41. What is meant by infinite bus-bars?
The source or supply lines with non-variable voltage and frequency are called infinite bus-bars. The source lines are said to have zero source impedance and infinite rotational inertia.
42. How does increase in excitation of the Alternator connected to infinite bus-bars affect this operation?
Increase in excitation level of the synchronous generator will effectively increase the reactive component of the current supplied by the generator and hence the active power delivered.
43. In what respect does a i-phase Induction motor differ from a 3-phase Induction motor?
Construction wise a plain i-phase Induction motor is more or less similar to a
3-phase squirrel-cage Induction motor except that its stator is provided with only i-phase winding.
44. What are the inherent characteristics of plain i-phase Induction motor ?
A plain i-phase Induction motor is not used in practice due to the following inherent characteristics
• A plain i-phase Induction motor does not have any starting torque
• However, if the rotor is initially given a starting torque, by some means, the motor can pick up its speed in a direction at which the initial torque is given and deliver the required output.
45. Name the two different theories with which principle of i-phase induction motors are explained.
The two different theories are
• Double revolving field theory
• Cross field theory
46. State double revolving field theory.
Double revolving theory, formulated by Ferrari, states that a single pulsating magnetic field m as its maximum value can be resolved into two rotating magnetic fields of ( m/2) as their magnitude rotating in opposite direction at synchronous speed proportional to the frequency of the pulsating field.
47. Name any four types of i-phase induction motors.
Based on the method of starting arrangement provided, the i-phase Induction motors are classified as follows
(i) Split-phase motor
(ii) Capacitor start motor
(iii) Capacitor start and run motor
(iv) Shaded pole motor
(v) Repulsion start Induction run motor
48. Why are centrifugal switches provided on many i-phase Induction motors?
Centrifugal switches are provided on many i-phase Induction motors to disconnect the starting / auxiliary winding from the supply when the motor reaches about 70% of its synchronous speed.
49. How is the direction of a capacitor start Induction motor be reversed?
The direction of rotation can be reversed by interchanging the terminals of either the main winding or the starting winding.
50. State the principle of 3 phase IM?
While starting, rotor conductors are stationary and they cut the revolving magnetic field and so an emf is induced in them by electromagnetic induction. This induced emf produces a current if the circuit is closed. This current opposes the cause by Lenz’s law and hence the rotor starts revolving in the same direction as that of the magnetic field.
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