Which of the following factors improve commutation in a dc machine ? 1. High contact resistance of brushes 2. High inductance of the coils undergoing commutation 3. Shifting of brushes in the direction of rotation for a generator and in the opposite direction For a motor 4. Narrow width for the commutation bars Select the correct answer using the codes given below : Codes :

Electrical Machines
Which of the following factors improve commutation in a dc machine ? 1. High contact resistance of brushes 2. High inductance of the coils undergoing commutation 3. Shifting of brushes in the direction of rotation for a generator and in the opposite direction For a motor 4. Narrow width for the commutation bars Select the correct answer using the codes given below : Codes :

1 and 3

1, 2 and 4

2, 3 and 4

1, 2, and 3

1 and 3

1, 2 and 4

2, 3 and 4

1, 2, and 3

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Electrical Machines
In a loaded DC motor, if the brushes are given a shift from the interpolar axis in the direction of rotation, then the commutation will

improve and the speed rises.

deteriorate and the speed rises.

deteriorate and the speed falls.

improve and the speed falls.

improve and the speed rises.

deteriorate and the speed rises.

deteriorate and the speed falls.

improve and the speed falls.

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Electrical Machines
The air gap between the yoke and armature is dc machine is kept very small

to achieve a stronger magnetic field.

All of these

to avoid over heating.

to avoid locking of the armature.

to achieve a stronger magnetic field.

All of these

to avoid over heating.

to avoid locking of the armature.

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Electrical Machines
Compensating windings are used in DC generators

mainly to reduce the eddy currents by providing local short circuit.

None of these

to neutralize the cross magnetizing effect of the armature reaction.

to provide path for the circulation of cooling air.

mainly to reduce the eddy currents by providing local short circuit.

None of these

to neutralize the cross magnetizing effect of the armature reaction.

to provide path for the circulation of cooling air.

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Electrical Machines
In shunt dc motor the direction of rotation of the motor will reversed. When

only field terminals are changed.

none of above.

only armature terminals are reversed.

either field terminals are reversed or armature terminals are reversed.

only field terminals are changed.

none of above.

only armature terminals are reversed.

either field terminals are reversed or armature terminals are reversed.

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Electrical Machines
In DC generators, the cause of rapid brush wear may be

rough commutator surface.

any of above.

imperfect contact.

severe sparking.

rough commutator surface.

any of above.

imperfect contact.

severe sparking.

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Electrical Machines

Electrical Machines In a loaded DC motor, if the brushes are given a shift from the interpolar axis in the direction of rotation, then the commutation will

Electrical Machines The air gap between the yoke and armature is dc machine is kept very small

Electrical Machines Compensating windings are used in DC generators

Electrical Machines In shunt dc motor the direction of rotation of the motor will reversed. When

Electrical Machines In DC generators, the cause of rapid brush wear may be

Electrical Machines
In a loaded DC motor, if the brushes are given a shift from the interpolar axis in the direction of rotation, then the commutation will

Electrical Machines
The air gap between the yoke and armature is dc machine is kept very small

Electrical Machines
Compensating windings are used in DC generators

Electrical Machines
In shunt dc motor the direction of rotation of the motor will reversed. When

Electrical Machines
In DC generators, the cause of rapid brush wear may be