Two Marks Questions with Answers

Two Marks Questions with Answers

Q. 1 Mention the factors on which hysteresis loss depends.

AU : Dec.-08

Ans. :

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.

 

Q. 2 Distinguish between statically induced and dynamically induced e.m.f.s.AU : May-08, 09, 15, Dec.-10, 11, 14

Ans.:

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.                  

 

Q.3  Define torque. AU : May-10      

Ans.:

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

 

Q. 4 Give the analogy between electric circuit and magnetic circuit.

 [Refer section 1.14]  AU : Dec.-10

 

Q. 5 A conductor 80 cm long moves at right angle to its length at a constant speed of 30 m/s in a uniform magnetic field of flux density 1.2 T. Find the emf induced when the conductor motion is normal to the field flux.    AU : May-11, Dec.-19

Ans.:

e = B l v sin θ and θ = 90°

e = 1.2 × 80 × 10-2 × 30 = 28.8 V

 

Q. 6 Write Lorentz force equation. AU : May-11   

Ans. :

If a particle carrying charge + Q and moving with velocity ???? is present in a region where both electric and magnetic fields are present, then the particle in a motion experiences forces ???. and ???? due to the electric and magnetic fields respectively and the resultant force experienced by a charged particle in motion is given by,

Above equation is called Lorentz Force Equation which relates mechanical force to the electrical force.  

 

Q. 7 Define Flux Linkage. AU : Dec.-16   

Ans. :

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

 

Q. 8 Define relative permeability. AU : May-17  

Ans. :

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,

µ_r = B/B₀  where H is same.

It is dimensionless and has no units.                                                                           

Q. 9 Define m.m.f. and reluctance. AU : May-14, Dec.-14

OR

Write down the expression for reluctance. What is its unit ? AU : May-19

Ans. :

The m.m.f. 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       

The reluctance 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 = 1/ µa = 1/ µ₀ µ_ra A/Wb

 

Q. 10 What are the causes of core loss ? What are the components of core loss ?

Ans. :

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 (12R) in the core. Such loss is called as eddy current loss. Thus hysteresis and eddy current are the two components of core loss.

 

Q. 11 Why the core of an electrical machines are laminated ?                             

Ans. :

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.

 

Q. 12 What is the method used to reduce the eddy current loss ? AU : Dec.-13    

Ans. :

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.

 

Q. 13 Define fringing ? AU: Dec.-18                      

Ans. :

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.

 

Q. 14 State the factors on which the magnitude of the force experienced by the conductor depends.

Ans. :

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/m' i.e. Tesla.

 2) Magnitude of the current I passing through the conductor in Amperes

3) Active length 'l' of the conductor in metres.    

 

Q. 15 State Fleming's left hand rule. AU : Dec.-15

Ans. :

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'.                              

 

Q. 16 State right hand thumb rule.                                                                      

Ans. :

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.

 

Q. 17 Define magnetic flux density. AU : Dec.-17

Ans. :

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/m² or Tesla

 

Q. 18 What is magnetic flux ? State its unit.          AU : Dec.-12       

Ans. :

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)      

 

Q. 19 State Coulomb's law.                                                                             

Ans. :

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 = KM₁ M₂/d²

where K depends on the nature of the surroundings and called permeability.              

 

Q. 20 State the factors necessary for the dynamically induced e.m.f.                

Ans. :

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.)    

 

 

Q. 21 State the Faraday's laws of electromagnetic induction.

Ans. :

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 x Turns of coil).

 

Q. 22 Define self inductance. State its various expressions. AU : Dec.-17        

Ans. :

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,

 

Q. 23 What is mutually induced e.m.f. ?

Ans. :

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.

 

Q. 24 What is leakage flux ? How it is incorporated in machine models ? AU : May-16

Ans. :

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.    

 

Q.25 What is hysteresis loss and how can this loss be minimized ? AU : Dec.-11,13,16

Ans. :

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.

 

Q. 26 Clearly define the MMF and EMF.  AU : May-12, Dec.-14

Ans. :

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.                   

 

Q. 27 What are the core losses and how can this loss be minimzed ? AU : May-12

Ans. :

For core losses, refer Q.15. The hysteresis loss is minimized by selecting material with small hysteresis loop area while eddy current loss is minimized by using laminated construction.                                                                              

 

Q. 28 A coil of 1500 turns carrying a current of 5 amps produces a flux of 2.5 mWb. Find the self inductance of the coil. AU : Dec.- 12   

Ans. :

 

Q. 29 Draw the typical magnetization curve of ferromagnetic material. AU : May-13

Ans. :

The ferromagntic materials are iron, nickel, cobalt and their alloys. The hysteresis loop area for such materials is very small as shown in the Fig. 1.27.1.

 

Q. 30 What are quasi-static fields ? AU : May-14, Dec.-15

Ans. :

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.                                                                                       

 

Q. 31 Name the main magnetic quantities with their symbols having the following units : Webers, Tesla, AT/Wb, H/m. AU : Dec.-13  

Ans. :

 i) The flux denoted as o 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 u measured in H/m

 

Q. 32 Mention the materials suitable for fabrication of permanent magnets. 

AU : May-15, Dec.-19                                                                                               

Ans. :

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

 

Q. 33 Define Stacking factor. AU : Dec.-15   

Ans. :

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.

 

Q. 34 State Ampere's Law. AU : May-16   

Ans. :

The Ampere's law states that the circulation of magnetic field strength H around a closed path C is equal to the current enclosed by that path. Mathematically it is expressed as,

 

Q. 35 Give the expression for hysteresis losses and eddy current losses. AU : May-17   

Ans. :

The Hystesis loss is given by,

Hysteresis loss = K_h (Bm)^1.6 f × Volume watts

where

K_h = Characteristic constant of the material        

B_m = Maximum flux density and

f = Frequency in cycles per second

The eddy current loss is given by,

Eddy current loss = K_e (B_m)² f2× t² × Volume watts

where,

K_e = A characteristic constant of material

B_m = Maximum flux density

f = Frequency

t = Thickness of the lamination

 

Q. 36 Why the operating point of the magnetic systems is not selected in the saturation zone of the B.H. Characteristic ? AU : May-19   

Ans. :

In the saturation region, the flux density B remains constant though H is increased. The B-H relation is nonlinear in the saturation region. If current is increased to increase H, B remains constant and material gets heated causing high losses. Hence operating point is not selected in the saturation region.