MCQs on Electrical Machines

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01․ There are two 2-pole DC machines. Main field axis of machine A is horizontal and that of B is vertical. It is preferable to purchase
machine A
machine B
any of the two machines
none of these

If we place field winding vertically then there may arise a problem of stability of the machine but if we place field winding in horizontal plane then it will be more stable structure of the machine. So preferred machine would be having horizontal main field axis.

02․ A compound dc generator is delivering full load current at a terminal voltage of 230 V. Its series-field winding gets short circuited. If its terminal voltage 1. becomes more than 230 V, it is overcompounded generator 2. becomes more than 230 V, it is differentially compounded generator 3. becomes less than 230 V, it is overcompounded generator 4. becomes less than 230 V, it is differentially compounded generator 5. becomes less than 230 V, it is level compounded generator From these, the correct answer is
1, 4, 5
2, 3, 5
1, 4
2,3

In question terminal voltage of the compound generator is given 230 V. If series field winding gets short circuited then effect of series field flux will be lost. Now if terminal voltage decreases then it implies that before short circuiting of the series field winding, series field flux was aiding the main field flux.Hence we have two options in hand, in which series field flux aids the main field flux. Either it may be over compounded or level compounded generator. So, if terminal voltage becomes less than 230 V then either it may be over compounded or level compounded generator where series field flux aids the main field flux. On the other hand if terminal voltage increases then it implies that before short circuiting of the series-field winding, series field flux was opposing the main field flux which is the characteristic of none another than under compounded generator. So, if terminal voltage becomes greater than 230 V then without hesitation we can point it out as under compounded generator.

03․ DC generators are usually designed to develop armature voltages not exceeding 650 V because of the limitations imposed by
field winding
armature winding
commutator
starters

Due to the limitation of commutator dc generators are not usually designed beyond 650 V.

04․ In a DC series generator, the terminal voltage with increase in load will
decrease
increase gradually and then stay at rated voltage
increase to rated voltage and then may decrease
remain nearly constant

As field is connected in series with the load so with increase in the load current ,field current also increases i.e. terminal voltage rises to rated voltage. Further increase in the load may cause excessive loss in the series field which may decrease the terminal voltage.

05․ Consider the following statements : For a level compounded dc generator to run at constant voltage, the series field mmf must effectively compensate 1. armature reaction mmf 2.armature resistance drop 3. brush contact voltage drop Which of these statements is/are correct ?
2 alone
1 and 2
1 and 3
1, 2 and 3

In case of level compounded dc generator no load terminal voltage and full load terminal voltage should be same. So, series field mmf must nullify the causes of voltage drop from armature to the load end. Here, 1,2 and 3 are causes for voltage drop which should be compensated by series field mmf.

06․ Under which of the following conditions is a dc motor provided with compensating winding used ? 1. Wide range of speed control above normal 2. Wide range of steady load variation with no speed control 3.Wide range of rapid variation in load Select the correct answer using the codes given below : Codes :
1, 2 and 3
2 and 3
1 and 2
1 and 3

Wide range of speed control above normal is possible by field weakening method. Here compensating winding must be provided so that armature mmf remains under control. In case of steady or rapid load variation armature current varies accordingly. To compensate the armature reaction phenomena due to variation in armature current we must provide compensating winding.

07․ The following statements relate to compensating winding (CW) and interpole winding (IW) of a dc machine : 1. CW neutralizes the armature reaction under a pole pitch 2. IW is connected in series with armature circuit 3. IW produces mmf in the commutating zone only 4. CWmmf is directed along the brush axis 5. Mmf produced by IW is equal to armature mmf From these, the correct statements are
1, 2, 3, 4 and 5
1, 3, 4, 5
2, 3, 4, 5
2, 3, 4

IW is connected in series with the armature winding so that affect of interpole can vary with the armature current variation. Interpoles are designed in a manner such that it only producemmf in the commutating zone only. CW is directed along the direct axis i.e. brush axis.

08․ If the speed of a dc motor increases with load torque, then it is a
series motor
permanent magnet motor
differentially compounded motor
comulatively compounded motor

In case of differentially compounded motor with the increase in armature current flux decreases which in-turn increases the speed (Nα1/Φ).

09․ If the field of a dc shunt motor gets opened while the motor is running, then the
motor will become slow.
motor will attain dangerously high speed
armature current will drop
armature will oscillate about original speed as the mean speed

If the field of a dc shunt motor gets open then field flux will become zero. As we know Nα1/Φ. So, motor will attain dangerously high speed.

10․ A DC cumulatively compounded motor delivers rated load torque at rated speed. If the series field is short-circuited, then the armature current and speed will
both increase
both decrease
increase and decrease respectively
decrease and increase respectively

If series field is short-circuited then series field resistance (very small) will become zero i.e. armature current will increase negligibly. Due to short circuited series field total flux will also reduce i.e. speed will increase as Nα1/Φ. Most preferable option should be armature current remains constant and speed increases. As no such option is given, most preferred option will be (a).

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