Electrical Machines - Mixed Bag

Answer the following in brief:
1. Why is in a normal synchronous machine Xq smaller than Xd?
2. Resistance of dampers for synchronous generators is lower than that of dampers for synchronous motors. Why?
3. What is meant by an infinite bus?
4. Distribution transformers are designed for lower iron losses. Why?
5. What are the functions of transformer oil?
6. Do we use laminations for all iron parts of an electrical machine. If not, why?
7. Explain why in an induction motor a high value of rotor resistance is preferred at starting?
8. Why is an ordinary induction motor not started at normal voltage?
9. Why is the rotor core loss almost absent in normal operating condition of an induction motor?
10. Why is star-delta starter not suitable for high voltage induction motor?

Answers

  1. The quadrature axis reactance in a salient pole machine is smallest than the direct axis reactance, because a given current component in that axis gives rise to smaller flux, the reluctance of the magnetic path including the interpolar spaces.
  2. In synchronous generators, damper winding is provided to suppress the negative sequence field and to damp the oscillations when the machine starts hunting, hence, damper winding has lowest value. In synchronous motors the function of damper winding is to provide starting torque and to develop damping power when the machine starts hunting hence, it is designed to have high resistance.
  3. The terminal voltage and its frequency is held constant independent of load drawn from the bus. then the bus is called infinite bus.
  4. Distribution transformer are kept on for 24 hours even if load is there or not. hence continuously iron losses takes place, therefore these are designed for low value of iron losses.
  5. Functions of transformer oil are (a) To create insulation to insulated conductors and coils. (b) To provide cooling medium to dissipate or carryover heat to the tank walls.
  6. We make use of laminations for those iron pans only in which fluctuations of flux takes place. For all those parts, where, there are no fluctuations, no eddy currents are included, hence laminations arc not provided.
  7. By adding external resistance to the rotor circuit, the starting torque of the motor is increased as a result of improved power factor.
  8. An ordinary motor if. started at normal voltage, will draw very high starting current, which may be 5 to 7 times the full load current and hence motor may be damaged.
  9. In normal operating conditions of an induction motor, frequency of induced rotor emf and hence, rotor current is small. This will have very small value of iron losses because hysteresis and copper losses are both dependent on frequency.
  10. Star-delta starter is not suitable for voltage exceeding 3000 V because of the excessive number of stator turns needed for delta connection.
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