Free Electrical Machines MCQs with Answers

• A. d.c. series motor
• B. shaded pole motor
• C. capacitor-start capacitor-run motor
• D. reluctance motor

Explanation: Compressors require high starting torque and then efficient, quiet continuous operation, which is provided by a capacitor-start…

• A. improves the efficiency
• B. helps the rotor teeth to remain under the stator teeth
• C. helps in reducing the tendency of the rotor teeth to remain under the stator teeth
• D. improves the power factor

Explanation: Rotor-slot skewing prevents rotor and stator teeth from remaining directly aligned, thereby reducing cogging, magnetic locking, noise, and…

• A. secondary winding surrounds the primary winding
• B. primary winding surrounds the secondary winding
• C. both are usual arrangements
• D. none of the above

Explanation: In the usual ceiling-fan capacitor-run motor arrangement, the auxiliary or secondary winding is placed around the main or primary winding.

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

Explanation: A universal motor is a series motor that can operate on either AC or DC, so its speed-load characteristic resembles that of a DC series…

• A. high starting torque
• B. low starting torque
• C. medium starting torque
• D. very high starting torque

Explanation: A shaded-pole motor develops only a weak rotating magnetic field because the shaded coil produces a small delayed flux.

• A. damage to the starting winding
• B. damage to the centrifugal switch
• C. overloading of running winding
• D. none of the above

Explanation: If the centrifugal switch remains closed, the starting winding stays energized after the motor has accelerated.

• A. shunt motor
• B. series motor
• C. compound motor
• D. separately excited motor

Explanation: A repulsion-induction motor combines high starting torque with better running speed regulation than a simple repulsion motor, giving a…

• A. thin wire placed at the top of the slots
• B. thin wire placed at the bottom of the slots
• C. thick wire placed at the bottom of the slots
• D. thick wire placed at the top of the" slots

Explanation: The main winding carries the running current, so it uses comparatively thick wire and is placed at the bottom of the slots.

• A. brush axis is at 45° electrical to the field axis
• B. brush axis coincides with the field axis
• C. brush axis is at 90° electrical to the field axis
• D. none of the above

Explanation: Repulsion-motor torque is proportional to the interaction of the relevant fluxes and is maximum when the brush axis is approximately 45…

• A. aluminium
• B. cast iron
• C. chrome steel
• D. copper

Explanation: The hysteresis rotor is made from hard magnetic material such as chrome steel, which has a wide hysteresis loop and retains magnetism.

• A. pulsating
• B. uniform
• C. none of the above
• D. nil

Explanation: At standstill, the forward and backward rotating field components produce equal and opposite torques, so their net starting torque is…

• A. friction
• B. varying the resistance
• C. tapping the field
• D. centrifugal mechanism

Explanation: A sewing-machine universal motor commonly controls speed by inserting variable resistance in series with the motor, reducing the applied…

• A. capacitor run motor
• B. hysteresis motor
• C. universal motor
• D. repulsion motor

Explanation: Portable drills need a compact motor with high starting torque and high speed, which are characteristic advantages of the universal motor.

• A. retentivity
• B. resistivity
• C. susceptibility
• D. none of the above

Explanation: The rotor of a hysteresis motor requires high retentivity so it retains magnetism and develops hysteresis torque as the rotating field…

• A. equal to full load current
• B. less than full load current
• C. slightly more than full load current
• D. several times the full load current

Explanation: A shaded-pole motor has relatively high leakage impedance, which limits its locked-rotor current; it is therefore only slightly greater…

• A. It requires more maintenance of commutator and other mechanical devices
• B. It makes quite a bit of noise on starting
• C. In fractional horse power motors, it has replaced the capacitor motors
• D. It is not easily reversed

Explanation: Repulsion-start induction motors have high starting torque but require commutator maintenance, produce noise, and are difficult to…

• A. running winding
• B. rotor winding
• C. field winding
• D. compensating winding

Explanation: The starting winding is used only to create the initial rotating-field effect and is disconnected after the motor accelerates.

• A. smaller brush width
• B. less number of field turns
• C. more number of armature turns
• D. less air gap
• E. all of the above

Explanation: An AC series motor uses fewer field turns, more armature turns, a smaller air gap, and narrower brushes than a comparable DC series motor…

• A. it revolves synchronously
• B. it is not subject to any magnetic or mechanical vibrations
• C. it can be easily manufactured in extremely small sizes of up to 1 W output
• D. it develops hysteresis torque which is extremely steady both in amplitude and phase

Explanation: A hysteresis motor produces a nearly constant hysteresis torque, steady in both magnitude and phase, while running synchronously.

• A. It requires only one winding
• B. It can rotate in one direction only
• C. It is self-starting
• D. It is not self-starting

Explanation: A single-phase induction motor has a pulsating rather than rotating starting field, so its starting torque is zero.