DC NETWORK THEOREM - ELECTRICAL ENGINEERING BASIC

Current: “Rate of flow of electric charge.”

time

Charge

I = Coulombs/Sec or Ampere

Note:-

1. Direction of current is same as the direction of motion of +Ve charge or opposite to the direction of

motion of –Ve charge.

Voltage: “Energy required in transferring a charge of one coulomb from one point to

another point.”

Charge(Q)

Energy(W)

V = Joule/Coulomb or Volts

EMF (Electromotive force): “The EMF of a voltage source is the energy imparted by

the source to each coulomb of the charge passing through it.”

Charge(Q)

Energy(W)

E = Joule/Coulomb or Volts

Potential Difference: “The pd between two points is the energy required in transferring a

charge of coulomb from one point to another point.”

Charge(Q)

Energy(W)

pd = Joule/Coulomb or Volts

Voltage drop: “The voltage drop between two points is the decrease in energy required

in transferring a charge of coulomb from one point to another point.”

Charge(Q)

Energy(W)

Voltage drop = Joule/Coulomb or Volts

Resistance: “Electric resistance is the property of material which offers opposition to the

flow of current and dissipates energy.”

a

l

R = ñ Ohm or _ (Law of resistance)

Where l = Length of the wire

a = cross − sectional area of the wire

ñ = Resistivity or Specific Resistance of the material

Note:-

1. Resistance also depends on temperature.

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MACHINE DESIGNING - QUESTIONS FOR PRACTICES

1.      Obtain expression for output equation of a rotating machine. Apply this for a synchronous, induction and dc machine.

2.      Discuss factors affecting size of rotating machine and how the separation of main dimensions is done.

3.      Explain SCR in synchronous machines.

4.      Find the main dimensions of a 20 MVA, 11kV, 50Hz, 150r.p.m., 3-phase water wheel generator. The average gap density is 0.6 Wb /m² and ampere conductors per meter are 35,000. The peripheral speed should not exceed 65 m/s at normal running speed in order to limit the run-away speed.

5.      Give procedure for designing the field system of a three-phase turbo generator from the given design data. Also indicate how much excitation power may be required for such a generator of about 100 MW capacity.

6.      A 1500 kVA, 3-phase, star-connected, 3300 V, 250 rpm water wheel generator has the following design data:

Effective gap length                           = 7mm

Effective gap area/pole                       = 0.075 m²

Winding factor                                    = 0.955

Field mmf per pole under  rated condition                 = 7850 A

No. of field turns/pole                                    = 180

The peak value of fundamental flux density distribution B…= 0.94 T and peak value of the actual flux density distribution B…=0.9T.

The permissible value of current density in field winding is 3 A/mm². Determine:

(i)     Turns  / phase

(ii)   MMF for air gap

(iii) Armature MMF /pole

(iv) Field current

(v)   Sectional area of field conductor.  

7.      Design the suitable values of diameter and length of a 75 MVA, 11 KV, 50 Hz, 3000 rpm, 3-phase star connected alternator. Also determine the value of flux, conductor per slot, no of turns per phase, and size of armature conductor. Given

Average gap density               = 0.6    T

Amp conductor per m             = 50,000

Peripheral speed                      = 180 m/s

Current density                       = 6 A/m²

8.      Calculate the diameter, core length, number of conductors of the stator, size of conductor, number of stator slots of a 30 MVA, 11KV, 3000 r.p.m. 50Hz star connected turbo alternator. Assume suitable data:

Bav =0.55 Wb/m²,         ac =55000 A/m,          Kw = 0.955,

Peripheral velocity = 160 m/s.

9.      If two synchronous machines running at the same speed and having the same number of poles, the physical dimensions are in the ratio 3 : 2. Compare the outputs, armature copper losses and iron losses in the two machines. Assume specific magnetic loading and current density to be same for both the machines. 

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MECHANISM OF BREAK DOWN OF GASES

At normal temperature and pressure, the gases are excellent insulators. The current conduction is of the order of 10–10 A/cm2. This current conduction results from the ionisation of air by the cosmic radiation and the radioactive substances present in the atmosphere and the earth. At higher fields, charged particles may gain sufficient energy between collision to cause ionisation on impact with neutral molecules.
It is known that during an elastic collision, an electron loses little energy and rapidly builds up its
kinetic energy which is supplied by an external electric field. On the other hand, during elastic collision, a large part of the kinetic energy is transformed into potential energy by ionising the molecule struck by the electron. Ionisation by electron impact under strong electric field is the most important process leading to break-down of gases.

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