What is the main advantage of using a squirrel-cage rotor in an induction motor?
A Simple and rugged design
B High starting torque
C Requires external resistance for speed control
D Better efficiency at high speeds
The squirrel-cage rotor is known for its simple and rugged construction. It consists of conductive bars connected by end rings, which makes it resistant to mechanical damage and reliable under harsh operating conditions. This design does not require external excitation or additional components like brushes or slip rings, making it more cost-effective and low-maintenance.
Which method is commonly used for starting high-power induction motors?
A Star-delta starting
B Capacitor start
C Direct-on-line (DOL) starting
D Autotransformer starting
For high-power induction motors, autotransformer starting is commonly used. It reduces the starting current by using a portion of the supply voltage during startup. This minimizes stress on the motor and electrical supply system, particularly for large motors that draw significant inrush current during startup.
In a synchronous motor, what determines the speed of operation?
A The load on the motor
B The supply frequency and the number of poles
C The rotor resistance
D The rotor’s magnetic field strength
The speed of a synchronous motor is determined by the supply frequency and the number of poles in the motor. The formula for synchronous speed is \( N_s = \frac{120 \times f}{P} \), where \(N_s\) is the synchronous speed in RPM, \(f\) is the supply frequency, and \(P\) is the number of poles. The motor operates at this fixed speed, independent of load.
What is the primary effect of increasing the number of poles in an induction motor?
A Decreases the motor’s speed
B Increases the motor’s speed
C Decreases the starting current
D Increases the power factor
Increasing the number of poles in an induction motor decreases the synchronous speed. The synchronous speed is inversely proportional to the number of poles, so more poles lead to a lower speed, which is useful in applications where slower speeds are required, such as in fans and pumps.
What is the primary function of the excitation system in a synchronous motor?
A To produce mechanical power
B To provide field current for the rotor
C To create the rotating magnetic field
D To regulate the motor’s speed
The excitation system in a synchronous motor provides the necessary current to the rotor’s windings, creating a magnetic field that interacts with the stator’s rotating magnetic field. This interaction allows the motor to operate at synchronous speed.
Which of the following motors operates on the principle of electromagnetic induction?
A DC motor
B Induction motor
C Synchronous motor
D Permanent magnet motor
Induction motors operate on the principle of electromagnetic induction, where the rotor current is induced by the rotating magnetic field produced by the stator. This current creates its own magnetic field that interacts with the stator field, causing the rotor to rotate.
What is the primary purpose of using laminated cores in the construction of induction motors?
A To increase motor speed
B To reduce core losses (eddy currents)
C To make the motor lighter
D To improve power factor
Laminated cores in induction motors are used to reduce core losses, especially eddy current losses. By using thin layers of material, the formation of circulating currents (eddy currents) is minimized, which helps to improve motor efficiency and reduce heat generation.
In an induction motor, what happens to the torque as the slip increases?
A The torque decreases
B The torque remains constant
C The torque increases
D The torque fluctuates
As the slip increases in an induction motor, the relative motion between the rotor and the magnetic field increases, which induces a larger current in the rotor and results in an increase in torque. However, excessive slip can lead to inefficiency and motor overheating.
What is the effect of adding external resistance to the rotor circuit of a wound-rotor induction motor?
A Increases the motor’s efficiency
B Decreases the starting current
C Reduces the motor’s speed
D Increases the torque at startup
Adding external resistance to the rotor circuit of a wound-rotor induction motor increases the starting torque by reducing the current in the rotor during startup. The added resistance helps to limit the initial current, preventing damage to the motor and electrical system.
What is the key advantage of a wound rotor compared to a squirrel-cage rotor in an induction motor?
A Higher efficiency
B Reduced noise and vibration
C Better speed control
D More compact design
A wound rotor offers better speed control because it allows external resistances to be added to the rotor circuit, enabling control over the motor’s torque and speed. This is particularly useful in applications that require variable-speed operation.
What is the purpose of using a capacitor in a single-phase induction motor?
A To improve the starting torque
B To reduce the core losses
C To decrease the power factor
D To increase the speed of the motor
A capacitor is used in single-phase induction motors to create a phase difference between the current in the two windings, providing the necessary starting torque. Without the capacitor, the motor would not be able to start on its own.
What happens when the rotor of an induction motor reaches synchronous speed?
A The motor continues to run
B The motor stops
C The rotor current becomes zero
D The motor loses torque
When the rotor of an induction motor reaches synchronous speed, the slip becomes zero, meaning there is no relative motion between the stator’s rotating magnetic field and the rotor. As a result, no current is induced in the rotor, and no torque is produced. The motor stops producing power and cannot operate at synchronous speed.
What is the primary characteristic of a synchronous motor that distinguishes it from an induction motor?
A It has a constant speed
B It operates on direct current
C It requires brushes and slip rings
D It produces its own magnetic field
The key characteristic of a synchronous motor is that it operates at a constant speed, determined by the supply frequency and the number of poles. Unlike induction motors, which vary their speed depending on load, synchronous motors maintain a fixed speed under all conditions.
What happens to the power factor of a synchronous motor when it is under-excited?
A The power factor becomes leading
B The power factor becomes lagging
C The power factor is unity
D The power factor becomes zero
When a synchronous motor is under-excited, the field current is too low, and the motor draws reactive power from the supply. This results in a lagging power factor, as the current lags the voltage in an inductive load.
What is the primary purpose of the air gap in an induction motor?
A To allow for cooling of the rotor
B To reduce friction between the rotor and stator
C To provide insulation and allow magnetic flux transfer
D To prevent the rotor from touching the stator
The air gap between the stator and rotor provides electrical insulation and allows the magnetic flux generated by the stator to pass through to the rotor. This interaction creates the induced current in the rotor, enabling the motor to function.
What type of motor is typically used in high-precision speed control applications?
A Synchronous motor
B Induction motor
C Universal motor
D Permanent magnet motor
Synchronous motors are often used in high-precision speed control applications because they operate at a constant speed, independent of the load. This makes them ideal for applications such as clocks, timers, and other equipment requiring precise and stable speed.
What is the effect of harmonic currents on the performance of an induction motor?
A They increase efficiency
B They reduce torque and cause heating
C They improve the power factor
D They have no effect on the motor
Harmonic currents distort the current waveform and increase losses in the motor, leading to higher operating temperatures and reduced efficiency. Harmonics can also result in mechanical vibrations and reduced torque output.
In a three-phase system, what is the typical relationship between the voltage and current waveforms?
A They are in phase with each other
B The voltage leads the current by 90 degrees
C The voltage lags the current by 90 degrees
D The voltage and current waveforms are out of phase
In a balanced three-phase system, the voltage and current waveforms are typically in phase with each other. This balanced condition ensures efficient power transfer and operation of the motor, as the phases complement each other and provide continuous power delivery.
What is the purpose of voltage regulation in a synchronous motor?
A To maintain a constant motor speed
B To maintain a constant voltage output
C To reduce motor efficiency
D To prevent overload conditions
Voltage regulation in a synchronous motor involves adjusting the excitation system to maintain a constant output voltage under varying load conditions. This ensures that the motor operates efficiently and that the power supplied to the load remains stable.
What is the typical cooling method used in large industrial induction motors?
A Air cooling
B Water cooling
C Oil cooling
D Hybrid cooling
Large industrial induction motors typically use water cooling due to its higher heat dissipation capability compared to air cooling. Water-cooled systems are more effective at maintaining a safe operating temperature, which helps prevent overheating and extends the motor’s lifespan.