What type of transformer is used for electrical power distribution?
A Step-up transformer
B Step-down transformer
C Isolation transformer
D Auto transformer
Step-down transformers are used in electrical power distribution to reduce the high transmission voltage to a usable level.
What is the function of damping windings in synchronous machines?
A To reduce eddy current losses
B To generate torque
C To suppress oscillations during transient conditions
D To step down voltage
Damping windings are used to suppress oscillations and stabilize the rotor during transient conditions.
Which loss in electrical machines is constant and independent of load?
A Core loss
B Copper loss
C Mechanical loss
D Stray loss
Core losses, comprising hysteresis and eddy current losses, are constant and occur irrespective of the load.
What does an increase in slip in an induction motor indicate?
A Increased speed
B Decreased rotor speed
C Improved efficiency
D Reduced load
An increase in slip indicates a decrease in rotor speed relative to the synchronous speed.
Why is an autotransformer more efficient than a two-winding transformer?
A It uses a single winding for energy transfer
B It has better insulation
C It uses less copper and iron
D It operates at a higher frequency
An autotransformer is more efficient because it uses a single winding for both primary and secondary, reducing material requirements.
What does the term “no-load current” refer to in a transformer?
A Current when load is maximum
B Current drawn by the transformer with no external load
C Current flowing through the secondary winding
D Current due to winding resistance
No-load current is the small amount of current drawn by the transformer to maintain the core’s magnetic field when no load is connected.
What is the role of insulation in electrical machines?
A To increase torque
B To prevent electrical leakage and short circuits
C To reduce vibration
D To improve speed
Insulation prevents electrical leakage, ensures safe operation, and protects the machine from short circuits.
What is the primary advantage of a brushless DC motor?
A Low cost
B High starting torque
C Low maintenance and longer lifespan
D Simple control mechanism
Brushless DC motors eliminate the need for brushes, reducing maintenance and increasing their operational lifespan.
What is the primary purpose of oil in transformers?
A To increase voltage
B To provide insulation and cooling
C To improve power factor
D To reduce copper losses
Transformer oil provides insulation and helps cool the transformer by dissipating heat.
Which type of motor is commonly used in electric vehicles?
A Universal motor
B Stepper motor
C Brushless DC motor
D Synchronous motor
Brushless DC motors are commonly used in electric vehicles because of their high efficiency, reliability, and torque characteristics.
What is the primary role of an electrical machine?
A To store energy
B To convert energy from one form to another
C To transfer power without losses
D To increase current
Electrical machines either convert mechanical energy into electrical energy (generators) or electrical energy into mechanical energy (motors). Transformers transfer energy by stepping voltage up or down without changing the energy form.
What does the armature winding in a DC machine do?
A Creates the magnetic field
B Generates or carries the main current
C Reduces friction
D Steps up voltage
The armature winding in a DC machine is responsible for either generating the EMF (in a generator) or carrying the current that interacts with the magnetic field to produce torque (in a motor).
What is the purpose of a commutator in a DC machine?
A To convert AC generated in the armature into DC
B To generate magnetic flux
C To reduce losses
D To step up current
The commutator in a DC machine reverses the current in the armature winding to ensure that the output voltage remains unidirectional (DC).
What determines the synchronous speed of a machine?
A Frequency of supply and the number of poles
B Resistance of the rotor
C Core material
D Type of winding
Synchronous speed (Ns) is determined by the supply frequency (f) and the number of poles (P) as per the formula: \(N_s = \frac{120f}{P}\).
What is the main loss reduced by laminating the core of electrical machines?
A Hysteresis loss
B Eddy current loss
C Copper loss
D Mechanical loss
Laminations reduce the path for circulating currents induced in the core, minimizing eddy current losses while maintaining magnetic properties.
Why is the rotor of an induction motor not at synchronous speed?
A To allow induction of current in the rotor
B To reduce losses
C To create a uniform magnetic field
D To prevent overheating
The rotor must have a relative motion to the stator’s magnetic field to induce current, which generates torque. This is achieved by running slightly slower than synchronous speed.
What does slip in an induction motor indicate?
A The speed of the motor
B The difference between synchronous speed and rotor speed
C The power factor of the motor
D The load on the motor
Slip is the percentage difference between the synchronous speed and the rotor speed. Without slip, no current would be induced in the rotor.
What is the primary purpose of field windings in electrical machines?
A To produce the magnetic field
B To carry current
C To insulate the core
D To reduce losses
Field windings generate the magnetic field necessary for the operation of machines such as motors and generators.
What is the function of brushes in a DC motor?
A To transfer current to and from the rotating commutator
B To insulate the rotor
C To step up voltage
D To reduce torque
Brushes in a DC motor ensure electrical contact with the rotating commutator, transferring current between the stationary and moving parts.
What type of current is induced in the rotor of an induction motor?
A Direct current
B Alternating current
C Pulsating current
D Constant current
The changing magnetic field of the stator induces alternating current in the rotor, which interacts with the stator field to produce torque.
Which component in a transformer minimizes magnetic flux leakage?
A Windings
B Core
C Oil
D Bushings
The core in a transformer ensures a low-reluctance path for the magnetic flux, minimizing leakage and improving efficiency.
What is the typical power source for stepper motors?
A DC supply with pulses
B Pure AC
C Constant DC voltage
D High-frequency AC
Stepper motors operate using DC pulses, which allow precise control of angular movement by sequentially energizing the stator windings.
What is the main purpose of excitation in synchronous machines?
A To supply the magnetic field
B To start the machine
C To increase speed
D To reduce losses
Excitation provides the necessary current to the field windings of synchronous machines, creating the magnetic field required for operation.
What is the primary cause of core losses?
A Copper resistance
B Hysteresis and eddy currents
C Friction in moving parts
D Improper insulation
Core losses are caused by hysteresis (magnetic property changes) and eddy currents (circulating currents in the core material).
What type of machine is used in elevators?
A Universal motor
B Induction motor
C Stepper motor
D Synchronous motor
Induction motors are widely used in elevators due to their robustness, reliability, and ability to handle varying loads efficiently.
What is the main reason for using silicon steel in transformer cores?
A To reduce hysteresis losses
B To increase resistance
C To improve mechanical strength
D To enhance cooling
Silicon steel has low hysteresis loss, high magnetic permeability, and minimizes core losses, making it suitable for transformers.
What does a flywheel in a machine help with?
A To store rotational energy and smooth speed fluctuations
B To step up torque
C To generate power
D To reduce heat
Flywheels store rotational energy and smooth out fluctuations in speed or torque, ensuring stable operation.
Which motor is most suitable for low-power appliances like mixers?
A Universal motor
B Stepper motor
C Induction motor
D Synchronous motor
Universal motors can operate on both AC and DC and are suitable for compact, high-speed applications like mixers.
What is the main advantage of brushless motors?
A Longer lifespan due to no brushes
B Simpler construction
C Higher losses
D Low efficiency
Brushless motors eliminate the need for brushes, reducing wear and maintenance while improving durability and efficiency.
Why are cooling systems necessary in electrical machines?
A To increase efficiency
B To dissipate heat and prevent overheating
C To improve insulation
D To improve power factor
Cooling systems dissipate heat generated due to electrical and mechanical losses, protecting the machine and enhancing its lifespan.
What is the primary role of an electrical machine?
A To store energy
B To convert energy from one form to another
C To transfer power without losses
D To increase current
Electrical machines either convert mechanical energy into electrical energy (generators) or electrical energy into mechanical energy (motors). Transformers transfer energy by stepping voltage up or down without changing the energy form.
What does the armature winding in a DC machine do?
A Creates the magnetic field
B Generates or carries the main current
C Reduces friction
D Steps up voltage
The armature winding in a DC machine is responsible for either generating the EMF (in a generator) or carrying the current that interacts with the magnetic field to produce torque (in a motor).
What is the purpose of a commutator in a DC machine?
A To convert AC generated in the armature into DC
B To generate magnetic flux
C To reduce losses
D To step up current
The commutator in a DC machine reverses the current in the armature winding to ensure that the output voltage remains unidirectional (DC).
What determines the synchronous speed of a machine?
A Frequency of supply and the number of poles
B Resistance of the rotor
C Core material
D Type of winding
Synchronous speed is given by the formula: \(N_s = \frac{120f}{P}\), where \(f\) is the supply frequency and \(P\) is the number of poles. It determines the speed at which the magnetic field rotates in synchronous machines.
What is the main loss reduced by laminating the core of electrical machines?
A Hysteresis loss
B Eddy current loss
C Copper loss
D Mechanical loss
Laminations reduce the path for circulating currents induced in the core, minimizing eddy current losses while maintaining magnetic properties.
Why is the rotor of an induction motor not at synchronous speed?
A To allow induction of current in the rotor
B To reduce losses
C To create a uniform magnetic field
D To prevent overheating
The rotor must have a relative motion to the stator’s magnetic field to induce current, which generates torque. This is achieved by running slightly slower than synchronous speed.
What does slip in an induction motor indicate?
A The speed of the motor
B The difference between synchronous speed and rotor speed
C The power factor of the motor
D The load on the motor
Slip is the percentage difference between the synchronous speed and the rotor speed. Without slip, no current would be induced in the rotor.
What is the primary purpose of field windings in electrical machines?
A To produce the magnetic field
B To carry current
C To insulate the core
D To reduce losses
Field windings generate the magnetic field necessary for the operation of machines such as motors and generators.
What is the function of brushes in a DC motor?
A To transfer current to and from the rotating commutator
B To insulate the rotor
C To step up voltage
D To reduce torque
Brushes in a DC motor ensure electrical contact with the rotating commutator, transferring current between the stationary and rotating parts of the motor.
What type of current is induced in the rotor of an induction motor?
A Direct current
B Alternating current
C Pulsating current
D Constant current
The changing magnetic field of the stator induces alternating current in the rotor, which generates torque to drive the motor.
Which component in a transformer minimizes magnetic flux leakage?
A Windings
B Core
C Oil
D Bushings
The core in a transformer ensures a low-reluctance path for the magnetic flux, minimizing leakage and improving efficiency.
What type of machine is used in electric vehicles for high torque and efficiency?
A Stepper motor
B Brushless DC motor
C Synchronous motor
D Induction motor
Brushless DC motors are used in electric vehicles for their high efficiency, excellent torque characteristics, and low maintenance requirements.
What is the main reason for using silicon steel in transformer cores?
A To reduce hysteresis losses
B To increase resistance
C To improve mechanical strength
D To enhance cooling
Silicon steel has low hysteresis loss, high magnetic permeability, and minimizes core losses, making it ideal for transformer cores.
What happens to the rotor speed of an induction motor when the load increases?
A It decreases
B It increases
C It remains constant
D It becomes equal to synchronous speed
When the load increases, the rotor speed decreases, leading to an increase in slip to generate more torque.
What is the primary cause of core losses?
A Copper resistance
B Hysteresis and eddy currents
C Friction in moving parts
D Improper insulation
Core losses in electrical machines occur due to hysteresis (magnetization and demagnetization of the core) and eddy currents (circulating currents in the core).
Why are centrifugal switches used in single-phase induction motors?
A To reduce core losses
B To disconnect the starting winding after achieving speed
C To step up voltage
D To improve power factor
Centrifugal switches disconnect the starting winding once the motor reaches a certain speed, preventing overheating and improving efficiency.
What is the role of a shading coil in a single-phase induction motor?
A To create a phase difference for starting torque
B To reduce losses
C To increase speed
D To improve power factor
A shading coil creates a phase difference in the magnetic field, producing the necessary starting torque for single-phase induction motors.
What is the main reason for using silicon steel in transformer cores?
A To reduce hysteresis losses
B To increase resistance
C To improve mechanical strength
D To enhance cooling
Silicon steel has low hysteresis loss, high magnetic permeability, and minimizes core losses, making it ideal for transformer cores.
What happens to the rotor speed of an induction motor when the load increases?
A It decreases
B It increases
C It remains constant
D It becomes equal to synchronous speed
When the load increases, the rotor speed decreases, leading to an increase in slip to generate more torque.
What is the primary cause of core losses?
A Copper resistance
B Hysteresis and eddy currents
C Friction in moving parts
D Improper insulation
Core losses in electrical machines occur due to hysteresis (magnetization and demagnetization of the core) and eddy currents (circulating currents in the core).