EE8301 - Electrical Machines - I - Unit-4 Two marks

Subject Name: Electrical Machines - I

Subject Code: EE8301

B.E. EEE Dept - semester 03

Anna University Regulation 2017 B.E. EEE Dept 3^rd semester EE8301(Electrical Machines - I) 2 marks for 4th unit is provided below. EE8301 all units 2 marks, Electrical Machines - I all units notes & syllabus are listed down for students to utilize it for scoring maximum marks with our study materials.

1.State the basic operation required for generator action to exist.

Answer

A generating action requires following basic components to exist, i) The conductor or a coil ii) The flux iii) The relative motion between conductor and flux.

2.State Fleming's right hand rule.

Answer

If three fingers of a right hand, namely thumb, index finger and middle finger are outstretched so that everyone of them is at right angles with the remaining two, and if in this position index finger is made to point in the direction of lines of flux, thumb in the direction of the relative motion of the conductor with respect to flux then the outstretched middle finger gives the direction of the e.m.f. induced in the conductor.

3.What is the nature of the induced e.m.f. in a d.c. generator ? How it is converted to d.c. ?

Answer

The basic nature of the induced e.m.f. in a d.c. generator is alternating (a.c.). It is converted to d.c. using a device called commutator.

4.What are the functions of yoke ? What is the choice of material for the yoke ?

Answer

1. It serves the purpose of outermost cover of the d.c. machine. So that the insulating materials get protected from harmful atmospheric elements like moisture, dust and various gases like SO₂ , acidic fumes etc. 2. It provides mechanical support to the poles. 3. It forms a part of the magnetic circuit. It provides a path of low reluctance for magnetic flux. It is prepared by using cast iron. For large machines rolled steel, cast steel, silicon steel is used which provides high permeability.

5.Why pole shoe has been given a specific shape ?

Answer

It is necessary that maximum area of the armature comes across the flux produced by the field winding. Pole shoe enlarges the area of armature core to come across the flux, which is necessary to produce larger induced e.m.f. To achieve this, pole shoe has been given a particular shape.

6.State the function of field winding in a d.c. machine.

Answer

To carry current due to which pole core, on which the field winding is placed behaves as an electromagnet, producing necessary flux.

7.What are the functions of armature core ? What is the choice of material for the armature core ?

Answer

1. Armature core provides house for armature winding i.e. armature conductors. 2. To provide a path of low reluctance to the magnetic flux produced by the field winding. As it has to provide a low reluctance path to the flux, it is made up of magnetic material like cast iron or cast steel.

8.State the functions of armature winding.

Answer

1. Generation of e.m.f. takes place in the armature winding in case of generators. 2. To carry the current supplied in case of d.c. motors. 3. To do the useful work in the external circuit.

9.State the functions of commutator.

Answer

1. To facilitate the collection of current from the armature conductors. 2. To convert internally developed alternating e.m.f. to unidirectional (d.c.) e.m.f. 3. To produce unidirectional torque in case of motors.

10.What is the function of brushes ? Why brushes are made up of soft material ?

Answer

The function of brushes is to collect current from commutator and make it available to the stationary external circuit. It connects the the staionary external circuit to rotating commutator. Brushes are stationary and resting on the surface of the commutator just making a contact with it. Thus as commutator rotates there is friction between brushes and commutator. To avoid wear and tear of commutator which is costly, the brushes are made up of soft material like carbon.

11.Which are the two types of armature windings used in d.c. machines ? Explain in brief.

Answer

The two types of armature windings used in d.c. machines are lap winding and wave winding. In lap winding, if connection is started from conductor in slot 1 then connections overlap each other as winding proceeds. It produces P number of parallel paths. In wave type of connection, winding always travels ahead avoiding overlapping. It travels like a progressive wave hence called wave winding. Irrespective of number of poles it always produces 2 number of parallel paths.

12.State the e.m.f. equation of a d.c. machine.

Answer

The e.m.f. equation of a d.c. machine is E = (ϕPNZ / 60A) where P = Number of poles, N = Speed in r.p.m., A = Number of parallel paths, Z = Number of conductors and ϕ = Flux per pole.

13.State the advantages of double layer winding.

Answer

1. It provides neat arrangement as all coils are identical. 2. Greater flexibility can be achieved with double layer winding as coil span can be easily selected.

14.Define a pole pitch.

Answer

It may be defined as the distance between the two adjacent poles i.e. the periphery of the armature divided by the number of poles. It may also be defined as the number of armature conductors or number of armature slots per pole.

15.Define back pitch and front pitch.

Answer

The distance which is measured interms of armature conductors i.e. between top and bottom coil sides of a coil measured around the back of the armature i.e. away from the commutator is called back pitch. It is defined as the distance measured between two coilsides which are connected to the same commutator segment.

16.What is armature reaction ? What are its effects ?

Answer

When the generator is loaded, the armature carries the current. The armature current sets up its own magnetic field. The effect of this armature flux on the distribution of main field flux is called armature reaction. Its two effects are, i) It reduces the generated voltage by distorting the main flux. ii) It causes sparking at the brushes. It has two effects on the main flux. One effect is to produce the flux which is in direct opposition with field flux reducing it. This is called demagnetising component. Second effect is to produce component which is at right angles to the main field flux distorting it .This is called cross magnetising component.

17.What is MNA and GNA ?

Answer

The axis along which there is no e.m.f. induced in the armature conductors is called Magnetic Neutral Axis (MNA). The Geometric Neutral Axis (GNA) is nothing but the axis of symmetry between the poles.

18.Define commutation and commutation period.

Answer

A process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the short coil remains circuited is known as commutation period.

19.What is linear, under and over commutation ?

Answer

In commutation reversal of current takes place. If current varies uniformly along straight line then the commutation is said to be linear commutation. If current takes more time than linear commutation then it is called under or delayed commutation. If reversal of current in the coil is faster than ideal or linear commutation then it is said to be over commutation or accelerated commutation.

20.State the methods of improving commutation.

Answer

These methods are, 1. Resistance commutation 2. Giving a brush shift 3. Use of interpoles. The methods 2 and 3 are the parts of E.M.F. commutation.

21.How the generators are classified based on method of excitation ?

Answer

There are two methods of excitation used for d.c. generators, 1. Separate excitation 2. Self excitation. In separately excited generator, a separate external d.c. supply is used to provide exciting current through the field winding. The d.c. generator produces d.c. voltage. If this generated voltage itself is used to excite the field winding of the same d.c. generator, it is called self excited generator. Based on how field winding is connected to the armature to derive its excitation, this type is further divided into following three types : i) Shunt generator ii) Series generator and iii) Compound generator.

22.Explain how self excited d.c. generator builds its voltage ?

Answer

Thus when the generator is started, due to the residual flux, it develops a small e.m.f. which drives a small current through the field winding. This tends to increase the flux produced. This in turn increases the induced e.m.f. This further increases the field current and the flux. The process is cumulative and continues till the generator develops rated voltage across its armature. This is voltage building process in self excited generators.

23.State the applications of various types of generators.

Answer

* Shunt Generators : Commonly used in battery charging and ordinary lighting purposes. * Series generators : Commonly used as boosters on d.c. feeders, as a constant current generators for welding generator and arc lamps. * Cumulatively compound generators : These are used for domestic lighting purposes and to transmit energy over long distance. * Differential compound generators : The use of this type of generators is very rare and it is used for special application like electric arc welding.

24.Define critcal field resistance.

Answer

The critical field resistance as that resistance of the field circuit at a given speed at which generator just excites and starts voltage building while beyond this value generator fails to excite.

25.Define critical speed.

Answer

The speed for which the given field resistance acts as critical resistance is called the critical speed.

26.Why generators are operated in parallel ?

Answer

The generators are operated in parallel to obtain, 1. Continuity and reliability of service without interruption. 2. Ease from maintenance and repair point of view. 3. Operating each generator efficiently . 4. Increase in plant capacity.

27.The series field winding has low resistance while the shunt field winding has high resistance. Why ?

Answer

The series field winding is always connected in series with the armature and hence has to carry the armature current which directly gets decided by the load. Thus the current passing through series field winding is of high level. The voltage drop across series field winding gets added to the voltage drop across armature winding while deciding the back e.m.f. This voltage drop must be very small. Hence as the current through series field winding is high, in order to keep voltage drop across it to a small value, its resistance is very low. The shunt field winding is directly connected across the rated supply voltage hence to limit current through it, resistance is very high.

28.What are the conditions for parallel operation of D.C. generators ?

Answer

i) The voltages of both the generators must be equal. ii) The polarities of the generators must be same or the connections must be interchanged till they become same. iii) The change of voltage with change of load should be of same character. iv) The prime movers driving the generators should have stable and similar rotational speed characteristics. v) The positive nad negative terminals of generators must be connected properly to the positive and negative of busbar.

29.What are the major parts of a D.C. generators ?

Answer

The major parts of d.c. machines are yoke, poles, field winding, armature, armature winding, commutator and brushes.

30.What is a commutator ?

Answer

The commutator is a device which converts alternating induced e.m.f. in a generator to d.c. e.m.f. In case of motor it produces unidirectional torque.

31.Why the air gap between the pole pieces and the armature is kept very small ?

Answer

The air gap between the pole pieces and the armature is kept very small to keep the field strength at its maximum value. This is because the flux lines can pass through iron very easily than the air gap. The small air gap generates high voltage. Larger the air gap means stronger must be the magnetising force to produce the required voltage hence the air gap is kept very small.

32.Why does curving the pole faces in a d.c. machines contributes to a smoother d.c. output voltage from it ?

Answer

For having smoother output voltage from d.c. machines its commutation must be as ideal as possible. In order to have good commutation the flux density in the air gap must decrease gradually from. maximum value under the centre of the pole to zero on the interpolar axis. The flux distribution which drops abruptly from maximum to zero leads to commutation difficulties and magnetic noise. Thus to achieve good air gap flux distribution the pole faces are curved which improves the commutation and contributes to a smoother d.c. output voltage.

33.What is meant by reactance voltage ?

Answer

An amature coil has certain self inductance. When coil undergoes commutation, there is change in current (di) in that coil and the commutation takes certain time (dt). Thus there is self induced e.m.f. in a coil undergoing commutation given by L(di/dt). This e.m.f. induced in the coil undergoing commutation is called reactance voltage.

34.Specify the role of Interpoles in DC machine ?

Answer

The interpoles are used to induce the reversing e.m.f. in the coil undergoing commutation required to neutralise the reactance voltage. The interpoles are placed in between the main poles. The interpoles make the communication sparkless and with interpoles the sparkless commutation upto 20 to 30 % overload with fixed brush position can be obtained. They also help in neutralising cross-magnetising effect of armature reaction.

35.What is meant by residual emf in DC generator ?

Answer

When supply to the field winding in generators is removed, the field current becomes zero. But still there exists some magnetic flux associated with the poles. This is called residual flux. The e.m.f. induced in the generators by cutting the residual flux by armature is called residual of e.m.f.

Important links:-

EE8301 - Electrical Machines - I - Unit-1 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-2 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-3 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-4 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-5 Two marks (Click here)
EE8301 - Electrical Machines - I - All unit notes (Click here)

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