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

• between 5 to 10 mm

• between 10 to 15 mm

• between 15 to 20 mm

• between 25 to 35 mm

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

• Increase in length of air gap at pole tips

• Increasing reluctance of pole tips

• Compensating windings

• Interpoles

• All of the above

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

• quiet

• noisy

• either of the above

• none of the above

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

• increase

• decrease

• either of the above

• none of the above

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

• equal to that of armature mmf

• less in comparison with the armature mmf

• large in comparison with the armature mmf

• none of the above

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

• worst, ventilation

• poor ventilation

• better ventilation

• none of the above

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

• 10 m/s

• 20 m/s

• 30 m/s

• 40 m/s

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

• increase

• decrease

• either of the above

• none of the above

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

• less than

• equal to

• greater than

• none of the above

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

• Metal graphite brushes

• Hard carbon brushes

• Electro-graphitic brushes

• Natural graphite brushes