Electrical Motors MCQ Quiz - Objective Question with Answer for Electrical Motors - Download Free PDF

Last updated on Mar 9, 2025

Latest Electrical Motors MCQ Objective Questions

Electrical Motors Question 1:

The frequency of the rotor current in an induction motor is equal to:

  1. slip × input frequency
  2. slip × stator current
  3. half of the input frequency
  4. the stator current

Answer (Detailed Solution Below)

Option 1 : slip × input frequency

Electrical Motors Question 1 Detailed Solution

Concept

The frequency of the rotor current in an induction motor is given by:

\(f_r=sf_s\)

where, fr = Rotor frequency

fs = Supply frequency

s =Slip

The slip of the induction motor is given by:

\(s={N_s-N_r\over N_s}\)

where, Ns = Synchronous speed

Nr = Rotor speed

Electrical Motors Question 2:

The motor used in household refrigerators is

  1. single phase induction motor
  2. Hysteresis motor
  3. dc series motor
  4. Reluctance motor

Answer (Detailed Solution Below)

Option 1 : single phase induction motor

Electrical Motors Question 2 Detailed Solution

Single phase induction motor is used in household refrigerators.

Applications of different 1ϕ induction motor

Single phase motor

Application

Split phase motor

Shop grinders, Washing machine, Belt driven conveyors

Capacitor run motor

Compressors, Conveyors, Refrigerators, Air Conditioners, Ceiling fans

Shaded pole motor

Hair dryers, Toys, Record players, Small fans, Electric clocks

AC series motor

Sewing machine, Table fans, Food mixer

Additional Information

Motor

Application

DC series motor

Traction system, Cranes, Air compressor

DC shunt motor

Lathe machines, Pumps, Blowers, Conveyors belts

DC compound motor

Presses, Shears, Elevators, Rolling mills

Stepper motor

3D printing equipment, CNC milling machines

Electrical Motors Question 3:

Which of the following devices does NOT use an electric motor?

Electric fan, Electric mixer, Galvanometer, Computer

  1. Galvanometer
  2. Electric mixer
  3. Computer
  4. Electric fan

Answer (Detailed Solution Below)

Option 1 : Galvanometer

Electrical Motors Question 3 Detailed Solution

The correct answer is Galvanometer.Key Points

  • A galvanometer is a device used to detect and measure small electric currents, but it does not use an electric motor to function.
  • An electric fan uses an electric motor to rotate its blades and create air movement.
  • An electric mixer uses an electric motor to turn its beaters or blades and mix ingredients.
  • A computer does not use an electric motor as its primary means of operation, but it may contain fans or other components that do.
  • An electric motor is a device that converts electrical energy into mechanical energy and is used in many different types of machines and devices.

Additional Information

  • Galvanometers are often used in scientific experiments to measure electrical currents in circuits and can also be found in some types of measuring instruments.
  • Electric fans come in a variety of sizes and styles, from small desk fans to large industrial fans used in factories and warehouses.
  • Electric mixers are commonly used in kitchens to mix and prepare ingredients for cooking and baking.
  • Computers rely on a variety of components to function, including processors, memory, storage devices, and input/output devices such as keyboards and monitors.
  • While these components may use electricity to operate, they do not typically rely on electric motors.

Electrical Motors Question 4:

In an electric motor, the _________ act(s) as a contact between the commutator and the terminals of the battery.

  1. plug key
  2. carbon brushes
  3. split ring
  4. magnetic poles

Answer (Detailed Solution Below)

Option 2 : carbon brushes

Electrical Motors Question 4 Detailed Solution

The correct answer is carbon brushes.

Key Points

  • Carbon Brushes:
    • Carbon brushes provide the electrical connection between the stationary part of the motor and the rotating commutator.
    • They are made of graphite or a mixture of graphite and copper.
    • Carbon is chosen for its conductivity, self-lubricating properties, and softness.
    • The softness prevents excessive wear on the commutator.
    • Brushes are spring-loaded to maintain consistent contact.
    • They wear down over time and are replaceable.
    • Proper brush maintenance is crucial for motor performance.
    • Worn brushes can cause poor contact, sparking, and reduced performance.
    • Brush design and material are chosen based on motor requirements.
    • Different grades of carbon optimize performance and longevity.
    • They are essential for transferring electrical power to the rotating armature.

Additional Information

  • Plug Key:
    • A plug key (or switch) opens or closes an electrical circuit.
    • It acts as a simple on/off switch.
    • It controls the flow of electricity to the entire motor circuit.
    • It is not directly involved in the dynamic connection between the power source and the rotating armature.
    • The plug key's state remains constant once the motor is running.
    • It is separate from the motor and not involved in the internal workings of rotation.
    • It's like a gate controlling electricity to the motor.
  • Split Ring (Commutator):
    • A split ring (commutator) is part of a DC motor.
    • It's a ring of conductive material, split into insulated segments.
    • Segments are connected to the rotor coils (armature).
    • It reverses the current direction in the rotor coils every half rotation.
    • This ensures continuous rotation of the motor.
    • Without it, the rotor would oscillate instead of rotating.
    • It works with the carbon brushes.
    • Brushes make electrical contact with the rotating commutator.
    • This allows current to flow to the rotor coils.
    • As the rotor turns, brushes move between commutator segments, causing current reversal.
    • The commutator design (number of segments) is specific to the motor.
  • Magnetic Poles:
    • Magnetic poles (north and south) are essential for motor operation.
    • The interaction of magnetic fields creates the torque for rotation.
    • The stator's magnetic field interacts with the rotor's field.
    • This interaction creates forces on the rotor coils.
    • These forces create torque, causing the rotor to spin.
    • Pole strength and arrangement influence motor speed and torque.
    • The magnetic field interacts with the current in rotor coils.
    • This results in forces on the rotor coils.
    • These forces create torque, which causes the rotor to rotate.
    • The design and strength of the magnetic poles are crucial for efficient operation.
    • They determine the magnetic field strength and the torque produced.

Electrical Motors Question 5:

The electrical device which receives energy at the electrical port and delivers it at the mechanical port is ____________

  1. Electrical Motors
  2. Thermocouples
  3. Crystal oscillators
  4. Thermistors

Answer (Detailed Solution Below)

Option 1 : Electrical Motors

Electrical Motors Question 5 Detailed Solution

1.) Electrical Motors

  • The electrical device that receives energy at the electrical port and delivers it at the mechanical port is the electrical motor.
  • The main principle behind all electric motors is Faraday’s Law of induction.
  • Most electric motors develop their mechanical torque by the interaction of conductors carrying current in a direction at right angles to a magnetic field. 
  • The types of electrical motors include DC Motors, AC Motors like Synchronous and Induction Motors, and specialized motors such as Stepper and Servo Motors.

 

2.) Thermocouples

  • A thermocouple is a device that converts temperature differences into an electric voltage, based on the principle of the thermoelectric effect.
  • The thermocouple working principle is based on the Seeback Effect.
  • This effect states that when a closed circuit is formed by jointing two dissimilar metals at two junctions, and junctions are maintained at different temperatures then an electromotive force (e.m.f.) is induced in this closed circuit.

 

3.) Thermistors

  • A thermistor is a resistor whose electrical resistance varies significantly with temperature changes.
  • Thermistors are used in digital thermometers, automotive applications, household appliances, and circuit protection.

 

4.) Crystal oscillators

  • A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. 
  • These oscillators are usually made of Quartz crystal. Although Rochelle salt and Tourmaline also exhibit the piezoelectric effect, Quartz is preferred because it is inexpensive, readily available, and mechanically strong.
  • Crystal oscillators operate on the principle of the inverse piezoelectric effect. When an alternating voltage is applied to the crystal, it vibrates at its natural frequency. These vibrations are then converted into oscillations.

Top Electrical Motors MCQ Objective Questions

The motor used in household refrigerators is

  1. single phase induction motor
  2. Hysteresis motor
  3. dc series motor
  4. Reluctance motor

Answer (Detailed Solution Below)

Option 1 : single phase induction motor

Electrical Motors Question 6 Detailed Solution

Download Solution PDF

Single phase induction motor is used in household refrigerators.

Applications of different 1ϕ induction motor

Single phase motor

Application

Split phase motor

Shop grinders, Washing machine, Belt driven conveyors

Capacitor run motor

Compressors, Conveyors, Refrigerators, Air Conditioners, Ceiling fans

Shaded pole motor

Hair dryers, Toys, Record players, Small fans, Electric clocks

AC series motor

Sewing machine, Table fans, Food mixer

Additional Information

Motor

Application

DC series motor

Traction system, Cranes, Air compressor

DC shunt motor

Lathe machines, Pumps, Blowers, Conveyors belts

DC compound motor

Presses, Shears, Elevators, Rolling mills

Stepper motor

3D printing equipment, CNC milling machines

A DC servomotor is similar to a regular DC motor except that its design is modified to cope with _________

  1. Electronic switching
  2. Slow speeds
  3. Static conditions 
  4. Both (b) & (c)

Answer (Detailed Solution Below)

Option 4 : Both (b) & (c)

Electrical Motors Question 7 Detailed Solution

Download Solution PDF

A servomotor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration

A DC servomotor is similar to a regular DC motor except that its design is modified to cope with slow speeds and static conditions

The frequency of the rotor current in an induction motor is equal to:

  1. slip × input frequency
  2. slip × stator current
  3. half of the input frequency
  4. the stator current

Answer (Detailed Solution Below)

Option 1 : slip × input frequency

Electrical Motors Question 8 Detailed Solution

Download Solution PDF

Concept

The frequency of the rotor current in an induction motor is given by:

\(f_r=sf_s\)

where, fr = Rotor frequency

fs = Supply frequency

s =Slip

The slip of the induction motor is given by:

\(s={N_s-N_r\over N_s}\)

where, Ns = Synchronous speed

Nr = Rotor speed

Electrical Motors Question 9:

In d.c. generator, which loss varies with the load?

  1. Copper loss
  2. Eddy current loss
  3. Hysteresis loss
  4. Windage loss

Answer (Detailed Solution Below)

Option 1 : Copper loss

Electrical Motors Question 9 Detailed Solution

As the armature core is made of iron and it rotates in a magnetic field, a small current gets induced in the core. Due to this current, eddy current loss and hysteresis loss occur in the armature iron core. Iron losses are also called as core losses or magnetic losses. These are constant losses and independent of load.

The copper losses are the winding losses taking place during the current flowing through the winding. Copper losses are directly proportional to square of the current. These are variable losses dependent on load current.

Windage losses are the losses sustained by a machine due to the resistance offered by air to the rotation of the shaft. These are mechanical losses and independent of load.

Electrical Motors Question 10:

The rotating part of a DC motor is known as:

  1. Pole
  2. Stator
  3. Rotor
  4. Pole shoe
  5. None of these

Answer (Detailed Solution Below)

Option 3 : Rotor

Electrical Motors Question 10 Detailed Solution

Electrician 15 20Q DC Machines HIndi - Final images Q13

Pole: Poles are the structures on the stator on which the field windings are wound.

Stator: it is a stationary part of a motor that creates a magnetic field.

Rotor: The rotor is the rotating part of DC machine, it is used to transmit the motor forces to the output shaft.

Electrical Motors Question 11:

Speed of synchronous motor on increasing load

  1. Becomes infinity
  2. Remains constant
  3. Increases
  4. Decreases

Answer (Detailed Solution Below)

Option 2 : Remains constant

Electrical Motors Question 11 Detailed Solution

  • In a synchronous motor, the rotating rotor poles are being interlocked with rotating stator pole
  • So the speed is constant irrespective of the load
  • Hence if the load on the synchronous motor is increased, then the speed of the motor will remain the same

Electrical Motors Question 12:

In a stepper motor, the speed of rotation determined by________

  1. By the current supplied to the motor
  2. By the magnitude of the voltage applied
  3. By the load applied to the motor
  4. By the frequency of the waveforms used

Answer (Detailed Solution Below)

Option 4 : By the frequency of the waveforms used

Electrical Motors Question 12 Detailed Solution

In stepper motor rotation speed is determined by pulse frequency (Hz), and motor rotation can be easily adjusted by changing the pulse number and frequency input to the driver.

Stepper motor special type of synchronous motor which is designed to rotate a specific number of degree for every electric pulse received by its control unit.

In stepper motor there is no brushes. This motor does not rotate continuously, instead it rotates in form of pluses or in discrete steps. That’s why it is called stepper motor.

Electrical Motors Question 13:

The motor used in household refrigerators is

  1. single phase induction motor
  2. Hysteresis motor
  3. dc series motor
  4. Reluctance motor

Answer (Detailed Solution Below)

Option 1 : single phase induction motor

Electrical Motors Question 13 Detailed Solution

Single phase induction motor is used in household refrigerators.

Applications of different 1ϕ induction motor

Single phase motor

Application

Split phase motor

Shop grinders, Washing machine, Belt driven conveyors

Capacitor run motor

Compressors, Conveyors, Refrigerators, Air Conditioners, Ceiling fans

Shaded pole motor

Hair dryers, Toys, Record players, Small fans, Electric clocks

AC series motor

Sewing machine, Table fans, Food mixer

Additional Information

Motor

Application

DC series motor

Traction system, Cranes, Air compressor

DC shunt motor

Lathe machines, Pumps, Blowers, Conveyors belts

DC compound motor

Presses, Shears, Elevators, Rolling mills

Stepper motor

3D printing equipment, CNC milling machines

Electrical Motors Question 14:

Which of the following devices does NOT use an electric motor?

Electric fan, Electric mixer, Galvanometer, Computer

  1. Galvanometer
  2. Electric mixer
  3. Computer
  4. Electric fan

Answer (Detailed Solution Below)

Option 1 : Galvanometer

Electrical Motors Question 14 Detailed Solution

The correct answer is Galvanometer.Key Points

  • A galvanometer is a device used to detect and measure small electric currents, but it does not use an electric motor to function.
  • An electric fan uses an electric motor to rotate its blades and create air movement.
  • An electric mixer uses an electric motor to turn its beaters or blades and mix ingredients.
  • A computer does not use an electric motor as its primary means of operation, but it may contain fans or other components that do.
  • An electric motor is a device that converts electrical energy into mechanical energy and is used in many different types of machines and devices.

Additional Information

  • Galvanometers are often used in scientific experiments to measure electrical currents in circuits and can also be found in some types of measuring instruments.
  • Electric fans come in a variety of sizes and styles, from small desk fans to large industrial fans used in factories and warehouses.
  • Electric mixers are commonly used in kitchens to mix and prepare ingredients for cooking and baking.
  • Computers rely on a variety of components to function, including processors, memory, storage devices, and input/output devices such as keyboards and monitors.
  • While these components may use electricity to operate, they do not typically rely on electric motors.

Electrical Motors Question 15:

Electrical braking preferred in industries is

  1. Dynamic braking
  2. Plugging
  3. Regenerative braking
  4. None of these

Answer (Detailed Solution Below)

Option 3 : Regenerative braking

Electrical Motors Question 15 Detailed Solution

A running motor may be brought to rest quickly either by mechanical braking or electric braking.

The electric braking of a DC motor is of three types,

(i) Rheostatic or dynamic braking

(ii) Plugging or reverse current braking

(iii) Regenerative braking
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