EE8301 - Electrical Machines - I - Unit-3 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 3rd 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.Why the i-λ relationship of a magnetic circuit is almost linear ?

Answer

As i is similar to H while λ i.e. Nϕ is similar to flux density B hence i-λ relationship is similar to B-H relationship which is linear till saturation.

2.What is a multiply excited magnetic field system ?

Answer

A magnetic field system in which more that one windings are excited by an external source to produce magnetic fields is called multiply excited magnetic field system.

3.Give any four examples for singly excited magnetic systems.

Answer

The examples for singly excited magnetic systems are electromagnetic relays, reluctance motor, hysteresis motors, solenoidal coil, toroid coil etc.

4.What are the causes for irrecoverable energy loss when the flux in the magnetic circuit undergoes a cycle ?

Answer

The hysteresis and eddy current effects are the main causes for irrecoverable energy loss when the flux in the magnetic circuit undergoes a cycle.

5.Give examples for multiply excited magnetic field systems and explain the term multiply excited systems.

Answer

A magnetic field system in which more that one windings are excited by an external source to produce magnetic fields is called multiply excited magnetic field system. The examples are alternators, synchronous motors, d.c. shunt motors etc.

6.Why do all practical energy conversion devices make use of the magnetic field as a coupling medium rather than an electric field ?

Answer

i) The magnetic field is most suited for practical devices. ii) The energy storing capacity of magnetic field is much higher than that of electrical field. iii) The force density on the surface of magnetic field is more than that on electric field.

7.Give an example for each of single and multiple excited system.

Answer

Singly excited systems are used in electromagnetic relays, reluctance motor, hysteresis motors, solenoidal coil, toroid coil etc. While alternators, synchronous motors, d.c. shunt machines and loudspeakers are the examples of doubly excited systems.

8.State the principle of electromechanical energy conversion.

Answer

The process which involves the transfer of energy between electrical and mechanical systems, via the electric field or magnetic field is called electromechanical energy conversion.

9.In a magnetic circuit with a small air gap, in which part the maximum energy is stored and why ? (or) Predominant energy storage occurs in the air gap of an electromechanical energy conversion device. Is this statement correct ?

Answer

In a magnetic circuit with a small air gap, the maximum energy is stored in the air gap. The reluctance of the air gap is much larger than the other parts hence the major part of m.m.f. is required to overcome the air gap reluctance due to which most of the magnetic energy is stored in the air gap.

10.What are the three basic principles for the electromechanical energy conversion ?

Answer

i) The use of small energy signals for converting electrical energy to mechanical or vice versa. This principle uses vibrational motion for the energy conversion. Such devices are transducers, loud speakers, telephone receivers, microphones etc. ii) The principle of producing the mechanical force or torque based on the translatory motion. Such devices are electromagnets, solenoids, relays etc. iii) The principle which uses rotational motion for the continuous energy conversion. Such devices include generators and motors which handle large energy signals.

11.What is Magnetic saturation ?

Answer

When a magnetic material is magnetised, the flux density increases as magnetic field intensity applied increases, upto certain point. After a certain value reached by flux density, there can not be any change in its value though applied magnetic field intensity is increased. This condition is called magnetic saturation.

12.Write the principle of energy conservation.

Answer

According to the principle of conservation of energy, the energy cannot be created or destroyed but it can be transformed from one form to another.

13.Give examples for continuous energy conversion equipment and force producing devices.

Answer

The continuous energy conversion devices are generators, motors, alternators while force producing devices are relays, solenoids, toroids, linear actuators etc.

14.Define coil span factor or pitch factor.

Answer

The factor by which there is a reduction in the e.m.f. due to short pitching of the coils is called coil span factor. It is defined as the ratio of the resultant e.m.f. when coils are short pitched to the resultant e.m.f. when coils are full pitched.

15.Define distribution factor or winding factor.

Answer

The factor by which there is a reduction in the e.m.f. due to distribution of coils is called distribution factor. The distribution factor is defined as the ratio of the resultant e.m.f. when coils are distributed to the resultant e.m.f. when coils are concentrated. It is also called as winding factor or breadth factor.

16.Define the term pole pitch.

Answer

It is centre to centre distance between the two adjacent poles. 1 pole is responsible for 180° electrical of induced e.m.f. So 180° electrical is also called one pole pitch. Practically how many slots are under one pole which are responsible for 180° electrical, are measured to specify the pole pitch.

17.Define the term synchronous speed.

Answer

The speed of the synchronous machine for which it produces alternating e.m.f. at a specified rated frequency for a fixed number of poles is called its synchronous speed. It is denoted as Nₛ and given by Nₛ = 120f/P where f is the frequency and P is number of poles.

18.What is the function of an exciter ?

Answer

The exciter is used to excite the field winding of synchronous machine by a d.c. supply.

19.What are the advantages when the stator coils are short pitched ?

Answer

a) The length required for the end connections of coils is less i.e. inactive length of winding is less. So less copper is required. Hence economical. b) Short pitching eliminates high frequency harmonics which distort the sinusoidal nature of e.m.f. Hence waveform of an induced e.m.f. is more sinusoidal due to short pitching. c) As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimised. This increases the efficiency. d) It reduces the tooth ripples. e) Mechanical strength of the coils is increased.

20.Write an expression for synchronous speed.

Answer

It is denoted as Nₛ and given by Nₛ = 120f/P where f is the frequency and P is number of poles.

21.Explain the following terms with respect to rotating electrical machines. a) Pole pitch b) Chording angle

Answer

a) It is centre to centre distance between the two adjacent poles. 1 pole is responsible for 180° electrical of induced e.m.f. So 180° electrical is also called one pole pitch. Practically how many slots are under one pole which are responsible for 180° electrical, are measured to specify the pole pitch. b) The angle by which the coils are short pitched is called angle of short pitch or chording angle. It is denoted by α. It is given by, α = 180° - Actual coil span of the coils.

22.State the necessary conditions for production of steady torque by the interaction of stator and rotor fields in an electric machine.

Answer

The necessary conditions are, 1. The two fields, stator and rotor, must have same number of poles. 2. The relative motion between the two fields must be zero. 3. The space phase angle δ which is angle between the two field axes, must remain constant with time.

23.Define rotating magnetic field.

Answer

A magnetic field having constant amplitude but whose axis continuously rotates in a plane with a certain speed is called rotating magnetic field.

24.Explain the concept of electrical degree. Give the relation between electrical angle and mechanical angle.

Answer

The angle through which an induced alternating e.m.f. travels for mechanical rotation of conductors is called an electrical degree. For 2 pole machine, electrical angle is same as mechanical angle of rotation but for P pole machine the relation between the two is given by θₑ = (P/2)θₘ.

25.Define coil span or coil pitch.

Answer

The coil span is the distance on the periphery of the armature between two coil sides of a coil. It is usually expressed in terms of number of slots or degrees electrical.

26.What is full pitch and short pitch coils ?

Answer

If the distance on the periphery of the armature between the two coil sides is equal to 180° electrical i.e. 1 pole pitch then the coils are called full pitch coils. If the distance is less than a pole pitch then the coils are called short pitch coils.

27.How rotating field can be obtained ?

Answer

The rotating field can be obtained by exciting the set of three phase stationary winding by a three phase supply.

28.What is two layer winding ?

Answer

If there are two coilsides per armature slot in an armature winding, one at the bottom and other on the top then the winding is called double layer winding.

29.State the assumptions made while obtaining m.m.f. space wave.

Answer

1. It is cylindrical rotor machine. 2. The armature and rotor are made up of high grade magnetic material hence permeability of these parts is much higher than air. Hence reluctance is low so entire reluctance can be assumed to be due to two air gaps. 3. Thus if total m.m.f. is Ni then half the m.m.f. is required to create flux from rotor to stator in the air gap while half is required to create flux from stator to rotor in the air gap.

30.What is slot angle ? How it is accounted for ?

Answer

The phase difference contributed by one slot in degrees electrical is called slot angle β. As slots per pole contributes 180° electrical which is denoted as 'n’ i.e. slots per pole, β = 180°/n

31.Why is the efficiency of a three phase induction motor is less than that of a three phase transformer ?

Answer

Three phase induction motor is considered as rotating transformer. But due to rotating feature there are friction and windage losses associated with it. While three phase transformer is a static device and there are no rotational losses present it. Hence the efficiency of a three phase induction motor is less than that of a three phase transformer.

32.What is meant by mechanical angle ? Explain.

Answer

The angle through which the rotor or conductors of the machine rotates physically is called mechanical angle. While the angle through which an induced e.m.f. in the conductors travels is called an electrical angle.

33.What is meant by SPP ? What is its significance ?

Answer

SPP means slots per pole per phase. It is important in designing the winding of an electrical machines. It indicates that how many slots per pole per phase are available to distribute the winding. It determines the distribution factor of the winding.

34.What is meant by distributed winding ?

Answer

In practice all the 'm' slots per pole per phase are used to place the winding. Thus all the coils belonging to a phase are not placed in a single slot under a pole but they are distributed in all the available slots per pole. Such a winding is called distributed winding.

35.Write the equation, which relates rotor speed in electrical and mechanical radian/second.

Answer

The relation is through number of pairs of poles and given by, ω = (P/2)ωₘ where P = Number of poles, ω = Electrical speed and ωₘ = Mechanical speed in rad/sec.

36.What are the requirements of Excitation system ?

Answer

The requirements of excitation system are, 1) Very quick response. 2) High sensitivity. 3) It must be able to produce a rapid fall of voltage by demagnetising the field when a fault takes place. 4) Its power requirement must be low. 5) It should be able to operate with any kind of regulator.

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)

Students searching keywords / Search terms:-

* EE8301 Electrical Machines - I 2 marks with answers
* EE8301 Electrical Machines - I all units Notes
* EE8301 Electrical Machines - I Syllabus
* EE8301 Electrical Machines - I Anna university notes
* Anna University Regulation 2017 EE8301 Electrical Machines - I Notes
* Regulation 2017 EE8301 Electrical Machines - I All units notes
* Regulation 2017 EE8301 Electrical Machines - I All units two marks notes
* Regulation 2017 EE8301 Electrical Machines - I Part-A 2 Marks with answers
* EE8301 unit 1 two marks
* EE8301 unit 2 two marks
* EE8301 unit 3 two marks
* EE8301 unit 4 two marks
* EE8301 unit 5 two marks
* EE8301 All units notes
* Anna University Regulation 2017 two marks with answers
* Anna University Regulation 2017 notes