Free Electrical Machines MCQs with Answers

• A. direction of induced e.m.f.
• B. direction of flux
• C. direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
• D. direction of motion of conductor, if forefinger points along the lines of flux

Explanation: With the forefinger along the flux lines, Fleming’s right-hand rule places the thumb in the direction of conductor motion and the middle…

• A. rivets
• B. counter sunk screws
• C. brazing
• D. welding

Explanation: Pole shoes are normally secured to the pole cores with countersunk screws, which provide a firm mechanical connection while keeping the…

• A. magnetic flux, direction of current flow and resultant force
• B. magnetic flux, direction of motion and the direction of e.m.f. induced
• C. magnetic field strength, induced voltage and current
• D. magnetic flux, direction of force and direction of motion of conductor

Explanation: Fleming’s right-hand rule relates the magnetic-flux direction, the motion of the conductor, and the direction of induced e.m.f.

• A. current
• B. voltage
• C. speed
• D. none of above

Explanation: Mechanical losses consist mainly of friction and windage, both of which increase with rotational speed.

• A. where low voltage and high currents are involved
• B. where high voltage and small cur-rents are involved
• C. in both of the above cases
• D. in none of the above cases

Explanation: Copper brushes have low contact resistance and are therefore suitable for low-voltage, high-current operation.

• A. Lap winding
• B. Wave winding
• C. Either of A. and B. above
• D. Depends on other features of design

Explanation: Generated e.m.f. is inversely related to the number of parallel paths. Wave winding normally has only two parallel paths, whereas lap…

• A. severe sparking
• B. rough commutator surface
• C. imperfect contact
• D. any of the above

Explanation: Severe sparking, a rough commutator surface, and imperfect brush contact can all damage the brush surface and increase wear.

• A. ball bearings
• B. bush bearings
• C. magnetic bearmgs
• D. needle bearings

Explanation: Ball bearings are commonly used to support the rotor shaft in smaller and general-purpose DC machines because they provide low friction…

• A. double the number of poles
• B. same as the number of poles
• C. half the number of poles
• D. two

Explanation: In a simplex lap winding, the number of parallel paths is equal to the number of poles, so one brush is normally provided for each pole.

• A. all the four poles are north poles
• B. alternate poles are north and south
• C. all the four poles are south poles
• D. two north poles follow two south poles

Explanation: Adjacent poles in a DC machine must have opposite polarities so that the armature conductors experience alternating magnetic fields.

• A. is amenable to better voltage control
• B. is more stable
• C. has exciting current independent of load current
• D. has all above features

Explanation: A separately excited generator has an independent field supply, allowing better voltage control and making its field current independent…

• A. field control method
• B. armature controlled method
• C. field diverter
• D. frequency control method

Explanation: The Ward Leonard system varies the DC motor armature voltage by supplying it from a separately driven generator, while the field is…

• A. decrease in speed
• B. increase in speed
• C. short circuit
• D. open circuit

Explanation: Armature reaction weakens and distorts the main flux in a DC motor. For a given back EMF requirement, the weakened flux causes the motor…

• A. series with field winding
• B. series speed armature winding
• C. parallel big field winding
• D. parallel with armature winding

Explanation: Interpoles must produce a magnetic field proportional to armature current so they can counteract armature reaction during commutation.

• A. not develop starting torque
• B. fail to start without load
• C. run at dangerously high speed
• D. draw more current without producing sufficient torque

Explanation: For a DC series motor, flux decreases sharply when the load is removed, while torque is not needed to sustain rotation.

• A. voltage control method
• B. flux control method
• C. rheostat control method
• D. all of the above

Explanation: A DC shunt motor can be controlled by varying armature voltage, varying field flux, or inserting resistance in the armature circuit.

• A. If the field winding resistance of a DC shunt motor is increases, its speed will
• B. decreases
• C. increases
• D. remains same
• E. none of the above

Explanation: Increasing the shunt-field resistance reduces field current and therefore weakens the flux.

• A. decreases
• B. increases
• C. remain same
• D. none of the above

Explanation: A temperature rise increases the resistance of the series field and armature windings, causing a larger internal voltage drop and…

• A. decreases.
• B. increases
• C. remain same
• D. none of the above

Explanation: Heating increases the shunt-field winding resistance, so field current and flux decrease.

• A. dc generator
• B. dc motor
• C. transformer
• D. all of the above

Explanation: In a DC motor, the generated back emf opposes the applied voltage and armature current, which is why it is called back emf.