What is the primary purpose of a stator in a synchronous motor?
A Supply current
B Generate flux
C Rotate with load
D Control speed
The stator in a synchronous motor is responsible for generating the rotating magnetic field, which interacts with the rotor’s magnetic field to maintain synchronization and keep the motor running at a constant speed.
Which rotor type is used in most industrial induction motors?
A Squirrel cage rotor
B Slip ring rotor
C Wound rotor
D Permanent magnet rotor
Squirrel cage rotors are commonly used in industrial induction motors due to their simplicity, reliability, and low maintenance. They provide a robust design and are ideal for general purpose applications.
How is torque produced in an induction motor?
A Through rotor resistance
B By rotor speed
C Through magnetic attraction
D By electromagnetic induction
Torque in an induction motor is produced due to electromagnetic induction. The rotating magnetic field in the stator induces a current in the rotor, creating a magnetic field that interacts with the stator field to produce torque.
What happens if the excitation of a synchronous motor is too high?
A Decreased torque
B Increased speed
C Loss of synchronization
D Increased efficiency
If the excitation of a synchronous motor is too high, the motor may lose synchronization, causing it to fall out of sync with the stator’s rotating magnetic field. This loss of synchronization affects its stability and performance.
What is the typical use of a wound rotor in an induction motor?
A To start the motor
B To produce high torque
C To provide cooling
D To regulate speed
The wound rotor in an induction motor is often used to provide high starting torque. By connecting external resistances to the rotor circuit, the starting current is reduced, improving the motor’s ability to start under load.
What defines the synchronous speed of a motor?
A The speed at zero load
B The speed of the magnetic field
C The speed of the rotor
D The speed at full load
The synchronous speed of a motor is defined as the speed at which the magnetic field generated by the stator rotates. It is determined by the supply frequency and the number of poles in the motor.
Which starting method is used for reducing the starting current in an induction motor?
A Rotor resistance
B Capacitor start
C Star delta
D Direct on line
The star delta starting method is used to reduce the starting current in an induction motor. Initially, the motor is connected in a star configuration, reducing the voltage and current, and then it switches to the delta configuration for normal operation.
What is the impact of increased slip in an induction motor?
A No impact on torque
B Increases efficiency
C Increases torque
D Decreases torque
Increased slip in an induction motor means that the rotor is moving slower compared to the rotating magnetic field. This allows the motor to generate more torque to handle heavier loads, although it also increases power loss.
Which motor type is commonly used in applications where precise speed control is not required?
A Universal motor
B Synchronous motor
C Induction motor
D Stepper motor
Induction motors are commonly used in applications where precise speed control is not critical. They are simple, cost effective, and capable of handling varying loads efficiently, making them suitable for many industrial applications.
What is the primary function of the slip rings in a wound rotor motor?
A Supply power to the rotor
B Provide external resistance
C Increase efficiency
D Generate flux
The slip rings in a wound rotor motor provide a continuous connection to external resistances, which are used during the motor’s startup. These resistances help control the starting torque and reduce the inrush current.
What type of power supply is commonly used to run induction motors?
A Three phase AC
B DC power
C AC power
D Single phase DC
Induction motors are most commonly powered by three phase AC supply. This provides the necessary rotating magnetic field to the stator, allowing the motor to operate efficiently with balanced power distribution.
How is the efficiency of an induction motor affected by core losses?
A Enhances torque
B Increases efficiency
C No effect on efficiency
D Decreases efficiency
Core losses, which consist of eddy currents and hysteresis losses, cause heat generation in the motor’s core and reduce its overall efficiency. These losses consume energy that could otherwise be used for useful work.
What causes a synchronous motor to operate at a constant speed?
A Rotor speed variation
B Load on the motor
C Magnetic field interaction
D Rotor resistance
A synchronous motor operates at a constant speed because the rotor is magnetically locked to the rotating magnetic field of the stator. The rotor speed is synchronized with the stator’s field, ensuring constant speed under varying loads.
What is the main drawback of operating a synchronous motor at low excitation?
A Loss of synchronism
B Lower voltage
C Increased efficiency
D Higher torque
If the excitation in a synchronous motor is too low, the rotor will fail to maintain synchronism with the stator’s rotating magnetic field, causing the motor to lose its constant speed and become unstable.
What happens if the load on an induction motor is reduced?
A The rotor speed increases
B The slip increases
C The torque remains the same
D The rotor speed decreases
When the load on an induction motor is reduced, the slip decreases, and the rotor speed increases towards synchronous speed. This is because less torque is needed to maintain the motor’s operation under lower load conditions.