Free Electrical Machines MCQs with Answers

• A. high voltage, high current
• B. low voltage, high current
• C. high voltage, low current
• D. low voltage, low current

Explanation: A lap winding has as many parallel paths as poles, so it can carry high current at relatively low voltage.

• A. abrasion from dust
• B. excessive spring pressure
• C. rough commutator bars
• D. high mica insulation between com-mutation bars
• E. all of the above factors

Explanation: Brush wear can result from abrasive dust, excessive spring pressure, a rough commutator, or mica projecting above the copper bars.

• A. Dummy coils
• B. Commutator
• C. Eye bolt
• D. Equilizer rings

Explanation: The commutator mechanically rectifies the alternating emf induced in the armature conductors, delivering unidirectional current at the…

• A. mainly to reduce the eddy currents by providing local short-circuits
• B. to provide path for the circulation of cooling air
• C. to neutralise the cross-magnetising effect of the armature reaction
• D. none of the above

Explanation: Compensating windings are placed in the pole faces and carry armature current, producing an opposing mmf that neutralizes the…

• A. Insulation failure between two commutator bars
• B. Insulation failure between two turns of a coil
• C. Two of more turns of the same coil getting grounded
• D. All of the above

Explanation: A short circuit can result from insulation failure between commutator bars, between turns of a coil, or from turns of the same coil…

• A. speed of armature
• B. type of winding
• C. voltage
• D. amount of current to be collected

Explanation: The number of brushes is selected mainly according to the armature current that must be collected, since multiple brushes reduce current…

• A. electromagnets
• B. permanent magnets
• C. both A. and B.
• D. none of the above

Explanation: In the conventional DC generator, the magnetic field is produced by field-winding electromagnets, whose excitation can be separately…

• A. the brushes of opposite polarity should track each other
• B. the brushes of same polarity should track each other
• C. brush position has no effect on the commutator grooving
• D. None

Explanation: Brushes of opposite polarity are arranged to track the same commutator path, so their combined action helps prevent uneven wear and groove…

• A. reduces generator e.m.f.
• B. increases armature speed
• C. reduces interpoles flux density
• D. results in sparking trouble

Explanation: The demagnetizing component of armature reaction weakens the main field flux, so the generated emf decreases according to E ∝ ΦN.

• A. cumulatively compounded long shunt
• B. deferentially compounded long shunt
• C. cumulatively compounded short shunt
• D. differentially compounded short shunt

Explanation: When a compound generator operates as a motor, the armature current reverses relative to the series-field connection, reversing the…

• A. along neutral axis
• B. along field axis
• C. in any of the above positions
• D. in none of the above positions

Explanation: A coil is short-circuited by the brush while it is passing through the magnetic neutral axis, which is the commutation zone.

• A. lap winding
• B. wave moving
• C. duplex winding
• D. any of the above

Explanation: A DC welding generator requires high current at relatively low voltage, which is provided by a lap winding.

• A. brushes
• B. field
• C. armature
• D. load

Explanation: Critical resistance is the maximum field-circuit resistance at which a shunt generator can just build up its voltage.

• A. 640 V
• B. 620 V
• C. 600 V
• D. 580 V

Explanation: For a generator, terminal voltage is approximately E minus the armature voltage drop: 600 − (200 × 0.1) = 580 V. The correct option is d.

• A. multiplication of front and back pitches
• B. division of front pitch by back pitch
• C. sum of front and back pitches
• D. difference of front and back pitches

Explanation: In a simplex lap winding, the resultant pitch equals the difference between the back pitch and front pitch, with the sign depending on the…

• A. to reduce the reluctance of the mag¬netic path
• B. to spread out the flux to achieve uniform flux density
• C. to support the field coil
• D. to discharge all the above functions

Explanation: Pole shoes reduce magnetic reluctance, spread flux over the armature for a more uniform flux density, and provide mechanical support for…

• A. neutralising residual magnetism
• B. creating residual magnetism by a D.C. source
• C. making the magnetic losses of forces parallel
• D. increasing flux density by adding extra turns of windings on poles

Explanation: Flashing the field means applying an external DC supply to the field winding so that residual magnetism is restored or strengthened.

• A. rotary converter
• B. mercury are rectifier
• C. induction motor D.C. generator set
• D. synchronous motor D.C. generator set

Explanation: An induction-motor-driven DC generator set provides a convenient, controllable DC supply for a DC machines laboratory.

• A. number of armature coils
• B. number of armature coil sides
• C. number of armature conductors
• D. number of armature turns

Explanation: Each armature coil is connected to one commutator segment pair or segment connection, so the number of commutator segments corresponds to…

• A. lap winding
• B. wave winding
• C. delta winding
• D. duplex wave winding

Explanation: A welding generator must deliver high current at low voltage, so it uses a lap winding with multiple parallel paths.