Free Electrical Machines MCQs with Answers

• A. Hysteresis losses
• B. Eddy current losses
• C. Copper losses
• D. Windage losses

Explanation: Laminating the magnetic core increases the resistance of paths available for circulating currents, sharply reducing eddy-current loss.

• A. Mechanical loss
• B. Core loss
• C. Copper loss
• D. All of the above

Explanation: Mechanical losses, core losses and copper losses all ultimately appear as heat: friction and windage heat moving parts, core losses heat…

• A. Providing laminations in armature core
• B. Providing laminations in stator
• C. Using non-magnetic material for frame
• D. Using material of low hysteresis co-efficient for armature core material

Explanation: Hysteresis loss depends on the magnetic material and is reduced by using armature-core material with a low hysteresis coefficient.

• A. over load relay is connected in series and no volt relay in parallel with the load
• B. over load relay is connected in parallel and no volt relay in series with the load
• C. over load relay and no volt relay are both connected in series with the load
• D. over load relay and no volt relay are both connected in parallel with the load

Explanation: The overload relay is placed in series with the motor circuit so excessive current trips it, while the no-volt release coil is connected…

• A. trains
• B. cranes
• C. hoists
• D. machine tools

Explanation: DC shunt motors provide nearly constant speed with good speed regulation, making them suitable for machine tools.

• A. magnetic field
• B. active length of the conductor
• C. current flow through the conductors
• D. number of conductors
• E. radius of armature
• F. all above factors

Explanation: Motor torque follows the force on the armature conductors, with T proportional to flux, current, conductor length, number of conductors…

• A. Field copper loss
• B. Windage loss
• C. Armature copper loss
• D. None of the above

Explanation: Armature copper loss is Iₐ²Rₐ, so it changes substantially as load current changes.

• A. hysteresis loss
• B. field copper loss
• C. armature copper loss
• D. eddy current loss

Explanation: At rated load, armature current is high and the armature copper loss, Iₐ²Rₐ, rises accordingly.

• A. Loss of efficiency
• B. Excessive heating of core
• C. Increase in terminal voltage
• D. Rise in temperature of ventilating air

Explanation: Iron losses convert electrical energy into heat, reducing efficiency and increasing the core and ventilating-air temperature.

• A. Frequency of magnetic reversals
• B. Maximum value of flux density
• C. Volume and grade of iron
• D. Rate of flow of ventilating air

Explanation: Hysteresis loss depends mainly on frequency of reversal, maximum flux density, and the volume and magnetic quality of the iron.

• A. high initial cost
• B. high maintenance cost
• C. low efficiency at Hght loads
• D. all of the above

Explanation: Ward-Leonard control requires a motor-generator set, giving high initial and maintenance costs and poor efficiency at light loads because…

• A. residual magnetism of the generator
• B. core losses of motor
• C. mechanical losses of motor and generator together
• D. all of the above

Explanation: At very low speed, the motor needs only a small applied voltage, but the Ward-Leonard generator cannot reduce its output below the voltage…

• A. Field control
• B. Armature voltage control
• C. Shunt armature control
• D. Mechanical loading system

Explanation: Armature-voltage control changes speed while keeping the field flux nearly constant, so motor torque can remain constant over the control…

• A. voltage control method
• B. field divertor method
• C. field control method
• D. armature resistance control method

Explanation: In Ward-Leonard control, a motor-generator set varies the voltage supplied to the controlled motor armature.

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

Explanation: A series motor must not run without load because its speed can rise dangerously when the load current and hence field flux become very…

• A. D.C. shunt motor
• B. D.C. series motor
• C. Induction motor
• D. Reluctance motor

Explanation: Reluctance motors provide nearly constant-speed operation and are commonly used in clocks, timers, signalling devices and other small…

• A. D.C. shunt motor
• B. D.C. series motor
• C. Single phase induction motor (split phase start or induction run motor)
• D. Reluctance motor
• E. Synchronous motor

Explanation: Household refrigerator compressors commonly use single-phase induction motors, especially split-phase or capacitor-start types, because…

• A. increase
• B. decrease
• C. remain same
• D. become zero

Explanation: For a shunt motor, field flux is nearly constant and back emf is approximately proportional to speed, since Eb is proportional to flux…

• A. D.C. shunt motor
• B. D.C. series motor
• C. Universal motor
• D. Synchronous motor

Explanation: A synchronous motor is suitable for driving a rotary compressor because it runs at constant speed and can provide efficient continuous…

• A. regenerative
• B. plugging
• C. dynamic braking
• D. none of the above
• E. any of the above

Explanation: Reversing the armature connections while keeping the field connections unchanged reverses the electromagnetic torque against the existing…