Two Marks Questions with Answers

Two Marks Questions with Answers

Q. 1 Define voltage regulation of alternator.

Ans. :

The voltage regulation of an alternator is defined as the change in its terminal voltage when full load is removed, keeping field excitation and speed constant, divided by the rated terminal voltage.

So if  V_ph = Rated terminal voltage and E_ph = No load induced e.m.f.

then voltage regulation is defined as,

% Regulation = E_ph - V_ph / × 100

Q. 2 What are the factors that contribute reduction in terminal voltage of a loaded alternator ?

Ans. :

1. Armature reaction

2. Load current and

3. Load power factor.

Q. 3 Define voltage regulation. Name two methods used to determine voltage regulation of alternators.

Ans. :

The voltage regulation of an alternator is defined as the change in its terminal voltage when full load is removed, keeping field excitation and speed constant, divided by the rated terminal voltage.

The two methods are :

1. Synchronous impedance method or E.M.F. method and

2. Ampere-turns method or M.M.F. method

Q. 4 List the three important parameters of an armature winding of an alternator.

Ans. :

1. Armature resistance R_a,

2. Armature leakage reactance X_L and

3. Reactance corresponding to armature reaction denoted as X_ar.

Q. 5 Define leakage flux. State its effect.

Ans. :  When armature carries a current, it produces its own flux. Some part of this flux completes its path through the air around the conductors itself. Such a flux is called leakage flux. This leakage flux makes the armature winding inductive in nature. So winding possesses a leakage reactance, in addition to the resistance due to which there is additional voltage drop across the armature winding when an alternator is loaded. 

Q. 6 Define armature reaction in an alternator.

Ans. : When the load is connected to the alternator, the armature winding of the alternator carries a current. So there are two fluxes present in the air gap, one due to armature current while second is produced by the field winding called main flux. The flux produced by the armature is called armature flux. So effect of the armature flux on the main flux affecting its value and the distribution is called armature reaction.

Q. 7 State the nature of armature reaction for the various types of load power factors.

Ans. : 1. Distorting effect of armature reaction under unity p.f. condition of the load is called cross magnetising effect of armature reaction.

2. Armature flux tries to cancel the main flux when the load power factor is lagging. Such an effect of armature reaction is called demagnetising effect of the armature reaction.

3. An effect of armature reaction due to leading power factor of the load is to assist the field flux which is called magnetising effect of the armature reaction.

Q. 8 What is armature reaction reactance and synchronous reactance of an alternator.

Ans. : To quantify the voltage drop due to the armature reaction, armature winding is assumed to have a fictitious reactance. This fictitious reactance of the armature is called armature reaction reactance denoted as X_ar Ω/ph. And the drop due to armature reaction can be accounted as the voltage drop across this reactance as I_a X_ar.

The sum of the fictitious armature reaction reactance accounted for considering armature reaction effect and the leakage reactance of the armature is called synchronous reactance of the alternator denoted as X_s.

So X_s = X_L + X_AR Ω/ph

Q. 9 Define synchronous impedance of an alternator.

Ans. : An impedance obtained by combining per phase values of synchronous reactance and armature resistance is called synchronous impedance of the alternator denoted as Z_s.

 So 

Q. 10 Draw the equivalent circuit of an alternator.

(Refer section 2.7)

Q. 11 State the voltage equation of an alternator.

Ans. : The voltage equation of an alternator is,

Q. 12 Draw the vector diagram of loaded alternator with lagging power factor.

Q. 13 Draw the vector diagram of loaded alternator with leading power factor.

Q. 14 Draw the vector diagram of loaded alternator with unity power factor.

Q.15 State the expression for induced e.m.f. in terms of winding parameters for any power factor condition.

Ans. : In general for any power factor condition induced e.m.f. per phase is given by,

(E_ph)² = (V_ph cos ϕ + I_aR_a)² + (V_ph sin ϕ ± I_aX_s)²

+ Sign for lagging p.f. loads and - Sign for leading p.f. loads

V_ph = Per phase rated terminal voltage and I_a = Per phase armature current

Q. 16 Draw and comment on load characteristics of an alternator.

(Refer section 2.10)

Q. 17 Draw the circuit diagram for open circuit and short circuit test on alternator.

(Refer section 2.13)

Q. 18 What data is required to calculate regulation by synchronous impedance method 7

Ans. :

1. The armature resistance per phase (Ra).

2. Open circuit characteristics which is the graph of open circuit voltage against the field current. This is possible by conducting open circuit test on the alternator.

3. Short circuit characteristics which is the graph of short circuit current against field current. This is possible by conducting short circuit test on the alternator.

Q. 19 How to obtain synchronous impedance from E.M.F. method ?

Ans. :

From E.M.F. method, synchronous impedance is given by,

Q. 20 State the procedure to conduct open and short circuit tests on an alternator.

(Refer sections 2.13.1 and 2.13.2)

Q. 21 State the advantages and limitations of synchronous impedance method. 

Ans. :

The advantages are : 1. The value of synchronous impedance Zs for any load condition can be calculated. 2. Regulation of the alternator at any load condition and load power factor can be determined. 3. Actual load need not be connected to the alternator. 4. The method can be used for very high capacity alternators.

The main limitation of this method is that the method gives large values of synchronous reactance. This leads to high values of percentage regulation than the actual results. Hence this method is called pessimistic method.

Q. 22 For which two purposes m.m.f. is required for an alternator ?

Ans. :

1. It must have an m.m.f. necessary to induce the rated terminal voltage on open circuit.

2. It must have an m.m.f. equal and opposite to that of armature reaction m.m.f.

Q. 23 What is the use of Potier triangle ?

Ans. :

In alternators, armature leakage reactance is the voltage drop while the armature reaction is m.m.f. quantity. The Potier triangle is used to separate the armature leakage reactance and the armature reaction m.m.f. from each other. Hence the regulation obtained by the Potier method is more accurate than the other methods.

Q. 24 Define short circuit ratio for an alternator.

Ans. :

The short circuit ratio is the ratio of the excitation required to produce open circuit voltage equal to the rated voltage to the excitation required to produce rated full load current under short circuit.

Mathematically, SCR (Short Circuit Ratio) = I_f for rated open circuit voltage / I_f for rated short circuit current

Q. 25 Explain why an alternator with low value of short circuit ratio has lower limit of stability.

Ans. :

The synchronous power is inversely proportional to Xs. This is the power which keeps alternators in synchronism during parallel operation and maintains the stability. Any disturbances from equilibrium conditions are compensated by synchronizing power. For low value of SCR, Xs is very large and synchronizing power is very low. As synchronizing power decreases, tendency of alternators to remain in synchronism decreases. This decreases the stability. Thus low SCR puts the stability limit.

Q. 26 State the assumption made in the Potier method and explain the effect of these assumptions on the accuracy of the voltage regulation.

Ans. :

1. In the entire calculation procedure of Potier method, the armature resistance is assumed to be negligibly small.

2. In a zero power factor test, inductors are assumed to be a perfect zero power factor condition. 

3. The leakage reactance of the machine remains unchanged at all the conditions including zero power factor and short circuit conditions.

Q. 27 Why is the synchronous impedance method used to determine voltage regulation of synchronous machine called pessimistic ?

Ans. :

The synchronous impedance is practically variable and not constant. If there is saturation, it remains constant but it decreases towards the saturation while it increases for the low saturation region. In synchronous impedance method, in short circuit test, the field current required is very small to pass the short circuit current hence the flux density is low and the region is low saturation region. Hence the synchronous impedance is much higher than its normal value. Thus the drop IaZsis high due to which the regulation is also high than the actual value, by synchronous impedance method. Hence the method is called pessimistic.

Q. 28 Write down the causes for reduction in terminal voltage of alternator from it's no load value Eo to V for a lagging power factor.

Ans. :

On no load armature current is zero so there are no voltage drops existing. But on lagging power factor load, the armature current flows which causes the voltage drop across the armature resistance and armature leakage reactance. Similarly there is demagnetising armature reaction existing for lagging power factor load which deceases the main flux and causes the reduction in the terminal voltage.

Q. 29 What are the reasons for drop in voltage from no load to full load?    

Ans. :

As armature current increases as load changes from no load to full load, the following voltage drops take place,

1. The voltage drop across the armature resistance, IaRa.

2. The voltage drop across the armature leakage reactance.

3. The drop in voltage due to the armature reaction effect.

Q. 30 Under what condition the terminal voltage of an alternator will be greater than induced e.m.f. ?

Ans. :

On leading power factor loads, the armature reaction is magnetising. Due to this, the main flux increases which is responsible for the increase in the terminal voltage. Hence under loading power factor loads the terminal voltage is greater than the induced e.m.f.

Q. 31 What are the experimental data required for Potier method ?

Ans. :

1. It requires open circuit characteristics by conducting open circuit test.

2. The graph of terminal voltage against excitation (field current) when delivering full load zero power factor current. This is achieved by conducting zero power factor test.

Q. 32 What is the necessity for predetermination of voltage regulation ?

Ans. :

Most of the alternators are manufactured with large power rating, hundreds of kW or MW, and also with large voltage ratings upto 33 kV. For Alternators of such power and voltage ratings, conducting direct load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.

Q. 33 Name the various methods for predetermining the voltage regulation of alternators.

Ans. :

1. Synchronous impedance or EMF method.

2. Ampere-turn or MMF method.

3. Potter's triangle or zero power factor method.

4. A.S.A. modification of MMF method.