Free Electrical Machines MCQs with Answers

• A. external current
• B. armature current
• C. shunt current
• D. load current

Explanation: In a short-shunt compound generator, the series field is connected in series with the external load, so it carries load current.

• A. is always present
• B. is always absent
• C. may be sometimes present
• D. none of the above

Explanation: At rated load, armature current flows in the separately excited generator, producing armature reaction.

• A. slots
• B. armature conductors
• C. winding elements
• D. poles

Explanation: Each winding element is connected between two adjacent commutator bars, so the number of commutator bars equals the number of winding…

• 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: A simplex wave winding requires an even number of conductors, with the number arranged so that the average pitch is integral, commonly…

• A. 2
• B. 4
• C. 6
• D. 8

Explanation: For a duplex lap winding, the number of parallel paths is A = mP, where m = 2 and P = 2. Thus A = 2 × 2 = 4.

• A. external resistance = internal char-acteristic - armature reaction
• B. internal characteristic = magnetisation characteristic - ohmic drop
• C. external characteristic = magnetisation characteristic - ohmic drop - armature reaction
• D. magnetisation characteristic = external characteristic

Explanation: The external characteristic is below the magnetisation characteristic because armature reaction reduces generated emf and armature…

• A. increasing its field resistance
• B. decreasing its field resistance
• C. increasing its speed
• D. decreasing its speed

Explanation: Increasing speed raises the generated emf for a given field current, making the open-circuit characteristic steeper.

• A. Series generator
• B. Shunt generator
• C. Over compound generator
• D. Flat compound generator

Explanation: An over-compound generator raises its generated voltage with increasing load, compensating for the voltage drop in a long transmission…

• A. Series field
• B. Compensating field
• C. Inter pole field
• D. Shunt field

Explanation: An energized shunt field is highly inductive, so opening its circuit can produce a large induced voltage that damages insulation and…

• A. demagnetisation of leading pole tip and magnetisation of trailing pole tip
• B. demagnetisation of trailing pole tip and magnetisation of leading pole tip
• C. damagnetising the center of all poles
• D. magnetising the center of all poles

Explanation: In a generator, armature reaction weakens the leading pole tip and strengthens the trailing pole tip.

• A. zero
• B. small
• C. the same as rated voltage
• D. high

Explanation: Residual magnetism can provide flux, but generated emf also depends on speed, as given by E = kΦN.

• A. to reduce eddy current losses
• B. to enhance flux density
• C. to amplify voltage
• D. to provide mechanical balance for the rotor

Explanation: Dummy coils are placed in unused armature slots and connected mechanically, not electrically.

• A. zero
• B. about 2 V
• C. about 50 V
• D. 220 V

Explanation: With no field excitation, only residual magnetism produces voltage, so the open-circuit voltage is a small residual value rather than zero…

• A. 150 V
• B. 175 V
• C. 240 V
• D. 290 V

Explanation: For approximately constant flux, generated emf is proportional to speed: E₂/E₁ = N₂/N₁. Thus E₂ = 200 × 1200/1000 = 240 V.

• A. sinusoidal
• B. triangular
• C. pulsating
• D. flat topped

Explanation: At no load, the pole faces produce an approximately uniform flux density over much of the pole arc.

• A. armature reaction
• B. armature resistance drop
• C. field weakening due to armature reaction and armature
• D. commutation

Explanation: Load voltage drops result from armature resistance, armature reaction, and the resulting reduction in field flux.

• A. interpoles
• B. dummy coils
• C. compensating winding
• D. shifting of axis of brushes

Explanation: Interpoles, compensating windings, and suitable brush shifting help neutralize the reactance emf during commutation.

• A. there is no load on|he generator
• B. the generator runs on full load
• C. the generator runs on overload
• D. the generator runs on designed speed

Explanation: With no armature current, armature reaction is absent, so the magnetic flux axis is not displaced.

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

Explanation: A level-compounded generator is designed so that the series-field voltage rise compensates for armature-resistance and other internal…

• A. is the same as that of the main pole ahead
• B. is the same as that of the immediately preceding pole
• C. is opposite to that of the main pole ahead
• D. is neutral as these poles do not play part in generating e.m.f.

Explanation: In a generator, each interpole has the same polarity as the main pole ahead of it in the direction of rotation.