EE8301 - Electrical Machines - I - Unit-1 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 1st 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.Define the term synchronous speed.

Answer

For synchronous machines there exists a fixed relationship between number of poles P, frequency f and the speed of the machine. The speed of the synchronous machine for the given number of poles and the rated frequency is called the synchronous speed denoted as Nₛ.

2.Write an expression for synchronous speed.

Answer

An expression for synchronous speed is Nₛ = 120f/P where f is the frequency and the P is the number of poles of the machine.

3.What does speed voltage mean ?

Answer

When the magnetic flux is constant and stationary and the coil rotates to cut the flux then e.m.f. gets induced due to relative speed between flux and coil. This e.m.f. is called speed e.m.f., rotational e.m.f., motional e.m.f. or dynamically induced e.m.f.

4.Mention the factors on which hysteresis loss depends.

Answer

1. The hysteresis loss is directly proportional to the area under the hysteresis curve i.e. area of the hysteresis loop. 2. It is directly proportional to frequency i.e. number of cycles of magnetization per second. 3. It is directly proportional to volume of the material.

5.Distinguish between statically induced and dynamically induced e.m.f.s.

Answer

An induced e.m.f. which is due to physical movement of coil, conductor with respect to flux or movement of magnet with respect to stationary coil, conductor is called dynamically induced e.m.f. or motional induced e.m.f. The change in flux lines with respect to coil can be achieved without physically moving the coil or the magnet. Such induced e.m.f. in a coil which is without physical movement of coil or a magnet is called statically induced e.m.f.

6.Define torque.

Answer

A turning or a twisting force about an axis is defined as torque.

7.What are the three types of basic rotating electric machines ?

Answer

i) D.C. machines ii) Induction machines iii) Synchronous machines

8.Define Flux Linkage.

Answer

The product of the flux and number of turns of the coil to which the flux is linked, is called flux linkage.

9.Define relative permeability.

Answer

The relative permeability is defined as the ratio of flux density produced in a medium (other than free space) to the flux density produced in free space, under the influence of same magnetic field strength and under identical conditions. Thus if the magnetic field strength is H which is producing flux density B in the medium while flux density B₀ in free space then the relative permeability is defined as, μᵣ = B/B₀ where H is same. It is dimensionless and has no units.

10.Define m.m.f. and reluctance.

Answer

It is given by the product of the number of turns of the magnetizing coil and the current passing through it. Mathematically it can be expressed as, m.m.f. = NI ampere turns where N = Number of turns of magnetising coil and I = Current through coil It is defined as the resistance offered by the material to the flow of magnetic flux through it. It is denoted by 'S'. It is given by the expression, S = l/μa = l/μ₀μᵣa A/Wb

11.What are the causes of core loss ? What are the components of core loss ?

Answer

When the core is subjected to an alternating flux then it undergoes the cycles of magnetisation and demagnetisation. This produces hysteresis effect which causes hysteresis loss in the core. Similarly core is under the influence of the changing flux and under such condition according to the Faraday's law of electromagnetic induction, e.m.f. gets induced in the core. Now if core is solid, then such induced e.m.f. circulates currents through the core. Such currents in the core which are due to induced e.m.f. in the core are called eddy currents. Due to such currents there is power loss (I²R) in the core. Such loss is called as eddy current loss. Thus hysteresis and eddy current are the two components of core loss.

12.Why the core of an electrical machines are laminated ?

Answer

When the core is laminated, it gets divided into thin laminations. The path of the eddy currents is broken due to the insulating sheets present between the laminations. This prevents the flow of eddy currents through the core and reduces the eddy current losses. The laminated structure is also easy from construction point of view. Hence the core of an electrical machines are laminated.

13.State the two types of three phase induction motors.

Answer

i) Squirrel cage induction motor ii) Wound rotor or slip ring induction motor

14.What is the method used to reduce the eddy current loss ?

Answer

The use of laminated core construction reduces the eddy current loss. This divides the solid iron core into thin laminations. The path of the eddy currents is broken due to the insulating material sheets between the laminations. Hence the eddy currents and the corresponding losses are minimised.

15.Define fringing ?

Answer

When flux enters into the air gap, it passes through the air gap in terms of parallel flux lines. There exists a force of repulsion between the magnetic lines of force which are parallel and having same direction. Due to this repulsive force there is tendency of the magnetic flux to bulge out (spread out) at the edge of the air gap. This tendency of flux to bulge out at the edges of the air gap is called magnetic fringing.

16.State the factors on which the magnitude of the force experienced by the conductor depends.

Answer

The magnitude of the force experienced by the conductor depends on, 1) Flux density (B) of the magnetic field in which the conductor is placed measured in Wb/m² i.e. Tesla. 2) Magnitude of the current I passing through the conductor in Amperes. 3) Active length 'l' of the conductor in metres.

17.State Fleming's left hand rule.

Answer

The rule states that, 'Outstretch the three fingers of the left hand namely the first finger, middle finger and thumb such that they are mutually perpendicular to each other. Now point the first finger in the direction of magnetic field and the middle finger in the direction of the current then the thumb gives the direction of the force experienced by the conductor'.

18.State right hand thumb rule.

Answer

It states that, hold the current carrying conductor in the right hand such that the thumb pointing in the direction of current and parallel to the conductor, then curled fingers point in the direction of the magnetic field or flux around it.

19.Define magnetic flux density.

Answer

The flux per unit area (a) in a plane at right angles to the flux is known as 'flux density'. Mathematically, B = ϕ/a = Wb/mμ Tesla

20.What is magnetic flux ? State its unit.

Answer

The total number of lines of force existing in a particular magnetic field is called magnetic flux. Lines of force can be called lines of magnetic flux. The unit of flux is weber and flux is denoted by symbol (ϕ) .

21.State Coulomb's law.

Answer

The force (F) exerted by one pole on the other pole is, a) Directly proportional to the product of the pole strengths, b) Inversely proportional to the square of the distance between them, and c) Nature of medium surrounding the poles. Mathematically this law can be expressed as, F = KM₁M₂/d² where K depends on the nature of the surroundings and called permeability.

22.Which are the two types of rotors used in synchronous machines ?

Answer

i) Salient pole or projected pole type rotor ii) Nonsalient or smooth cylindrical rotor

23.State the factors necessary for the dynamically induced e.m.f.

Answer

1) A coil or conductor. 2) A magnetic field (permanent magnet or electromagnet). 3) Relative motion between conductor and magnetic flux (achieved by moving conductor with respect to flux or moving with respect to conductor.)

24.State the Faraday's laws of electromagnetic induction.

Answer

i) Whenever the number of magnetic lines of force (flux) linking with a coil or circuit changes, an e.m.f. gets induced in that coil or circuit. ii) The magnitude of the induced e.m.f. is directly proportional to the rate of change of flux linkages (Flux * Turns of coil).

25.Define self inductance. State its various expressions.

Answer

The property of the coil which opposes any change in the current passing through it is called Self Inductance or Only Inductance. The various expression for self inductance are, L = Nϕ/l, L = N²/S and L = N²μa/l = N²μ₀μᵣa/l

26.What is mutually induced e.m.f. ?

Answer

Any change in current through coil A produces e.m.f. in coil B, this phenomenon is called mutual induction and e.m.f. is called mutually induced e.m.f.

27.What is leakage flux ? How it is incorporated in machine models ?

Answer

Part of the primary flux as well as the secondary flux completes the path through air and links with the respecting winding only. Such a flux is called leakage flux. Leakage fluxes link with the respective windings only and not to both the windings. It is incorporated as leakage inductance in the machine models.

28.What is hysteresis loss and how can this loss be minimized ?

Answer

When a magnetic material is subjected to repeated cycles of magnetization and demagnetization, it results into disturbance in the alignment of the various domain. Now energy gets stored when magnetic field is established and energy is returned when field collapses. But due to hysteresis, all the energy is never returned though field completely collapses. This loss of energy appears as heat in the magnetic material. This is called as hysteresis loss. The hysteresis loss can be minimized by selecting a material having very small hysteresis loop area.

29.Clearly define the MMF and EMF.

Answer

An electrical effort required to drift the free electrons in one particular direction to constitute a current is called electromotive force (EMF). The driving force in a magnetic circuit which causes the production of flux in a magnetic circuit is called magnetomotive force (MMF). It is the product of number of turns of the magnetising coil (N) and current (I) passing through it.

30.What are the core losses and how can this loss be minimzed ?

Answer

When the core is subjected to an alternating flux then it undergoes the cycles of magnetisation and demagnetisation. This produces hysteresis effect which causes hysteresis loss in the core. Similarly core is under the influence of the changing flux and under such condition according to the Faraday's law of electromagnetic induction, e.m.f. gets induced in the core. Now if core is solid, then such induced emf. circulates currents through the core. Such currents in the core which are due to induced e.m.f. in the core are called eddy currents. Due to such currents there is power loss (I²R) in the core. Such loss is called as eddy current loss. The hysteresis loss is minimized by selecting material with small hysteresis loop area while eddy current loss is minimized by using laminated construction.

31.What are quasi-static fields ?

Answer

The time invariant fields are called static fields. The quasi-static field theory is applicable at low frequencies when the dimensions of the region of interest are small compared to the wavelength of the electromagnetic field that permeates it. The field which is slowly varying i.e. the time required by electromagnetic field wave needs to propagate through a typical dimension of the system of interest is small compared with the time scale of field evolution of the system then the field is called quasi-staic field.

32.Name the main magnetic quantities with their symbols having the following units : Webers, Tesla, AT/Wb, H/m.

Answer

i) The flux denoted as ϕ measured in webers. ii) The flux density denoted as B measured in tesla. iii) The reluctance denoted as S measured in AT/Wb. iv) The permeability denoted as μ measured in H/m

33.Mention the materials suitable for fabrication of permanent magnets.

Answer

The materials suitable for permanent magnets are alnico, rare earth cobalt, iron, nickel, lodestone, samarium-cobalt combinations, neodymium etc.

34.Define Stacking factor.

Answer

Magnetic cores are generally constructed by laminations arranged in stack which are coated with insulation layers. Hence there is small gap present in between the successive laminations. Hence effective magnetic area is less than the overall area of stack of laminations. The stacking factor is defined as the ratio of the volume occupied by magnetic material to the total volume of the core. It is also called lamination factor or space factor.

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)
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EE8301 - Electrical Machines - I - Unit-5 Two marks (Click here)
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