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WorksheetsUnit 12 - ELT 502
Total questions: 99
Worksheet time: 15hrs 5mins
How can the direction of a capacitor start single phase motor be reversed?
Reverse the connection to the capacitor
Reverse the connection to the start winding
Reverse the polarity of the supply
Reverse the connection of the start and run windings
When are single phase series universal motors used?
When high starting torque is required
When starting on no-load is required
When constant speeds are required
When low speed drives are required
When is the stator winding of a three phase squirrel-cage induction motor connected in star?
when in full load
when running at rated speed
when on light load
when starting the motor
What is the percentage slip of a motor at full load speed where the nameplate details are: f= 50Hz, poles = 4, full load speed = 1440 rpm?
s = 3%
s = 4%
s = 5%
s = 6%
What is the shape of the laminations used in the magnetic circuit of a single phase shell-type transformer?
LL
TT
LE
EI
What type of transformer is used for interconnecting high voltage transmission systems operating at different voltage levels?
Current transformer
wound transformer
Voltage transformer
Autotransformer
What conditions must be satisfied before connecting two transformers in parallel to share a common load?
Same inductance, same kVA rating
Same polarity, same phase sequence
Same polarity resistance, same current ratio
Same number of polarity turns, same voltage regulation
What is the relationship between the VA rating of a transformer and the internal percentage impedance Z?
VA ∝1/ Z
VA ∝Z
VA ∝2Z
VA ∝ Z/2
What two power losses occur in a transformer when operating on no-load?
Mechanical losses, core losses
Frictional losses, heat losses
Rotational losses, copper losses
Copper losses, core losses
What is the ideal electrical angle of separation of the start and run windings to create a rotating magnetic field in a split phase induction motor?
30
60
90
120
At what speed three-phase cylindrical two-pole synchronous alternator rotates when connected to a 50 Hz infinite busbar system?
1000 rpm
1500 rpm
3000 rpm
6000 rpm
What is the first action to be taken when a thermal overload trips an induction motor?
Short circuit the thermal trip.
Disconnect the thermal trip.
Replace the thermal trip.
Reset the thermal trip.
What is the purpose of the Buchholz relay used with large distribution transformers?
To record operating times.
To check voltage levels
To monitor gas levels
To check current flow.
What is the secondary current for a single-phase transformer which has a primary current of 50 A and a turns ratio of 2:1?
25 A
50 A
100 A
200 a
What is the essential property of the oil used in transformer tanks to facilitate cooling?
High viscosity.
Low hydrogen content
High carbon content
Low viscosity
Which property of a transformer is measured by the change in output voltage from no-load to full-load conditions?
voltage ratio
transformer efficiency
voltage regulation
power transfer
Why are power transformers operated in parallel?
To increase the current rating of the supply.
To maximize power supply flexibly.
To improve voltage regulation.
To reduce heat loss.
What is the danger of opening the secondary circuit of an energized current transformer?
High voltage appears across the secondary.
Large current flows in the primary.
High voltage appears across the primary.
Large current flows in the secondary.
What are the required operating characteristics of transformers used in welding machines?
Low secondary current, low secondary voltage.
High secondary current, low secondary voltage.
High secondary current, high secondary voltage
Low secondary current, high secondary voltage.
What type of magnetic field is produced in the stator winding of a three-phase induction motor when it is energized?
oscillating
static
rotating
pulsating
What type of rotor is used in a slip-ring induction motor?
solid rotor
wound rotor
squirrel cage rotor
double cage rotor
What is the condition for maximum starting torque in an induction motor?
Rotor reactance is twice the rotor resistance.
Rotor reactance is half the rotor resistance.
Rotor reactance is equal to the rotor resistance.
Rotor reactance is zero
What is a typical application of split phase induction motors?
Domestic appliances.
Large constant speed machines.
Driving heavy loads requiring high starting torque.
Driving industrial loads for long periods of time.
How can the direction of rotation of a series universal motor be reversed?
Change the polarity of the supply
Reverse the armature or field winding connections.
Reverse the armature and field winding connections
Change the voltage
What type of motor is used when operating in a corrosive atmosphere?
Drip-proof.
Semi-enclosed.
Fan-cooled.
Pipe-ventilated.
How can a three-phase synchronous motor be started?
By using a pony motor.
By using a squirrel-cage rotor.
By using a star delta motor starter.
By using a rotor resistance starter.
What prime mover is used with a low-speed portable three-phase alternator?
Reciprocating engine.
Water turbine.
Steam turbine.
Diesel engine.
The centrifugal switch of a split phase motor closes upon reaching _____ of its synchronous speed.
85%
65%
55%
75%
Wound rotor induction motors has windings connected via _________.
slip ring
stator
rotor
winidng
The transformer has no load output voltage of 220 Volts. During full load this value drops to 212 Volts. Calculate the voltage regulation.
3.63 %
3.63 V
3.63 A
3.63 W
What is the purpose of a tap changer in transformer?
to regulate the output voltage
to regulate the input voltage
to monitor gas level
to regulate the magnetic flux
The transformer is connected WYE in the secondary side with a line to line voltage of 380 Volts. What is the value of line to neutral voltage?
220 V
210 V
230 V
200 V
The 200 kVA transformer has a power factor of 0.8 with the following losses:
Core losses = 1000 Watts
Copper Losses = 2000 Watts
What is the efficiency of the transformer?
98.125%
98.5 %
98.25 %
98.75 %
What is the purpose of oil in the transformer?
used for cooling
used for lubrication
used to reduce the voltage
used to increase the voltage
All important information of a transformer can be found in its ______________.
name plate
core
windings
casing
If the turns ratio of the transformer is more than 1, the transformer is _______.
step up
step down
either step up or step down
step equal
If the turns ratio of the transformer is less than 1, the transformer is _______.
step up
step down
either step up or step down
step equal
The average efficiency of a transformer is _____.
98 %
88 %
78 %
68 %
The losses of the transformer during no-load condition will only be ___________.
iron losses
copper losses
copper and iron losses
no lossed
The transformer capacity rating is normally expressed in _________?
kVA
kVAR
kW
power factor
In a step up transformer, the value of the electric current in the secondary side is ___________ compare to the primary side.
lower
higher
equal
either higher or lower
What is the purpose of decreasing the voltage in the distribution circuits?
to minimize losses
to increase the speed of the current
for safety purposes
to protect the cable
What is the purpose of increasing the voltage level in the transmission line?
to minimize losses
to increase the speed of the current
for safety purposes
to protect the cable
When an AC supply is applied in the primary side of the transformer a _____________________ will be developed and move around the core of the transformer.
magnetic flux
electric current
induced voltage
inductive reactance
____________ is a static electrical device that changes AC voltage level from one circuit to another without changing the frequency
transformer
motor
generator
resistor
Losses due to reversal of magnetization in the transformer core are:
mechanical losses
hysteresis losses
eddy current losses
copper losses
Losses due to ohmic resistance of the transformer windings are:
Eddy current losses
Mechanical losses
hysteresis losses
copper losses
The equation relating the speed of magnetic field rotation (Ns) to the number of poles (P) and electrical frequency ( f) is:
Ns = 120P / f
Ns = 60f / P
Ns = 120f / P
Ns = f / 120P
At what speed will a three-phase cylindrical six-pole synchronous alternator rotate when connected to a 60 Hz infinite busbar system?
1000 rpm
1200 rpm
1500 rpm
2000 rpm
What is a common use of a portable alternator?
To boost the grid during heavy demand
As a standby supply if the main supply fails
Where a low-voltage dc supply is required
Where a variable frequency supply is required
What is a common use of an engine-generator or portable generator?
Where a low-voltage dc supply is required
To boost the grid during heavy demand
In the event of a power outage
Where a variable frequency supply is required
What is the shape of the laminations used in the magnetic circuit of a single-phase shell-type transformer?
E-I, E-E
E-I, E-E
L-L, U-I
L-L, E-E
What is the shape of the laminations used in the magnetic circuit of a single-phase core-type transformer?
E-I, E-E
E-I, E-E
L-L, U-I
L-L, E-E
The purpose that the transformer core is made up of laminations is to _____________ the flux leakage and _____________ the iron losses
reduce ; increase
increase ; reduce
increase ; increase
reduce ; reduce
Universal Motors are so named because:
they can run AC
they can run DC
they can run both AC and DC
they can be used universally
Universal Motor is basically:
A combined shunt and series DC motor
A combined shunt and series DC motor
A parallel DC motor
A series DC motor
When is the stator winding of a three phase squirrel-cage induction motor connected in star?
when in full load
when running at rated speed
when on light load
when starting the motor
Star-delta starter is used in the motors, which are designed to run on:
delta or star
star
delta
singe-phase induction motor
In a star-delta starter, When the stator winding is star connected, voltage over each phase in motor will be reduced by a factor of _______________ of that would be for delta connected winding.
Note:
sqrt = square root
1/3
2/3
1/ sqrt (3)
2/ sqrt (3)
In a star-delta starter, When the stator winding is star connected, voltage over each phase in motor will be reduced by a percentage of ___________ of that would be for delta connected winding.
48%
58%
68%
78%
In a star-delta starter, The starting torque as the connection is star will be ___________ times that it will be for delta connected winding.
2/3
1/ sqrt (3)
2 / sqrt (3)
1/3
What is the percentage slip of a motor at full load speed where the nameplate details are: f= 60Hz, poles = 6, full load speed = 1116 rpm?
s = 1%
s = 3%
s = 5%
s = 7%
Where is the synchronous speed in rpm and is the rotational speed in rpm, the formula for the in an induction motor is:
slip = { (Ns - N / Ns } x 100%
slip = { Ns / (Ns - N) } x 100%
slip = { (N - Ns / N } x 100%
slip = { N / (N - Ns) } x 100%
In Buchholz relay, in case of minor faults:
The hinged float at the top of the chamber moves upwards and downwards alternately.
The hinged float at the top of the chamber remains unchanged.
The hinged float at the top of the chamber moves upwards.
The displacements of oil tilts the hinged float at the top of the chamber thereby the mercury switch closes the contacts of the alarm circuit.
In Buchholz relay, in case of minor faults:
The float at the bottom of the chamber moves upwards.
The float at the bottom of the chamber remains unaffected because the gases produced will not be sufficient to operate it
The float at the bottom of the chamber moves downwards.
The float at the bottom of the chamber moves upwards and downwards alternately.
In Buchholz relay, in case of severe faults such as phase to earth faults and faults in tap changing gear
The float at the bottom of the chamber is tilted and the trip circuit is closed. This trip circuit will operate the circuit breaker and isolates the transformer.
The float at the bottom of the chamber remains unaffected because the gases produced will not be sufficient to operate it.
The float at the bottom of the chamber moves upwards.
The float at the bottom of the chamber moves upwards and downwards alternately.
Which of the following transformer ratios is true?
Np/Ns = Vs/Vp = Is/Ip
Np/Ns = Vp/Vs = Ip/Is
Np/Ns = Vp/Vs = Is/Ip
Np/Ns = Vs/Vp = Ip/Is
What is the secondary current for a single-phase transformer which has a primary current of 90 A and a turns ratio of 5:3?
Hint:
Np/Ns = Is/Ip
60 A
80 A
120 A
150 A
What is the primary current for a single-phase transformer which has a secondary current of 140 A and a turns ratio of 7:4?
Hint:
Np/Ns = Is/Ip
70 A
80 A
90 A
100 A
What is the primary voltage for a single-phase transformer which has a secondary voltage of 40 V and a turns ratio of 3:1?
Hint:
Ns/Np = Vs/Vp
120 V
150 V
180 V
200 V
What is the secondary voltage for a single-phase transformer which has a primary voltage of 50 V and a turns ratio of 5:1?
Hint:
Np/Ns = Vp/Vs
250 V
50 V
10 V
5 V
The purposes of the transformer oil are:
Conducting electricity
Cooling, insulating and protecting from oxidization
Protecting from cold weather
Protecting from dirt
The dielectric strength of transformer oil that is also known as breakdown voltage of transformer oil or BDV of transformer oil should be:
Medium, not high and not low
As low as possible
As high as possible
That depends on the temperature of the transformer
Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer, is considered as:
30 kV
20 kV
10 kV
5 kV
The presence of moisture content and conducting substances in the transformer oil ____________ the value of the breakdown voltage of the transformer (BDV).
increases much
increases slightly
does not affect
reduces
With increase in temperature the resistivity of oil:
increases rapidly
increases slowly
decreases rapidly
does not get affected
Moisture or water content in transformer oil is:
Hint:
desirable = something you want
little desirable
moderately desirable
highy desirable
highly undesirable
Acidity of transformer oil:
is beneficial property
is harmful property
Prevents oxidization
Prevents rusting
Flash point of transformer oil is the temperature at which oil gives enough vapors to produce a flammable mixture with air. So it is desirable to have ____________ flash point of transformer oil.
Very high
Very low
Slightly low
Medium value
Pour point of transformer oil is:
The minimum temperature at which oil just start to vaporize under standard test condition.
The minimum temperature at which oil just start to boil under standard test condition.
The minimum temperature at which oil just start to flow under standard test condition.
The minimum temperature at which oil just start to become intolerably hot under standard test condition.
With the decrease in temperature, the viscosity of the transformer oil will:
Decrease little
increase
decrease much
does not get affected
Which property of a transformer is measured by the change in output voltage from no-load to full-load conditions?
voltage ratio
transformer efficiency
voltage regulation
power transformer
If E represents the no-load secondary voltage and represents the full-load secondary voltage, the percentage regulation of a transformer is given as:
VR = [ (E - V) / E ] x 100%
VR = [ (E - V) / v ] x 100%
VR = [ (E / E - V) ] x 100%
VR = [ (V / E - V) ] x 100%
A transformer delivers 150 volts at no-load and the voltage drops to 141 volts at full load, the regulation would be:
Hint:
VR = { (VNL - VFL) / VNL } x 100%
4%
5%
6%
7%
All the followings are advantages of operating transformers in parallel except for:
To maximize the voltage rating of the supply.
To maximize electrical power system efficiency.
To maximize electrical power system availability.
To maximize power system reliability.
What conditions must be satisfied before connecting two transformers in parallel to share a common load?
Same inductance, same kVA rating
Same polarity, same phase sequence
Same polarity resistance, same current ratio
Same number of polarity turns, same voltage regulation
What conditions must be satisfied before connecting two transformers in parallel to share a common load?
Same inductance, same kVA rating
Same number of polarity turns, same voltage regulation
Same polarity resistance, same current ratio
Same voltage ratio, same percentage impedance
What type of magnetic field is produced in the rotor winding of a three-phase synchronous motor when it is energized?
oscillating
constant
rotating
pulsating
As compared to the squirrel-cage motor, the wound-rotor (slip-ring) induction motor has __________ inrush current and _____________ starting torque.
higher ; lower
lower ; lower
higher ; higher
lower ; higher
The type of motor that is used in applications such as ceiling fans, food mixers, refrigerators, vacuum cleaners, portable drills, hair driers is:
Single-phase induction motor.
Three-phase squirrel-cage induction motor.
Three-phase wound-rotor induction motor.
Three-phase synchronous motor
The wattage for the single-phase induction motor is in the range of:
Multiple kilowatts.
Fractional kilowatt.
Tens kilowatts.
Hundreds kilowatts
Single phase induction motors are extensively used in:
Large machines of high speed.
Heavy-duty machines.
Low power applications such as domestic and light-industrial appliances.
Heavy industrial machines
In single phase induction motors, the starting winding is made ______________ whereas the running winding is made with ______________.
highly capacitive ; highly inductive
highly inductive ; highly resistive
highly resistive ; highly capacitive
highly resistive ; highly inductive
When there are minimum two alternating fluxes, having a phase difference between them, then the resultant magnetic field is.:
static
rotating
alternating
pulsating
Split phase motors have _________ starting torque and ________ power factor.
high ; poor
high ; high
poor ; poor
poor ; high
When comparing the capacitor start induction motor with the spilt phase one, the capacitor start motor has ___________ starting torque and _________ inrush current.
higher ; higher
lower ; lower
lower ; higher
higher ; lower
When comparing the capacitor run induction motor with the capacitor start one, the capacitor run motor has _____________ starting torque and ___________ power factor and efficiency.
higher ; worse
lower ; better
lower ; worse
higher ; better
When comparing the capacitor start, capacitor run induction motor with other types of single phase induction motor, it has ______________ starting torque and ___________ power factor and efficiency.
higher ; worse
lower ; better
lower ; worse
higher ; better
All are methods of starting three-phase synchronous motor except:?
By using a damper winding.
By using a slip-ring induction motor.
By using a star delta motor starter.
By using a small D.C machine.
