What does voltage regulation measure in an electrical machine?
A Load current
B Torque change
C Speed variation
D Voltage change
Voltage regulation measures the change in output voltage when the load on an electrical machine changes from no‐load to full‐load conditions. It indicates the machine’s ability to maintain a stable voltage despite variations in load.
In an electrical machine, what is the effect of increasing harmonics in the supply voltage?
A Increased speed
B Reduced efficiency
C Improved torque
D Increased efficiency
Harmonics in the supply voltage cause distortion, which results in additional losses in the machine, especially in the form of heat. These losses reduce the overall efficiency and can lead to overheating and damage to the machine.
What is the main cause of harmonic distortion in electrical machines?
A Overloading
B Increased voltage
C Non‐linear loads
D Excessive cooling
Harmonic distortion is primarily caused by non‐linear loads such as rectifiers, variable frequency drives, and computers. These loads generate harmonics, which can lead to distorted voltage and current waveforms, affecting machine performance.
What is the impact of increasing slip in an induction motor?
A Reduced efficiency
B Increased torque
C Increased speed
D Reduced voltage
In an induction motor, slip represents the difference between the synchronous speed and the rotor speed. As slip increases (with higher load), the motor operates less efficiently, with more power dissipated as heat and greater losses.
What is the purpose of voltage regulation in electrical machines?
A To adjust speed
B To improve torque
C To prevent overheating
D To maintain constant voltage
Voltage regulation ensures that an electrical machine maintains a stable output voltage despite changes in the load. It is essential for maintaining consistent operation of equipment powered by the machine, especially in power generation and distribution systems.
How does an induction motor achieve torque production?
A By direct current supply
B By electromagnetic induction
C By external excitation
D By mechanical force
Induction motors generate torque through electromagnetic induction. The stator produces a rotating magnetic field, which induces a current in the rotor. The interaction between the rotor’s magnetic field and the stator’s field creates the torque that drives the motor.
What is the effect of increasing the number of poles on the synchronous speed of a motor?
A Increases torque
B Decreases speed
C No effect
D Increases speed
In a synchronous motor, the synchronous speed is inversely proportional to the number of poles. Increasing the number of poles reduces the synchronous speed, making it suitable for applications that require lower speeds.
What is the primary role of the field winding in a synchronous motor?
A To control load
B To maintain speed
C To produce magnetic field
D To generate torque
In a synchronous motor, the field winding is responsible for generating the magnetic field on the rotor. The interaction of this field with the rotating magnetic field of the stator enables the rotor to lock with the stator and rotate at synchronous speed.
How does slip affect the torque‐speed characteristic of an induction motor?
A Torque increases with slip
B Speed increases with slip
C Torque remains constant
D Torque decreases with slip
As the slip in an induction motor increases, the motor’s torque also increases. This occurs because the relative motion between the stator’s magnetic field and the rotor increases, resulting in more induced current and greater torque generation.
What is the impact of low voltage on the operation of electrical machines?
A Better cooling
B Increased torque
C Increased speed
D Decreased efficiency
Low voltage can reduce the efficiency of electrical machines. The reduced voltage leads to a lower magnetic field strength, which decreases the motor’s ability to generate torque and increases energy losses due to greater current draw.
What is the main cause of core losses in electrical machines?
A Overloading
B Eddy currents and hysteresis
C Resistance in windings
D Mechanical friction
Core losses in electrical machines are caused by eddy currents and hysteresis within the core material. Eddy currents generate heat and waste energy, while hysteresis loss occurs due to the continuous magnetization and demagnetization of the core material.
What type of motor is commonly used in variable‐speed applications?
A Induction motor
B Stepper motor
C Synchronous motor
D DC motor
DC motors are often used in variable‐speed applications because their speed can be easily controlled by varying the input voltage. This characteristic makes DC motors ideal for applications such as electric vehicles, conveyors, and elevators.
In an induction motor, how is the torque generated?
A By external mechanical force
B By electrical excitation
C By induced current in the rotor
D By changing the speed
Torque in an induction motor is generated when the rotating magnetic field produced by the stator induces current in the rotor. This current creates its own magnetic field, which interacts with the stator’s field, resulting in torque production.
What is the effect of increasing the frequency in a synchronous motor?
A Decreases speed
B Increases speed
C No effect
D Increases torque
The synchronous speed of a motor is directly proportional to the supply frequency. As the frequency increases, the synchronous speed increases, making the motor rotate faster. This is important for controlling the motor speed in AC applications.
Which of the following methods is commonly used to control the speed of an induction motor?
A Varying supply frequency
B Changing supply voltage
C Adjusting rotor resistance
D Using a commutator
The speed of an induction motor can be controlled by varying the supply frequency. This is commonly done using a variable frequency drive (VFD), which adjusts the frequency to control the motor speed for various applications.