WorksheetsEMA Revision A
Total questions: 30
Worksheet time: 15mins
The direction of the magnetic field produced by a current in a solenoid is determine by
Left Hand Grip Rule.
Right Hand Grip Rule.
Fleming’s Left Hand Rule.
Fleming’s Right Hand Rule.
What is the direction force acting on the conductor?
Up.
Down.
Clockwise.
Anti-clockwise.
When a current of 10 A is pass through a solenoid having 150 turns, the magnetomotive force produced equal
10A
150A
750A
1500A
The part of a dc machine that supports the field coils and increases the cross- sectional area of the air gap is
armature
pole shoes.
commutator.
brush assembly.
When the currents flowing in the shunt field and series field of a dc compound machine are in the same direction, the connection is known as
cumulative.
differential.
long shunt.
short shunt.
Which of the following 3-phase transformer connections is suitable for general supplies where neutral point is available for single-phase supply?
Star/star.
Delta/delta.
Star/delta
Delta/star.
During operation the primary and secondary winding of a transformer are linked by
self-induction.
mutual induction.
electrical connection.
mechanical connection.
Which of the following is not a part of a split-phase induction motor?
Stator
Endplates
Wound rotor
Starting winding
Figure shows a schematic diagram of a centrifugal switch of a single-phase induction motor. At this instant, the motor is
stationary.
running at 40% rated speed.
running at 60% rated speed.
running at 80% rated speed.
Which of the following single-phase motor has its armature and field coil connected in series?
Servo motor
Universal motor
Split-phase induction motor
Capacitor-start induction motor
The part of the single-phase induction motor label X is
fan.
rotor.
stator.
endplate.
The speed of the rotating magnetic field of a 3-phase induction motor is
slip speed.
motor speed.
phase speed.
synchronous speed.
The synchronous speed of a 3-phase induction motor depends on
supply power.
supply voltage.
supply current.
supply frequency.
A 3-phase induction motor has an input power of 10 kW and an output power of
8.5kW.What is the efficiency of themotor?
15%
18%
85%
118%
When a 3-phase induction motor winding is shorted to the stator core, the insulation resistance tester will read
zero.
infinity.
5.5 M Ω
500M Ω
A technician would like to check the insulation resistance of a 3-phase induction motor. Which of the following measuring instrument is suitable?
Ohmmeter
Oscilloscope
Insulation tester
Clip-on ammeter
A 2-pole, 3-phase, 50 Hz induction motor runs a speed of 2750 rpm. What is the value of the slip.
0.083
0.092
0.833
0.917
A remote start button and the start button of the main circuit is always connected in
start.
delta.
series.
parallel.
In order to prevent the motor from restarting unexpectedly, all motors shall be provided with
no-volt tripping.
overload tripping.
short-circuit tripping.
earth-leakage tripping.
The fuse blown when the start button of the direct-on-line starter is pressed. The likely cause could be due to
open-coil.
shorted-coil.
defective stop button.
defective start button.
Which of the following starters is selected to reduce the starting voltage of a 3- phase induction motor to 70% its rated voltage:
Star-delta starter
Direct-on-line starter
Auto-transformer starter
Rotor resistance starter
Figure 12 shows the control circuit of a starter. The part name M is
contactor coil.
start pushbutton.
stop pushbutton
overload contact
Which of the following methods is used to control the speed of a 3-phase induction motor by varying its supply frequency?
Rotary converter
External rotor resistance
Having two stator windings
Pole amplitude modulation
The equipment that reduces the effect of harmonics in the electrical supply is the
line filter.
capacitor bank.
synchronous motor.
transient suppressor.
In figure, identify the function of the section A.
It saves energy.
It filters the noise.
It rectifies the alternating current to direct current.
It converts the direct current to alternating current.
State the function of the acceleration in the setting menu of a variable speed drive.
It enables the reduction of harmonics
It enables the reduction of short circuit.
It enables the reduction of power consumption.
It enables the reduction of water hammering effect.
When a variable speed drive is used to control a 3-phase induction motor, the current drawn from the supply is
in pulses.
a square wave.
a smooth sine wave
a pulse modulated wave.
State the AC drive that uses frequency control by pulse width modulation without feedback.
Close-loop drive.
Open-loop drive.
Cascading drive.
Sequential drive.
When the mechanical load of a synchronous motor is reduced,
the motor will stop.
the motor will runs faster.
the motor will runs slower.
the motor speed remains unchanged.
The field exciter of a 3-phase alternator is connected to a
DC supply.
AC supply.
3-phase supply.
single-phase supply.
