Which statement is correct for AC machine comparative to dc machine?

Question

Question

This question was previously asked in

PGCIL NER DT (Electrical) 27 Feb 2022 Official Paper

Answer 1

The correct answer is option 2):(Motor speed control can be done by changing the frequency. )

Concept:

The most common way of controlling AC motor speed is by changing the motor's frequency.
DC motor speed control cannot be done by changing the frequency
A carbon Brush is a sliding contact used to transmit electrical current from a static to a rotating part. It present in DC Motor
The DC motor is 30% more efficient than AC motors due to the secondary magnetic field being generated from the permanent magnets rather than copper windings
The DC motors require more maintenance for those designs that use brushes and commutators which feature mechanical wear.
So option 2 is correct.