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

Concept

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

\(f_r=sf_s\)

where, f_r = Rotor frequency

f_s = Supply frequency

s =Slip

The slip of the induction motor is given by:

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

where, N_s = Synchronous speed

N_r = Rotor speed

Single phase induction motor is used in household refrigerators.

Applications of different 1ϕ induction motor

Additional Information

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.

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.

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.

Single phase induction motor is used in household refrigerators.

Applications of different 1ϕ induction motor

Additional Information

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

Concept

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

\(f_r=sf_s\)

where, f_r = Rotor frequency

f_s = Supply frequency

s =Slip

The slip of the induction motor is given by:

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

where, N_s = Synchronous speed

N_r = Rotor speed

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.

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.

• 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

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.

Single phase induction motor is used in household refrigerators.

Applications of different 1ϕ induction motor

Additional Information

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.

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