Free Electrical Machines MCQs with Answers

• A. of lower H.P. should be selected
• B. of higher H.P. should be selected
• C. can be used for 50°C ambient temperature also
• D. is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

Explanation: A higher ambient temperature reduces the permissible temperature rise and therefore the motor's safe output rating.

• A. constant voltage drive
• B. constant current drive
• C. constant torque drive
• D. none of the above

Explanation: Armature-voltage control changes speed while keeping the field flux approximately constant, so torque is proportional to armature current…

• A. zero torque braking
• B. smallest torque braking
• C. highest torque braking
• D. none of the above

Explanation: Plugging applies reverse torque while the motor is still rotating in its original direction, producing a large braking current and…

• A. torque will remain constant
• B. torque will change but power will remain constant
• C. torque and power both will change
• D. torque, power and speed, all will change

Explanation: Changing shunt-field excitation changes flux and speed; under the usual field-control assumption, the motor operates approximately at…

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

Explanation: A series motor has very poor speed control because its flux changes substantially with load, causing large speed variations.

• A. independent of Ia
• B. proportional to la
• C. varies as (Ia)
• D. varies as la

Explanation: In a shunt motor, the field flux is nearly constant, so speed is approximately independent of armature current.

• A. increasing the resistance in armature circuit
• B. increasing the resistance in field circuit
• C. reducing the resistance in the field circuit
• D. reducing the resistance in the armature circuit

Explanation: Increasing field-circuit resistance weakens the field flux, and since motor speed is approximately inversely proportional to flux, the…

• A. a resistance is connected in series with armature
• B. a resistance is connected parallel to the armature
• C. armature is temporarily open circuited
• D. a high value resistor is connected across the field winding

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

• A. 2.36%
• B. 4.76%
• C. 6.77%
• D. 8.84%

Explanation: Using rated-load speed as the reference, speed regulation is (1100 − 1050)/1050 × 100 = 4.76%.

• A. Voltage control method
• B. Field control method
• C. Armature control method
• D. All above methods

Explanation: Armature-control speed regulation uses a series resistance, which wastes substantial power as heat and therefore has low efficiency.

• A. field flux only
• B. armature current only
• C. both A. and B.
• D. none of the above

Explanation: Motor torque follows T ∝ ΦIa; in a shunt motor the field flux Φ is nearly constant, so torque is proportional mainly to armature current…

• A. reduce slightly
• B. increase slightly
• C. increase proportionately
• D. remains unchanged

Explanation: Increasing load increases armature current and the voltage drop in the armature circuit, slightly reducing back e.m.f. and speed.

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

Explanation: A series motor has poor speed regulation because its flux varies with load, producing a large change in speed between light-load and…

• A. D.C. series motor
• B. D.C. shunt motor
• C. induction motor
• D. all of above motors

Explanation: A D.C. series motor develops very high starting torque because the series field flux increases with armature current.

• A. the motor will first stop and then run in opposite direction as series motor
• B. the motor will work as series motor and run at slow speed in the same direction
• C. the motor will work as series motor and run at high speed in the same direction
• D. the motor will not work and come to stop

Explanation: Opening the shunt field removes the main flux, leaving the series field whose flux is opposite to the original shunt-field flux in a…

• A. Single phase capacitor start
• B. Induction motor
• C. Synchronous motor
• D. D.C. motor
• E. None of the above

Explanation: A D.C. motor provides high starting torque and its speed can be varied over a wide range by controlling the armature voltage or field…

• A. level compounded
• B. under compounded
• C. cumulatively compounded
• D. differentially compounded

Explanation: A cumulatively compounded motor combines the high starting torque of a series motor with better speed regulation, making it suitable for…

• A. in one direction
• B. in both directions
• C. below normal speed only
• D. above normal speed only

Explanation: Ward-Leonard control varies the generator output voltage continuously, giving smooth speed control from zero through the rated speed and…

• A. it increases the input power con-sumption
• B. commutator gets damaged
• C. both A. and B.
• D. none of the above

Explanation: Sparking at the brushes produces intense local heating and arcing, which burns and pits the commutator surface.

• A. series motor only
• B. shunt motor only
• C. compound motor only
• D. both shunt and compound motor

Explanation: A three-point starter has terminals for the line, shunt field and armature, so it is used with shunt and cumulatively compounded motors.