Single Phase Induction Motors — Part 1 MCQs with Explanations

In a capacitor start single-phase motor, when the capacitor is replaced by a resistance, the motor will continue to run in the same direction . The capacitor in a single-phase motor is used to create a phase shift for starting torque. If replaced by a resistance, the phase shift required for starting will not be achieved, resulting in lower starting torque. However, once the motor is running, it will continue to operate in the same direction as before. The motor's performance might be less efficient, but the direction of rotation will remain unchanged.

The speed-torque characteristic of a repulsion induction motor is similar to that of a D.C. series motor. This similarity is due to the fact that both motors exhibit high starting torque and a significant drop in speed with an increase in load. Such characteristics make these motors suitable for applications requiring high starting torque, like traction systems.

Reluctance motor is suitable for timing and control purposes . Reluctance motors are known for their precise control and high efficiency at synchronous speeds. They have a simple construction with a rotor that does not have any windings or permanent magnets, which makes them reliable and easy to maintain. The precise timing and control capabilities of reluctance motors make them ideal for applications such as clocks, timers, and other control devices. Their ability to maintain a constant speed under varying loads further enhances their suitability for timing and control applications.

Shaded pole motors are commonly used in applications like time phonographs due to their simple construction and reliable operation. They have low starting torque, which is suitable for devices that require consistent and low-speed operation. These motors are also inexpensive and have a long lifespan, making them ideal for such applications.

In a single phase induction motor , the starting winding is used only during starting to provide the necessary starting torque . Once the motor reaches about 70–80% of its rated speed , the starting winding must be disconnected to prevent overheating and energy loss. This disconnection is achieved automatically by a centrifugal switch , which operates due to the centrifugal force developed as the rotor speed increases. A magnetic relay and a thermal relay are mainly used for protection purposes, not for disconnecting the starting winding in this application. Hence, the starting winding of a refrigerator motor is disconnected by a centrifugal switch , making Option C the correct answer.

In A.C. series motors, compensating winding is used to reduce sparking at the brushes . When a series motor operates, the armature current and field winding current are in series. This leads to a higher chance of sparking at the brushes due to the commutation process. The compensating winding is placed in the pole faces of the motor and produces a magnetic field opposite to that produced by the armature current during commutation. This counteracts the armature reaction, reducing sparking at the brushes. Therefore, option C is the correct choice.

In an A.C. series motor (also known as a universal motor), the armature coils are connected directly to the commutator without any intermediate components like resistors, reactances, or capacitors. The term "solidly connected" means that the connection is direct and does not involve any impedance (resistance or reactance) or energy-storing elements (like capacitors). Reason: Since the same current flows through the field and the armature in a series motor, any additional impedance would affect the motor's performance, efficiency, and torque characteristics. Option A: through resistance – This would cause unnecessary power loss and is not standard in practical design. Option B: through reactances – Reactances would distort the current and reduce torque; hence not used. Option C: through capacitors – Capacitors are not used in the armature-commutator connection of an A.C. series motor. Therefore, the armature coils are connected to the commutator solidly, making Option D the correct answer.

the starting winding is connected through a centrifugal switch is the correct answer to “In a split phase motor”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

secondary winding surrounds the primary winding is the correct answer to “In a ceilingfan employing capacitor run motor”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Reversing, heavy start is the correct answer to “Which type of load is offered by cranes and hoists”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Hysteresis motor is the correct answer to “Which of the following motors is preferred for tape-recorders”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Universal motor is the correct answer to “Which of the following motors is used in mixies”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

hysteresis loss is the correct answer to “A hysteresis motor works on the principle of”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

300 uF is the correct answer to “The value of starting capacitor of a fractional horse power motor will be”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

paper spaced oil filled type is the correct answer to “In a two value capacitor motor, the capacitor used for running purposes is”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

current in the starting winding leads the voltage is the correct answer to “In capacitor start single-phase motors”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

retentivity is the correct answer to “In a hysteresis motor, the rotor must have”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Capacitor run motor is the correct answer to “Which of the following motors will operate at high power factor”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

0.6 power factor lagging is the correct answer to “Single phase induction motor usually operates on”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

low resistance and high inductance is the correct answer to “In a split phase motor, the running winding should have”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Damage to the starting winding is the correct answer to “If the centrifugal switch does not open at 70 to 80 percent of synchronous speed of motor, it would result in”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

all of the above is the correct answer to “A.C. series motor as compared to D.C. series motor has”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

all of the above is the correct answer to “If a D.C. series motor is operated on A.C. supply, it will”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

all of the above is the correct answer to “A schrage motor can run on”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Hysteresis motor is the correct answer to “Which motor is normally free from mechanical and magnetic vibrations”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

100 to 150 W is the correct answer to “The wattage of motor for driving domestic sewing machine will be around”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Repulsion start induction run motor is the correct answer to “Short-circuiter is used in”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

polarity marking is the correct answer to “The capacitors used in single-phase capacitor motors have no”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

it has low starting as well as running currents at relatively high power factor is the correct answer to “Two-value capacitor motor finds increased application as compressor motor in small home air-conditioners because”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Hysteresis motor is the correct answer to “Which of the following motors is used in tape-recorders”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Three-phase series motor is the correct answer to “Which of the following motors is used in a situation where load increases with speed”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Repulsion induction motor is the correct answer to “Which of the following motors has two separate windings on the motor”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

disconnects auxiliary winding of the motor is the correct answer to “In a single phase motor the centrifugal switch”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

inherently non-self-starting with low torque is the correct answer to “A single-phase induction motor is”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Stator is the correct answer to “The starting winding of a single-phase motor is placed in”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Balancing of rotor is the correct answer to “In case of high speed universal motor which of the following needs more attention”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Universal motor is the correct answer to “Which of the following motors can be run on AC. as well as D.C. supply”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

Pistol drills is the correct answer to “Which of the following applications always have some load whenever switched on”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

improve power factor and provide better speed regulation is the correct answer to “The purpose of stator winding in the compensated repulsion motor is to”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.

sound equipment is the correct answer to “As hysteresis motors are free from mechanical and magnetic vibrations therefore these are considered as suitable for”. This question is standard in competitive examination syllabi for Single Phase Induction Motors.