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WorksheetsElectrical Machines Set 1
Total questions: 20
Worksheet time: 2hrs 40mins
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
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.
