Free Electrical Machines MCQs with Answers

• A. Slip rings
• B. Commutator
• C. Field winding
• D. Armature winding

Explanation: The field winding is stationary and generally has better heat dissipation than the rotating armature and commutator assembly.

• A. series motors
• B. shunt motors
• C. compound motors
• D. none of the above

Explanation: A shunt motor has nearly constant flux and comparatively low starting torque, while series and compound motors develop higher starting…

• A. 100%
• B. around 90%
• C. anywhere between 75% and 90%
• D. less than 50%

Explanation: Maximum mechanical power occurs when the back e.m.f. is approximately half the supply voltage, making the developed power about half the…

• A. shunt motors
• B. shunt as well as compound motors
• C. shunt, compound and series motors
• D. all D.C. motors

Explanation: A three-point starter has terminals for the line, armature, and shunt field, so it is suitable for shunt motors and compound motors.

• A. to improve cooling
• B. to reduce copper losses
• C. to increase the generated e.m.f.
• D. to reduce the sparking

Explanation: Fractional-pitch coils reduce the e.m.f. induced in a coil during commutation, making current reversal easier and reducing sparking at the…

• A. Starter for a car
• B. Drive for a water pump
• C. Fan motor
• D. Motor operation in A.C. or D.C

Explanation: A D.C. series motor develops very high starting torque, which is required to crank a car engine through the starter mechanism.

• A. Frame
• B. Shaft
• C. Commutator
• D. Stator

Explanation: The commutator is the distinctive mechanical rectifier of a conventional D.C.

• A. pumping sets
• B. air compressors
• C. electric traction
• D. machine shops

Explanation: D.C. motors are especially valuable in electric traction because their high starting torque and easy speed control suit frequent starting…

• A. almost negligible
• B. rated full-load current
• C. less than full-load current
• D. more than full-load current

Explanation: For maximum mechanical power in a shunt motor, the armature current is approximately half the short-circuit current, which is normally far…

• A. back e.m.f. is equal to armature drop
• B. armature drop is negligible
• C. flux is proportional to armature current
• D. flux is practically constant in D:C. shunt motors

Explanation: Motor speed is approximately proportional to back e.m.f. divided by flux, and the shunt field maintains nearly constant flux.

• A. The motor will stop
• B. The motor will continue to run
• C. The armature may burn
• D. The motor will run noisy

Explanation: If back e.m.f. disappears while the supply remains connected, the opposing voltage is lost and the armature current can rise to a…

• A. up to 5 H.P
• B. up to 10 H.P
• C. up to 15 H.P
• D. up to 20 H.P

Explanation: A direct-on-line starter applies the full supply voltage at starting, causing a high inrush current, so it is generally used for small…

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

Explanation: In a shunt motor, the field winding is connected across the supply, so its flux remains nearly constant.

• A. decreasing the field current
• B. increasing the field current
• C. decreasing the armature current
• D. increasing the armature current

Explanation: Motor speed is approximately proportional to the back-emf divided by field flux.

• A. Ward Leonard control
• B. rheostatic control
• C. any of the above method
• D. none of the above method

Explanation: Ward-Leonard control varies the armature voltage using a motor-generator set, allowing speed reduction with little energy wastage compared…

• A. run at normal speed
• B. not run
• C. run at lower speed
• D. burn due to heat produced in the field winding by .eddy currents

Explanation: An ordinary DC shunt motor on AC has a highly inductive field and armature reaction, producing poor torque and excessive losses, including…

• A. shunt motor
• B. series motor
• C. differential compoutid D.C. motor
• D. cumulative compound D.C. motor

Explanation: Shears and punches need a high starting and overload torque, which is supplied by a cumulative compound motor through the series-field…

• A. it limits the speed of the motor
• B. it limits the starting current to a safe value
• C. it starts the motor
• D. none of the above

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

• A. has its field winding consisting of thick wire and less turns
• B. has a poor torque
• C. can be started easily without load
• D. has almost constant speed

Explanation: The series field carries the full armature current and therefore uses thick wire with relatively few turns to keep its resistance low.

• A. the speed will increase abruptly
• B. the speed will increase in proportion to reduction in load
• C. the speed will remain almost/constant
• D. the speed will reduce

Explanation: A shunt motor has nearly constant field flux, and its speed automatically changes only slightly as load varies.