SESSION:19

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1)       What is basic principle of operation of alternators/DC generators?

They are working on the fundamental principle based on Faraday's Laws of Electromagnetic Induction.
This law states that, when a current carrying conductor moves in magnetic field, it induces an EMF.

2)       How to minimize the eddy current losses?

When the core is laminated and insulated from each other with paper or varnish the eddy current loss is minimized.

3)       Define Pole Pitch.

The distance between the centres of two adjacent poles is called pole pitch. One pole pitch is equals to 180 electrical degrees. It is also defined as the number of slots per pole.

4)       Define coil span.

The distance between the two coil sides of a coil is called as coil span. It may be expressed in electrical degrees or in number of slots.

5)       What is meant by full pitched winding?

If the coil span is equal to pole pitch, the winding is called as full pitched winding.

6)       What is meant by short pitched winding?

If the coil span is less than the pole pitch, the winding is called as shot-pitched winding. It is also known as short-chorded winding.

7)       What are the advantages and disadvantages of short-pitched winding?

The advantages are

·         They save copper for end connections

·         They improve the waveform of the generated EMF.

·         The generated EMF can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced.

The disadvantages are

·         The total voltage around the coils is somewhat reduced. Because the voltage induced in the two coil sides are slightly out of phase.

8)       What is superposition theorem?

Superposition Theorem:

In any network made up of linear resistances and containing more than one source of EMF, the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that branch if each source was considered separately, all other sources being replaced at that time by their respective internal resistances.

9)       Explain Maximum Power Transfer Theorem. What should be the value of R_Thevenin in the figure for Maximum Power Transfer?

The power transferred from a supply source to a load is at its maximum when the resistance of the load is equal to the internal resistance of the source.

   According to Maximum Power Transfer Theorem, R_Thevenin = R_Load

  So, R_Thevenin = 0.8 Ohm

10)    Explain Nortan Theorem.

The current that flows in any branch of a network is the same as that which would flow in the branch if it were connected across a source of electrical energy, the short-circuit current of which is equal to the current that would flow in a short-circuit across the branch, and the internal resistance of which is equal to the resistance which appears across the open-circuited branch terminals.

11)    Tell me something about Kirchhoff’s Current and Voltage Law.

Current Law:

 At any junction in an electric circuit the total current flowing towards that junction is equal to the total current flowing away from the junction. In other words there is no accumulation of charge at any point in the circuit.

Voltage Law:

 In any closed loop in a network, the algebraic sum of the voltage drops (i.e. products of current and resistance) taken around the loop is equal to zero.

 

12)    Define Power and Energy?

The rate at which work is done in an electric circuit is known as Electric power.
Power = Voltage X Current

The basic unit for electric power is Watts.

The instrument used to measure the power is known as watt meter.

The total amount of work done in an electric circuit is called as electric energy. 

Energy = Power X Time

The basic unit is Joule or watt-sec.

The practical unit is Kilo watt-hour

The instrument used to measure the energy is known as Energy meter.

13)    What is meant by cycle in ac waveform? What is period? Define frequency?

One complete set of change in value is known as waveform.
If you any basic question on AC waveform, then first draw an ac waveform and explain with reference to the waveform.

14)    Explain about various AC Values?

Instantaneous value:

Instantaneous values are the values of the alternating quantities at any instant of time. 

Peak value:

The largest value reached in a half cycle is called the peak value or the maximum value or the amplitude of the waveform. Such values are represented by Vm, Im etc.

Peak-to-peak value is the difference between the maximum and minimum values in a cycle.

Average value:

The average or mean value of a symmetrical alternating quantity (such as a sine wave), is the average value measured over a half cycle (since over a complete cycle the average value is zero). For a sine wave, average value = 0.637 times maximum value 

RMS Value: 

The effective value of an alternating current is that current which will produce the same heating effect as an equivalent direct current. The effective value is called the root mean square (rms) value and whenever an alternating quantity is given, it is assumed to be the rms value.

15)    What is form factor and peak factor?

The ratio of rms value to average value of an ac quantity is called form factor.

Form Factor = RMS value/ Average value

The ratio of maximum value to the RMS value of an ac quantity is called as peak factor.

Peak Factor = Maximum Value/RMS value