WorksheetsEMA Revision B
Total questions: 30
Worksheet time: 18mins
The magnetomotive force (mmf) of a coil is 1000 A. If the current through the coil is decreased by half, its mmf will be
halved.
doubled.
quartered.
the same.
A conductor of length 50 cm is carrying a current of 5 A as shown in Figure. The flux density required to produce a force of 2 N is
0.08 T
0.8 T
50T
500 T
A conductor carries a current of 5 A placed in a magnetic field having a uniform flux density of 10 T experiences a force of 100 N. The length of the conductor is
1 m.
2 m.
100 m.
200 m.
For a dc machine to operate as a generator, it has to be driven by a
AC supply.
DC supply.
transformer.
prime mover.
The alternating current induced in the armature conductors of a dc machine is made unidirectional in the external load circuit by means of
slip rings.
commutator.
carbon brushes.
compensating winding.
A transformer has 100 primary turns and 150 secondary turns, the transformation ratio is
0.33
0.5
0.67
1.5
Which of the following 3-phase transformer connection is suitable for low voltage and large load:
star/star.
delta/delta.
star/delta.
delta/star.
Which of the following single-phase motor has one capacitor and no centrifugal switch
Universal motor.
Split-phase induction motor.
Capacitor-start induction motor.
Permanent split capacitor motor
The direction of rotation of the split-phase induction motor can be reversed by
reversing the two supplies lines.
reversing the starting winding only.
reversing the running winding only.
reversing the starting winding or running winding.
The part of the single-phase induction motor label X is
fan.
rotor.
stator.
endplate.
The synchronous speed of a 6-pole, 3-phase 50 Hz, 940 rpm induction motor is
500 rpm
940 rpm
1000 rpm
1500 rpm
The two capacitors used in a two-value capacitor motor are
paper and electrolytic.
oil-impregnated and electrolytic.
paper and oil-impregnated.
oil-impregnated and mica.
The running speed of a 3-phase induction motor is
constant irrespective of the mechanical load.
faster than the speed of rotating magnetic field.
slower than the speed of rotating magnetic field.
the same as the speed of rotating magnetic field.
A 2.2 kW, 400 V, 50 Hz 3-phase induction motor has an efficiency of 88%, the input power is
1.9kW
6.3kW
5.5kW
2.5kW
The speed of a motor can be measured using
ohm-meter.
tachometer.
oscilloscope.
Clip-on meter.
The direction of rotation of a 3-phase induction motor can be reversed by
interchange all the three phase leads.
interchange any 2 of the three phase leads.
change the star connection to delta connection.
change the delta connection to star connection.
Which of the following terms refer to the open circuit in one of the 3-phase supply to the 3-phase induction motor?
overload trip.
open-phasing.
three-phasing.
single-phasing.
A remote stop button and the stop button of the main circuit is always connected in
star.
delta.
series.
parallel.
Soft-starting of 3-phase induction motor is preferred nowadays. Which one of the following methods can suitably be replaced?
Direct-on-line starting.
Forward-reverse starting.
Auto-transformer starting.
Rotor resistance starting.
Figure shows the main circuit of a star-delta starter. Which contactor/s should operate when the START pushbutton is being pressed?
Star contactor only
Main contactor only
Main and star contactors
Main and delta contactors
Which of the following starters is selected to reduce the starting voltage of a 3-
phase induction motor to 1/ 3 its rated voltage:
Star-delta starter
Direct-on-line starter
Auto-transformer starter
Rotor resistance starter
Figure shows the control circuit of a starter. The part name M is
contactor coil.
holding contact.
stop pushbutton.
start pushbutton.
Which of the following braking methods using an induction motor as a motor and as a generator alternatively?
Plugging
Friction braking
Dynamic braking
Regenerative braking
Which of the following methods cannot be used to control speed of a 3-phase induction motor?
Varying the supply voltage
Varying the rotor resistance
Varying the supply frequency
Varying the number of stator poles
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 purpose of acceleration of motor using a variable speed drive.
It reduces leakage.
It helps to reduce fault.
It eliminates harmonics.
It reduces surge due to transient.
The pulse-width modulated waveform in a variable speed drive is produced by
rectifier.
filtering capacitor.
free-wheeling diode.
Insulated gate bipolar transistor.
State the AC drive that uses flux control by pulse width modulation with feedback
Close-loop drive.
Open-loop drive.
Cascading drive.
Sequential drive.
Identify the statement that is true for a synchronous motor when its excitation and electrical supply are switched on at the same time to its stator winding.
The motor will not run.
The motor will run slowly.
The motor will run at a high speed.
The motor will develop a full-load torque.
The rotor used in an alternator that has a large axial diameter and short axial length is the
wound rotor.
salient pole rotor.
squirrel cage rotor.
smooth cylindrical rotor.
