A transformer is used to increase (step up) or decrease (step down) voltage levels in an AC system through electromagnetic induction.
What is the main purpose of field windings in a DC motor?
A To cool the motor
B To produce a magnetic field
C To generate current
D To reduce torque
Field windings generate the magnetic field required for the operation of the DC motor.
Which type of current flows through the rotor of an induction motor?
A Induced current
B DC current
C Direct supply current
D Alternating current
The current in the rotor of an induction motor is induced by the relative motion between the rotor and the magnetic field of the stator.
What is the typical frequency of an AC supply used in electrical machines in most countries?
A 25 Hz
B 100 Hz
C 50 Hz or 60 Hz
D 75 Hz
Most countries use an AC supply frequency of either 50 Hz or 60 Hz for electrical machines.
Which type of motor is used in electric trains?
A Stepper motor
B DC series motor
C Synchronous motor
D Universal motor
DC series motors are used in electric trains because of their high starting torque and ability to handle varying loads.
What is the function of a centrifugal switch in a single-phase induction motor?
A To control speed
B To disconnect the starting winding
C To step up voltage
D To increase torque
The centrifugal switch disconnects the starting winding after the motor reaches a certain speed to improve efficiency.
What does the term “voltage regulation” mean in a synchronous generator?
A The generator’s speed stability
B The change in voltage from no-load to full-load
C The insulation level of the windings
D The efficiency of the generator
Voltage regulation refers to the change in terminal voltage of a generator as the load varies from no-load to full-load.
What is a universal motor capable of operating on?
A Both AC and DC
B Only AC
C Only DC
D High-frequency current
Universal motors can operate on both AC and DC power supplies, making them suitable for portable tools and appliances.
What does the term “starting torque” refer to?
A Torque required to maintain speed
B Torque required to start the motor
C Torque generated at synchronous speed
D Torque that causes slip
Starting torque is the torque produced by a motor when it starts from rest to overcome inertia and any load.
Which part of a transformer ensures efficient magnetic flux transfer?
A Windings
B Core
C Insulation
D Brushes
The core of a transformer ensures efficient magnetic flux transfer between the primary and secondary windings.
What happens to the power factor of an induction motor as the load increases?
A Decreases
B Increases
C Remains constant
D Becomes zero
The power factor of an induction motor improves as the load increases because the reactive component of the current decreases relative to the total current.
What is the primary use of a synchronous condenser?
A To improve power factor
B To increase voltage
C To convert AC to DC
D To reduce load
Synchronous condensers are used to improve the power factor by providing reactive power to the grid.
What type of machine is a dynamo?
A A DC generator
B An AC generator
C A stepper motor
D A synchronous motor
A dynamo is a type of DC generator that converts mechanical energy into DC electrical energy.
What is the primary purpose of oil in transformers?
A To reduce winding resistance
B To provide insulation and cooling
C To increase core efficiency
D To step down voltage
Transformer oil is used for insulation and cooling to prevent overheating and ensure safe operation.
In an induction motor, slip is required to:
A Maintain constant speed
B Induce current in the rotor
C Improve efficiency
D Reduce core losses
Slip is necessary in an induction motor to create relative motion between the stator’s rotating magnetic field and the rotor, which induces current.
What type of machine is commonly used for elevators?
A Universal motor
B Stepper motor
C Induction motor
D Synchronous motor
Induction motors are used for elevators due to their robustness, reliability, and ability to handle varying loads efficiently.
Which component of a DC motor prevents sparking at the commutator?
A Brushes
B Armature winding
C Core
D Field winding
Brushes in a DC motor ensure proper contact with the commutator and reduce sparking by maintaining a clean electrical connection.
What type of machine is typically used in wind turbines?
A Stepper motor
B Synchronous generator
C Universal motor
D DC generator
Synchronous generators are commonly used in wind turbines to convert mechanical energy from wind into electrical energy.
What is the main cause of mechanical losses in electrical machines?
A Core saturation
B Friction and windage
C Eddy currents
D Copper resistance
Mechanical losses in electrical machines are caused by friction in bearings and windage due to air resistance.
What is the function of a damping winding in synchronous machines?
A To reduce oscillations during starting
B To increase efficiency
C To maintain constant load
D To improve power factor
Damping windings reduce oscillations and stabilize the rotor during starting or load changes in synchronous machines.
What type of energy conversion occurs in a motor?
A Electrical to heat
B Mechanical to electrical
C Electrical to mechanical
D Heat to electrical
A motor converts electrical energy into mechanical energy to perform mechanical work.
Which of the following is a rotating electrical machine?
A Motor
B Transformer
C Capacitor
D Inductor
Motors are rotating electrical machines that convert electrical energy into mechanical energy.
What is the main cause of hysteresis loss in electrical machines?
A Repeated magnetization and demagnetization of the core
B Resistance in the windings
C Heat generated by the rotor
D Air gap in the machine
Hysteresis loss occurs due to repeated magnetization and demagnetization of the core material.
What is the purpose of laminations in a transformer core?
A To reduce eddy current losses
B To increase core weight
C To provide mechanical strength
D To step up voltage
Laminations in the transformer core reduce eddy current losses by restricting the flow of circulating currents.
Which electrical machine operates on the principle of electromagnetic induction?
A Battery
B Generator
C Capacitor
D Resistor
Generators work on the principle of electromagnetic induction, where a conductor moving in a magnetic field generates an EMF.
What type of current is induced in the rotor of an induction motor?
A Direct current
B Alternating current from the supply
C Alternating current induced by the stator field
D Pulsating current
The rotor current in an induction motor is induced by the rotating magnetic field of the stator.
What is the function of brushes in a DC machine?
A To conduct current between the rotating and stationary parts
B To insulate the winding
C To generate the magnetic field
D To step up voltage
Brushes conduct current between the stationary parts and the rotating commutator in a DC machine.
What is the purpose of slip in an induction motor?
A To control voltage
B To induce current in the rotor
C To reduce losses
D To maintain speed
Slip allows relative motion between the rotor and the stator’s magnetic field, inducing current in the rotor.
Which type of machine is commonly used for high-speed applications?
A Stepper motor
B Synchronous motor
C Universal motor
D DC shunt motor
Synchronous motors are commonly used for high-speed applications as they operate at a constant speed determined by the supply frequency.
What is the typical role of cooling systems in electrical machines?
A To dissipate heat generated by losses
B To increase power output
C To maintain insulation
D To reduce torque
Cooling systems remove heat generated due to electrical and mechanical losses, preventing overheating and damage.
What is the main difference between a generator and a motor?
A A generator requires a battery to operate
B A generator converts mechanical energy to electrical energy; a motor does the opposite
C A motor generates higher voltage
D A generator does not use windings
Generators convert mechanical energy into electrical energy, while motors convert electrical energy into mechanical energy.
What is the main purpose of a commutator in a DC motor?
A To reverse the direction of current in the armature winding
B To step up the voltage
C To create the magnetic field
D To increase torque
The commutator reverses the current direction in the armature winding to ensure unidirectional torque in a DC motor.
What does the term “synchronous speed” refer to?
A The speed of the rotor in an induction motor
B The speed of the rotor in a stepper motor
C The speed of the stator magnetic field
D The speed of the brushes
Synchronous speed is the speed at which the stator magnetic field rotates, determined by the supply frequency and the number of poles.
What is the purpose of a stepper motor?
A High-speed operation
B Precise position control
C High torque generation
D Constant speed operation
Stepper motors are used for precise position control, making them ideal for robotics, CNC machines, and printers.
What causes eddy current losses in electrical machines?
A Circulating currents induced in the core
B Friction between moving parts
C Resistance in windings
D Improper cooling
Eddy current losses occur due to circulating currents induced in the core material by the changing magnetic field.
What is the role of excitation in a synchronous machine?
A To step up voltage
B To control the load current
C To provide the necessary magnetic field
D To improve cooling
Excitation provides the necessary magnetic field in synchronous machines for their operation.
What type of machine is typically used in washing machines?
A DC motor
B Universal motor
C Stepper motor
D Synchronous motor
Universal motors are used in washing machines because they can run on both AC and DC power and provide high speed.
What is the function of a rotor in electrical machines?
A To rotate and interact with the magnetic field
B To provide insulation
C To step up voltage
D To dissipate heat
The rotor is the rotating part of an electrical machine that interacts with the stator’s magnetic field to produce torque.
Why is silicon steel used in the cores of electrical machines?
A To reduce hysteresis and eddy current losses
B To increase weight
C To improve cooling
D To decrease speed
Silicon steel has high magnetic permeability and low hysteresis loss, making it ideal for reducing core losses.
What does “load” refer to in an electrical machine?
A The current flowing through the machine
B The power demand on the machine
C The cooling requirement
D The speed of the rotor
Load refers to the power demand placed on the machine, which determines the machine’s performance and efficiency.
What is the basic principle of operation of a transformer?
A Electromechanical energy conversion
B Electromagnetic induction
C Electrical resistance
D Mechanical resonance
A transformer works on the principle of electromagnetic induction, where a changing current in one winding induces a voltage in another winding.
What type of losses are caused by the resistance of the windings in an electrical machine?
A Copper losses
B Core losses
C Eddy current losses
D Hysteresis losses
Copper losses are caused by the resistance of the windings when current flows through them, resulting in heat.
Which component of a DC machine provides mechanical support to the armature?
A Commutator
B Brushes
C Shaft
D Bearings
The shaft provides mechanical support to the armature and transfers mechanical energy in or out of the machine.
In a three-phase induction motor, how many sets of windings are present in the stator?
A One
B Two
C Three
D Four
A three-phase induction motor has three sets of windings in the stator, each displaced by 120° electrically.
What is the primary purpose of an armature winding in a generator?
A To create a magnetic field
B To generate electrical power
C To step up voltage
D To reduce losses
The armature winding in a generator generates electrical power by inducing an EMF when the magnetic field changes.
What is “slip” in the context of an induction motor?
A Speed difference between two motors
B The difference between synchronous speed and rotor speed
C Voltage difference between stator and rotor
D Current lag in the rotor
Slip is the difference between the synchronous speed of the stator’s magnetic field and the actual speed of the rotor, expressed as a percentage.
What is the primary advantage of using a synchronous motor?
A High starting torque
B Constant speed operation
C Low cost
D Simple construction
Synchronous motors run at a constant speed, determined by the supply frequency and the number of poles.
Which type of electrical machine is used for precision control in robotics?
A Induction motor
B Universal motor
C Stepper motor
D Synchronous motor
Stepper motors are used for precise position control, making them ideal for robotics, CNC machines, and printers.
What is the role of a stator in an electrical machine?
A To store energy
B To produce a magnetic field
C To conduct current to the rotor
D To reduce losses
The stator is the stationary part of an electrical machine that produces the magnetic field required for operation.
What is the main component responsible for torque generation in an induction motor?
A Voltage across the stator winding
B Resistance of the rotor
C Interaction of rotor current and stator magnetic field
D Rotor speed
Torque is generated in an induction motor due to the interaction between the rotor current and the stator’s rotating magnetic field.