Free Electrical Machines MCQs with Answers

• A. almost zero
• B. less than no load terminal voltage
• C. more than no load terminal voltage
• D. equal to no-load terminal voltage

Explanation: An over-compounded generator has a series-field voltage rise greater than the internal voltage drop as load increases.

• A. Series generator
• B. Shunt generator
• C. Compound generator
• D. None of the above

Explanation: Self-excited series, shunt, and compound generators need residual magnetism to initiate the initial generated emf and field current.

• A. Series generator
• B. Shunt generator
• C. Compound generator
• D. Separately excited generator

Explanation: A series generator has its field winding in series with the load, so at no load its field current is nearly zero and only a small residual…

• A. r.p.m. is more than 300
• B. r.p.m. is less than 300
• C. number of poles is 4
• D. number of poles is 2

Explanation: Electrical angle equals mechanical angle multiplied by P/2, where P is the number of poles.

• A. series generator
• B. shunt generator
• C. long shunt compound generator
• D. any of'the above

Explanation: A long-shunt compound generator provides a comparatively steady charging voltage while supplying the varying current needed by an…

• A. voltage generated is less than the rated voltage
• B. generated voltage is proportional to the load on the generator
• C. voltage remains constant irrespective of the load
• D. speed varies in proportion to the load on the generator

Explanation: In a flat-compounded generator, the series-field effect is adjusted to compensate for internal voltage drops as load changes.

• A. neutralise the commutating self induced e.m.f.
• B. neutralise the armature reaction flux
• C. neutralise both the armature reaction flux as well as commutating e.m.f. induced in the coil
• D. perform none of the above functions

Explanation: Interpoles produce a commutating flux that opposes the armature-reaction flux in the commutation zone and induces a reversing emf to…

• A. separately excited generator
• B. shunt generator
• C. series generator
• D. compound generator

Explanation: A turbo-alternator commonly uses a DC shunt generator as its exciter because it supplies the field current required by the alternator.

• A. to supply traction load
• B. to supply industrial load at constant voltage
• C. voltage at the toad end of the feeder
• D. for none of the above purpose

Explanation: A series generator has a nearly constant current characteristic and is therefore used as a series booster to compensate for voltage drop…

• A. increase
• B. decrease
• C. remain unaffected
• D. fluctuate heavily

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

• A. identified by the scarring of the commutator segment to which open circuited coil is connected
• B. indicated by a spark completely around the commutator
• C. both A. and (b)
• D. none of the above

Explanation: An open armature coil interrupts current transfer at its associated commutator bars, producing scarring or burning there and abnormal…

• A. any even number of conductors
• B. any odd number of conductors
• C. that even number which is exact multiple of poles + 2
• D. that even number which is exact multiple of poles

Explanation: In a simplex lap winding, the coil and conductor arrangement requires an even total number of armature conductors so that the winding can…

• A. brushes should be of proper grade and size
• B. brushes should smoothly run in the holders
• C. smooth, concentric commutator properly undercut
• D. all of the above

Explanation: Good commutation depends on correctly sized and graded brushes, free brush movement in the holders, and a smooth, concentric, properly…

• A. total copper loss and mechanical loss
• B. armature copper loss and iron loss
• C. shunt field copper loss and mechanical loss
• D. iron loss and mechanical loss

Explanation: Stray losses in a DC machine comprise core or iron losses and mechanical losses such as friction and windage.

• A. silicon steel
• B. copper
• C. non-ferrous material
• D. cast-iron

Explanation: Silicon steel is used because its high magnetic permeability provides an efficient flux path, while laminations reduce eddy-current loss…

• A. rotor
• B. stator
• C. field
• D. armature

Explanation: The armature is the part containing conductors in which the useful e.m.f. is induced, whether it is the rotor or stator.

• A. E/2
• B. 2E
• C. slightly less than E
• D. E

Explanation: Generated e.m.f. follows E ∝ ΦN. With pole flux Φ constant, doubling speed N doubles the e.m.f., changing E to 2E.

• A. rectifier
• B. primemover
• C. rotating amplifier
• D. power pump

Explanation: A D.C. generator is called a rotating amplifier because a relatively small change in field power can control a much larger armature…

• A. Graphite brushes
• B. Carbon brushes
• C. Metal graphite brushes
• D. None of the above

Explanation: Metal-graphite brushes have a lower contact resistance and hence a smaller brush voltage drop than ordinary carbon or graphite brushes.

• A. number of pole pairs
• B. number of poles
• C. number of parallel paths
• D. number of commutator segments

Explanation: In a simplex lap winding, equalizer connections are made between points of equal potential, with one tapping arrangement for each pair of…