What does the efficiency of an electrical machine depend on?
A Power losses
B Torque
C Voltage
D Speed
Efficiency in an electrical machine is determined by the ratio of mechanical output power to the electrical input power. It is primarily affected by power losses, which include core losses, copper losses, and mechanical losses.
What is the effect of increasing load on the speed of a DC motor?
A Speed fluctuates
B Speed increases
C Speed remains constant
D Speed decreases
As the load on a DC motor increases, the speed decreases. This is because more current is required to maintain the torque, which leads to a drop in the back EMF, thereby reducing the speed of the motor.
In which type of motor is torque directly proportional to the square of the current?
A Stepper motor
B DC motor
C Induction motor
D Synchronous motor
In an induction motor, torque is proportional to the square of the current. As the load increases, the current increases, which results in a greater torque. This is a key characteristic of induction motor behavior.
What type of loss occurs due to the resistance of the windings in an electrical machine?
A Copper loss
B Mechanical loss
C Eddy current loss
D Core loss
Copper loss occurs because of the resistance in the windings of an electrical machine. This loss results in heat generation and reduced efficiency, especially in the armature and field windings where current flows.
How does the torque in a synchronous motor behave under varying load?
A Fluctuates unpredictably
B Remains constant
C Increases linearly
D Decreases linearly
In a synchronous motor, the torque remains constant regardless of load. This is because the motor operates at synchronous speed, and the torque is determined by the excitation and the load, but it does not vary with load changes.
What is the purpose of using a cooling system in electrical machines?
A To reduce vibration
B To maintain insulation
C To increase efficiency
D To prevent overheating
Cooling systems in electrical machines are essential to prevent overheating, which can degrade insulation and cause damage. Cooling methods like air, water, and oil cooling help maintain the operating temperature within safe limits, ensuring optimal performance.
What is the primary cause of core losses in electrical machines?
A Friction
B Hysteresis
C Eddy currents
D Copper resistance
Core losses are caused by eddy currents that form in the magnetic core of electrical machines. These currents create heat and reduce efficiency. Laminating the core material helps minimize these losses by restricting the path of the eddy currents.
How does the speed of a synchronous motor relate to the supply frequency?
A Inversely proportional
B Directly proportional
C Independent of frequency
D Constant at all frequencies
The speed of a synchronous motor is directly proportional to the supply frequency. The relationship is defined by the formula: speed = 120 × frequency / number of poles. As the frequency increases, the synchronous speed also increases.
What is the role of the stator in an induction motor?
A Control speed
B Reverse current
C Produce magnetic field
D Generate mechanical power
The stator in an induction motor produces a rotating magnetic field when a three-phase current passes through its windings. This field induces current in the rotor, causing it to rotate and produce mechanical power to drive the load.
What does “slip” in an induction motor refer to?
A Difference in speed
B Voltage drop
C Frequency variation
D Difference between rotor and stator speed
Slip in an induction motor refers to the difference between the synchronous speed of the rotating magnetic field and the actual speed of the rotor. Slip is necessary for torque production and is proportional to the load on the motor.
What is the function of the armature in a DC motor?
A Produce magnetic field
B Control speed
C Convert electrical power to mechanical power
D Generate torque
The armature in a DC motor is responsible for converting electrical power into mechanical power. As current flows through the armature windings, it interacts with the magnetic field produced by the field windings, generating a mechanical force (torque).
In a synchronous motor, what happens if the load increases beyond a certain point?
A The motor stalls
B The motor accelerates
C The motor maintains constant speed
D The motor slows down
A synchronous motor maintains a constant speed, even when the load increases. The speed is determined by the supply frequency and the number of poles, and the motor adjusts the torque to handle the load without changing speed.
What is the typical starting method for a squirrel-cage induction motor?
A Slip-ring starting
B Direct-on-line starting
C Star-delta starting
D Autotransformer starting
The direct-on-line (DOL) starting method is commonly used for squirrel-cage induction motors. It provides the motor with full voltage at startup, making it suitable for small to medium-sized motors where a high starting torque is not required.
What happens to the torque in a motor if the armature resistance increases?
A Torque remains unchanged
B Torque increases
C Torque decreases
D Torque fluctuates
As the armature resistance increases in a motor, it reduces the current flowing through the armature for a given voltage, which results in a decrease in the torque produced. Lower resistance improves motor performance and efficiency.
What is the main disadvantage of a DC motor compared to an induction motor?
A Requires frequent maintenance
B More torque
C Higher cost
D Higher efficiency
DC motors require more frequent maintenance than induction motors due to the presence of brushes and a commutator, which wear out over time. In contrast, induction motors have no brushes and are simpler and more durable.