Question 1

Two generators are running in parallel. One of the generators may run as motor for which of the following reasons ?
A)The direction of that generator is reversed
B)The speed of that generator is increased
C)The field of that generator is weakened
D)That generator takes large share of loads

Accepted Answer

D

Question 2

A D.C. generator works on the principle of
A)Lenz's law
B)Ohm's law
C)Faraday's law of electromagnetic induction
D)none of the above

Accepted Answer

A D.C. generator works on the principle of Faraday's law of electromagnetic induction, which states that a voltage is induced in a conductor when it is exposed to a changing magnetic field.

Question 3

A series generator can self-excite
A)only if the load current is zero
B)only if the load current is not zero
C)irrespective of the value of load current
D)none of the above

Accepted Answer

A series generator can self-excite only if there is some load current flowing through it, as the field winding is in series with the armature. Without load current, there would be no magnetic field generated to induce a voltage in the armature.

Question 4

A shunt generator can self-excite
A)only if the resistance of the field circuit is less than critical value
B)only if the resistance of the field circuit is greater than critical value
C)irrespective of the value of the resis-tance in the field circuit
D)none

Accepted Answer

A shunt generator can self-excite only if the resistance of the field circuit is less than the critical value. This is because the field circuit needs to allow enough current to flow to generate the initial magnetic field necessary for the generator to start producing electricity. If the resistance is too high, it will limit the current too much, preventing the generator from self-exciting.

Question 5

The terminal voltage of a series generator is 150 V when the load current is 5 A. If the load current is increased to 10 A, the terminal voltage will be
A)150 V
B)less than 150 V
C)greater than 150 V
D)none of the above

Accepted Answer

In a series generator, the terminal voltage increases with an increase in load current due to the increased field excitation, which enhances the generated EMF.

Question 6

The open circuit voltage of a compound generator is 250 V. At full load the terminal voltage
A)will be less than 250 V
B)will always be 250 V
C)may be greater or less than 250 V
D)none of the above

Accepted Answer

The terminal voltage of a compound generator at full load may be greater or less than 250 V depending on the compounding. If it's over-compounded, voltage increases with load; if under-compounded, voltage decreases; if flat-compounded, it attempts to maintain constant voltage.

Question 7

Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be
A)270 V
B)267.5V
C)265 V
D)257.4V

Accepted Answer

The bus bar voltage is calculated by taking the average of the two generator voltages, adjusted for the voltage drop across the armature resistance due to the load current shared by the generators. Since the exact current distribution depends on the internal characteristics of the generators and the external circuit, without specific information on how the 1000 A is divided, a precise calculation involving the armature resistance can't be provided. However, the bus bar voltage will generally be slightly less than the average of the two EMFs due to the voltage drop in the armature resistance. The closest answer, without performing a detailed calculation that accounts for the division of current, is the average of the two EMFs, which is (270V + 265V) / 2 = 267.5V. This assumes an ideal or simplified scenario where the voltage drop across the armature resistance is minimal or equally affects both generators in a way that the average voltage remains the most representative value.

Question 8

The essential condition for parallel operation of two D.C. generators is that they have '
A)same kW rating
B)the same operation r.p.m.
C)the same drooping voltage charac-teristics
D)same percentage regulation

Accepted Answer

For parallel operation of DC generators, it is essential that they have the same drooping voltage characteristics to ensure stable load sharing and avoid circulating currents.

Question 9

When two D.C. generators are running in parallel an equilizer bar is used
A)to increase the series flux
B)to increase the generated e.m.f.
C)to reduce the combined effect of ar-mature reaction of both the machines
D)so that the two identical machines will pass approximately equal currents to the load

Accepted Answer

The equilizer bar ensures that two identical DC generators running in parallel will share the load equally, distributing the current approximately equally between them. This helps in maintaining stability and efficiency in the power supply system.

Question 10

With a D.C. generator which of the following regulation is preferred ?
A)100% regulation
B)infinite regulation
C)50% regulation
D)1% regulation

Accepted Answer

1% regulation in a D.C. generator is preferred because it indicates very little change in voltage from no load to full load, which is desirable for stable operation.

Question 11

Which generator would you prefer for feeding long D.C. transmission lines ?
A)Series generator
B)Shunt generator
C)Over compound generator
D)Flat compound generator

Accepted Answer

Over compound generators are preferred for feeding long DC transmission lines because they can maintain a constant voltage over a wide range of load conditions, compensating for the voltage drop in the transmission lines.

Question 12

In a D.C. generator the critical resistance can be increased by
A)increasing its field resistance
B)decreasing its field resistance
C)increasing its speed
D)decreasing its speed

Accepted Answer

Increasing the speed of a D.C. generator increases the generated voltage, which in turn increases the critical resistance.

Question 13

The number of armature parallel paths in a two-pole D.C. generator having duplexlap winding is
A)2
B)4
C)6
D)8

Accepted Answer

In a duplex lap winding, the number of parallel paths, $A$, is equal to twice the number of poles, $P$, because it is a lap winding and "duplex" means there are two circuits per segment. Since it's a two-pole generator, $A = 2P = 2 \times 2 = 4$.

Question 14

For both lap and wave windings, there are as many commutator bars as the numberof
A)slots
B)armature conductors
C)winding elements
D)poles

Accepted Answer

In both lap and wave windings, the number of commutator bars is equal to the number of winding elements.

Question 15

The series field of a short-shunt D.C. generator is excited by
A)external current
B)armature current
C)shunt current
D)load current

Accepted Answer

The answer of The series field of a short-shunt D.C....

Question 16

As a result of armature reaction, the reduction in the total mutual air gap flux in aD.C. generator is approximately
A)40 percent
B)25 percent
C)10 percent
D)5 percent

Accepted Answer

The answer of As a result of armature reaction, the...

Question 17

Shunt generators are most suited for stable parallel operation because of their
A)rising voltage characteristics
B)identical voltage characteristics
C)drooping voltage characteristics
D)linear voltage characteristics

Accepted Answer

Shunt generators have drooping voltage characteristics, which means that as the load increases, the voltage slightly decreases. This characteristic is beneficial for stable parallel operation because it helps in load sharing among generators. When one generator's load increases, its voltage drops slightly, discouraging further increase in load, while the other generators compensate, thus maintaining system stability.

Question 18

The main factor which leads to unstable parallel operation of flat and over compounded generators is
A)their rising voltage characteristics
B)unequal number of turns in their series field windings
C)unequal speed regulation of their primemovers
D)unequal series field resistances

Accepted Answer

The main factor leading to unstable parallel operation of flat and over compounded generators is their rising voltage characteristics. This causes a mismatch in voltage regulation, leading to instability when these generators are operated in parallel.

Quiz 15: Direct Current Generators and Parallel Operation

Two generators are running in parallel. One of the generators may run as motor for which of the following reasons ?

A D.C. generator works on the principle of

A series generator can self-excite

A shunt generator can self-excite

The terminal voltage of a series generator is 150 V when the load current is 5 A. If the load current is increased to 10 A, the terminal voltage will be

The open circuit voltage of a compound generator is 250 V. At full load the terminal voltage

Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be

The essential condition for parallel operation of two D.C. generators is that they have '

When two D.C. generators are running in parallel an equilizer bar is used

With a D.C. generator which of the following regulation is preferred ?

Which generator would you prefer for feeding long D.C. transmission lines ?

In a D.C. generator the critical resistance can be increased by

The number of armature parallel paths in a two-pole D.C. generator having duplexlap winding is

For both lap and wave windings, there are as many commutator bars as the numberof

The series field of a short-shunt D.C. generator is excited by

As a result of armature reaction, the reduction in the total mutual air gap flux in aD.C. generator is approximately

Shunt generators are most suited for stable parallel operation because of their

The main factor which leads to unstable parallel operation of flat and over compounded generators is