The rotor is the rotating part of an induction motor, which moves due to the torque produced by electromagnetic induction.
Which machine is commonly used for small home appliances like mixers?
A Synchronous motor
B DC motor
C Universal motor
D Stepper motor
Universal motors are used in home appliances because they are compact, lightweight, and can run on both AC and DC.
What is “excitation” in electrical machines?
A Supplying current to the field winding
B Increasing core losses
C Controlling rotor speed
D Reducing slip
Excitation refers to supplying current to the field winding of an electrical machine to create the required magnetic field.
What is the primary use of stepper motors?
A High-speed operations
B Precise position control
C Heavy torque applications
D High power generation
Stepper motors are used for precise position control in applications like robotics, printers, and CNC machines.
What is the main difference between a single-phase and three-phase induction motor?
A Three-phase motors have better efficiency and starting torque
B Single-phase motors have three stator windings
C Three-phase motors operate on DC supply
D Single-phase motors have higher power output
Three-phase motors are more efficient and provide better starting torque compared to single-phase induction motors.
What is “slip” in an induction motor expressed as?
A Percentage of synchronous speed
B Torque divided by speed
C Efficiency multiplied by load
D Voltage divided by current
Slip is the difference between synchronous speed and rotor speed, expressed as a percentage of the synchronous speed.
Why is a squirrel cage rotor preferred in most induction motors?
A Simple and robust construction
B High efficiency in low-load conditions
C Ease of speed control
D Low core losses
The squirrel cage rotor has a simple and robust design, making it durable and cost-effective for most induction motors.
What is the primary reason for winding copper losses in electrical machines?
A Resistance of the winding
B Magnetic flux
C Core vibration
D Voltage regulation
Copper losses occur due to the resistance of the winding, which causes energy loss in the form of heat.
Which cooling method is used in large electrical machines?
A Convection only
B Forced air or oil cooling
C Radiation cooling
D Water vapor cooling
Large electrical machines use forced air or oil cooling to efficiently dissipate heat generated by losses.
What is the role of a permanent magnet in some electrical machines?
A To reduce slip
B To increase resistance
C To provide a constant magnetic field
D To reduce voltage losses
Permanent magnets provide a constant magnetic field in electrical machines, eliminating the need for field windings or external excitation.
What is the main function of a motor?
A To store electrical energy
B To convert electrical energy into mechanical energy
C To regulate voltage
D To generate electricity
A motor’s primary function is to convert electrical energy into mechanical energy, which is used for performing mechanical work.
Which component of a transformer prevents flux leakage?
A Core
B Stator
C Rotor
D Windings
The core in a transformer helps confine the magnetic flux to prevent leakage and ensure efficient energy transfer between windings.
What does “rotor” refer to in an electrical machine?
A Stationary magnetic part
B Rotating part of the machine
C The winding for generating voltage
D The frame for supporting the machine
The rotor is the rotating part of an electrical machine that interacts with the stator to produce torque or generate power.
What is the synchronous speed of a machine dependent on?
A Resistance of windings
B Supply frequency and number of poles
C Rotor mass
D Core material
The synchronous speed of an electrical machine depends on the supply frequency and the number of poles as per the formula: \( N_s = \frac{120f}{P} \).
What is the purpose of a starter in a motor?
A To limit the initial current
B To increase speed
C To reduce torque
D To maintain constant temperature
A starter limits the initial current drawn by a motor during starting, preventing damage to the motor and the power supply system.
Which loss is caused by alternating magnetization of the core?
A Friction loss
B Copper loss
C Hysteresis loss
D Eddy current loss
Hysteresis loss occurs due to the repeated magnetization and demagnetization of the core material in an alternating magnetic field.
Which material is commonly used for the core of electrical machines?
A Aluminum
B Silicon steel
C Copper
D Carbon
Silicon steel is used for cores in electrical machines because of its high magnetic permeability and low hysteresis loss.
What does an increase in slip in an induction motor indicate?
A Increased speed
B Decreased rotor speed
C Improved efficiency
D Reduced torque
As slip increases, the rotor speed decreases relative to the synchronous speed, which can result in higher torque but lower efficiency.
What is the main advantage of a three-phase motor over a single-phase motor?
A Higher efficiency and smoother operation
B Easier starting without additional components
C Lower weight for the same power rating
D Less complex construction
Three-phase motors are more efficient and provide smoother operation due to the uniform power delivery from three-phase currents.
What is the main characteristic of a synchronous generator?
A Produces DC power
B Operates without excitation
C Operates at constant speed regardless of load
D Produces variable voltage
A synchronous generator operates at a constant speed (synchronous speed) irrespective of the load, as its speed is determined by the supply frequency and the number of poles.
What causes eddy current losses in electrical machines?
A Improper insulation
B Circulating currents in the core
C Friction between moving parts
D High-frequency operation
Eddy current losses occur due to circulating currents induced in the core by the changing magnetic field, resulting in heat generation.
What is the role of bearings in electrical machines?
A To generate power
B To control magnetic flux
C To support the rotating shaft
D To increase voltage
Bearings provide mechanical support to the rotating shaft, reducing friction and ensuring smooth operation.
What is the main advantage of brushless motors?
A Low torque output
B Low-speed operation
C Maintenance-free design
D Simple control mechanism
Brushless motors eliminate the need for brushes and commutators, reducing wear and maintenance requirements, making them more durable.
What is the purpose of a flywheel in electrical machines?
A To store rotational energy
B To reduce slip
C To step up voltage
D To improve insulation
A flywheel stores rotational energy and helps maintain smooth operation by reducing fluctuations in speed.
Which part of a DC motor experiences commutation?
A Stator
B Field winding
C Armature winding
D Core
Commutation occurs in the armature winding of a DC motor, where the direction of current in conductors is reversed to ensure unidirectional torque.
Why are permanent magnets used in some motors?
A To control speed
B To reduce resistance
C To provide a fixed magnetic field
D To improve cooling
Permanent magnets create a fixed magnetic field, eliminating the need for external field excitation and simplifying motor design.
What is the primary function of field winding in a synchronous machine?
A To produce the main magnetic field
B To reduce vibrations
C To increase speed
D To improve cooling efficiency
The field winding in a synchronous machine is responsible for producing the magnetic field required for operation.
What type of cooling is used in high-power electrical machines?
A Natural air cooling
B Forced air or liquid cooling
C Radiation cooling
D Magnetic cooling
High-power electrical machines use forced air or liquid cooling to efficiently dissipate heat and maintain operating temperatures.
Which of the following is an essential step in electrical machine maintenance?
A Increasing speed periodically
B Reducing power supply voltage
C Inspecting insulation and connections
D Operating the machine without load
Regular inspection of insulation, connections, and components is crucial to ensure the machine’s proper functioning and safety.
Which type of motor is commonly used in computer hard drives?
A Stepper motor
B Brushless DC motor
C Universal motor
D Induction motor
Brushless DC motors are used in computer hard drives due to their high precision, reliability, and maintenance-free design.
What type of energy conversion occurs in a generator?
A Electrical to mechanical
B Mechanical to electrical
C Electrical to heat
D Mechanical to thermal
A generator converts mechanical energy into electrical energy using the principle of electromagnetic induction.
What is the stationary part of an induction motor called?
A Rotor
B Armature
C Stator
D Commutator
The stator is the stationary part of an induction motor that contains the winding connected to the power supply.
What is the main function of a slip ring in an AC generator?
A To transfer current to or from the rotating coil
B To rectify AC to DC
C To reduce resistance
D To increase torque
Slip rings provide a continuous electrical connection between the stationary circuit and the rotating coil.
What is the frequency of the induced EMF in the rotor of a squirrel cage induction motor at synchronous speed?
A Zero
B Equal to the supply frequency
C Half of the supply frequency
D Twice the supply frequency
At synchronous speed, the relative motion between the rotor and the magnetic field is zero, so no EMF is induced.
Which type of motor is commonly used for conveyor belts?
A Stepper motor
B Induction motor
C DC motor
D Synchronous motor
Induction motors are commonly used for conveyor belts due to their ruggedness, reliability, and ability to handle varying loads.
What is the primary cause of copper losses in electrical machines?
A Resistance in the windings
B Hysteresis in the core
C Eddy currents in the core
D Magnetic flux leakage
Copper losses occur due to the resistance of the windings, leading to heat generation when current flows through them.
What is the primary role of a capacitor in single-phase motors?
A To increase power output
B To create a phase shift for starting torque
C To reduce losses
D To provide insulation
The capacitor in single-phase motors creates a phase shift between the currents in the start and run windings, enabling the motor to develop starting torque.
What does the term “core saturation” refer to in electrical machines?
A Increased slip in the rotor
B Magnetic core operating at maximum flux density
C Loss of rotor alignment
D Overheating of the windings
Core saturation occurs when the magnetic core operates at its maximum flux density and cannot accommodate additional magnetic flux.
What is the main advantage of using a brushless motor?
A Higher starting torque
B Less maintenance and longer lifespan
C Simplified construction
D Reduced manufacturing cost
Brushless motors eliminate the need for brushes and commutators, reducing maintenance and increasing durability.
What determines the torque produced by an induction motor?
A Voltage applied to the stator
B Rotor material
C Interaction between rotor current and stator magnetic field
D Size of the stator core
Torque in an induction motor is produced by the interaction between the rotor current and the rotating magnetic field of the stator.
What happens to the speed of a DC motor when load increases?
A Increases
B Decreases
C Remains constant
D Becomes zero
The speed of a DC motor decreases with an increase in load due to the drop in terminal voltage caused by increased armature current.
What is the primary function of laminating the core of a transformer?
A To reduce eddy current losses
B To improve cooling efficiency
C To reduce core weight
D To improve voltage regulation
Laminating the core reduces eddy current losses by limiting the flow of induced currents in the core material.
What is the main characteristic of a stepper motor?
A Rotates in discrete steps
B Operates at synchronous speed
C Produces high starting torque
D Requires brushes for operation
Stepper motors rotate in discrete steps, allowing precise control over position and speed.
What does a flywheel help maintain in electrical machines?
A Low power consumption
B Smooth rotational motion
C Reduced current losses
D Stable voltage
A flywheel helps smooth out rotational motion by storing rotational energy and reducing speed fluctuations.
Which part of a synchronous motor determines the rotor speed?
A Supply frequency
B Load torque
C Rotor resistance
D Stator winding
The rotor speed of a synchronous motor is directly proportional to the supply frequency and remains constant.
What is the function of commutators in DC machines?
A To ensure unidirectional current flow
B To reduce core losses
C To step up voltage
D To control slip
Commutators in DC machines reverse the direction of current in the armature winding, ensuring unidirectional torque.
Why are ball bearings used in electrical machines?
A To reduce electrical losses
B To minimize mechanical friction
C To increase magnetic flux
D To maintain constant speed
Ball bearings reduce mechanical friction in rotating parts, improving the machine’s efficiency and longevity.
What is the primary purpose of insulation in electrical machines?
A To prevent electrical leakage
B To increase current
C To regulate speed
D To improve magnetic field strength
Insulation prevents electrical leakage, ensuring safety and proper operation of electrical machines.
Which loss is directly proportional to the square of the load current?
A Core loss
B Copper loss
C Friction loss
D Windage loss
Copper losses are proportional to the square of the load current because they depend on the resistance of the windings and the current flowing through them.
What is the typical cooling method for large electrical machines?
A Natural convection
B Forced air or liquid cooling
C Radiation
D Thermoelectric cooling
Large electrical machines often use forced air or liquid cooling systems to dissipate the heat generated by losses efficiently.