Free Electrical Machines MCQs with Answers

• A. varying field flux with tapped field windings
• B. connecting rheostat in series
• C. applying variable voltage by means of silicon controlled rectifier
• D. applying variable voltage by means of variable auto-transformer
• E. all of the above methods

Explanation: Universal-motor speed can be controlled by changing field flux, adding series resistance, or varying the applied voltage using an SCR or…

• A. It is extremely sensitive to fluctuations in supply voltage
• B. Its high starting torque is due to its high rotor hysteresis loss
• C. It is extremely quiet in operation
• D. It accelerates from rest to full-speed almost instantaneously

Explanation: A hysteresis motor is relatively insensitive to supply-voltage fluctuations, while its hysteresis torque gives smooth and rapid…

• A. armature
• B. field
• C. rotor
• D. stator

Explanation: In a single-phase induction motor, the starting winding is an auxiliary stator winding placed 90 electrical degrees from the main winding.

• A. gearing
• B. belts
• C. brakes
• D. chains

Explanation: A universal motor runs at very high speed, so a gear train is commonly used to reduce speed and increase the available torque.

• A. Split phase motor
• B. Universal motor
• C. Hysteresis motor
• D. Shaded pole motor

Explanation: The hysteresis motor has a smooth, non-toothed rotor and produces nearly uniform torque, so it operates with exceptionally low mechanical…

• A. fans
• B. blowers
• C. sound equipment
• D. mixer grinders

Explanation: The quiet, vibration-free operation of a hysteresis motor makes it suitable for sound equipment, record players and similar precision…

• A. armature winding
• B. field winding
• C. either armature winding or field winding
• D. none of the above

Explanation: Reversing either the armature-current direction or the field-current direction reverses the torque, because torque depends on the…

• A. high inductance of field and armature circuits
• B. induced current in rotor due to variations of flux
• C. fine copper wire winding
• D. none of the above

Explanation: When a DC series motor operates on AC, its field and armature windings have substantial inductive reactance, causing a large lagging…

• A. Split phase motor
• B. Reluctance motor
• C. Hysteresis motor
• D. Universal motor

Explanation: A hysteresis motor uses a smooth cylindrical rotor made of hard magnetic material, with no rotor teeth, slots or electrical winding.

• A. gear trains
• B. V-belts
• C. brakes
• D. chains

Explanation: A gear train reduces the output speed while allowing the universal motor to continue operating at its efficient high speed.

• A. Universal motor
• B. Repulsion motor
• C. Capacitor motor
• D. All single phase motors have zero starting torque

Explanation: A capacitor-run motor, often loosely called a capacitor motor, has relatively poor starting torque because its small running capacitor…

• A. Capacitor run motor
• B. Shaded pole motor
• C. Hysteresis motor
• D. Schrage motor

Explanation: The Schrage motor permits control of its power factor through injected voltage, so it can be operated at approximately unity power factor.

• A. Gradually varying load
• B. Non-reversing, no-load start
• C. Reversing, light start
• D. Reversing, heavy start

Explanation: Cranes and hoists frequently start with heavy loads and must reverse direction for lifting and lowering operations.

• A. 200 to 250 W
• B. 250 to 500 W
• C. 50 to 150 W
• D. 10 to 20 W

Explanation: Domestic ceiling fans use small motors, commonly rated roughly from 50 to 150 W depending on fan size and design.

• A. End play
• B. Air gap
• C. Insulation in rotor
• D. Balancing of rotor

Explanation: A universal motor runs at very high speed, so even slight rotor unbalance produces substantial centrifugal vibration and stress.

• A. good power factor
• B. high efficiency
• C. minimum cost
• D. high starting torque

Explanation: The repulsion-start induction-run motor combines repulsion-motor starting with induction-motor running, giving it a high starting torque.

• A. from main pole to shaded pole
• B. from shaded pole to main pole
• C. either of the above depending on voltage
• D. either of the above depending on power factor

Explanation: The shaded portion reaches its induced magnetic effect later than the unshaded portion, producing a sweeping field from the main, unshaded…

• A. Resistance start
• B. Capacitor start capacitor run
• C. Shaded pole
• D. Universal

Explanation: Shaded-pole motors are simple, inexpensive and suitable for the very small power needed in phonographs and similar timing or household…

• A. Universal motor
• B. Repulsion motor
• C. Synchronous motor
• D. Reluctance motor

Explanation: A universal motor is designed so that reversing the supply polarity reverses both armature and field currents, keeping the torque…

• A. air capacitor
• B. paper spaced oil filled type
• C. ceramic type
• D. a.c. electrolytic type

Explanation: The running capacitor remains connected continuously, so it is commonly a reliable paper-spaced, oil-filled capacitor.