Consider the following statements regarding dc machines : 1. possible resistance of shunt-field winding is 1 Ω 2. possible resistance of series-field winding is 1 Ω 3. electrographite brushes are used for 230 V, 10 A machines 4. Copper-graphite brushes are used for 230 V, 10 A machines 5. Possible armature-circuit resistance is 1 Ω From these, the correct statements are

Electrical Machines
Consider the following statements regarding dc machines : 1. possible resistance of shunt-field winding is 1 Ω 2. possible resistance of series-field winding is 1 Ω 3. electrographite brushes are used for 230 V, 10 A machines 4. Copper-graphite brushes are used for 230 V, 10 A machines 5. Possible armature-circuit resistance is 1 Ω From these, the correct statements are

1, 4

1, 3, 5

2, 4 ,5

2, 3 , 5

1, 4

1, 3, 5

2, 4 ,5

2, 3 , 5

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Electrical Machines
DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of

Heavy sparking at the first stud of starting resistance steps.

Both of above.

None of these.

Heavy sparking at brushes.

Heavy sparking at the first stud of starting resistance steps.

Both of above.

None of these.

Heavy sparking at brushes.

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

dc machines are comparatively low rating machines.

commutator action is not possible otherwise

it gives more uniform air-gap flux distribution

it reduces field structure iron losses

dc machines are comparatively low rating machines.

commutator action is not possible otherwise

it gives more uniform air-gap flux distribution

it reduces field structure iron losses

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Electrical Machines
Iron loss in a transformer mainly occurs in

steel core in transformer.

transformer insulating oil.

winding of transformer.

iron made tank of transformer.

steel core in transformer.

transformer insulating oil.

winding of transformer.

iron made tank of transformer.

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Electrical Machines
A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because

there was a break in the armature circuit

field winding was wrongly connected

field resistance was higher than critical resistance

there was no residual magnetism in the field circuit

there was a break in the armature circuit

field winding was wrongly connected

field resistance was higher than critical resistance

there was no residual magnetism in the field circuit

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Electrical Machines
A DC shunt machine generates 250 V on open circuit at 1000 RPM. Armature resistance is 0.5 Ω, field resistance 250 Ω. When running as a motor takes 4 A at 250 V. Calculate the efficiency when running as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4%.

82.5%

78.5%

72.5%

67.5%

82.5%

78.5%

72.5%

67.5%

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Electrical Machines

Electrical Machines DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of

Electrical Machines In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

Electrical Machines Iron loss in a transformer mainly occurs in

Electrical Machines A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because

Electrical Machines A DC shunt machine generates 250 V on open circuit at 1000 RPM. Armature resistance is 0.5 Ω, field resistance 250 Ω. When running as a motor takes 4 A at 250 V. Calculate the efficiency when running as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4%.

Electrical Machines
DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of

Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

Electrical Machines
Iron loss in a transformer mainly occurs in

Electrical Machines
A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because

Electrical Machines
A DC shunt machine generates 250 V on open circuit at 1000 RPM. Armature resistance is 0.5 Ω, field resistance 250 Ω. When running as a motor takes 4 A at 250 V. Calculate the efficiency when running as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4%.