AC AND DC MACHINES

Back emf is the phenomena occurred in _________

• DC generator

• DC motor

• AC motor

• DC Machines

The main advantage of dc motors compared to ac motors is

• Less maintenance

• Low starting torque

• Low cost

• Speed control

In DC Motor speeds above the rated speed can be attained by varying which of the following winding current

• Field winding

• Armature winding

• Either field or armature winding

• Not possible

Starter is used in DC Motor for which of the following purpose:

• Improve efficiency

• Reduce starting current

• Improving starting torque

• Improving Back EMF

For machine tools, which DC motor can be used?

• DC Series motor

• DC Shunt motor

• DC cumulative compound motor

• DC differential compound motor

In a DC shunt motor, speed is related to armature current as _____________

• Directly proportional to the armature current

• Proportional to the square of the current

• Independent of armature current

• Inversely proportional to the armature current

If the supply frequency of a transformer increases, the secondary output voltage of the transformer

• Increase

• Decrease

• Remain the same

• Increase as a square of it

Transformer ratings are given in _____________

• HP

• KW

• KVA

• KVAR

The purpose of the transformer core is to provide ____________

• Low reluctance path

• High inductive path

• High capacitive path

• High reluctance path

Which type of flux does transformer action need?

• Alternating electric flux

• Constant magnetic flux

• Increasing magnetic flux

• Alternating magnetic flux