High School

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C23. The field-weakening with permanent magnet DC machines would:

(a) Increase the speed beyond rated at full armature voltage
(b) Decrease the speed
(c) Increase mechanical power developed
(d) Decrease the torque
(e) Neither of the above

C24. The rotor of a conventional 3-phase induction motor rotates:

(a) Faster than the stator magnetic field
(b) Slower than the stator magnetic field
(c) At the same speed as the stator magnetic field
(d) At about 80% speed of the stator magnetic field
(e) Both (b) and (d) are true

C25. Capacitors are often connected in parallel with a 3-phase cage induction generator for fixed-speed wind turbines in order to:

(a) Consume reactive power
(b) Improve power factor
(c) Increase transmission efficiency
(d) Improve power quality
(e) Both (b) and (c) are correct answers

C26. A cage induction machine itself:

(a) Always absorbs reactive power
(b) Supplies reactive power if over-excited
(c) Neither consumes nor supplies reactive power
(d) May provide reactive power under certain conditions
(e) Neither of the above

C27. The ratio of the rotor copper losses and mechanical power of a 3-phase induction machine having a slip \(s\) is:

(a) \((1-s): s\)
(b) \(s: (1-s)\)
(c) \((1+s): (1-s)\)
(d) Not slip dependent
(e) 2:1

Answer :

C23. (a) Increase the speed beyond rated at full armature voltage

C24. (c) At the same speed as the stator magnetic field.

C25. (b) Improve power factor

C26. (c) Neither consumes nor supplies reactive power

C27. (a) (1-5): s

C23. In field-weakening with permanent magnet DC machines, increasing the armature voltage beyond the rated value allows the machine to operate at a higher speed than its rated speed. This is achieved by weakening the magnetic field produced by the permanent magnets, enabling the rotor to spin faster. Learn more about field-weakening in permanent magnet DC machines to increase speed beyond rated at full armature voltage.

C24. The rotor of a conventional 3-phase induction motor rotates at the same speed as the stator magnetic field. The rotating magnetic field produced by the stator induces currents in the rotor, creating a torque that drives the rotor to rotate. The rotor speed matches the speed of the rotating magnetic field, ensuring efficient operation of the induction motor.The speed relationship between the rotor and stator magnetic field in a 3-phase induction motor.

C25. Capacitors are connected in parallel with a 3-phase cage induction generator for fixed-speed wind turbines to improve power factor. The reactive power generated by the induction generator is compensated by the capacitors, leading to a higher power factor. This helps in reducing the amount of reactive power supplied by the generator, improving the overall efficiency of the system. Learn more about the role of capacitors in improving power factor in 3-phase cage induction generators for fixed-speed wind turbines.

C26. A cage induction machine neither consumes nor supplies reactive power under normal operating conditions. The machine's operation is primarily focused on converting electrical power into mechanical power. Reactive power consumption or supply depends on the machine's load and operating conditions. Learn more about the reactive power behavior of cage induction machines.

C27. The ratio of the rotor copper losses to the mechanical power of a 3-phase induction machine is approximately given by (1-5):s, where 's' represents the slip of the induction machine. This ratio indicates the proportion of copper losses in the rotor compared to the mechanical power output of the machine. As the slip increases, the rotor copper losses become a larger fraction of the mechanical power. Learn more about the relationship between rotor copper losses and mechanical power in a 3-phase induction machine with slip.

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