Free Electrical Machines MCQs with Answers

• A. Series motor
• B. Shunt motor
• C. Cumulatively compounded motor
• D. Differentially compounded motor

Explanation: A D.C. series motor gives very high starting torque, which is essential for lifting heavy loads from rest in cranes and hoists.

• A. 2 to 3 per cent
• B. 10 to 15 per cent
• C. 20 to 25 per cent
• D. 50 to 75 per cent

Explanation: Residual magnetism in a D.C. machine is normally only a small fraction of the rated flux, approximately 2 to 3 percent.

• A. Low speed operation
• B. High speed operation
• C. Variable speed operation
• D. Fixed speed operation

Explanation: D.C. motors are especially preferred where continuously variable speed is needed because their armature voltage and field flux can be…

• A. 4.0
• B. 2.0
• C. 1.0
• D. 0.5

Explanation: Mechanical power is proportional to E_b(V - E_b), which is maximum when E_b = V/2. Therefore, the ratio E_b/V is 0.5.

• A. To reduce eddy current loss
• B. To support the field coils
• C. To spread out flux for better uniformity
• D. To reduce the reluctance of the magnetic path

Explanation: Pole shoes support the field coils, spread the flux over the armature, and reduce magnetic reluctance.

• A. Starting torque
• B. Operating speed
• C. Full-load current
• D. All of the above

Explanation: For a shunt motor, increasing supply voltage raises speed and usually raises starting torque.

• A. Series motor
• B. Shunt motor
• C. Air motor
• D. Battery operated motor

Explanation: An air motor has no electrical sparks, hot electrical windings, or commutator, so it is safer in an explosive atmosphere.

• A. damage to commutator segments
• B. damage to commutator insulation
• C. increased power consumption
• D. all of the above

Explanation: Commutator sparking produces intense local heating, which can damage both the copper segments and their insulation, while also increasing…

• A. as separately wound unit
• B. in parallel with armature winding
• C. in series with armature winding
• D. in parallel with field winding

Explanation: A conductively compensated series motor requires the compensating winding to carry the armature current so that its flux opposes armature…

• A. shunt
• B. series
• C. differentially compounded
• D. cumulativelyc'ompounded

Explanation: A differentially compounded motor has series flux opposing the shunt flux, so its resultant flux and starting torque can be low.

• A. nothing will happen to the motor
• B. this will make armature to take heavy current, possibly burning it
• C. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses D. motor will run at very slow speed

Explanation: If the shunt field opens, the motor flux collapses while the applied voltage remains, causing a dangerous rise in speed.

• A. Series motor
• B. Shunt motor
• C. Cumulatively compounded motor
• D. Differentially compounded motor

Explanation: A cumulatively compounded motor combines the shunt motor's good speed regulation with the series motor's high starting torque.

• A. Series motor
• B. Shunt motor
• C. Cumulatively compounded 'motor
• D. Differentially compounded motor

Explanation: A differentially compounded motor has opposing series and shunt fluxes, so its flux falls as load increases.

• A. the power drawn in kW
• B. the power drawn in kVA
• C. the gross power
• D. the output power available at the shaft

Explanation: A motor nameplate normally specifies its rated mechanical output, measured at the shaft.

• A. the torque required
• B. the speed of the motor
• C. the voltage across the terminals
• D. none of the above

Explanation: For a motor with constant field flux, electromagnetic torque is proportional to armature current, as given by T = kφIa.

• A. 100% of the previous value
• B. 50% of the previous value
• C. 25% of the previous value
• D. 10% of the previous value
• E. none of the above

Explanation: Before magnetic saturation, a series motor has flux proportional to armature current, so T ∝ φIa ∝ Ia².

• A. proportional to the armature current
• B. proportional to the square of the armature current
• C. proportional to field current
• D. inversely proportional to the armature current

Explanation: In the unsaturated region of a series motor, flux is approximately proportional to armature current, while speed varies approximately as…

• A. low
• B. around 500 Q
• C. 1000 Q
• D. infinitely large

Explanation: The motor's armature resistance is normally very low, so a starting resistor is inserted only to limit the large current produced when…

• A. Blowers
• B. Conveyors
• C. Air compressors
• D. Centrifugal pumps

Explanation: Conveyors often require high starting torque because the motor must overcome static friction and start a loaded belt.

• A. Lenz's law
• B. Faraday's law
• C. Coloumb's law
• D. Fleming's left-hand rule

Explanation: Fleming's left-hand rule gives the direction of force on a current-carrying conductor in a magnetic field, and therefore determines the…