What happens to the torque in an induction motor when the load increases?
A Torque decreases
B Torque remains constant
C Torque increases
D Torque becomes zero
When the load increases on an induction motor, the slip increases, and this induces more current in the rotor. This results in increased torque being produced to handle the higher load. Higher torque is necessary to overcome the resistance from the load.
What is the primary function of the stator windings in an induction motor?
A To create the rotating magnetic field
B To reduce the motor’s starting current
C To supply power to the rotor
D To control the motor’s speed
The stator windings in an induction motor generate a rotating magnetic field when energized by the power supply. This field interacts with the rotor to induce current, generating torque and allowing the rotor to rotate.
What is the main advantage of using squirrel-cage rotors in induction motors?
A Reduced maintenance costs
B Improved speed control
C Higher efficiency at low loads
D Increased torque output
The squirrel-cage rotor is simple, rugged, and does not require external electrical connections, such as slip rings. This makes the motor reliable and reduces maintenance requirements, making it ideal for general-purpose industrial applications.
How does increasing the number of poles in a synchronous motor affect its speed?
A Speed increases
B Speed decreases
C Speed remains constant
D Speed becomes variable
The synchronous speed of a motor is inversely proportional to the number of poles. Increasing the number of poles decreases the motor’s synchronous speed, which is beneficial for applications requiring lower speeds and higher torque.
What happens when the slip of an induction motor is zero?
A The motor reaches maximum torque
B The rotor speed matches the synchronous speed
C The motor runs without producing power
D The rotor current becomes zero
When the slip is zero, the rotor speed matches the synchronous speed of the rotating magnetic field, meaning the motor is not producing any torque and effectively stops.
What is the primary reason for using squirrel-cage rotors in induction motors?
A To improve the motor’s speed control
B To reduce maintenance costs
C To improve the power factor
D To reduce mechanical wear
Squirrel-cage rotors are simple and durable with no external connections, such as slip rings or brushes, which reduces maintenance costs. These rotors are ideal for applications where low maintenance and high reliability are essential.
What is the role of the excitation system in a synchronous motor?
A To control the motor’s speed
B To supply the field current to the rotor
C To reduce mechanical vibrations
D To improve the motor’s power factor
The excitation system provides the necessary field current to the rotor, generating the magnetic field that interacts with the stator’s rotating magnetic field, allowing the motor to run at synchronous speed.
What happens to the rotor current in an induction motor as the slip increases?
A The rotor current decreases
B The rotor current increases
C The rotor current remains constant
D The rotor current becomes zero
As the slip increases, the relative speed between the rotor and the rotating magnetic field increases. This induces more current in the rotor, which results in higher torque production to handle the increased load.
What is the typical use of wound rotors in induction motors?
A High-efficiency applications
B Variable-speed applications
C Constant-speed applications
D Low-torque applications
Wound rotors allow for better control of the motor’s starting torque and speed. By adding external resistance, wound-rotor motors are ideal for applications requiring variable speed, such as cranes or mills.
What is the typical use of squirrel-cage rotors in industrial applications?
A High-torque applications
B Constant-speed applications
C High-power applications
D Low-efficiency applications
Squirrel-cage rotors are ideal for applications where constant speed is required, such as in pumps, fans, and compressors. They are robust, reliable, and require minimal maintenance, making them suitable for general-purpose use.
What is the effect of increasing the rotor resistance in a wound-rotor induction motor during normal operation?
A The motor operates more efficiently
B The motor’s slip increases
C The motor’s efficiency decreases
D The motor’s power factor improves
Increasing the rotor resistance in a wound-rotor induction motor causes an increase in slip. This leads to greater power loss in the rotor, which reduces the efficiency of the motor under normal operation, though it helps improve starting torque.
What is the role of insulation materials in electric motors?
A To prevent electrical breakdown and short circuits
B To increase the power factor
C To reduce mechanical wear
D To increase motor speed
Insulation materials are used in electric motors to protect the windings from electrical breakdown, preventing short circuits. They ensure safe operation under high voltage and maintain the integrity of the motor’s electrical system.
What happens when the excitation of a synchronous motor is reduced?
A The motor’s power factor becomes leading
B The motor’s power factor becomes lagging
C The motor’s torque output increases
D The motor’s speed increases
Reducing the excitation in a synchronous motor weakens the rotor’s magnetic field, causing the motor to draw more reactive power. This results in a lagging power factor, which is typical when under-excitation occurs.
What is the effect of increasing the air gap between the stator and rotor in an induction motor?
A It reduces the magnetic coupling and decreases efficiency
B It increases the motor’s efficiency
C It increases the motor’s torque production
D It improves the motor’s power factor
Increasing the air gap in an induction motor reduces the efficiency because the magnetic coupling between the stator and rotor weakens. A larger air gap results in lower flux density and increased losses, which reduces torque production.
What is the purpose of a voltage regulator in a synchronous motor?
A To regulate the rotor’s excitation and maintain stable voltage
B To increase the motor’s speed
C To reduce mechanical vibrations
D To control the motor’s power factor
The voltage regulator in a synchronous motor adjusts the excitation of the rotor to maintain a stable output voltage under varying load conditions. This ensures that the motor can operate efficiently and without voltage fluctuations.
What is the effect of over-excitation in a synchronous motor?
A It leads to a leading power factor
B It leads to a lagging power factor
C It increases the motor’s efficiency
D It reduces the motor’s speed
Over-excitation in a synchronous motor strengthens the rotor’s magnetic field, causing the motor to supply more reactive power to the system. This results in a leading power factor, which can help improve the overall power factor of the grid.
What happens to the rotor current in an induction motor as the slip increases?
A The rotor current increases
B The rotor current decreases
C The rotor current remains constant
D The rotor current becomes zero
As the slip increases, the difference between the rotor speed and the stator’s rotating magnetic field increases. This increases the induced current in the rotor, which leads to higher torque production. The rotor current is proportional to the slip.
What is the effect of under-excitation in a synchronous motor?
A It leads to a leading power factor
B It leads to a lagging power factor
C It increases the motor’s efficiency
D It causes the motor to lose synchronism
Under-excitation reduces the rotor’s magnetic field, causing the motor to consume more reactive power. This results in a lagging power factor, which is typically undesirable as it indicates poor power usage efficiency.
What is the primary effect of increasing the number of poles in a synchronous motor?
A It increases torque at lower speeds
B It reduces torque at lower speeds
C It increases the motor’s speed
D It decreases the motor’s efficiency
Increasing the number of poles in a synchronous motor results in a lower synchronous speed but increases the torque production at lower speeds. This is beneficial for applications requiring high torque and lower speeds.
What is the effect of core losses in an induction motor?
A They generate heat and reduce efficiency
B They improve the power factor
C They increase the motor’s efficiency
D They increase torque production
Core losses, caused by hysteresis and eddy currents, generate heat and reduce the efficiency of an induction motor. These losses consume power without contributing to useful work, making them detrimental to motor efficiency.