Free Electrical Machines MCQs with Answers

• A. these motors have high starting torque
• B. these motors are not self-starting
• C. back e.m.f. of these motors is zero initially
• D. to restrict armature current as there is no back e.m.f. while starting

Explanation: At starting, the motor has zero back e.m.f., so the armature would draw an excessive current limited only by its very low resistance.

• A. the speed of motor will be reduced %
• B. the armature current will reduce
• C. the motor will attain dangerously high speed 1
• D. the motor will continue to nuvat constant speed

Explanation: Opening the shunt field greatly reduces flux, and motor speed varies approximately inversely with flux for a given back e.m.f.

• A. current in the conductor aovtaat of conductor
• B. resultant force on conductor
• C. none of the above

Explanation: Fleming’s left-hand rule assigns the forefinger to magnetic field, the middle finger to current, and the thumb to force or motion.

• A. Shunt motor
• B. Series motor
• C. Differential compound motor
• D. Cumulative compound motor

Explanation: Elevators require high starting torque and reasonably controlled speed, which is provided by the series-field torque of a cumulative…

• A. high starting torque
• B. low starting torque
• C. variable speed
• D. frequent on-off cycles

Explanation: In a differential compound motor, the series field opposes the shunt field, reducing the net flux as load increases and giving…

• A. Series motor
• B. Shunt motor
• C. Cumulative compound motor
• D. Differential compound motor

Explanation: Machine tools generally need nearly constant speed despite load changes, which is the main characteristic of a shunt motor.

• A. Lathe machine
• B. Centrifugal pump
• C. Locomotive
• D. Air blower

Explanation: A locomotive must start by moving a heavy train, so it requires very high starting torque.

• A. interchanging supply terminals
• B. interchanging field terminals
• C. either of A. and B. above
• D. None of the above

Explanation: Reversing only the field connections reverses the field relative to the armature current and hence reverses torque.

• A. Shunt motor
• B. Series motor
• C. Cumulative compound motor
• D. Differentiate compound motor

Explanation: A series motor has very high no-load speed because its flux becomes small when armature current and series-field current are small.

• A. the pressure in the turbine rotor is approximately same as in con¬denser
• B. the pressure in the turbine rotor is higher than pressure in the con¬denser
• C. the pressure in the turbine rotor gradually decreases from inlet to exit from condenser
• D. none from the above

Explanation: A De Laval turbine is an impulse turbine, so nearly the entire pressure drop occurs in the nozzle and the rotor pressure remains almost…

• A. is same
• B. is different
• C. increases from one side to the other side
• D. decreases from one side to the other side

Explanation: In an impulse wheel, the moving blades only change the velocity and direction of steam; ideally, they produce no pressure drop.

• A. there is enthalpy drop in fixed and moving blades
• B. there is enthalpy drop only in moving blades
• C. there is enthalpy drop in nozzles
• D. none of the above

Explanation: For an impulse turbine, the pressure and enthalpy drop occur in the fixed nozzles, which convert enthalpy into steam velocity.

• A. increase in dryness fraction of exit steam
• B. decrease in dryness fraction of exit steam
• C. no change in the quality of exit steam
• D. decrease or increase of dryness fraction of exit steam depending upon inlet quality

Explanation: Nozzle friction increases the actual exit enthalpy at the same pressure, and for wet steam this corresponds to a higher dryness fraction.

• A. increase in exit velocity from the nozzle
• B. decrease in exit velocity from the nozzle
• C. no change in exit velocity from the nozzle
• D. increase or decrease depending upon the exit quality of steam

Explanation: Friction dissipates part of the available enthalpy drop inside the nozzle, leaving less energy for conversion into kinetic energy.

• A. 0.555
• B. 0.578
• C. 0.5457
• D. 0.6

Explanation: For superheated steam, using an isentropic exponent of about 1.3 gives a critical pressure ratio near 0.546.

• A. 0.6
• B. 0.578
• C. 0.555
• D. 0.5457

Explanation: For initially dry-saturated steam, the critical pressure ratio is approximately 0.577 to 0.578. Therefore, option b is the closest value.

• A. pvls=C
• B. pv1126 = C
• C. pv1A = C
• D. pv = C

Explanation: Superheated steam behaves approximately like a gas during nozzle expansion, so its isentropic relation is commonly written as p v^1.3 = C.

• A. series wound generator
• B. shunt wound generator
• C. synchronous generator
• D. none of the above

Explanation: A booster is normally a series-wound generator connected in series with a feeder to compensate for voltage drop.

• A. reactors
• B. generators
• C. transformers
• D. all of the above

Explanation: Electromechanical voltage regulators, such as regulators used with alternators, adjust generator excitation to maintain terminal voltage.

• A. using reactors
• B. increasing the load
• C. adjusting the governor
• D. reducing the terminal voltage
• E. none of the above

Explanation: Generator frequency follows f = PN/120, so it increases when rotor speed is increased.