Free Electrical Machines MCQs with Answers

• A. synchronous speed
• B. below synchronous speed
• C. above synchronous speed
• D. any of the above

Explanation: An induction motor must have positive slip to induce rotor current and produce torque, so its rotor speed remains slightly below…

• A. high starting torque is required
• B. load torque is heavy
• C. heavy pull out torque is required
• D. all of the above

Explanation: A slip-ring motor permits external rotor resistance to be added during starting, allowing high starting torque with controlled current.

• A. slip ring motor only but not with squirrel cage induction motor
• B. squirrel cage induction motor only but not with slip ring motor
• C. squirrel cage as well as slip ring induction motor
• D. none of the above

Explanation: Reduced-voltage starting methods can be applied to both squirrel-cage and slip-ring induction motors, although slip-ring motors also allow…

• A. initial cost is the main consideration
• B. maintenance cost is to be kept low
• C. higher starting torque is the main consideration
• D. all above considerations are involved

Explanation: A squirrel-cage motor has low starting torque because its rotor resistance cannot be externally increased.

• A. slip ring induction motor
• B. squirrel cage induction motor
• C. both A. and (b)
• D. none of the above

Explanation: The squirrel-cage rotor has no slip rings, brushes, or external starting equipment, so it needs less maintenance than a slip-ring motor.

• A. dependent on the shaft loading
• B. dependent on the number of slots
• C. slip times the stand still e.m.f. induced in the rotor
• D. none of the above

Explanation: Rotor frequency and induced rotor emf are proportional to slip; therefore, rotor emf at running condition is s times the standstill rotor…

• A. to short circuit the rotor at slip rings
• B. to short circuit the starting resistances in the starter
• C. to short circuit the stator phase of motor to form star
• D. none of the above

Explanation: After the motor accelerates, the starting resistance of a slip-ring motor is removed by short-circuiting the rotor slip rings.

• A. slip ring type
• B. squirrel cage type
• C. any of the above
• D. none of the above

Explanation: A slip-ring induction motor is preferred for high-inertia loads because external rotor resistance provides high starting torque with…

• A. decreases the rotor current
• B. increases the rotor current
• C. rotor current becomes zero
• D. rotor current rernains same

Explanation: For a specified torque, the required rotor current is essentially fixed; adding rotor resistance mainly increases slip and lowers the…

• A. it is run on load
• B. it is run in reverse direction
• C. it is run on voltage higher than the rated voltage
• D. e.m.f. is injected in the rotor circuit

Explanation: At synchronous speed, slip and the naturally induced rotor emf are zero, so an ordinary induction motor cannot produce torque there.

• A. rotor leakage reactance
• B. stator reactance
• C. the reactive lagging magnetizing current necessary to generate the magnetic flux
• D. all of the above

Explanation: The dominant cause is the lagging magnetizing current needed to establish air-gap flux; stator and rotor leakage reactances also add…

• A. supply frequency
• B. supply voltage
• C. copper losses in motor
• D. none of the above

Explanation: Slip equals rotor copper loss divided by rotor input power, so it changes with the rotor copper loss as the load changes.

• A. sR2X2 = 1
• B. sR2 = X2
• C. R2 = sX2
• D. R2 = s2X2

Explanation: For maximum torque during running, the slip-dependent rotor reactance equals the rotor resistance: sX2 = R2.

• A. high inductance arid low resistance
• B. low inductance and high resistance
• C. low inductance and low resistance
• D. high inductance and high resistance

Explanation: The inner cage is designed for running performance, so it uses low resistance and deep bars that give high leakage inductance.

• A. the power factor will be low
• B. windage losses will be more
• C. bearing friction will reduce
• D. copper loss will reduce In an induction motor

Explanation: A larger air gap increases the magnetizing current required to establish flux, making the induction motor draw more reactive current and…

• A. rotor current
• B. rotor power factor
• C. rotor e.m.f.
• D. shaft diameter

Explanation: Induction-motor torque is proportional to rotor current, rotor emf, and rotor power factor, but shaft diameter is only a mechanical design…

• A. Air motor
• B. Induction motor
• C. D.C. shunt motor
• D. Synchronous motor

Explanation: An air motor is preferred in explosive atmospheres because it has no electrical sparking contacts or windings that could ignite gas.

• A. graphite
• B. grease
• C. mineral oil
• D. molasses

Explanation: Grease is normally packed into ball bearings because it remains in place and provides both lubrication and sealing against dust.

• A. leakage reactance
• B. power factor on short circuit
• C. short-circuit current under rated voltage
• D. all of the above

Explanation: The blocked-rotor test gives the equivalent leakage impedance and hence leakage reactance, directly gives the short-circuit power factor…

• A. noload test
• B. blocked rotor test
• C. stator resistance test
• D. all of the above

Explanation: The no-load test supplies the magnetizing and core-loss data, the blocked-rotor test supplies series impedance data, and the…