In self cooled motors the cooling time constant is about ______ than the heating time
constant because cooling conditions are worse at standstill.
(a) 2 to 3 times greater
(b) 3 to 4 times greater
(c) 4 to 5 times greater
(d) none of the above

1 Answer

Answer :

(a) 2 to 3 times greater

Related questions

Description : The time taken by the machine to attain 0.632 of its final steady temperature rise is called (a) heating time constant (b) cooling time constant (c) either (a) or (b) (d) none of the above

Last Answer : (a) heating time constant

Description : Machines cooled by direct cooling method may be called (a) "supercharged" (b) "inner cooled" (c) "conductor cooled" (d) any of the above

Last Answer : (d) any of the above

Description : For large power transformers, self oil cooled type or air blast type which of the following values of current density may be used ? (a) 1.0 to 1.2 A/mm2 (b) 1.5 to 2.0 A/mm2 (c) 2.2 to 3.2 A/mm2 (d) 3.2 to 4.2 A/mm2

Last Answer : (c) 2.2 to 3.2 A/mm2

Description : The noise produced by a ______ cooled machine is less as the rotor moves in a medium of smaller density. (a) air (b) hydrogen (c) either (a) or (b) (d) none of the above

Last Answer : (b) hydrogen

Description : Direct water cooling of rotor winding presents (a) no mechanical difficulties (b) lesser mechanical difficulties (c) greater mechanical difficulties (d) none of the above

Last Answer : (c) greater mechanical difficulties

Description : In direct cooled system using hydrogen both stator and rotor conductors are made (a) solid (b) hollow (c) perforated (d) any ofthe above

Last Answer : (b) hollow

Description : 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) Change in motor parameters (e) All of the above

Last Answer : (e) All of the above

Description : Which of the following in an application of D.C. motors? (a) Traction (b) Drives for process industry (c) Battery driven vehicles (d) Automatic control (e) All of the above

Last Answer : (e) All of the above

Description : Which ofthe following materials is used in the manufacture of resistance grids to be used in the starters of large motors ? (a) Copper (b) Aluminium (c) Steel (d) Cast-iron

Last Answer : (d) Cast-iron

Description : In induction motors, the length of air gap in kept as small as mechanically possible is order to have (a) low power factor (b) high power factor (c) high over load capacity (d) any of the above

Last Answer : (b) high power factor

Description : The thermal conductivity of hydrogen is ______ times that of air (a) 2 (b) 3 (c) 5 (d) 7

Last Answer : (d) 7

Description : A current density of _____ is ilsed for large power transformers with forced circulation of oil or with water cooling coils (a) 1.5 to 2.5 A/mm2 (b) 3.5 to 4.5 A/mm2 (c) 4.0 to 5.0 A/mm2 (d) 5.4 to 6.2 A/mm2

Last Answer : (d) 5.4 to 6.2 A/mm2

Description : Transformers with a capacity of upto _____ have a cooling radiator system with natural cooling (a) 2 MVA (b) 5 MVA (c) 7.5 MVA (d) 10 MVA

Last Answer : (d) 10 MVA

Description : With direct water cooling it is possible to have ratings of about (a) 200 MW (b) 300 MW (c) 400 MW (d) 600 MW

Last Answer : (d) 600 MW

Description : cooling is the process of dissipating the armature and field winding losses to a cooling medium circulating within the winding insulation wall (a) Direct (b) Indirect (c) Conventional (d) Any ofthe above

Last Answer : (a) Direct

Description : With conventional hydrogen cooling it is possible to increase the rating of a single unit to (a) 50 MW (b) 100 MW (c) 200 MW (d) none ofthe above

Last Answer : (c) 200 MW

Description : Which ofthe following is an advantage of hydrogen cooling ? (a) Increase in efficiency (b) Increase in ratings (c) Increase in life (d) Smaller size of coolers (e) All ofthe above

Last Answer : (e) All ofthe above

Description : Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating upto (a) 10 MW (b) 30 MW (c) 60 MW (d) 150 MW

Last Answer : (c) 60 MW

Description : Which ofthe following methods is used for air cooling of turbo-alternators ? (a) One sided axial ventilation (b) Two sided axial ventilation (c) Multiple inlet system (d) All ofthe above

Last Answer : (d) All ofthe above

Description : The design of electromagnets is based upon which of the following fundamental equations ? (a) Force equation (b) Magnetic circuit equation (c) Heating equation (d) Voltage equation (e) All of the above

Last Answer : (e) All of the above

Description : In transformers using hot rolled steel, the cross-section of the yoke is made about _____ greater than that of the core (a) 5 percent (b) 10 percent (c) 15 percent (d) 30 percent (e) none of the above

Last Answer : (c) 15 percent

Description : The cold rolled grain oriented steel has ______ permeability in the direction of the grain orientation. (a) minimum (b) maximum (c) nil (d) none of the above

Last Answer : (b) maximum

Description : Hot rolled sheets have ______ value of permeability (a) zero (b) low (c) high (d) none ofthe above

Last Answer : (b) low

Description : Materials exhibiting zero value of resistivity are known as ______ . (a) conductors (b) semiconductors (c) insulators (d) superconductors

Last Answer : (d) superconductors

Description : Aluminium when adopted as a conductor material in ______ transformers, decreases the overall cost of the transformer (a) small size (b) medium size (c) large size (d) any of the above size

Last Answer : (a) small size

Description : Commercial available medium size machines have a speed range of ______ . (a) 200 to 400 r.p.m. (b) 600 to 1000 r.p.m. (c) 1000 to 1500 r.p.m. (d) 2000 to 2500 r.p.m.

Last Answer : (d) 2000 to 2500 r.p.m.

Description : The length cf air gap to be provided in ______ is primarily determined by power factor consideration. (a) d.c. series motor . (b) d.c. shunt motor (c) induction motor (d) synchronous motor

Last Answer : (c) induction motor

Description : In ______ machines, the size of the shaft is decided by the critical speed which depends on the deflection of the shaft. (a) small (b) medium (c) large (d) any of the above.

Last Answer : (c) large

Description : The efficiency of a machine should be as ______ as possible to reduce the operating cost. (a) high (b) low (c) either of the above (d) none of the above

Last Answer : (a) high

Description : 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

Last Answer : (b) equal to

Description : Which of the following methods does not take into account the maximum temperature rise under variable load conditions ? (a) Equivalent power method (b) Equivalent current method (c) Method of average losses (d) Equivalent torque method

Last Answer : (c) Method of average losses

Description : Pick out the wrong statement (A) A horizontal line on the humidity chart indicates the temperature changes at constant molal humidity (B) When water is cooled from 80 to 70°C by exposure ... & air cannot be increased by either reducing the mixture temperature or by increasing the total pressure

Last Answer : (D) Relative saturation of unsaturated mixture of water vapor & air cannot be increased by either reducing the mixture temperature or by increasing the total pressure

Description : The change in flux linkages can be caused in which ofthe following ways ? (a) The flux is constant with respect to time and is stationary and the coil moves through it (b) The coil is stationary with ... above occure together, i.e., the coil moves through a time varying field (d) All ofthe above

Last Answer : (d) All ofthe above

Description : On which of the following variables heat convected depends ? (a) Power density (b) Temperature difference between heated surface and coolant (c) Thermal resistivity, density, specific heat (d) Gravitational constant (e) All ofthe above

Last Answer : (e) All ofthe above

Description : Hydrogen has a heat transfer co-efficient _____ times that of air (a) 1.5 (b) 2.5 (c) 3.5 (d) 4.5

Last Answer : (a) 1.5

Description : The density of hydrogen is _____ times the density of air. (a) 0.07 (b) 1.5 (c) 2.5 (d) 3.5

Last Answer : (a) 0.07

Description : is universally used for windings of electrical machines because it is easily workable without any possibility of fracture. (a) Silver (b) Steel (c) Aluminium (d) Copper

Last Answer : (d) Copper

Description : In a D.C. machine the current per brush arm should not be more than (a) 100 A (6) 200 A (e) 300 A (d) 400 A

Last Answer : (d) 400 A

Description : The flash point of transformer oil should be higher than (a) 40°C (b) 60°C (c) 80°C (d) 104°C

Last Answer : (d) 104°C

Description : The heat dissipating capability of transformers of ratings higher than 30 kVA in increased by providing which of the following ? (a) Corrugations (b) Fins (c) Tubes (d) Radiator tanks (e) All of the above

Last Answer : (e) All of the above

Description : The resistivity of water should not be less than (a) 10 Q m (b) 100 Q m (c) 1000 Q m (d) 2000 Q m

Last Answer : (d) 2000 Q m

Description : 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

Last Answer : (d) between 25 to 35 mm

Description : 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

Last Answer : (d) 0.4 to 0.5 mm

Description : A practical formula for determining the thickness of insulation between low voltage and high voltage windings is (a) 1 + 0.2 kVmm (6) 2 + 0.5 kVmm (c) 4 + 0.7 kV mm (d) 5 + 0.9 kV mm

Last Answer : (d) 5 + 0.9 kV mm

Description : The usual values of maximum flux densities for distribution transformers using hot rolled silicon steel are (a) 0.5 to 0.8 Wb/m2 (b) 0.8 to 1.0 Wb/m2 (c) 1.1 to 1.35 Wb/m2 (d) 1.4 to 1.8 Wb/m2

Last Answer : (c) 1.1 to 1.35 Wb/m2

Description : The forced oil and air circulation method is usual one for transformers of capacities (a) upto 5 MVA (b) upto 10 MVA (c) upto 20 MVA (d) 30 MVA upwards

Last Answer : (d) 30 MVA upwards

Description : Noise produced by cooling fans (employed in air cooled heat exchangers or cooling tower) is mainly caused due to the turbulence created by blade passage through air. It can be reduced by use of a slower fan (A) With greater ... (B) Of increased diameter (C) Both (A) & (B) (D) Neither (A) nor (B)

Last Answer : (C) Both (A) & (B)

Description : 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

Last Answer : (c) between 9 to 16

Description : 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) Interpoles (e) All of the above

Last Answer : (e) All of the above

Description : 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

Last Answer : (a) quiet