What is the primary function of the stator winding in a synchronous motor?
A To supply current to the rotor
B To create the rotating magnetic field
C To provide mechanical power
D To regulate the motor’s speed
The stator winding in a synchronous motor generates the rotating magnetic field, which induces the rotor to rotate at synchronous speed. The rotor’s magnetic field interacts with this rotating field to maintain synchronism.
How does increasing the slip in an induction motor affect the torque?
A Torque increases
B Torque decreases
C Torque remains constant
D Torque becomes unstable
As the slip increases in an induction motor, the relative speed between the rotor and stator increases. This induces more current in the rotor, resulting in higher torque production. However, excessive slip reduces efficiency and can cause overheating.
What is the main effect of harmonic currents in induction motors?
A They increase motor efficiency
B They cause overheating and reduce efficiency
C They reduce the motor’s power factor
D They stabilize the motor’s speed
Harmonic currents distort the current waveform and lead to additional losses, primarily through increased heat generation. These effects reduce the motor’s overall efficiency, causing mechanical vibrations that affect the motor’s performance.
Which of the following is a common starting method for large induction motors?
A Direct-on-line (DOL) starting
B Star-delta starting
C Capacitor start
D Both A and B
Direct-on-line (DOL) starting and star-delta starting are common methods for large induction motors. DOL is used for smaller motors, while star-delta is preferred for larger motors to reduce inrush current during startup.
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, which means there is no difference between the speed of the rotor and the rotating magnetic field. In this condition, the motor is not producing any torque and would effectively stop.
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 increase starting torque
D To improve the power factor
Squirrel-cage rotors are preferred because they are simple, durable, and reliable. These rotors have no external brushes or slip rings, reducing maintenance requirements and costs. They are ideal for general-purpose applications.
What happens when the excitation of a synchronous motor is increased?
A The motor’s speed increases
B The power factor becomes leading
C The power factor becomes lagging
D The motor’s efficiency decreases
Increasing the excitation in a synchronous motor increases the rotor’s magnetic field strength, which results in the motor supplying more reactive power to the system. This causes the power factor to become leading.
How does increasing the slip in an induction motor affect the motor’s efficiency?
A The motor becomes more efficient
B The motor becomes less efficient
C The motor’s efficiency remains constant
D The motor becomes more powerful
As the slip increases, the motor becomes less efficient due to increased losses in the rotor and stator. Higher slip means more energy is converted into heat rather than mechanical work.
What is the primary role of the stator winding in an induction motor?
A To create the magnetic field that induces current in the rotor
B To provide mechanical power to the load
C To control the motor speed
D To connect to the external load
The stator winding in an induction motor generates the rotating magnetic field that induces current in the rotor. This magnetic interaction causes the rotor to turn and produce mechanical power.
What is the primary cause of core losses in an induction motor?
A Eddy currents and hysteresis in the core
B Friction in the bearings
C Overloading of the motor
D Insufficient insulation
Core losses, or iron losses, are caused by eddy currents and hysteresis within the iron core. These losses generate heat and reduce the overall efficiency of the motor.
What is the effect of increasing the rotor resistance in a wound-rotor induction motor?
A The starting torque decreases
B The starting torque increases
C The motor operates at higher speeds
D The motor efficiency increases
Increasing the rotor resistance in a wound-rotor induction motor improves the starting torque by limiting the initial current during startup, allowing the motor to start under higher loads.
What is the effect of under-excitation in a synchronous motor?
A It causes the motor to operate at a higher speed
B It causes the motor to operate with a lagging power factor
C It increases the motor’s efficiency
D It improves the motor’s torque
Under-excitation in a synchronous motor leads to a lagging power factor. The rotor field is weaker, and the motor consumes reactive power from the supply rather than generating it.
What happens when the load on an induction motor increases?
A The motor operates at synchronous speed
B The slip increases and the motor speed decreases slightly
C The motor speed increases
D The motor’s power factor improves
When the load on an induction motor increases, the rotor speed decreases slightly, and the slip increases. This allows the motor to produce more torque to handle the heavier load.
What is the purpose of a capacitor in a single-phase induction motor?
A To create a phase shift and improve starting torque
B To improve the motor’s power factor
C To reduce the motor’s power consumption
D To increase the motor’s speed
In single-phase induction motors, a capacitor is used to create a phase shift between the two windings. This improves the starting torque and allows the motor to start without external help.
What is the main advantage of using squirrel-cage rotors in induction motors?
A Low maintenance and rugged construction
B High starting torque
C Increased speed control
D Improved power factor
Squirrel-cage rotors are known for their simplicity and rugged construction. They require minimal maintenance, making them ideal for general-purpose applications where low maintenance and high reliability are required.
What happens when the excitation of a synchronous motor is reduced?
A The power factor becomes lagging
B The motor becomes more efficient
C The motor loses synchronism
D The motor produces more torque
Reducing the excitation in a synchronous motor causes the motor to draw more reactive power from the supply, resulting in a lagging power factor. This can also affect the motor’s voltage regulation and operational stability.
What is the effect of increasing the rotor resistance in a wound-rotor induction motor?
A The motor operates more efficiently at high speeds
B The starting torque increases
C The motor’s power factor improves
D The motor operates at synchronous speed
Increasing the rotor resistance in a wound-rotor induction motor improves the starting torque by limiting the initial current. This allows the motor to start under high load conditions, although it may reduce efficiency under normal operating conditions.
What is the effect of increasing the number of poles in a synchronous motor?
A The synchronous speed decreases
B The motor torque increases
C The power factor improves
D The rotor speed increases
Increasing the number of poles in a synchronous motor decreases the synchronous speed because synchronous speed is inversely proportional to the number of poles. More poles result in a lower speed, which is useful for applications requiring lower speeds and higher torque.
What is the primary role of insulation materials in electric motors?
A To prevent electrical breakdown and improve durability
B To reduce mechanical wear
C To reduce the motor’s power factor
D To improve the motor’s cooling efficiency
Insulation materials are used to prevent electrical breakdown by providing a barrier against high voltages. These materials protect the motor windings from short circuits and help improve the motor’s longevity and safe operation.
What is the primary reason for using an autotransformer starter for large induction motors?
A To reduce the inrush current during startup
B To increase the starting torque
C To reduce the motor speed
D To increase the efficiency at full load
An autotransformer starter is used to reduce the inrush current during the startup of large induction motors. By applying a reduced voltage during startup, it helps prevent damage to the motor and electrical components from high starting currents.