Deck 4: Design and Operation of DC Machines

Question 1

The high voltage winding is usually which of the following type ?

A)Cylindrical winding with circular conductors
B)Cross-over winding with either circular or small rectangular conductors
C)Continuous disc type winding with rectangular conductors
D)All of the above types

Answer 1

All of the above types

Question 2

Which of the following is the basic consideration in the design of insulation ?

A)Electrical considerations
B)Mechanical considerations
C)Thermal considerations
D)All of the above

Answer 2

All of the above

Question 3

A practical formula for determining the thickness of insulation between low voltage and high voltage windings is

A)1 + 0.2 kVmm
B)2 + 0.5 kVmm
C)4 + 0.7 kV mm
D)5 + 0.9 kV mm

Answer 3

5 + 0.9 kV mm

Question 4

The insulation between windings and grounded core and the insulation between the windings of the same phase is called

A)minor insulation
B)major insulation
C)either of the above
D)none of the above

Question 5

The cylindrical windings using circular conductors are used for current rating of

A)upto 20 A
B)upto 40 A
C)upto 60 A
D)upto 80 A

Question 6

The surge phenomenon is particularly important in case of

A)low voltage transformers
B)medium voltage transformers
C)high voltage transformers
D)any of the above

Question 7

Which of the following in an application of D.C. motors?

A)Traction
B)Drives for process industry
C)Battery driven vehicles
D)All of the above

Question 8

D.C. servomotors are used in

A)purely D.C. control systems
B)purely AC. control systems
C)both D.C. and AC. control systems
D)none of the above

Question 9

The stator of a D.C. machine comprises of

A)main poles
B)interpoles
C)frame
D)all of the above

Question 10

The laminations of the armature of a D.C. machine are usually _____ thick.

A)0.1 to 0.2 mm
B)0.2 to 0.3 mm
C)0.3 to 0.4 mm
D)0.4 to 0.5 mm

Question 11

is usually used for brush rockers

A)Mild steel
B)Copper
C)Aluminium
D)Cast-iron

Question 12

______ brushes are fragile and cause excessive wear of commutator,

A)Natural graphite
B)Hard carbon
C)Electro graphitic
D)Metal graphite

Question 13

Which of the following brushes can be used for high values of current density ?

A)Metal graphite brushes
B)Hard carbon brushes
C)Electro-graphitic brushes
D)Natural graphite brushes

Question 14

________ is the common method of applying brushes to the commntator.

A)Radial
B)Trailing
C)Reaction
D)All of the above

Question 15

Which of the following problem arises in D.C. motors which are fed from thyristor bridge circuits ?

A)Increased I R losses
B)Increased core losses
C)Poor commutation
D)All of the above

Question 16

The weight of copper of both armature and field windings decreases with _____ in number of poles.

A)increase
B)decrease
C)either of the above
D)none of the above

Question 17

In a D.C. machine the number of brush arms is _____ the number of poles.

A)less than
B)equal to
C)greater than
D)none of the above

Question 18

In a D.C. machine the current per brush arm should not be more than

A)100 A
B)200 A
C)300 A
D)400 A

Question 19

In a D.C. machine, the value of peripheral speed should not, normally, exceed

A)10 m/s
B)20 m/s
C)30 m/s
D)40 m/s

Question 20

In D.C. machines the width of the duct is usually

A)4 mm
B)6 mm
C)8 mm
D)10 mm

Question 21

D.C. machines designed with a large value of air gap length have

A)worst, ventilation
B)poor ventilation
C)better ventilation
D)none of the above

Question 22

In D.C. machines, ir order to prevent excessive distortion of field form by the armature reaction, the field mmf must be made

A)equal to that of armature mmf
B)less in comparison with the arma¬ture mmf
C)large in comparison with the arma-ture mmf
D)none of the above

Question 23

In D.C. machines, the ____ in field mmf results in increase in size and cost of machines.

A)increase
B)decrease
C)either of the above
D)none of the above

Question 24

The operation of a D.C. machine with large air gap lengths is comparatively

A)quiet
B)noisy
C)either of the above
D)none of the above

Question 25

Which of the following methods may be adopted to reduce the effects of armature reaction ?

A)Increase in length of air gap at pole tips
B)Increasing reluctance of pole tips
C)Compensating windings
D)All of the above

Question 26

In D.C. machines the usual limit of slot pitch is

A)between 5 to 10 mm
B)between 10 to 15 mm
C)between 15 to 20 mm
D)between 25 to 35 mm

Question 27

In D.C. machines the number of slots per pole usually lies

A)between 2 to 4
B)between 6 to 8
C)between 9 to 16
D)between 20 to 30