft bearings
Two types of Armature Windings according to the degree of closure
1. Open Coil Winding
2. Closed Coil Winding
Two Types of Closed Armature Winding
1. Ring Winding
2. Drum Winding
Two Types of Drum Winding
1. Lap winding – suitable for comparatively low voltage but high current generators.
The finish of each coil is connected to the start of the next coil so that winding or
commutator pitch is unity.
2. Wave winding – suitable for comparatively low current but high voltage generators.
The finish of coil is connected to the start of another coil electrical degrees away
from the first coil.
Types of DC Generators:
1. According to method of Excitation:
a. Separately excited generators – generators whose field magnets are
energized from an independent external source of DC current.
b. Self excited – generators whose field magnets are energized by the current
produced by the generators themselves.
2. According to how the field windings are connected: (Refer to Figure)
a. Shunt wound generators
b. Series woung generators
c. Compound wound generators
1. Short Shunt
2. Long Shunt
Losses in a DC generator:
1. Copper Losses
a. Armature Copper Loss Ia2Ra
Ia - armature current
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b. Shunt Copper Loss Ish Rsh or VtRsh
Ra – armature resistance
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c. Series Copper Loss Ise Rse
Ish – Shunt field current
d. Loss due to brush contact resistance
Rsh – shunt field resistance
Vt - terminal voltage
2. Iron Loss (Core Loss)
Rse – Series Field Resistance
1.6
a. Hysteresis loss Ph = Kh (Bmax) N watts
Ise - series field current
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b. Eddy Current Loss Pe = Ke(BmaxNt) watts
Pe – Eddy Current loss
c. Mechanical Loss – Friction and Windage
Ph – Hyteresis Loss
Kh , Ke – proportionality constant
3. Mechanical Loss – Friction & Windage
Bmax – maximum flux density
N – speed of armature
rotation
t – thickness of armature core
lamination
EFFICIENCIES
electrical power developed by the armature
1. Mechanical Efficiency
ŋm
=
------------------------------------------------------------total mechanical power input
Eg Ia
= ------------------------------------------------------------------B.H.P. of prime mover x 735.5
useful electrical power output
2.
Electrical Efficiency
VI
ŋe = ----------------------------------- = -------------electrical power developed
Eg Ia
useful electrical output
3. Over-all Efficiency
ŋg = ------------------------------- = ŋm x ŋe
Total mechanical power input
useful power output
VI
= ---------------------------------------------- = --------------------
-useful power output + total losses
losses
V I
+ total
Armature Reaction:
1. Demagnetizing component – armature reaction component which is in the same
phase with the main field flux. The direction is opposite to the direction of the main
field flux, resulting to the reduction of the effect of the maintained flux.
2. Cross-magnetizing or distorting component - armature reaction component which
is at right angle to the main field flux. This component crosses the main field flux.
Remedies for Neutralizing Armature Reaction
1. Increasing the length of the air gap
2. providing the machine with a compensating winding
3. Using commutating poles
4. reducing the cross-section of the pole faces.
Compensating winding – function is to neutralize the cross-magnetizing effect of the armature
reaction. Usually used in large DC machine which are subjected to large fluctuation in load.
Commutation –the process of current collection by the brush with respect to the changes
which takes place in a coil during the period of short-circuit by a brush.
1. Resistance commutation – replacing the low resisarbon brushes.tance copper
brushes with comparatively high resistance c
2. e.m.f commutation – method in which an arrangement is made to neutralize the
reactance voltage by producing a reversing e.m.f. in the short circuited coil under
the icommutation.
a. by giving the brush a forward lead sufficient enough to bring the shortcircuited coil under the influence of next pole of opposite polarity
b. by using interpoles or compoles.
Advantages of Carbon Brushes:
1. They have high contact resistance which is useful for having good communication.
2. They lubricate and polish the commutator as it rotates.
3. In the event of sparking, carbon brushes will damage the
commutator less in comparison to copper brushes.
Disadvantage of Carbon Brushes:
a) Due to their high contact resistance, a loss of approximately 2 V is caused. Hence they
are not much suitable for small machines where this voltage forms an appreciable
percentage loss.
b) Because of the above loss, the commutator has to be made somewhat larger than with
copper brushes so as to dissipate heat efficiently without greater rise of temperature.
c) Owing to their lower current density (about 7-8 A/cm2 as compared to 25-30 A/cm2 for
copper brushes) large holders are required.
DEFINITION OF TERMS:
a. Illumination– It is the density of the luminous flux on a surface.
b. Lumen – It is equal to the flux through a unit solid angle from a uniform point source of
one candle.
c. Footcandle – The illuminance produced by a luminous flux of one lumen uniformly
distributed over a surface of one square foot.
d. Luminance – The luminous intensity of any surface in a given direction per unit of
projected area of the surface as viewed from that direction (Photometric brightness)
e. Footlambert – A unit of luminance equal to 1/π candela per square foot.
f. Nit – SI unit of luminance where the meter is taken as the unit of length. Candela per
square meter.
g. Luminous efficacy – a quantity denoting the effectiveness of light sources.
h. It is the ratio of the total luminous flux (lumen) to the total power input (watts)
i. Reflectance – the ratio of reflected flux to incident flux.
j. Transmittance – the ratio of the transmitted flux to the incident flux
k. Absorptance – the ratio of the flux absorbed by a medium to the incident flux.
Reflectance + Transmittance + Absorptance = 1
l. Brightness – The intensity of sensation resulting from viewing light sources and surfaces.
m. Color – The quality of visual sensation which is associated with the spectral distribution
of light.
n. Violet – Color excited by shorter wavelengths of the visible spectrum.
o. Red – Color excited by longer wavelengths of the visible spectrum.
p. Color Matching – The process of adjusting the color of one area so that it is the same
color as another.
q. Color Rendering – The effect of a light source on the color appearance of objects in
conscious and subconscious comparison with their color appearance under a reference
light source.
r. Color rendering index – The measure of the degree of color shift which objects undergo
when illuminated by a reference source of comparable color temperature.
s. Correlated Color Temperature – The absolute temperature ( in Kelvins) of a blackbody
radiator whose chromacity most nearly resembles that of the light source.
t. Coefficient of Utilization – a measure of the efficiency of a luminaire.
u. It is the ratio of the light reaching the working surface to the light being emitted from the
luminaire.
v. Luminaire Dirt Depreciation Factor (LDD) – A factor due to the cleanliness of a
luminaire’s environment and on how often the luminaries is cleaned.
w. Lamp Lumen Depreciation Factor (LLD) – is a multiplier used with initial lamp lumens
to determine the lamp output depreciation due to aging.
x.
Glare – When the light source is in the line of vision.