Deck 15: Direct Current Generators and Parallel Operation

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

Answer 1

That generator takes large share of loads

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

Answer 2

Faraday's law of electromagnetic induction

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

Answer 3

only if the load current is not zero

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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