Electrical Machines - Mixed Bag

Answer the following in brief:
1. Why do round rotor synchronous generators have small diameters and large axial length of core?
2. Why does open circuit synchronous machine have curved shape?
3. Do slot harmonics affect the torque-speed curve of a three-phase induction motor?
4. Why are brushes of a d.c. machine made of carbon?
5. Why are interpoles used in a d.c. machines?
6. What is the all-day efficiency of a transformer?
7. Why are core-type transformer used in high-voltage circuits?
8. How are iron losses in a transformer made very small?

Answers
1. Synchronous generators of round rotor types are designed mostly of 2 pole type running al 3000 rpm. They are designed for lower values of diameter in order to limit the centrifugal forces which have a profound influence on the design, to get the designed value of {D^2}L for a given output. the length of core is therefore required to be kept large.

2. The o.c.c. of a synchronous machine have a curved shaped because of saturation of magnetic core. At low values If when iron is unsaturated. the o.c.c. is almost linear but high values of If magnetic core gels saturated, hence the o.c.c. becomes curved.

3. A three phase winding carrying a sinusoidal currents produces harmonics of the order of n = 6N  \pm  1, N is an integer. The movement of the harmonics is with or against the direction of the rotation depending upon the sign. These harmonics produce torque of the same, general torque slip shape as that of fundamental but with synchronous speed corresponding to that harmonics. Thus, the torque slip characteristics will get affected.

4. The function of brushes is to collect current from commutator.

5. Inter poles are used in d.c. machines to balance or to nullify the effect of cross-magneting component of armature reaction field.

6. All  Day Efficiency = output in kWh in 24 hours/input in kWh in 24 hours

7. High voltage transformers are designed with core type construction because this provides easy maintenance and tappings can be provided more conveniently.

8. Transformers are having been made of core of superior quality and thin laminations, has lesser eddy current losses and having selecting a soft magnetic material will have low hysteresis loss and hence iron losses can be made small.

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