Two Marks Questions with Answers

Two Marks Questions with Answers

Q. 1 What are the two types of synchronous generators ? Where are they used suitably ?

Ans. :

Two types of synchronous generators are salient pole or projected pole type and smooth cylindrical or nonsalient pole type. Salient pole type are used for low speed alternators with speeds ranging from 125 to 500 r.p.m. The nonsalient pole type are used for high speed alternators called turboaltemators with speeds ranging from 1500 to 3000 r.p.m.

 

Q. 2 Define pitch factor or coil span factor as applied to alternator.

Ans. :

The factor by which, induced e.m.f. gets reduced due to short pitching is called pitch factor or coil span factor denoted by Kc. It is defined as the ratio of resultant e.m.f. when coil is short pitch to the resultant e.m.f. when coil is full pitched. It is always less than one and given by,

 

 

 Q. 3 Define distribution factor as applied to alternator. State its expression.

 Ans. :

The factor by which there is a reduction in the e.m.f. due to distribution of coils is called distribution factor denoted as Ka- 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 always less than one.

where m = Slots per pole per phase, β = 180o / n and  n = slots per pole

 

Q. 4 Write down the e.m.f. equation of an alternator. AU : May-04, Dec.-lO

Ans. :

The e.m.f. equation of an alternator is,

E_ph = 4.44K_c K_d f ϕ T_ph volts

where f = Frequency, ϕ = Flux per pole, T_ph = Turns per phase, K_c = Pitch factor and

K_d = The distribution factor.

 

Q. 5 Write down any four advantages of short - pitch winding in synchronous machines.

OR What is the necessity of chording in the armature winding of a synchronous machine ?

Ans. :

1 The length required for the end connections of coils is less i.e. inactive length of winding is less. So less copper is required.

2) Due to less copper requirement, it is economical.

3) 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.

4) As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimised. This increases the efficiency.

 

Q. 6 Which type of synchronous machine is used with hydraulic turbines ? What is their operating speed range ?

Ans. :

Salient pole type alternators are used with hydraulic turbines. Their operating speed range is 125 to 500 r.p.m.

 

Q.7 Calculate the distribution factor for a 36 slots, 4 pole, single layer three phase winding of an alternator. AU : Dec.-07

Ans. :

 

Q.8 What are the advantages of brushless alternator ?

Ans :

1. The performance and design of the overall system can be optimized by selecting proper frequency and voltage for a.c. excitor.

2. It is not necessary to make arrangement for spare excitors, generator-field circuit breakers and field rheostats.

3. The arrangement is cheap and simple.

 

Q. 9 Why should an alternator run always at synchronous speed ? AU : May-03, 12, Dec.-10

Ans :

An alternator has fixed relation between speed, frequency and number of poles as,

N = 120 f/ P. Thus for fixed number of poles, alternator has to be rotated at a particular speed to keep the frequenvy of the generated e.m.f. constant at the required value. Such a speed is called synchronous speed of the alternator denoted as N_S. Thus to obtain constant frequency e.m.f. It is always rotated at synchronous speed

 

 

Q. 10 State any four advantages of ratating field and stationary armature.

Ans :

1) For stationary armature large space can be provided to accommodate large number of conductors and the insulation to obtain large e.m.f.

2) It is always better to protect high voltage winding from the centrifugal forces caused due to the rotation. This avoids the interaction of mechanical and electrical stresses.

3) It is easier to collect larger currents at very high voltages from a stationary member than from the slip ring and brush assembly.

4) The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage circuit and high voltage armature as stationary.

5) Due to low voltage level on the field side, the insulation required is less and hence field system has very low inertia. It is always better to rotate low inertia system than high inertia, as efforts required to rotate low inertia system are always less.

 

Q. 11 State the difference between salient and cylindrical type of rotors.

Ans :

Salient Pole Type

1. Poles are projecting out from the surface.       

2. Air gap is non uniform.      

3. Diameter is high and axial length is small.      

4. Mechanically weak.  

5. Preferred for low speed alternators.      

6. Prime mover used are water turbines, I.C. engines.  

7. For same size, the rating is smaller than cylindrical type.  

8. Separate damper winding is provided.  

Smooth Cylindrical Type

1. portion of the cylinder acts as poles hence poles are non projecting.

2. Air gap is uniform due to smooth cylindrical periphery.

Small diameter and large axial length is the feature.

4. Mechanically robust.

5. Preferred for high speed alternators .e. for turboaltemators.

6. Prime movers used are steam turbines, electric motors.

7. For same size, rating is higher than salient pole type.

8. Separate damper winding is not necessary.

 

Q. 12 What is the difference between degrees mechanical and degrees electrical. State the relation between them.

Ans. :

The degrees through which an alternator is rotated mechanically is called degrees mechanical while for the same mechanical rotation, the degrees through which an induced e.m.f. electrically travels is called degrees electrical. Both are related to each other through the number of poles of an alternator as,

1° mechanical = (P/2)^o electrical.

Thus for 4 pole machine, for 360° mechanical rotation of an alternator, 720° electrical i.e. 2 cycles of induced e.m.f. are produced.

 

Q. 13 Define a pole pitch.

Ans. :

It is center to center 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.

Pole pitch = 180° electrical = slots per pole (no. of slots/P) = n

 

Q. 14 Define slot angle.

Ans. :

The phase difference contributed by one slot in degrees electrical is called slot angle R. As slots per pole contributes 180° electrical which is denoted as 'n', we can write,

β = 1 slot angle = 180° / n

This means that if we consider an induced e.m.f. in the conductors which are placed in the slots which are adjacent to each other, there will exist a phase difference of β° in between them.

 

Q.15 State the various types of armature windings used in alternators.

Ans. :

In general following three types of armature windings are used in alternators : i) Single layer and double layer winding ii) Full pitch and short pitch winding iii) Concentrated and distributed winding.

 

Q. 16 Define a coil span and a full pitch coil.

Ans. :

A 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. If coil span is 'n' slots or 180° electrical the coil is called full pitch coil. 

 

Q. 17 What is the difference between concentrated and distributed winding. Which winding is preferred in alternator and why ?

Ans. :

In concentrated winding all conductors or coils belonging to a phase are placed in one slot under every pole. In distributed type of winding all the coils belonging to a phase are well distributed over the'm' slots per phase, under every pole.

The distributed winding is preferred because it makes the waveform of the induced e.m.f. more sinusoidal in nature and due to less number of conductors per slot, heat dissipation is better.

 

Q. 18 In a 8 pole, 3 phase alternator, armature has 72 slots. It is using an armature winding which is short pitched by one slot. Calculate its coil span factor.

Ans. :

n = Sots/pole = 72/8 = 9, β = -180/9 = 20° and coil is shorted by 1 slot i.e. by 20°

ɑ = angle of short pitch = 20° hence K_c = cos(ɑ/2) = cos (10) = 0.9848

 

Q. 19 Why practically most of the alternators are connected in star ?

Ans. :

Practically most of the alternators are star connected due to following reasons :

1. Neutral point can be earthed from safety point of view.

2. For the same phase voltage, voltage available across the terminal is more than delta connection.

3. For the same terminal voltage, the phase voltage in star is 1/^/3 times line value. This reduces strain on the insulation of the armature winding.

 

Q. 20 Write the causes of harmonics in the voltage and current waves of electrical machinery ?

Ans. :

1. Nonsinusoidal waveform of field flux.

2. Due to slotted periphery of armature, the air gap reluctance is varying which causes the harmonics called slot harmonics.

 

Q. 21 Explain how quick stopping of a synchronous machine may be achieved.

AU : May-08

Ans. :

The large synchronous machines may take long time about 30 to 40 minutes to stop, after prime mover is disconnected. Hence for quickly stopping the machine, it is provided with brakes so that the machine may not run for longer periods at low speeds. Such a running on low speeds is dangerous for the thrust bearings.

The brakes are designed in such a way that the complete energy of the rotating parts gets dissipated and the machine is brought to rest within 2 to 3 minutes. The air operated or oil operated brakes are used. 

 

Q. 22 How is the armature windings in alternators different from those used in d.c. machines ?

Ans. :

In d.c. machines, the armature winding is placed on rotor and is rotating part of the machine. In alternators, the armature winding is placed on stator and is stationary part of the machine. In d.c. machines, the armature winding is lap type or wave type while in alternators the armature winding is three phase, generally short pitch, double layer and distributed type.

 

Q. 23 What are the methods of reducing the space harmonics in a machine ?

AU : Dec.-05

Ans. :

1. Distribution of armature winding

2. Chording of armature winding

3. Using fractional winding

4. Using skewing

5. Using large length of air gap

 

Q. 24 Why the stator core is laminated?

Ans :

The sator core is laminated to reduce the eddy current losses. Due to the laminated construction, the path for the flow of eddy currents is obstructed and thus reduces the eddy current circulation and the corresponding losses.

 

Q. 25 What do you mean by single layer and double layer winding?

Ans :

In a single layer winding, a slot consists of only one coil side while in double layer it consists of two coil sides of two coil sides arranged one over the other in two layers. This is shown in the Fig.1.1

 

Q. 26 What are the various functions of damper winding provided with alternator ? AU : Dec.-ll

Ans. :

1. To reduce the overvoltage in abnormal conditions.

2. To reduce the subtransient reactance of an alternator.

3. For damping the rotor oscillations.

4. Tends to maintain balanced three phase voltages under unbalanced load conditions.

 

Q. 27 What is integral slot winding and fractional slot winding ?

Ans :

 If the number of slots per pole per phase is whole number then the winding is called integral slot winding and when it is fraction number then the winding is called fractional slot winding.

 

Q. 28 Why alternators are rated in kVA and not in kW ?

Ans :

The power rating of any machine is given by that continuous power which machine can deliver to the load without exceeding the temperature beyond safe limits. 

The temperature rise depends on the losses taking place in the machine. The core losses depends on the frequency and voltage and not on the power factor of the load. While the copper losses are I2R losses and the current is directly related to the apparent power of the machine and not to the power factor of the load. Thus the power rating is basically apparent power as the factors affecting it are independent of the power factor of the load. Hence the alternators are rated in kVA and not in kW.

 

Q. 29 What is damper winding ?

Ans :

The additional winding placed on the field pole faces in the form of short circuited conducting bars is called damper winding. The pole shoes are slotted to accommodate this damper winding.

 

Q. 30 State the effect of harmonic components on pitch factor.

Ans. :

If a is the angle of short pitch for fundamental flux wave then it changes to xa for xth harmonic component. Hence the pitch factor gets modified as,

K_cx = cos (xɑ/2) where x = Order of harmonic component.

 

Q. 31 What is the effect of harmonic components on distribution factor ?

Ans. :

Due to the harmonic components the distribution factor gets modified as,

 

Q. 32 What are the essential elements for generating emf in alternators ?

Ans. :

The essential elements are,

1. Armature winding i.e. stator wnding.

2. Field winding i.e. rotor winding.

3. Exciter to excite the field winding.

4. Prime mover to rotate the rotor.

 

Q. 33 How many slip rings are generally required for an alternator ? Why ?

Ans. :

The slip rings are required to establish the contact between rotating winding and stationary part. In alternators, the field winding is rotating to which d.c. supply is required. The stationary d.c. supply has two terminals : positive and negative. Hence two slip rings, one for each positive and negative are required to connect d.c. supply to the rotating field winding in alternators.

 

Q. 34 State the relation between frequency, speed and poles for an alternator. 

Ans. :

For an alternator,

N_S = 120f/P

where          N_S = Synchronous speed, f = Frequency, P = Number of poles.

 

Q. 35 For a 2 pole alternator, find the synchronous speed for a frequency of 50 Hz.

Ans. :

P = 2, f = 50 Hz.

N_S = 120f/P = 120 × 50 / 2 = 3000 r.p.m

 

Q. 36 What is synchronous speed of an alternator ?

Ans. :

For an alternator, there exists a fixed relation between speed, frequency and number of poles. For fixed number of poles and rated frequency, the alternator must be driven at specific speed. This speed which produces e.m.f. at a rated frequency for given number of poles is called synchronous speed of an alternator.

 

Q. 37 What is the difference between integral slot winding and fractional slot winding for an alternator ?

Ans. :

For an alternator winding, m is slots per pole per phase. If the value of m is integer, the winding is called integral slot winding while if the value of m is fraction then the winding is called fractional slot winding.

 

Q. 38 State the advantages of fractional slot winding used in alternators.

Ans. :

1. The number of slots need not be integral multiple of number of poles.

2. The number of slots may be selected for which notching gear is available, which is economical.

3. Saving in machine tools.

4. Easy to manufacture.

5. High frequency harmonics are reduced.

6. The available voltage waveform is sinusoidal.

 

Q. 39 How can you distinguish between the two types of large synchronous generator from their appearance ?

Ans. :

The slow speed salient pole alternators are larger in diameter and shorter in axial length.

The high speed cylindrical pole alternators are shorter in diameter and larger in axial length.

 

Q. 40 Distinguish between full-pitch coil and short-pitch coil.

Ans. :

If coil side in one slot is connected to a coil side in another slot which is one pole pitch (n slots) distance away from the first slot, then the coil is called full pitch coil. In full pitch coil, coil span is 180° electrical. 

If coil side in one slot is connected to other coil side in a slot which is at a distance less than the n slots, then coil is called short pitch coil. The coil span of short pitch coil is less than 180°. This is shown in the Fig. 1.15.1 (b)

(a) Full pitch coil (b) Short pitch coil

 

Q. 41 What are the advantages of salient pole type construction used for Synchronous machines.

Ans. :

The salient pole construction has advantages of less weight, small size, good performance and efficiency. It offers better ventilation. Due to special shape of pole faces, the flux distribution is sinusoidal, which helps to generate sinusoidal e.m.f. Due to variable reluctance, machine develops additional reluctance power which is independent of excitation.