PART-A 1.Define magnetic reluctance The opposition offered by the magnetic circuit for the magnetic flux path is known as magnetic reluctance. It is analogous to electric resistance. 2.List the different methods of speed control employed for dc series motor.  Field diverter method  Regrouping of field coils  Tapped field control  Armature resistance control  Armature voltage control for single motor  Series parallel control for multiple identical motors 3. Name the different methods of electrical breaking of dc motors. (i) Dynamic braking (ii) Regenerating braking (iii) Counter current braking or plugging 4. What are the typical uses of auto transformer ? (i)To give small boost to a distribution cable to correct for the voltage drop. (ii)As induction motor starters. (iii)As furnace transformers (iv)As interconnecting transformers (v)In control equipment for single phase and 3 phase elective locomotives. 5. Write down the advantages of short pitched coil. (i) The length required for the end connection of coils is less i.e., inactive length of winding is less. So less copper is required. Hence economical. (ii) Short pitching eliminated high frequency harmonics which distort the sinusoidal nature of emf. Hence waveform of an induced emf is more sinusoidal due to short pitching. (iii) As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimized. This increases the efficiency. 6. What is fringing? In the air gap the magnetic flux fringes out into neighboring air paths due to the reluctance of air gap which causes a non uniform flux density in the air gap of a machine. This effect is called fringing effect. 7.Give the emf equation of a transformer and define each term. Emf induced in primary coil E1 = 4.44 fΦ mN1 volt Emf induced in secondary coil E2 = 4.44fΦ mN2 volt Where f is the frequency of AC input Φ m is the maximum value of flux in the core

N1, N2 are the number of primary and secondary turns 8. Define voltage regulation of a transformer. `When a transformer is loaded with a constant primary voltage , the secondary voltage decreases for lagging power factor load, and increases for leading pf load because of its internal resistance and leakage reactance . The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage is termed as regulation 9.Why transformers are rated in kVA ? Copper loss of a transformer depends on current and iron loss on voltage . Hence total losses depends on Volt- Ampere and not on the power factor. That is why the rating of transformers are in kVA and not in kW 10.What is meant by electromagnetic torque? When the stator ad rotor windings of the machine both carry currents, they produce their own magnetic fields along their respective axes which sinusoidally distributed along the air-gaps. Torque results from the tendency of these two fields to align them 11.Define Quasi static field. The field surround an static electric charge is known as Static Electric Field .An important class of electromagnetic fields is quasi-static fields. These fields have the same spatial patterns as static fields but vary with time 12. What is the purpose of tertiary winding ? Apart from the Primary & Secondary windings, there sometimes placed a third winding in power transformers called "Tertiary Winding". Its purpose is to provide a cirulating path for the harmonics (especially third harmonics) produced in the transformers along with power frequency (50Hz. third harmonic means 150 Hz oscillations). 13. What is magnetostriction? When ferromagnetic materials are subjected to magnetizing mmf, these may undergo small changes in dimension; this phenomenon is known as magnetostriction. 14. What is back EMF in D.C motors ? As the motor armature rotates , the system of conductor come across alternate North and South pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductors . The direction of the emf induced is in the direction opposite to the current .As this emf always opposes the flow of current in motor operation it is called back emf.

15. Define leakage flux The flux setup in the air paths around the magnetic material is known as leakage flux.

16. What are the applications of step-up and step-down transformers? Step-up transformers are used in generating stations. Normally the generated voltage will be either 11 kV or 22 kV. This voltage is stepped up to 110 kV or 220 kV or 400 kV and transmitted through transmission lines. (In short it may be called as sending end). Step-down transformers are used in receiving stations. The voltage are again stepped down to 11 kV or 22 kV and transmitted through feeders. (In short it may be called as receiving end). Further these 11 kV or 22kV are stepped down to 3 phase 400 V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations and receiving stations are called power transformers.

17. State the torque equation for round rotor machine. . Where

P = No. pole D = Average diameter of air gap l= Axial length if air gap µ o = Permeability of free space = 4rr x 10-7 H/m g = air gap length F1 = Peak value of sinusoidal mmf stator wave F2 = peak value of sinusoidal mmf rotor wave Α = Angle between F1 and F2 called torque angle

18. Define rotating magnetic field. When a balanced three phase winding with phase distributed in space so that the relative space angle is 120 i s fed with balanced 3 phase current, resultant mmf rotates in air gap at speed.

19.Write the applications of singly excited and doubly excited magnetic system Singly excited magnetic system – EM Relays, Reluctance motor, MI instruments, Hysteresis motor. Double e x c i t e d m a g n e t i c s y s t e m – A l t e r n a t o r , S y n c h r o n o u s m o t o r , l o u d s p e a k e r s , tachometers, DC machines.

20. State the necessary conditions for the production of steady torque the interaction of stator and rotor fields in an electric machine. (i) The two fields must have the same number of poles (ii) The two fields shall be relatively stationary.

21. Define the term breadth factor The breadth factor is also called distribution factor or winding factor. The factor by which there is a reduction in the emf due to distribution of coil is called distribution factor denoted as kd.

22. Define statically induced emf. The coil remains stationary with respect to flux, but the flux through it changes with time. The emf induced is known as statically induced emf.

23. Distinguish between power transformer and distribution transformer. Power transformers have very high power ratings in the order of MVA. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in consumer side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed.

24. State stacking factor The stacking factor is defined as the ratio of the net cross sectional area of a magnetic core to the gross cross sectional area of the magnetic core. Due to lamination net cross sectional are will be always less than gross cross sectional area. Therefore the value of stacking factor is always less than unity

PART-B 1. Explain in detail about the steps involved in estimating the B-H characteristic curve and also derive the expression for the energy loss. (16)

2.Derive the expressions for self and mutual inductances. Also derive the expression for coefficient of coupling.

3.Develop an equation for induced emf in a transformer winding in terms of flux and frequency.

Let's say, T is number of turns in a winding, Φm is the maximum flux in the core in Wb. As per Faraday's law of electromagnetic induction, Where φ is the instantaneous alternating flux and represented as,

As the maximum value of cos2πft is 1, the maximum value of induced emf is, To obtain the rms value of induced counter emf, divide this maximum value of e by √2.

This is EMF equation of transformer. If E1 & E2 are primary and secondary emfs and T1 & T2 are primary and secondary turns then, voltage ratio or turns ratio of transformer is,

4. Explain the parallel operation of transformers in detail.(16)

5. Obtain the mechanical force developed in electromechanical energy conversion. (16)

6. a.Derive the EMF equation of a DC generator.(8)

6.b .Derive the torque equation of DC Motor It is seen that the turning or twisting force about an axis is called torque. Consider a wheel of radius R meters acted upon by a circumferential force F newtons as shown in the Fig. 1 The wheel is rotating at a speed of N r.p.m. Then angular speed of the wheel is, ω = (2πN)/60

rad/sec

So workdone in one revolution is, W = F x distance travelled in one revolution = F x 2 R joules And

P = Power developed = Workdone/Time = (F x 2πR) / (Time for 1 rev) = (F x 2πR) / (60/N) = (F x R) x (2πN/60)

.. .

P = T x ω watts Where T = Torque in N - m ω = Angular speed in rad/sec. Let Ta be the gross torque developed by the armature of the motor. It is also

called armature torque. The gross mechanical power developed in the armature is EbIa, as seen from the power equation. So if speed of the motor is N r.p.m. then, Power in armature = Armature torque x ω

.. .

Eb Ia = x (2N/60) but Eb in a motor is given by,

Eb = (ΦPNZ) / (60A) .. .

(ΦPNZ / 60A) x Ia = Ta x (2πN/60)

This is the torque equation of a d.c. motor. Types of Torque in the Motor Basically the torque is developed in the armature and hence gross torque produced is denoted as Ta.

Fig. 2 Type of torque

The mechanical power developed in the armature is transmitted to the load through the shaft of the motor. It is impossible to transmit the entire power developed by the armature to the load. This is because while transmitting the power through the shaft, there is a power loss due the friction, windage and the iron loss. The torque required to overcome theses losses is called lost torque, denoted as T f. These losses are also called stray losses. The torque which is available at the shaft for doing the useful work is known as load torque or shaft torque denoted as Tsh.

The shaft torque magnitude is always less than the armature torque, (Tsh < Ta). The speed of the motor remains same all along the shaft say N r.p.m. Then the product of shaft torque Tsh and the angular speed ω rad/sec is called power available at the shaft i.e. net output of the motor. The maximum power a motor can deliver to

the load safely is called output rating of a motor. Generally it is expressed in H.P. It is called H.P. rating of a motor.

7. Explain the construction and working of a DC generator.(16)

Hence the most basic tow essential parts of a generator are a) a magnetic field and b) conductors which move inside that magnetic field. Now we will go through working principle of dc generator. As, the working principle of ac generator is not in scope of our discussion in this section.

Single Loop DC Generator

In the figure above, a single loop of conductor of rectangular shape is placed between two opposite poles of magnet. Let's us consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis ab. When the loop rotates from its vertical position to its horizontal position, it cuts the flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an emf induced in these both of the sides (AB & BC) of the loop.

As the loop is closed there will be a current circulating through the loop. The direction of the current can be determined by Flemming's right hand Rule. This rule says that is you stretch thumb, index finger and middle finger of your right hand perpendicular to each other, then thumbs indicates the direction of motion of the conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and middle finger indicates the direction of flow of current through the conductor. Now if we apply this right hand rule, we will see at this horizontal position of the loop, current will flow from point A to B and on the other side of the loop current will flow from point C to D.

Now if we allow the loop to move further, it will come again to its vertical position, but now upper side of the loop will be CD and lower side will be AB (just opposite of the previous vertical position). At this position the tangential motion of the sides of the loop is parallel to the flux lines of the field. Hence there will be no question of flux cutting and consequently there will be no current in the loop. If the loop rotates further, it comes to again in horizontal position. But now, said AB side of the loop comes in front of N pole and CD comes in front of S pole, i.e. just opposite to the previous horizontal position as shown in the figure beside.

Here the tangential motion of the side of the loop is perpendicular to the flux lines, hence rate of flux cutting is maximum here and according to Flemming's right hand rule, at this position current flows from B to A and on other side from D to C. Now if the loop is continued to rotate about its axis, every time the side AB comes in front of S pole, the current flows from A to B and when it comes in front of N pole, the current flows

from B to A. Similarly, every time the side CD comes in front of S pole the current flows from C to D and when it comes in front of N pole the current flows from D to C. If we observe this phenomena in different way, it can be concluded, that each side of the loop comes in front of N pole, the current will flow through that side in same direction i.e. downward to the reference plane and similarly each side of the loop comes in front of S pole, current through it flows in same direction i.e. upwards from reference plane. From this, we will come to the topic of principle of dc generator. Now the loop is opened and connect it with a split ring as shown in the figure below. Split ring are made out of a conducting cylinder which cuts into two halves or segments insulated from each other. The external load terminals are connected with two carbon brushes which are rest on these split slip ring segments. Working Principle of DC Generator

It is seen that in the first half of the revolution current flows always along ABLMCD i.e. brush no 1 in contact with segment a. In the next half revolution, in the figure the direction of the induced current in the coil is reversed. But at the same time the position of the segments a and b are also reversed which results that brush no 1 comes in touch with that segment b. Hence, the current in the load resistance again flows from L to M. The wave from of the current through the load circuit is as shown in the figure. This current is unidirectional.