Two Marks Questions with Answers

Two Marks Questions with Answers

AU: Nov.-02

Q. 1 Why the i- λ relationship of a magnetic circuit is almost linear? AU : Nov. 02

 Ans.

As i is similar to H while λ i.e. No is similar to flux density B hence i - λ relationship is similar to B-H relationship which is linear till saturation.

 

Q. 2 Explain how flow of energy takes place in electromechanical device. [Refer section 2.1] AU: Dec.-13, Nov.-02

 

Q. 3 Mark the co-energy region in the i-λ graph. [Refer section 2.6.2]  AU : May -03

 

Q. 4 What is a multiply excited magnetic field system?AU: May-03

Ans.

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.

 

Q. 5 Give any four examples for singly excited magnetic systems.AU: Dec.-03

Ans.

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

 

Q. 6 How energy is stored in a magnetic system? [Refer section 2.6.2] AU : May-04

 

Q. 7 Draw the diagram of a magnetic field system with two electrical excitations. [Refer section 2.5] AU: May-04

 

Q. 8 What are the causes for irrecoverable energy loss when the flux in the magnetic circuit undergoes a cycle?AU: Dec.-04

Ans.

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

 

Q. 9 Write an expression for the mechanical energy output when the armature is move from one position to another with constant coil current. [Refer section 2.6.5]

AU : Dec- 04

 

Q. 10 Write the energy balance equation of a machine. AU : May-05, 11

Ans.

The energy balance equation for generating and motoring actions can be written as,

 

Q. 11 Give examples for multiply excited magnetic field systems and explain the term multiply excited systems. AU: Dec.-05, May-13

Ans.

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.

 

Q. 12 What is the significance of co-energy? [Refer section 2.6.2] AU: May-06

 

Q. 13 Why do all practical energy conversion devices make use of the magnetic field as a coupling medium rather than an electric field? AU : Dec-13, May-07,14,15

Ans.

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.

 

Q. 14 Give an example for each of single and multiple of all excited system.

AU: Dec.-06, 07, 19, May-09

Ans.

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.

 

Q. 15 State the principle of electromechanical energy conversion  AU: May-08, Dec.-08,17,18

Ans. :

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.

 

Q. 16 In a linear system prove that field energy and coenergy are equal.

[Refer section 2.6] AU: May-10

 

Q. 17 Write an expression for the stored energy in the magnetic field.

AU: May-10, Dec.-12

Ans. :

 

Q. 18 Draw the power flow diagram for motor and generator operation.

[Refer section 2.4] AU: Dec.-10

 

Q. 19 In a magnetic circuit with a small air gap, in which part the maximum energy is stored and why? AU: Dec.-10

OR Predominant energy storage occurs in the air gap of an electromechanical energy conversion device. Is this statement correct? AU: Dec.-17

Ans. :

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.

 

Q. 20 What are the three basic principles for the electromechanical energy conversion? AU: May-11

Ans. :

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.

 

Q. 21 Define field energy and coenergy.

[Refer section 2.6.2]  AU: Dec.-09, 14

 

Q. 22 What is Magnetic saturation? AU: Dec.-16

Ans. :

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.

 

Q. 23 Draw the i-λ characteristics of a nonlinear magnetic circuit when the armature is moved from X₁ to X₂. [Refer section 2.6.5]

 

Q. 24 Write the principle of energy conservation.

Ans.

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.

 

Q. 25 What are the advantages of field energy method? [Refer section 2.9] AU: Dec.-11

 

Q. 26 Draw the general block diagram of electromechanical energy conversion device. AU: Dec.-11

[Refer Fig. 2.1.1]

 

Q. 27 What do you mean by coenergy? AU: May-12, 13, 14, 16

Ans. :

 The field energy is a function of two variables λ and x, given by W_f  = W_f (λ,x) = W_f(i,x). The field energy is the area between the λ-axis and i-λ curve as shown in the Fig. 2.15.1.

Then the complementary area of the i – λ rectangle is calles co-energy given by,

W’_f(i,x) = i λ-W_f (λ,x)

It is also shown in the Fig. 2.15.1.

 

Q. 28 Draw the diagram indicating the flow of energy in electromechanical energy conversion via coupling medium. [Refer Fig. 2.4.1 and 2.4.2]

AU: Dec.-12, 14

 

Q. 29 Give examples for continuous energy conversion equipment and force producing devices.  AU: Dec.-18

Ans. :

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

 

Q. 30 What are the categories of electromechanical energy conversion devices ? AU: May-19

Ans. :

The categories of electromechnical energy conversion devices are,

1. Transducers such as microphones, loudspeakers, strain gauge, thermocouples etc.

2. Devices producing mechanical force or torque such as electromagnets, relays, solenoids etc.

3. Devices used for continuous energy conversion using rotational motion such as generators, motors etc.

 

Q. 31 Define the term pole pitch. AU: Dec.-03, May-05,17

Ans. :

It is centre to centre distance between the two adjacent poles. 1 pole is responsible for 180o 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.

 

Q. 32 Define the term synchronous speed. AU: Dec.-03, May-17

Ans. :

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 N_s and given by N_s = 120 f / p where f is the frequency and P is number of poles.

 

Q. 33 What are the advantages when the stator coils are short pitched? AU: Dec.-04,13,15, May-12, 13

Ans. :

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.

 

Q. 34 Define rotating magnetic field. AU: Dec.-08

 Ans. :

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

 

Q. 35 Explain the concept of electrical degree. Give the relation between electrical angle and mechanical angle. AU: May-03, Dec.-10

Ans.

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 θ ₌  P/w θ m.

 

Q. 36 Define coil span or coil pitch. AU : May-17

Ans. :

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.

 

Q. 37 What is full pitch and short pitch coils ?

Ans. :

If the distance on the periphery of the armature between the two coil sides is equal to 180o 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.

 

Q. 38 Draw the m.m.f. space wave of a single coil. [Refer section 2.12.1]

 

Q. 39 How rotating field can be obtained?

Ans. :

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

 

Q. 40 Write the equation of rotating m.m.f. wave.

Ans. :

The equation of rotating m.m.f. wave. is,

F_R  = 3/2 F_m cos(θ-ωt)

This shows that its magnitude is constant equal to 3/2 F_m while its phase angle changes linearily with time as wt. So its axis rotates in the air gap at a constant speed of ω rad(elect)/sec.

 

Q. 41 State the assumptions made while obtaining m.m.f. space wave. AU: May-10

Ans. :

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.

 

Q. 42 Why is the efficiency of a three phase induction motor is less than that of a three phase transformer? AU: May-11

Ans. :

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.

 

Q. 43 What is meant by distributed winding? AU: Dec.-14,16

Ans. :

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.

 

Q. 44 Write the equation, which relates rotor speed in electrical and mechanical radian/second.  AU : May-15

Ans. :

The relation is through number of pairs of poles and given by,

ω = ( P/2) ω_m

where P = Number of poles, ω = Electrical speed and ω_m = Mechanical speed in rad/sec.

 

Q. 45 Why fractional pitched winding is preferred over full pitched winding? AU: Dec.-19

Ans. :

1. Copper required is less due to reduced inactive length of the winding.

2. Eliminates high frequency harmonics making e.m.f. sinusiodal.

 3. Reduced eddy current and hysteresis loss.

4. Very much economical.