Smith Albert Edu world : May 2016

Monday, May 23, 2016

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.

(These questions are taken from AMIE exams.)

Tuesday, May 17, 2016

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

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.
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.
The terminal voltage and its frequency is held constant independent of load drawn from the bus. then the bus is called infinite bus.
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.
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.
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.
By adding external resistance to the rotor circuit, the starting torque of the motor is increased as a result of improved power factor.
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.
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.
Star-delta starter is not suitable for voltage exceeding 3000 V because of the excessive number of stator turns needed for delta connection.

(Author is associated with www.amiestudycircle.com)

Friday, May 13, 2016

Soil Mechanics - Permeability of Soil

Q.1. In terms of permeability of soil, following statement is true

(a) clays>silts>sands

(b) silts>sands>clays

(d) none of these

Q.2. Match List-I with List-II and select correct answer by using the codes given below the lists.

List-I

A. Gravels

B. Sand

C. Silt

D. Clays

List-II

1. 10^-7 cm/sec.

2. 10^-6 cm/sec.

3. 10^-1 cm/sec.

4. 10 cm/sec.

(a) A1,B2,C3,D4

(b) A3,B4,C1,D2

(d) A2,B4,C3,D1 (IES 1992)

Q.3. Constant head permeameter is used to test permeability of

(a) silt

(b) clay

(d) fine sand

Q.4. Falling head permeameter is used to test permeability of

(a) silt

(b) clay

(d) all these

Q.5. For practically impervious type of soil, the coefficient of permeability is determined using

(a) variable head test

(b) constant head test

(d) pumping test (IES 1993)

Q.6. Using falling head permeameter, value of permeability(k) is determined from the formula

(a) $\frac{{a.L}}{{A({t_2} - {t_1})}}.{\log _e}\frac{{{h_2}}}{{{h_1}}}$

(b) $\frac{{a.L}}{{A({t_2} - {t_1})}}.{\log _e}\frac{{{h_1}}}{{{h_2}}}$

(d) $\frac{{a.L}}{{A({t_2} - {t_1})}}.{\log _{10}}\frac{{{h_2}}}{{{h_1}}}$

Where t₂ - t₁ is the time taken for the head to fall from h₁ to h₂, A is cross-sectional area of sample, a is cross-sectional area of the standpipe and L is height of sample.

Q.7. If during a permeability test on a soil sample with a falling head permeameter, equal time intervals are noted for drop of head from h₁ to h₂ and again from h₂ to h₃, then which one of the following relations would hold good?

(a) h₁² = h₂h₃

(b) h₂² = h₁h₃

(d) none of these (IES 1995)

Q.8. In an unconfined flow pumping test, acquifer is

(a) underlain by permeable stratum

(b) underlain by impermeable stratum

(d) none of these

Q.9. In a confined flow pumping test, acquifer is

(a) underlain by impermeable stratum

(b) underlain by permeable stratum

(d) contained between impermeable strata, above and below

Q.10. In an unconfined flow pumping set, pumping generates a radial flow

(a) towards the filter well

(b) beyond filter well

(d) towards impermeable stratum

Answers:

1. c

2. c

3. c

4. d

5. d

6. b

7. b

8. b

9. d

10. a